ATmega16U4/32U4 - Complete

ATmega16U4/ATmega32U4
8-bit Microcontroller with 16/32K bytes of ISP Flash and
USB Controller
DATASHEET
Features
• High Performance, Low Power AVR® 8-Bit Microcontroller
• Advanced RISC Architecture
•
•
•
•
– 135 Powerful Instructions – Most Single Clock Cycle Execution
– 32 x 8 General Purpose Working Registers
– Fully Static Operation
– Up to 16 MIPS Throughput at 16MHz
– On-Chip 2-cycle Multiplier
Non-volatile Program and Data Memories
– 16/32KB of In-System Self-Programmable Flash
– 1.25/2.5KB Internal SRAM
– 512Bytes/1KB Internal EEPROM
– Write/Erase Cycles: 10,000 Flash/100,000 EEPROM
– Data retention: 20 years at 85C/ 100 years at 25C(1)
– Optional Boot Code Section with Independent Lock Bits
In-System Programming by On-chip Boot Program
True Read-While-Write Operation
Parts using external XTAL clock are pre-programed with a default USB bootloader
– Programming Lock for Software Security
JTAG (IEEE® std. 1149.1 compliant) Interface
– Boundary-scan Capabilities According to the JTAG Standard
– Extensive On-chip Debug Support
– Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface
USB 2.0 Full-speed/Low Speed Device Module with Interrupt on Transfer Completion
– Complies fully with Universal Serial Bus Specification Rev 2.0
– Supports data transfer rates up to 12Mbit/s and 1.5Mbit/s
– Endpoint 0 for Control Transfers: up to 64-bytes
– Six Programmable Endpoints with IN or Out Directions and with Bulk, Interrupt or
Isochronous Transfers
– Configurable Endpoints size up to 256 bytes in double bank mode
– Fully independent 832 bytes USB DPRAM for endpoint memory allocation
– Suspend/Resume Interrupts
– CPU Reset possible on USB Bus Reset detection
– 48MHz from PLL for Full-speed Bus Operation
– USB Bus Connection/Disconnection on Microcontroller Request
– Crystal-less operation for Low Speed mode
Peripheral Features
– On-chip PLL for USB and High Speed Timer: 32 up to 96MHz operation
– One 8-bit Timer/Counter with Separate Prescaler and Compare Mode
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•
•
•
•
•
– Two 16-bit Timer/Counter with Separate Prescaler, Compare- and Capture Mode
– One 10-bit High-Speed Timer/Counter with PLL (64MHz) and Compare Mode
– Four 8-bit PWM Channels
– Four PWM Channels with Programmable Resolution from 2 to 16 Bits
– Six PWM Channels for High Speed Operation, with Programmable Resolution from 2 to 11 Bits
– Output Compare Modulator
– 12-channels, 10-bit ADC (features Differential Channels with Programmable Gain)
– Programmable Serial USART with Hardware Flow Control
– Master/Slave SPI Serial Interface
– Byte Oriented 2-wire Serial Interface
– Programmable Watchdog Timer with Separate On-chip Oscillator
– On-chip Analog Comparator
– Interrupt and Wake-up on Pin Change
– On-chip Temperature Sensor
Special Microcontroller Features
– Power-on Reset and Programmable Brown-out Detection
– Internal 8MHz Calibrated Oscillator
– Internal clock prescaler and On-the-fly Clock Switching (Int RC / Ext Osc)
– External and Internal Interrupt Sources
– Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby, and Extended Standby
I/O and Packages
– All I/O combine CMOS outputs and LVTTL inputs
– 26 Programmable I/O Lines
– 44-lead TQFP Package, 10x10mm
– 44-lead QFN Package, 7x7mm
Operating Voltages
– 2.7 - 5.5V
Operating temperature
– Industrial (-40°C to +85°C)
Maximum Frequency
– 8MHz at 2.7V - Industrial range
– 16MHz at 4.5V - Industrial range
Note:
1. See “Data Retention” on page 8 for details.
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Pin Configurations
(INT.6/AIN0) PE6
UVcc
GND
AREF
PF0 (ADC0)
PF1 (ADC1)
PF4 (ADC4/TCK)
PF5 (ADC5/TMS)
PF6 (ADC6/TDO)
PF7 (ADC7/TDI)
GND
VCC
43
42
41
40
39
38
37
36
35
34
Pinout
AVCC
Figure 1-1.
44
1.
33 PE2 (HWB)
1
)
32 PC7 (ICP3/CLK0/OC4A
2
INDEX C
ORNER
D-
3
(
)
31 PC6 OC3A/OC4A
D+
4
30 PB6 (PCINT6/OC1B/OC4B/ADC
UGnd
5
29
UCap
6
VBus
7
20
21
(RXD1/INT2) PD2
(TXD1/INT3) PD3
(XCK1/CTS) PD5
22
19
(SDA/INT1) PD1
(OC0B/SCL/INT0) PD0
(PCINT7/OC0A/OC1C/RTS) PB7
2.
18
GND
17
23
XTAL1
11
16
AVCC
XTAL2
24
15
10
GND
25 PD4 (ICP1/ADC8)
14
9
12
(PDO/PCINT3/MISO
) PB3
26 PD6 (T1/OC4D/ADC9)
VCC
(PDI/PCINT2/MOSI) PB2
27 PD7 (T0/OC4D/ADC10)
8
13
(PCINT1/SCLK
) PB1
28 PB4 (PCINT4/ADC11)
ATmega32U4
ATmega16U4
44-pin QFN/T
QFP
RESET
(SS/PCINT0) PB0
PB5 (PCINT5/OC1A/OC4B/ADC
Overview
The ATmega16U4/ATmega32U4 is a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC
architecture. By executing powerful instructions in a single clock cycle, the device achieves throughputs
approaching 1 MIPS per MHz allowing the system designer to optimize power consumption versus processing
speed.
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Block Diagram
PF7 - PF4
VCC
PC7 PC6
PF1 PF0
PORTF DRIVERS
RESET
Block Diagram
XTAL2
Figure 2-1.
XTAL1
2.1
PORTC DRIVERS
GND
DATA REGISTER
PORTF
DATA DIR.
REG. PORTF
DATA REGISTER
PORTC
DATA DIR.
REG. PORTC
8-BIT DA TA BUS
POR - BOD
RESET
JTAG TAP
PROGRAM
COUNTER
STACK
POINTER
ON-CHIP DEBUG
PROGRAM
FLASH
SRAM
BOUNDARYSCAN
INSTRUCTION
REGISTER
INTERNAL
OSCILLATOR
INSTRUCTION
DECODER
TEMPERATURE
SENSOR
CONTROL
LINES
TIMING AND
CONTROL
MCU CONTROL
REGISTER
TIMERS/
COUNTERS
GENERAL
PURPOSE
REGISTERS
INTERRUPT
UNIT
UVcc
Y
Z
ON-CHIP
USB PAD 3V
REGULATOR
EEPROM
ALU
PLL
HIGH SPEED
ADC
TIMER/PWM
STATUS
REGISTER
AREF
UCap
1uF
AVCC
AGND
OSCILLATOR
WATCHDOG
TIMER
X
PROGRAMMING
LOGIC
CALIB. OSC
VBUS
DP
USB 2.0
ANALOG
COMPARATOR
DATA REGISTER
PORTE
DATA DIR.
REG. PORTE
PORTE DRIVERS
PE6
PE2
DATA REGISTER
PORTB
DATA DIR.
REG. PORTB
PORTB DRIVERS
PB7 - PB0
DM
TWO-WIRE SERIAL
INTERFACE
SPI
USART1
DATA REGISTER
PORTD
DATA DIR.
REG. PORTD
PORTD DRIVERS
PD7 - PD0
The AVR core combines a rich instruction set with 32 general purpose working registers. All the 32 registers are
directly connected to the Arithmetic Logic Unit (ALU), allowing two independent registers to be accessed in one
single instruction executed in one clock cycle. The resulting architecture is more code efficient while achieving
throughputs up to ten times faster than conventional CISC microcontrollers.
The device provides the following features: 16/32K bytes of In-System Programmable Flash with Read-WhileWrite capabilities, 512Bytes/1K bytes EEPROM, 1.25/2.5K bytes SRAM, 26 general purpose I/O lines (CMOS
outputs and LVTTL inputs), 32 general purpose working registers, four flexible Timer/Counters with compare
modes and PWM, one more high-speed Timer/Counter with compare modes and PLL adjustable source, one
USART (including CTS/RTS flow control signals), a byte oriented 2-wire Serial Interface, a 12-channels 10-bit
ADC with optional differential input stage with programmable gain, an on-chip calibrated temperature sensor, a
programmable Watchdog Timer with Internal Oscillator, an SPI serial port, IEEE std. 1149.1 compliant JTAG
test interface, also used for accessing the On-chip Debug system and programming and six software selectable
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power saving modes. The Idle mode stops the CPU while allowing the SRAM, Timer/Counters, SPI port, and
interrupt system to continue functioning. The Power-down mode saves the register contents but freezes the
Oscillator, disabling all other chip functions until the next interrupt or Hardware Reset. The ADC Noise
Reduction mode stops the CPU and all I/O modules except ADC, to minimize switching noise during ADC
conversions. In Standby mode, the Crystal/Resonator Oscillator is running while the rest of the device is
sleeping. This allows very fast start-up combined with low power consumption.
The device is manufactured using the Atmel® high-density nonvolatile memory technology. The On-chip ISP
Flash allows the program memory to be reprogrammed in-system through an SPI serial interface, by a
conventional nonvolatile memory programmer, or by an On-chip Boot program running on the AVR core. The
boot program can use any interface to download the application program in the application Flash memory.
Software in the Boot Flash section will continue to run while the Application Flash section is updated, providing
true Read-While-Write operation. By combining an 8-bit RISC CPU with In-System Self-Programmable Flash on
a monolithic chip, the device is a powerful microcontroller that provides a highly flexible and cost effective
solution to many embedded control applications.
The ATmega16U4/ATmega32U4 AVR is supported with a full suite of program and system development tools
including: C compilers, macro assemblers, program debugger/simulators, in-circuit emulators, and evaluation
kits.
2.2
Pin Descriptions
2.2.1
VCC
Digital supply voltage.
2.2.2
GND
Ground.
2.2.3
Port B (PB7..PB0)
Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port B output
buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port B pins
that are externally pulled low will source current if the pull-up resistors are activated. The Port B pins are tristated when a reset condition becomes active, even if the clock is not running.
Port B has better driving capabilities than the other ports.
Port B also serves the functions of various special features of the device as listed on page 74.
2.2.4
Port C (PC7,PC6)
Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port C output
buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port C pins
that are externally pulled low will source current if the pull-up resistors are activated. The Port C pins are tristated when a reset condition becomes active, even if the clock is not running.
Only bits 6 and 7 are present on the product pinout.
Port C also serves the functions of special features of the device as listed on page 77.
2.2.5
Port D (PD7..PD0)
Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port D output
buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port D pins
that are externally pulled low will source current if the pull-up resistors are activated. The Port D pins are tristated when a reset condition becomes active, even if the clock is not running.
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Port D also serves the functions of various special features of the ATmega16U4/ATmega32U4 as listed on
page 78.
2.2.6
Port E (PE6,PE2)
Port E is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port E output
buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port E pins
that are externally pulled low will source current if the pull-up resistors are activated. The Port E pins are tristated when a reset condition becomes active, even if the clock is not running.
Only bits 2 and 6 are present on the product pinout.
Port E also serves the functions of various special features of the ATmega16U4/ATmega32U4 as listed on
page 81.
2.2.7
Port F (PF7..PF4, PF1,PF0)
Port F serves as analog inputs to the A/D Converter.
Port F also serves as an 8-bit bi-directional I/O port, if the A/D Converter channels are not used. Port pins can
provide internal pull-up resistors (selected for each bit). The Port F output buffers have symmetrical drive
characteristics with both high sink and source capability. As inputs, Port F pins that are externally pulled low will
source current if the pull-up resistors are activated. The Port F pins are tri-stated when a reset condition
becomes active, even if the clock is not running.
Bits 2 and 3 are not present on the product pinout.
Port F also serves the functions of the JTAG interface. If the JTAG interface is enabled, the pull-up resistors on
pins PF7(TDI), PF5(TMS), and PF4(TCK) will be activated even if a reset occurs.
2.2.8
DUSB Full speed / Low Speed Negative Data Upstream Port. Should be connected to the USB D- connector pin
with a serial 22 resistor.
2.2.9
D+
USB Full speed / Low Speed Positive Data Upstream Port. Should be connected to the USB D+ connector pin
with a serial 22 resistor.
2.2.10 UGND
USB Pads Ground.
2.2.11 UVCC
USB Pads Internal Regulator Input supply voltage.
2.2.12 UCAP
USB Pads Internal Regulator Output supply voltage. Should be connected to an external capacitor (1µF).
2.2.13 VBUS
USB VBUS monitor input.
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2.2.14 RESET
Reset input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the
clock is not running. The minimum pulse length is given in Table 8-2 on page 53. Shorter pulses are not
guaranteed to generate a reset.
2.2.15 XTAL1
Input to the inverting Oscillator amplifier and input to the internal clock operating circuit.
2.2.16 XTAL2
Output from the inverting Oscillator amplifier.
2.2.17 AVCC
AVCC is the supply voltage pin (input) for all the A/D Converter channels. If the ADC is not used, it should be
externally connected to VCC. If the ADC is used, it should be connected to VCC through a low-pass filter.
2.2.18 AREF
This is the analog reference pin (input) for the A/D Converter.
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3.
About
3.1
Disclaimer
Typical values contained in this datasheet are based on simulations and characterization of other AVR
microcontrollers manufactured on the same process technology. Min. and Max. values will be available after the
device is characterized.
3.2
Resources
A comprehensive set of development tools, application notes and datasheets are available for download on
http://www.atmel.com/avr.
3.3
Code Examples
This documentation contains simple code examples that briefly show how to use various parts of the device. Be
aware that not all C compiler vendors include bit definitions in the header files and interrupt handling in C is
compiler dependent. Confirm with the C compiler documentation for more details.
These code examples assume that the part specific header file is included before compilation. For I/O registers
located in extended I/O map, "IN", "OUT", "SBIS", "SBIC", "CBI", and "SBI" instructions must be replaced with
instructions that allow access to extended I/O. Typically "LDS" and "STS" combined with "SBRS", "SBRC",
"SBR", and "CBR".
3.4
Data Retention
Reliability Qualification results show that the projected data retention failure rate is much less than 1PPM over
20 years at 85°C or 100 years at 25°C.
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4.
AVR CPU Core
4.1
Introduction
This section discusses the AVR core architecture in general. The main function of the CPU core is to ensure
correct program execution. The CPU must therefore be able to access memories, perform calculations, control
peripherals, and handle interrupts.
Architectural Overview
Figure 4-1.
Block Diagram of the AVR Architecture
Data Bus 8-bit
Flash
Program
Memory
Program
Counter
Status
and Control
32 x 8
General
Purpose
Registrers
Instruction
Decoder
Control Lines
Indirect Addressing
Instruction
Register
Direct Addressing
4.2
Interrupt
Unit
SPI
Unit
Watchdog
Timer
ALU
Analog
Comparator
I/O Module1
Data
SRAM
I/O Module 2
I/O Module n
EEPROM
I/O Lines
In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with separate
memories and buses for program and data. Instructions in the program memory are executed with a single level
pipelining. While one instruction is being executed, the next instruction is pre-fetched from the program memory.
This concept enables instructions to be executed in every clock cycle. The program memory is In-System
Reprogrammable Flash memory.
The fast-access Register File contains 32 x 8-bit general purpose working registers with a single clock cycle
access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typical ALU operation, two
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operands are output from the Register File, the operation is executed, and the result is stored back in the
Register File – in one clock cycle.
Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data Space addressing
– enabling efficient address calculations. One of the these address pointers can also be used as an address
pointer for look up tables in Flash program memory. These added function registers are the 16-bit X-, Y-, and Zregister, described later in this section.
The ALU supports arithmetic and logic operations between registers or between a constant and a register.
Single register operations can also be executed in the ALU. After an arithmetic operation, the Status Register is
updated to reflect information about the result of the operation.
Program flow is provided by conditional and unconditional jump and call instructions, able to directly address the
whole address space. Most AVR instructions have a single 16-bit word format. Every program memory address
contains a 16- or 32-bit instruction.
Program Flash memory space is divided in two sections, the Boot Program section and the Application Program
section. Both sections have dedicated Lock bits for write and read/write protection. The SPM instruction that
writes into the Application Flash memory section must reside in the Boot Program section.
During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the Stack. The
Stack is effectively allocated in the general data SRAM, and consequently the Stack size is only limited by the
total SRAM size and the usage of the SRAM. All user programs must initialize the SP in the Reset routine
(before subroutines or interrupts are executed). The Stack Pointer (SP) is read/write accessible in the I/O space.
The data SRAM can easily be accessed through the five different addressing modes supported in the AVR
architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional Global Interrupt Enable bit
in the Status Register. All interrupts have a separate Interrupt Vector in the Interrupt Vector table. The interrupts
have priority in accordance with their Interrupt Vector position. The lower the Interrupt Vector address, the
higher the priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, SPI, and other
I/O functions. The I/O Memory can be accessed directly, or as the Data Space locations following those of the
Register File, 0x20 - 0x5F. In addition, the ATmega16U4/ATmega32U4 has Extended I/O space from 0x60 0x0FF in SRAM where only the ST/STS/STD and LD/LDS/LDD instructions can be used.
4.3
ALU – Arithmetic Logic Unit
The high-performance AVR ALU operates in direct connection with all the 32 general purpose working registers.
Within a single clock cycle, arithmetic operations between general purpose registers or between a register and
an immediate are executed. The ALU operations are divided into three main categories – arithmetic, logical, and
bit-functions. Some implementations of the architecture also provide a powerful multiplier supporting both
signed/unsigned multiplication and fractional format. See “Instruction Set Summary” on page 418 for a detailed
description.
4.4
Status Register
The Status Register contains information about the result of the most recently executed arithmetic instruction.
This information can be used for altering program flow in order to perform conditional operations. Note that the
Status Register is updated after all ALU operations, as specified in the Instruction Set Reference. This will in
many cases remove the need for using the dedicated compare instructions, resulting in faster and more
compact code.
The Status Register is not automatically stored when entering an interrupt routine and restored when returning
from an interrupt. This must be handled by software.
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The AVR Status Register – SREG – is defined as:
Bit
Read/Write
Initial Value
7
I
R/W
0
6
T
R/W
0
5
H
R/W
0
4
S
R/W
0
3
V
R/W
0
2
N
R/W
0
1
Z
R/W
0
0
C
R/W
0
SREG
• Bit 7 – I: Global Interrupt Enable
The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual interrupt enable
control is then performed in separate control registers. If the Global Interrupt Enable Register is cleared, none of
the interrupts are enabled independent of the individual interrupt enable settings. The I-bit is cleared by
hardware after an interrupt has occurred, and is set by the RETI instruction to enable subsequent interrupts.
The I-bit can also be set and cleared by the application with the SEI and CLI instructions, as described in the
instruction set reference.
• Bit 6 – T: Bit Copy Storage
The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or destination for the
operated bit. A bit from a register in the Register File can be copied into T by the BST instruction, and a bit in T
can be copied into a bit in a register in the Register File by the BLD instruction.
• Bit 5 – H: Half Carry Flag
The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry Is useful in BCD
arithmetic. See “Instruction Set Summary” on page 418 for detailed information.
• Bit 4 – S: Sign Bit, S = N V
The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement Overflow Flag V.
See “Instruction Set Summary” on page 418 for detailed information.
• Bit 3 – V: Two’s Complement Overflow Flag
The Two’s Complement Overflow Flag V supports two’s arithmetic complements. See “Instruction Set
Summary” on page 418 for detailed information.
• Bit 2 – N: Negative Flag
The Negative Flag N indicates a negative result in an arithmetic or logic operation. See “Instruction Set
Summary” on page 418 for detailed information.
• Bit 1 – Z: Zero Flag
The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See “Instruction Set Summary” on
page 418 for detailed information.
• Bit 0 – C: Carry Flag
The Carry Flag C indicates a carry in an arithmetic or logic operation. See “Instruction Set Summary” on
page 418 for detailed information.
4.5
General Purpose Register File
The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve the required
performance and flexibility, the following input/output schemes are supported by the Register File:

One 8-bit output operand and one 8-bit result input

Two 8-bit output operands and one 8-bit result input
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
Two 8-bit output operands and one 16-bit result input

One 16-bit output operand and one 16-bit result input
Figure 4-2 shows the structure of the 32 general purpose working registers in the CPU.
Figure 4-2.
AVR CPU General Purpose Working Registers
7
General
Purpose
Working
Registers
R0
R1
R2
…
R13
R14
R15
R16
R17
…
R26
R27
R28
R29
R30
R31
0
Addr.
0x00
0x01
0x02
0x0D
0x0E
0x0F
0x10
0x11
0x1A
0x1B
0x1C
0x1D
0x1E
0x1F
X-register Low Byte
X-register High Byte
Y-register Low Byte
Y-register High Byte
Z-register Low Byte
Z-register High Byte
Most of the instructions operating on the Register File have direct access to all registers, and most of them are
single cycle instructions.
As shown in Figure 4-2, each register is also assigned a data memory address, mapping them directly into the
first 32 locations of the user Data Space. Although not being physically implemented as SRAM locations, this
memory organization provides great flexibility in access of the registers, as the X-, Y-, and Z-pointer registers
can be set to index any register in the file.
4.5.1
The X-register, Y-register, and Z-register
The registers R26..R31 have some added functions to their general purpose usage. These registers are 16-bit
address pointers for indirect addressing of the data space. The three indirect address registers X, Y, and Z are
defined as described in Figure 4-3.
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Figure 4-3.
The X-, Y-, and Z-registers
15
7
R27 (0x1B)
XH
X-register
15
7
R29 (0x1D)
YH
Y-register
Z-register
15
7
R31 (0x1F)
ZH
0
0
7
R26 (0x1A)
0
7
R28 (0x1C)
XL
0
0
YL
0
0
ZL
7
R30 (0x1E)
0
0
In the different addressing modes these address registers have functions as fixed displacement, automatic
increment, and automatic decrement (See “Instruction Set Summary” on page 418 for detailed information).
4.6
Stack Pointer
The Stack is mainly used for storing temporary data, for storing local variables and for storing return addresses
after interrupts and subroutine calls. The Stack Pointer Register always points to the top of the Stack. Note that
the Stack is implemented as growing from higher memory locations to lower memory locations. This implies that
a Stack PUSH command decreases the Stack Pointer.
The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt Stacks are located.
This Stack space in the data SRAM must be defined by the program before any subroutine calls are executed or
interrupts are enabled. The Stack Pointer must be set to point above 0x0100. The initial value of the stack
pointer is the last address of the internal SRAM. The Stack Pointer is decremented by one when data is pushed
onto the Stack with the PUSH instruction, and it is decremented by three when the return address is pushed
onto the Stack with subroutine call or interrupt. The Stack Pointer is incremented by one when data is popped
from the Stack with the POP instruction, and it is incremented by three when data is popped from the Stack with
return from subroutine RET or return from interrupt RETI.
The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually used
is implementation dependent. Note that the data space in some implementations of the AVR architecture is so
small that only SPL is needed. In this case, the SPH Register will not be present.
Bit
Read/Write
Initial Value
15
SP15
SP7
7
R/W
R/W
0
1
14
SP14
SP6
6
R/W
R/W
0
1
13
SP13
SP5
5
R/W
R/W
1
1
12
SP12
SP4
4
R/W
R/W
0
1
11
SP11
SP3
3
R/W
R/W
0
1
10
SP10
SP2
2
R/W
R/W
0
1
9
SP9
SP1
1
R/W
R/W
0
1
8
SP8
SP0
0
R/W
R/W
0
1
SPH
SPL
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4.6.1
Extended Z-pointer Register for ELPM/SPM - RAMPZ
Bit
Read/Write
Initial Value
7
RAMPZ7
R/W
0
6
RAMPZ6
R/W
0
5
RAMPZ5
R/W
0
4
RAMPZ4
R/W
0
3
RAMPZ3
R/W
0
2
RAMPZ2
R/W
0
1
RAMPZ1
R/W
0
0
RAMPZ0
R/W
0
RAMPZ
For ELPM/SPM instructions, the Z-pointer is a concatenation of RAMPZ, ZH, and ZL, as shown in Figure 4-4.
Note that LPM is not affected by the RAMPZ setting.
Figure 4-4.
Bit (Individually)
Bit (Z-pointer)
The Z-pointer used by ELPM and SPM
7
RAMPZ
23
0
16
7
ZH
15
0
7
ZL
7
8
0
0
The actual number of bits is implementation dependent. Unused bits in an implementation will always read as
zero. For compatibility with future devices, be sure to write these bits to zero.
4.7
Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The AVR CPU is driven by
the CPU clock clkCPU, directly generated from the selected clock source for the chip. No internal clock division is
used.
Figure 4-5 shows the parallel instruction fetches and instruction executions enabled by the Harvard architecture
and the fast-access Register File concept. This is the basic pipelining concept to obtain up to 1 MIPS per MHz
with the corresponding unique results for functions per cost, functions per clocks, and functions per power-unit.
Figure 4-5.
The Parallel Instruction Fetches and Instruction Executions
T1
T2
T3
T4
clkCPU
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
Figure 4-6 shows the internal timing concept for the Register File. In a single clock cycle an ALU operation using
two register operands is executed, and the result is stored back to the destination register.
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Figure 4-6.
Single Cycle ALU Operation
T1
T2
T3
T4
clkCPU
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
4.8
Reset and Interrupt Handling
The AVR provides several different interrupt sources. These interrupts and the separate Reset Vector each
have a separate program vector in the program memory space. All interrupts are assigned individual enable bits
which must be written logic one together with the Global Interrupt Enable bit in the Status Register in order to
enable the interrupt. Depending on the Program Counter value, interrupts may be automatically disabled when
Boot Lock bits BLB02 or BLB12 are programmed. This feature improves software security. See the section
“Memory Programming” on page 353 for details.
The lowest addresses in the program memory space are by default defined as the Reset and Interrupt Vectors.
The complete list of vectors is shown in “Interrupts” on page 63. The list also determines the priority levels of the
different interrupts. The lower the address the higher is the priority level. RESET has the highest priority, and
next is INT0 – the External Interrupt Request 0. The Interrupt Vectors can be moved to the start of the Boot
Flash section by setting the IVSEL bit in the MCU Control Register (MCUCR). Refer to “Interrupts” on page 63
for more information. The Reset Vector can also be moved to the start of the Boot Flash section by
programming the BOOTRST Fuse, see “Memory Programming” on page 353.
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. The user
software can write logic one to the I-bit to enable nested interrupts. All enabled interrupts can then interrupt the
current interrupt routine. The I-bit is automatically set when a Return from Interrupt instruction – RETI – is
executed.
There are basically two types of interrupts. The first type is triggered by an event that sets the Interrupt Flag. For
these interrupts, the Program Counter is vectored to the actual Interrupt Vector in order to execute the interrupt
handling routine, and hardware clears the corresponding Interrupt Flag. Interrupt Flags can also be cleared by
writing a logic one to the flag bit position(s) to be cleared. If an interrupt condition occurs while the
corresponding interrupt enable bit is cleared, the Interrupt Flag will be set and remembered until the interrupt is
enabled, or the flag is cleared by software. Similarly, if one or more interrupt conditions occur while the Global
Interrupt Enable bit is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the Global
Interrupt Enable bit is set, and will then be executed by order of priority.
The second type of interrupts will trigger as long as the interrupt condition is present. These interrupts do not
necessarily have Interrupt Flags. If the interrupt condition disappears before the interrupt is enabled, the
interrupt will not be triggered.
When the AVR exits from an interrupt, it will always return to the main program and execute one more
instruction before any pending interrupt is served.
Note that the Status Register is not automatically stored when entering an interrupt routine, nor restored when
returning from an interrupt routine. This must be handled by software.
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When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled. No interrupt will
be executed after the CLI instruction, even if it occurs simultaneously with the CLI instruction. The following
example shows how this can be used to avoid interrupts during the timed EEPROM write sequence.
Assembly Code Example
in
r16, SREG
;
store SREG value
cli
; disable interrupts during
timed sequence
sbi
EECR, EEMPE
;
start EEPROM write
sbi
EECR, EEPE
out
SREG, r16
;
restore SREG value (I-bit)
C Code Example
char cSREG;
cSREG = SREG;
store SREG value */
/* disable interrupts during timed sequence */
__disable_interrupt();
EECR |= (1<<EEMPE); /* start EEPROM write */
EECR |= (1<<EEPE);
SREG = cSREG; /* restore SREG value (I-bit) */
/*
When using the SEI instruction to enable interrupts, the instruction following SEI will be executed before any
pending interrupts, as shown in this example.
Assembly Code Example
sei
; set Global Interrupt Enable
sleep
; enter sleep, waiting for interrupt
; note: will enter sleep before any pending
; interrupt(s)
C Code Example
__enable_interrupt(); /* set Global Interrupt Enable */
__sleep(); /* enter sleep, waiting for interrupt */
/* note: will enter sleep before any pending
interrupt(s) */
4.8.1
Interrupt Response Time
The interrupt execution response for all the enabled AVR interrupts is five clock cycles minimum. After five clock
cycles the program vector address for the actual interrupt handling routine is executed. During these five clock
cycle period, the Program Counter is pushed onto the Stack. The vector is normally a jump to the interrupt
routine, and this jump takes three clock cycles. If an interrupt occurs during execution of a multi-cycle
instruction, this instruction is completed before the interrupt is served. If an interrupt occurs when the MCU is in
sleep mode, the interrupt execution response time is increased by five clock cycles. This increase comes in
addition to the start-up time from the selected sleep mode.
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A return from an interrupt handling routine takes five clock cycles. During these five clock cycles, the Program
Counter (three bytes) is popped back from the Stack, the Stack Pointer is incremented by three, and the I-bit in
SREG is set.
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5.
AVR Memories
This section describes the different memories in the device. The AVR architecture has two main memory
spaces, the Data Memory and the Program Memory space. In addition, the device features an EEPROM
Memory for data storage. All three memory spaces are linear and regular.
Table 5-1.
Memory Mapping
Memory
Mnemonic
Size
Start Address
Flash
End Address
32 Registers
I/O Registers
Ext I/O Registers
Internal SRAM
Notes:
5.1
1.
2.
ATmega16U4
32KB
16KB
0x7FFF(1)
0x3FFF(1)
0x3FFF(2)
0x1FFF(2)
Flash size
0x0000
Flash end
Size
-
32 bytes
32 bytes
Start Address
-
0x0000
0x0000
End Address
-
0x001F
0x001F
Size
-
64 bytes
64 bytes
Start Address
-
0x0020
0x0020
End Address
-
0x005F
0x005F
Size
-
160 bytes
160 bytes
Start Address
-
0x0060
0x0060
End Address
-
0x00FF
0x00FF
Size
ISRAM size
2.5KB
1.25KB
Start Address
ISRAM start
0x100
0x100
End Address
ISRAM end
0x0AFF
0x05FF
External Memory
EEPROM
ATmega32U4
Not Present.
Size
E2 size
1KB
512 bytes
End Address
E2 end
0x03FF
0x01FF
Byte address.
Word (16-bit) address.
In-System Reprogrammable Flash Program Memory
The device contains 16/32K bytes On-chip In-System Reprogrammable Flash memory for program storage.
Since all AVR instructions are 16 or 32 bits wide, the Flash is organized as 16K x 16. For software security, the
Flash Program memory space is divided into two sections, Boot Program section and Application Program
section.
The Flash memory has an endurance of at least 100,000 write/erase cycles. The device Program Counter (PC)
is 16 bits wide, thus addressing the 32K program memory locations. The operation of Boot Program section and
associated Boot Lock bits for software protection are described in detail in “Memory Programming” on
page 353. “Memory Programming” on page 353 contains a detailed description on Flash data serial
downloading using the SPI pins or the JTAG interface.
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Constant tables can be allocated within the entire program memory address space (see the LPM – Load
Program Memory instruction description and ELPM - Extended Load Program Memory instruction description).
Timing diagrams for instruction fetch and execution are presented in “Instruction Execution Timing” on page 14.
Figure 5-1.
Program Memory Map
Program Memory
0x00000
Application Flash Section
Boot Flash Section
0x7FFF (32KBytes)
5.2
SRAM Data Memory
Figure 5-2 on page 20 shows how the device SRAM Memory is organized.
The device is a complex microcontroller with more peripheral units than can be supported within the 64 location
reserved in the Opcode for the IN and OUT instructions. For the Extended I/O space from $060 - $0FF in
SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
The first 2,816 Data Memory locations address both the Register File, the I/O Memory, Extended I/O Memory,
and the internal data SRAM. The first 32 locations address the Register file, the next 64 location the standard
I/O Memory, then 160 locations of Extended I/O memory and the next 2,560 locations address the internal data
SRAM.
The five different addressing modes for the data memory cover: Direct, Indirect with Displacement, Indirect,
Indirect with Pre-decrement, and Indirect with Post-increment. In the Register file, registers R26 to R31 feature
the indirect addressing pointer registers.
The direct addressing reaches the entire data space.
The Indirect with Displacement mode reaches 63 address locations from the base address given by the Y- or Zregister.
When using register indirect addressing modes with automatic pre-decrement and post-increment, the address
registers X, Y, and Z are decremented or incremented.
The 32 general purpose working registers, 64 I/O registers, and the 1.25/2.5Kbytes of internal data SRAM in the
device are all accessible through all these addressing modes. The Register File is described in “General
Purpose Register File” on page 11.
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Figure 5-2.
Data Memory Map
Data Memory
32 Registers
64 I/O Registers
160 E xt I/O Reg.
$0000 - $001F
$0020 - $005F
$0060 - $00FF
ISRAM start : $0100
Internal S RAM
ISRAM end : $05FF / $0AFF
$FFFF
5.2.1
Data Memory Access Times
This section describes the general access timing concepts for internal memory access. The internal data SRAM
access is performed in two clkCPU cycles as described in Figure 5-3.
Figure 5-3.
On-chip Data SRAM Access Cycles
T1
T2
T3
clkCPU
Address
Compute Address
Address valid
Write
Data
WR
Read
Data
RD
Memory Access Instruction
5.3
Next Instruction
EEPROM Data Memory
The device contains 512Bytes/1K bytes of data EEPROM memory. It is organized as a separate data space, in
which single bytes can be read and written. The EEPROM has an endurance of at least 100,000 write/erase
cycles. The access between the EEPROM and the CPU is described in the following, specifying the EEPROM
Address Registers, the EEPROM Data Register, and the EEPROM Control Register.
For a detailed description of SPI, JTAG and Parallel data downloading to the EEPROM, see page 367,
page 371, and page 356 respectively.
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5.3.1
EEPROM Read/Write Access
The EEPROM Access Registers are accessible in the I/O space.
The write access time for the EEPROM is given in Table 5-3 on page 23. A self-timing function, however, lets
the user software detect when the next byte can be written. If the user code contains instructions that write the
EEPROM, some precautions must be taken. In heavily filtered power supplies, VCC is likely to rise or fall slowly
on power-up/down. This causes the device for some period of time to run at a voltage lower than specified as
minimum for the clock frequency used. See “Preventing EEPROM Corruption” on page 25. for details on how to
avoid problems in these situations.
In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. Refer to the
description of the EEPROM Control Register for details on this.
When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is executed.
When the EEPROM is written, the CPU is halted for two clock cycles before the next instruction is executed.
5.3.2
The EEPROM Address Register – EEARH and EEARL
Bit
Read/Write
Initial Value
15
–
EEAR7
7
R
R/W
0
X
14
–
EEAR6
6
R
R/W
0
X
13
–
EEAR5
5
R
R/W
0
X
12
–
EEAR4
4
R
R/W
0
X
11
EEAR11
EEAR3
3
R/W
R/W
X
X
10
EEAR10
EEAR2
2
R/W
R/W
X
X
9
EEAR9
EEAR1
1
R/W
R/W
X
X
8
EEAR8
EEAR0
0
R/W
R/W
X
X
EEARH
EEARL
• Bits 15..12 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
• Bits 11..0 – EEAR8..0: EEPROM Address
The EEPROM Address Registers – EEARH and EEARL specify the EEPROM address in the 512Bytes/1K
bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and E2_END. The initial
value of EEAR is undefined. A proper value must be written before the EEPROM may be accessed.
5.3.3
The EEPROM Data Register – EEDR
Bit
Read/Write
Initial Value
7
MSB
R/W
0
6
5
4
3
2
1
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
0
LSB
R/W
0
EEDR
• Bits 7..0 – EEDR7.0: EEPROM Data
For the EEPROM write operation, the EEDR Register contains the data to be written to the EEPROM in the
address given by the EEAR Register. For the EEPROM read operation, the EEDR contains the data read out
from the EEPROM at the address given by EEAR.
5.3.4
The EEPROM Control Register – EECR
Bit
Read/Write
Initial Value
7
–
R
0
6
–
R
0
5
EEPM1
R/W
X
4
EEPM0
R/W
X
3
EERIE
R/W
0
2
EEMPE
R/W
0
1
EEPE
R/W
X
0
EERE
R/W
0
EECR
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• Bits 7..6 – Res: Reserved Bits
These bits are reserved and will always read as zero.
• Bits 5, 4 – EEPM1 and EEPM0: EEPROM Programming Mode Bits
The EEPROM Programming mode bit setting defines which programming action that will be triggered when
writing EEPE. It is possible to program data in one atomic operation (erase the old value and program the new
value) or to split the Erase and Write operations in two different operations. The Programming times for the
different modes are shown in the table below. While EEPE is set, any write to EEPMn will be ignored. During
reset, the EEPMn bits will be reset to 0b00 unless the EEPROM is busy programming.
Table 5-2.
EEPROM Mode Bits
EEPM1
EEPM0
Programming Time
Operation
0
0
3.4ms
Erase and Write in one operation (Atomic Operation)
0
1
1.8ms
Erase Only
1
0
1.8ms
Write Only
1
1
–
Reserved for future use
• Bit 3 – EERIE: EEPROM Ready Interrupt Enable
Writing EERIE to one enables the EEPROM Ready Interrupt if the I bit in SREG is set. Writing EERIE to zero
disables the interrupt. The EEPROM Ready interrupt generates a constant interrupt when EEPE is cleared.
• Bit 2 – EEMPE: EEPROM Master Programming Enable
The EEMPE bit determines whether setting EEPE to one causes the EEPROM to be written. When EEMPE is
set, setting EEPE within four clock cycles will write data to the EEPROM at the selected address If EEMPE is
zero, setting EEPE will have no effect. When EEMPE has been written to one by software, hardware clears the
bit to zero after four clock cycles. See the description of the EEPE bit for an EEPROM write procedure.
• Bit 1 – EEPE: EEPROM Programming Enable
The EEPROM Write Enable Signal EEPE is the write strobe to the EEPROM. When address and data are
correctly set up, the EEPE bit must be written to one to write the value into the EEPROM. The EEMPE bit must
be written to one before a logical one is written to EEPE, otherwise no EEPROM write takes place. The
following procedure should be followed when writing the EEPROM (the order of steps 3 and 4 is not essential):
1. Wait until EEPE becomes zero.
2.
Wait until SELFPRGEN in SPMCSR becomes zero.
3.
Write new EEPROM address to EEAR (optional).
4.
Write new EEPROM data to EEDR (optional).
5.
Write a logical one to the EEMPE bit while writing a zero to EEPE in EECR.
6.
Within four clock cycles after setting EEMPE, write a logical one to EEPE.
The EEPROM can not be programmed during a CPU write to the Flash memory. The software must check that
the Flash programming is completed before initiating a new EEPROM write. Step 2 is only relevant if the
software contains a Boot Loader allowing the CPU to program the Flash. If the Flash is never being updated by
the CPU, step 2 can be omitted. See “Memory Programming” on page 353 for details about Boot programming.
Caution: An interrupt between step 5 and step 6 will make the write cycle fail, since the EEPROM Master Write
Enable will time-out. If an interrupt routine accessing the EEPROM is interrupting another EEPROM access, the
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EEAR or EEDR Register will be modified, causing the interrupted EEPROM access to fail. It is recommended to
have the Global Interrupt Flag cleared during all the steps to avoid these problems.
When the write access time has elapsed, the EEPE bit is cleared by hardware. The user software can poll this
bit and wait for a zero before writing the next byte. When EEPE has been set, the CPU is halted for two cycles
before the next instruction is executed.
• Bit 0 – EERE: EEPROM Read Enable
The EEPROM Read Enable Signal EERE is the read strobe to the EEPROM. When the correct address is set
up in the EEAR Register, the EERE bit must be written to a logic one to trigger the EEPROM read. The
EEPROM read access takes one instruction, and the requested data is available immediately. When the
EEPROM is read, the CPU is halted for four cycles before the next instruction is executed.
The user should poll the EEPE bit before starting the read operation. If a write operation is in progress, it is
neither possible to read the EEPROM, nor to change the EEAR Register.
The calibrated Oscillator is used to time the EEPROM accesses. The following table lists the typical
programming time for EEPROM access from the CPU.
Table 5-3.
EEPROM Programming Time
Symbol
EEPROM write
(from CPU)
Number of Calibrated RC Oscillator Cycles
Typ Programming Time
26,368
3.3ms
The following code examples show one assembly and one C function for writing to the EEPROM. The examples
assume that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts will occur during
execution of these functions. The examples also assume that no Flash Boot Loader is present in the software. If
such code is present, the EEPROM write function must also wait for any ongoing SPM command to finish.
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Assembly Code Example(1)
EEPROM_write:
; Wait for completion of previous write
sbic
EECR,EEPE
rjmp
EEPROM_write
; Set up address (r18:r17) in address register
out
EEARH, r18
out
EEARL, r17
; Write data (r16) to Data Register
out
EEDR,r16
; Write logical one to EEMPE
sbi
EECR,EEMPE
; Start eeprom write by setting EEPE
sbi
EECR,EEPE
ret
C Code Example(1)
void EEPROM_write(unsigned int uiAddress, unsigned char
ucData)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set up address and Data Registers */
EEAR = uiAddress;
EEDR = ucData;
/* Write logical one to EEMPE */
EECR |= (1<<EEMPE);
/* Start eeprom write by setting EEPE */
EECR |= (1<<EEPE);
}
Note:
1.
See “Code Examples” on page 8.
The next code examples show assembly and C functions for reading the EEPROM. The examples assume that
interrupts are controlled so that no interrupts will occur during execution of these functions.
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Assembly Code Example(1)
EEPROM_read:
; Wait for completion of previous write
sbic
EECR,EEPE
rjmp
EEPROM_read
; Set up address (r18:r17) in address register
out
EEARH, r18
out
EEARL, r17
; Start eeprom read by writing EERE
sbi
EECR,EERE
; Read data from Data Register
in
r16,EEDR
ret
C Code Example(1)
unsigned char EEPROM_read(unsigned int uiAddress)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set up address register */
EEAR = uiAddress;
/* Start eeprom read by writing EERE */
EECR |= (1<<EERE);
/* Return data from Data Register */
return EEDR;
}
Note:
5.3.5
1.
See “Code Examples” on page 8.
Preventing EEPROM Corruption
During periods of low VCC, the EEPROM data can be corrupted because the supply voltage is too low for the
CPU and the EEPROM to operate properly. These issues are the same as for board level systems using
EEPROM, and the same design solutions should be applied.
An EEPROM data corruption can be caused by two situations when the voltage is too low. First, a regular write
sequence to the EEPROM requires a minimum voltage to operate correctly. Secondly, the CPU itself can
execute instructions incorrectly, if the supply voltage is too low.
EEPROM data corruption can easily be avoided by following this design recommendation:
Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done by
enabling the internal Brown-out Detector (BOD). If the detection level of the internal BOD does not match the
needed detection level, an external low VCC reset Protection circuit can be used. If a reset occurs while a write
operation is in progress, the write operation will be completed provided that the power supply voltage is
sufficient.
5.4
I/O Memory
The I/O space definition of the device is shown in “Register Summary” on page 414.
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All I/Os and peripherals are placed in the I/O space. All I/O locations may be accessed by the LD/LDS/LDD and
ST/STS/STD instructions, transferring data between the 32 general purpose working registers and the I/O
space. I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI and CBI
instructions. In these registers, the value of single bits can be checked by using the SBIS and SBIC instructions.
Refer to the instruction set section for more details. When using the I/O specific commands IN and OUT, the I/O
addresses 0x00 - 0x3F must be used. When addressing I/O Registers as data space using LD and ST
instructions, 0x20 must be added to these addresses. The device is a complex microcontroller with more
peripheral units than can be supported within the 64 location reserved in Opcode for the IN and OUT
instructions. For the Extended I/O space from 0x60 - 0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD
instructions can be used.
For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory
addresses should never be written.
Some of the Status Flags are cleared by writing a logical one to them. Note that, unlike most other AVRs, the
CBI and SBI instructions will only operate on the specified bit, and can therefore be used on registers containing
such Status Flags. The CBI and SBI instructions work with registers 0x00 to 0x1F only.
The I/O and peripherals control registers are explained in later sections.
5.4.1
General Purpose I/O Registers
The device contains three General Purpose I/O Registers. These registers can be used for storing any
information, and they are particularly useful for storing global variables and Status Flags. General Purpose I/O
Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI, CBI, SBIS, and SBIC
instructions.
5.4.2
General Purpose I/O Register 2 – GPIOR2
Bit
Read/Write
Initial Value
5.4.3
6
5
4
3
2
1
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
0
LSB
R/W
0
GPIOR2
General Purpose I/O Register 1 – GPIOR1
Bit
Read/Write
Initial Value
5.4.4
7
MSB
R/W
0
7
MSB
R/W
0
6
5
4
3
2
1
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
0
LSB
R/W
0
GPIOR1
General Purpose I/O Register 0 – GPIOR0
Bit
Read/Write
Initial Value
7
MSB
R/W
0
6
5
4
3
2
1
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
0
LSB
R/W
0
GPIOR0
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6.
System Clock and Clock Options
6.1
Clock Systems and their Distribution
Figure 6-1 presents the principal clock systems in the AVR and their distribution. All of the clocks need not be
active at a given time. In order to reduce power consumption, the clocks to modules not being used can be
halted by using different sleep modes, as described in “Power Management and Sleep Modes” on page 43. The
clock systems are detailed below.
Figure 6-1.
Clock Distribution
General I/O
Modules
High Speed
Timer
ADC
clkTMR
clkUSB (48MHz)
USB
(1)
(2)
CPU Core
RAM
Flash and
EEPROM
clkADC
clk
AVR Clock
Control Unit
I/O
PLL Postcaler
clkCPU
clkFLASH
clkPLL
Reset Logic
PLL
Watchdog Timer
Source clock
System Clock
Prescaler
PLL Input
Multiplexer
PLL Clock
Prescaler
Watchdog
clock
clkPllPresc
Clock
Multiplexer
Clock Switch
Crystal
Oscillator
6.1.1
External Clock
Watchdog
Oscillator
Calibrated RC
Oscillator
CPU Clock – clkCPU
The CPU clock is routed to parts of the system concerned with operation of the AVR core. Examples of such
modules are the General Purpose Register File, the Status Register and the data memory holding the Stack
Pointer. Halting the CPU clock inhibits the core from performing general operations and calculations.
6.1.2
I/O Clock – clkI/O
The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and USART. The I/O clock is
also used by the External Interrupt module, but note that some external interrupts are detected by asynchronous
logic, allowing such interrupts to be detected even if the I/O clock is halted. Also, TWI address recognition is
handled in all sleep modes.
6.1.3
Flash Clock – clkFLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously with
the CPU clock.
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6.1.4
ADC Clock – clkADC
The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks in order to
reduce noise generated by digital circuitry. This gives more accurate ADC conversion results.
6.1.5
PLL Prescaler Clock – clkPllPresc
The PLL requires a 8MHz input. A prescaler allows user to use either a 8MHz or a 16MHz source (from a crystal
or an external source), using a divider (by 2) if necessary. The output of the prescaler goes into the PLL Input
multiplexer, that allows the user to select either the prescaler output of the System Clock Multiplexer, or the
Internal 8MHz Calibrated Oscillator.
6.1.6
PLL Output Clock – clkPll
When enabled, the PLL outputs one frequency among numerous choices between 32MHz and 96MHz. The
output frequency is determined by the PLL clock register. The frequency is independent of the power supply
voltage. The PLL Output is connected to a postscaler that allows user to generate two different frequencies
(clkUSB and clkTMR) from the common PLL signal, each on them resulting of a selected division ratio (/1, /1.5, /2).
6.1.7
High-Speed Timer Clock– clkTMR
When enabled, the PLL outputs one frequency among numerous choices between 32MHz and 96MHz, that
goes into the PLL Postcaler. The High Speed Timer frequency input is generated from the PLL Postcaler, that
proposes /1, /1.5 and /2 ratios. That can be determined from the PLL clock register. The High Speed Timer
maximum frequency input depends on the power supply voltage and reaches its maximum of 64MHz at 5V.
6.1.8
USB Clock – clkUSB
The USB hardware module needs for a 48MHz clock. This clock is generated from the on-chip PLL. The output
of the PLL passes through the PLL Postcaler where the frequency can be either divided by 2 or directly
connected to the clkUSB signal.
6.2
Clock Sources
The device has the following clock source options, selectable by Flash Fuse bits as shown below. The clock
from the selected source is input to the AVR clock generator, and routed to the appropriate modules.
Table 6-1.
Device Clocking Options Select(1)
Device Clocking Option
CKSEL[3:0] (or EXCKSEL[3:0])
Low Power Crystal Oscillator
1111 - 1000
Reserved
0111 - 0110
Low Frequency Crystal Oscillator
0101 - 0100
Reserved
0011
Calibrated Internal RC Oscillator
0010
External Clock
0000
Reserved
0001
Note:
1.
For all fuses “1” means unprogrammed while “0” means programmed.
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6.2.1
Default Clock Source ATmega16U4 and ATmega32U4
The device is shipped with Low Power Crystal Oscillator (8.0 - 16MHz) enabled and with the fuse CKDIV8
programmed, resulting in 1.0MHz system clock with an 8MHz crystal. See Table 28-5 on page 355 for an
overview of the default Clock Selection Fuse setting.
6.2.2
Default Clock Source ATmega16U4RC and ATmega32U4RC
The device is shipped with Calibrated Internal RC oscillator (8.0MHz) enabled and with the fuse CKDIV8
programmed, resulting in 1.0MHz system clock. See Table 28-5 on page 355 for an overview of the default
Clock Selection Fuse setting.
6.2.3
Clock Startup Sequence
Any clock source needs a sufficient VCC to start oscillating and a minimum number of oscillating cycles before it
can be considered stable.
To ensure sufficient VCC, the device issues an internal reset with a time-out delay (tTOUT) after the device reset is
released by all other reset sources. “On-chip Debug System” on page 46 describes the start conditions for the
internal reset. The delay (tTOUT) is timed from the Watchdog Oscillator and the number of cycles in the delay is
set by the SUTx and CKSELx fuse bits. The selectable delays are shown in the following table. The frequency of
the Watchdog Oscillator is voltage dependent as shown in this table.
Table 6-2.
Number of Watchdog Oscillator Cycles
Typ Time-out (VCC = 5.0V)
Typ Time-out (VCC = 3.0V)
Number of Cycles
0ms
0ms
0
4.1ms
4.3ms
512
65ms
69ms
8K (8,192)
Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum VCC. The delay will not
monitor the actual voltage and it will be required to select a delay longer than the VCC rise time. If this is not
possible, an internal or external Brown-Out Detection circuit should be used. A BOD circuit will ensure sufficient
VCC before it releases the reset, and the time-out delay can be disabled. Disabling the time-out delay without
utilizing a Brown-Out Detection circuit is not recommended.
The oscillator is required to oscillate for a minimum number of cycles before the clock is considered stable. An
internal ripple counter monitors the oscillator output clock, and keeps the internal reset active for a given
number of clock cycles. The reset is then released and the device will start to execute. The recommended
oscillator start-up time is dependent on the clock type, and varies from six cycles for an externally applied clock
to 32K cycles for a low frequency crystal.
The start-up sequence for the clock includes both the time-out delay and the start-up time when the device
starts up from reset. When starting up from Power-save or Power-down mode, VCC is assumed to be at a
sufficient level and only the start-up time is included.
6.3
Low Power Crystal Oscillator
Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for
use as an On-chip Oscillator, as shown in Figure 6-2. Either a quartz crystal or a ceramic resonator may be
used.
This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 output. It gives the
lowest power consumption, but is not capable of driving other clock inputs.
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C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors
depends on the crystal or resonator in use, the amount of stray capacitance, and the electromagnetic noise of
the environment. Some initial guidelines for choosing capacitors for use with crystals are given in the below
table. For ceramic resonators, the capacitor values given by the manufacturer should be used.
Figure 6-2.
Crystal Oscillator Connections
C2
XTAL2
C1
XTAL1
GND
The Low Power Oscillator can operate in three different modes, each optimized for a specific frequency range.
The operating mode is selected by the fuses CKSEL[3..1] as shown in this table.
Table 6-3.
Low Power Crystal Oscillator Operating Modes
Frequency Range(1) [MHz]
0.4 - 0.9
Notes:
1.
2.
CKSEL3..1
Recommended Range for Capacitors C1 and C2 [pF]
(2)
100
–
0.9 - 3.0
101
12 - 22
3.0 - 8.0
110
12 - 22
8.0 - 16.0
111
12 - 22
This option should not be used with crystals, only with ceramic resonators.
If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can be
programmed in order to divide the internal frequency by 8. It must be ensured that the resulting divided clock
meets the frequency specification of the device.
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in this table.
Table 6-4.
Start-up Times for the Low Power Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
Power-save
Additional Delay from
Reset
(VCC = 5.0V)
CKSEL0
SUT1..0
Ceramic resonator,
fast rising power
258CK
14CK + 4.1ms(1)
0
00
Ceramic resonator,
slowly rising power
258CK
14CK + 65ms(1)
0
01
Ceramic resonator,
BOD enabled
1K CK
14CK(2)
0
10
Ceramic resonator,
fast rising power
1K CK
14CK + 4.1ms(2)
0
11
Ceramic resonator,
slowly rising power
1K CK
14CK + 65ms(2)
1
00
Oscillator Source /
Power Conditions
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Table 6-4.
Start-up Times for the Low Power Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
Power-save
Additional Delay from
Reset
(VCC = 5.0V)
CKSEL0
SUT1..0
Crystal Oscillator,
BOD enabled
16K CK
14CK
1
01
Crystal Oscillator,
fast rising power
16K CK
14CK + 4.1ms
1
10
Crystal Oscillator,
slowly rising power
16K CK
14CK + 65ms
1
11
Oscillator Source /
Power Conditions
Notes:
1.
2.
These options should only be used when not operating close to the maximum frequency of the device, and
only if frequency stability at start-up is not important for the application. These options are not suitable for
crystals.
These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They
can also be used with crystals when not operating close to the maximum frequency of the device, and if
frequency stability at start-up is not important for the application.
Table 6-5.
Start-up times for the internal calibrated RC Oscillator clock selection
Start-up Time from Powerdown and Power-save
Additional Delay from
Reset (VCC = 5.0V)
SUT1..0
BOD enabled
6CK
14CK
00
Fast rising power
6CK
14CK + 4.1ms
Power Conditions
Slowly rising power
6CK
14CK + 65ms
01
(1)
10
Reserved
Note:
6.4
1.
11
The device is shipped with this option selected.
Low Frequency Crystal Oscillator
The device can utilize a 32.768kHz watch crystal as clock source by a dedicated Low Frequency Crystal
Oscillator. The crystal should be connected as shown in Figure 6-2 on page 30. When this Oscillator is selected,
start-up times are determined by the SUT Fuses and CKSEL0 as shown in the table below.
Table 6-6.
Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Power Conditions
BOD enabled
Start-up Time from
Power-down and
Power-save
Additional Delay from
Reset
(VCC = 5.0V)
CKSEL0
SUT1..0
1K CK
14CK(1)
0
00
(1)
Fast rising power
1K CK
14CK + 4.1ms
0
01
Slowly rising power
1K CK
14CK + 65ms(1)
0
10
0
11
Reserved
BOD enabled
32K CK
14CK
1
00
Fast rising power
32K CK
14CK + 4.1ms
1
01
Slowly rising power
32K CK
14CK + 65ms
1
10
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Table 6-6.
Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Power Conditions
Start-up Time from
Power-down and
Power-save
Additional Delay from
Reset
(VCC = 5.0V)
CKSEL0
SUT1..0
1
11
Reserved
Note:
6.5
1.
These options should only be used if frequency stability at start-up is not important for the application.
Calibrated Internal RC Oscillator
The calibrated internal RC Oscillator by default provides a 8.0MHz clock. This frequency is nominal value at 3V
and 25C. The device is shipped with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on
page 35 for more details. This clock may be selected as the system clock by programming the CKSEL Fuses as
shown in the table below. If selected, it will operate with no external components. During reset, hardware loads
the calibration byte into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. At 3V and
25C, this calibration gives a frequency of 8MHz ±1%. The oscillator can be calibrated to any frequency in the
range 7.3 - 8.1MHz within ±1% accuracy, by changing the OSCCAL register. When this Oscillator is used as the
chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for the Reset Time-out. For
more information on the pre-programmed calibration value, see the section “Calibration Byte” on page 356
Table 6-7.
Notes:
1.
2.
Internal Calibrated RC Oscillator Operating Modes
Frequency Range [MHz]
CKSEL[3:0]
7.3 - 8.1
0010
The device is shipped with this option selected.
If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can be
programmed in order to divide the internal frequency by 8.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in the table.
Table 6-8.
Start-up times for the internal calibrated RC Oscillator clock selection
Start-up Time from Powerdown and Power-save
Additional Delay from
Reset (VCC = 5.0V)
SUT1..0
BOD enabled
6CK
14CK
00
Fast rising power
6CK
14CK + 4.1ms
01
Slowly rising power
6CK
14CK + 65ms
10
Power Conditions
Reserved
6.5.1
11
Oscillator Calibration Register – OSCCAL
Bit
Read/Write
Initial Value
7
6
5
CAL7
CAL6
CAL5
R/W
R/W
R/W
Device Specific Calibration Value
4
CAL4
R/W
3
CAL3
R/W
2
CAL2
R/W
1
CAL1
R/W
0
CAL0
R/W
OSCCAL
• Bits 7..0 – CAL7..0: Oscillator Calibration Value
The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to remove process
variations from the oscillator frequency. The factory-calibrated value is automatically written to this register
during chip reset, giving an oscillator frequency of 8.0MHz at 25°C. The application software can write this
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register to change the oscillator frequency. The calibration range is ±40% and linear (calibration step ~0.4%).
With typical process at 25°C the code should be 127 for 8MHz. Input value of 0x00 gives the lowest frequency,
and 0xFF the highest.
The temperature sensitivity is quite linear but as said previously depends on the process. To determine its
slope, the frequency must be measured at two temperatures. The temperature sensor of the device allows such
an operation, that is detailed on “Sensor Calibration” on page 304. It is then possible to calibrate the oscillator
frequency in function of the temperature measured.
Note that this oscillator is used to time EEPROM and Flash write accesses, and these write times will be
affected accordingly. If the EEPROM or Flash are written, do not calibrate to more than 8.8 MHz. Otherwise, the
EEPROM or Flash write may fail.
6.5.2
Oscillator Control Register – RCCTRL
Bit
Read/Write
Initial Value
7
R
0
6
R
0
5
R
0
4
R
0
3
R
0
2
R
0
1
R
0
0
RCFREQ
R/W
0
RCCTRL
Bits 7..1 – Reserved
Do not set these bits. Bits should be read as ‘0’.
Bit 0– RCFREQ: RC Oscillator Frequency Select
When this bit is cleared (default value), the RC Oscillator output frequency is set to 8MHz. When the bit is set,
the RC output frequency is 1MHz. Note that the OSCCAL value has the same effect on both 8MHz and 1MHz
output modes (~0.4% / step).
6.6
External Clock
The device can utilize a external clock source as shown in Figure 6-3. To run the device on an external clock,
the CKSEL Fuses must be programmed as shown in Table 6-1 on page 28.
Figure 6-3.
External Clock Drive Configuration
NC
XTAL2
EXTERNAL
CLOCK
SIGNAL
XTAL1
GND
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in the table
below.
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Table 6-9.
Start-up Times for the External Clock Selection
Start-up Time from Powerdown and Power-save
Additional Delay from
Reset (VCC = 5.0V)
SUT1..0
BOD enabled
6CK
14CK
00
Fast rising power
6CK
14CK + 4.1ms
01
Slowly rising power
6CK
14CK + 65ms
10
Power Conditions
Reserved
11
When applying an external clock, it is required to avoid sudden changes in the applied clock frequency to
ensure stable operation of the MCU. A variation in frequency of more than 2% from one clock cycle to the next
can lead to unpredictable behavior. If changes of more than 2% is required, ensure that the MCU is kept in
Reset during the changes.
Note that the System Clock Prescaler can be used to implement run-time changes of the internal clock
frequency while still ensuring stable operation. Refer to “System Clock Prescaler” on page 35 for details.
6.7
Clock Switch
The device includes a Clock Switch controller, that allows user to switch from one clock source to another one
by software, in order to control application power and execution time with more accuracy.
6.7.1
Example of use
The modification may be needed when the device enters in USB Suspend mode. It then switches from External
Clock to Calibrated RC Oscillator in order to reduce consumption and wake-up delay. In such a configuration,
the External Clock is disabled. The firmware can then use the watchdog timer to be woken-up from power-down
in order to check if there is an event on the application. If an event occurs on the application or if the USB controller signals a non-idle state on the USB line (Resume for example), the firmware switches the Clock
Multiplexer from the Calibrated RC Oscillator to the External Clock. in order to restart USB operation.
This feature can only be used to switch between Calibrated 8MHz RC Oscillator, External Clock and Low Power
Crystal Oscillator. The Low Frequency Crystal Oscillator must not be used with this feature.
Figure 6-4.
Example of Clock Switching with Wake-up from USB Host
resume
1 Resume from Host
USB
CPU Clock
non-Idle
Idle
Ext
non-Idle
(Suspend)
RC
1
Ext
External
Oscillator
RC oscillator
3ms
w atchdog wake-up
from power-down
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Figure 6-5.
Example of Clock Switching with Wake-up from Device
upstream-resume
2 Upstream Resume from device
USB
non-Idle
CPU Clock
Idle
Ext
non-Idle
(Suspend)
RC
2
Ext
External
Oscillator
RC oscillator
3ms
6.8
w atchdog wake-up
from power-down
Clock Output Buffer
The device can output the system clock on the CLKO pin. To enable the output, the CKOUT Fuse has to be
programmed. This mode is suitable when the chip clock is used to drive other circuits on the system. The clock
also will be output during reset, and the normal operation of I/O pin will be overridden when the fuse is
programmed. Any clock source, including the internal RC Oscillator, can be selected when the clock is output on
CLKO. If the System Clock Prescaler is used, it is the divided system clock that is output.
6.8.1
System Clock Prescaler
The AVR USB has a system clock prescaler, and the system clock can be divided by setting the “CLKPR –
Clock Prescaler Register” on page 39. This feature can be used to decrease the system clock frequency and
the power consumption when the requirement for processing power is low. This can be used with all clock
source options, and it will affect the clock frequency of the CPU and all synchronous peripherals. clkI/O, clkADC,
clkCPU, and clkFLASH are divided by a factor as shown in Table 6-10 on page 40.
When switching between prescaler settings, the System Clock Prescaler ensures that no glitches occurs in the
clock system. It also ensures that no intermediate frequency is higher than neither the clock frequency
corresponding to the previous setting, nor the clock frequency corresponding to the new setting.
The ripple counter that implements the prescaler runs at the frequency of the undivided clock, which may be
faster than the CPU's clock frequency. Hence, it is not possible to determine the state of the prescaler - even if
it were readable, and the exact time it takes to switch from one clock division to the other cannot be exactly
predicted. From the time the CLKPS values are written, it takes between T1 + T2 and T1 + 2 * T2 before the
new clock frequency is active. In this interval, two active clock edges are produced. Here, T1 is the previous
clock period, and T2 is the period corresponding to the new prescaler setting.
To avoid unintentional changes of clock frequency, a special write procedure must be followed to change the
CLKPS bits:
1. Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits in CLKPR to zero.
2.
Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.
Interrupts must be disabled when changing prescaler setting to make sure the write procedure is not interrupted.
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6.9
PLL
The PLL is used to generate internal high frequency (up to 96MHz) clock for USB interface and/or High Speed
Timer module, the PLL input is supplied from an external low-frequency clock (the crystal oscillator or external
clock input pin from XTAL1).
6.9.1
Internal PLL
The internal PLL in the device generates a clock frequency between 32MHz and 96MHz from nominally 8MHz
input.
The source of the 8MHz PLL input clock is the output of the internal PLL clock prescaler that generates the
8MHz from the clock source multiplexer output (See “PLL Control and Status Register – PLLCSR” on page 40.
for PLL interface). The PLL prescaler allows a direct connection (8MHz oscillator) or a divide-by-2 stage for a
16MHz clock input.
The PLL output signal enters the PLL Postcaler stage before being distributed to the USB and High Speed
Timer modules. Each of these modules can choose an independent division ratio.
Figure 6-6.
PLL Clocking System
CKSEL3:0
PINDIV
PINMUX
PLOCK
PLLE
PLLTM1:0
/1.5
Lock
Detector
XTAL1
XTAL2
01
XTAL
OSCILLATOR
PLL clock
Prescaler
RC OSCILLATOR
8 MHz
10
clk TMR
11
0
1
clk
8MHz
PLL
/2
1
To System
Clock Prescaler
clk USB
PDIV3..0
0
PLLUSB
6.10
Clock switch Algorithm
6.10.1 Switch from External Clock to RC Clock
if (Usb_suspend_detected())
{
Usb_ack_suspend();
Usb_freeze_clock();
Disable_pll();
Enable_RC_clock();
while (!RC_clock_ready());
Select_RC_clock();
Disable_external_clock();
}
// if (UDINT.SUSPI == 1)
//
//
//
//
//
//
//
UDINT.SUSPI = 0;
USBCON.FRZCLK = 1;
PLLCSR.PLLE = 0;
CLKSEL0.RCE = 1;
while (CLKSTA.RCON != 1);
CLKSEL0.CLKS = 0;
CLKSEL0.EXTE = 0;
6.10.2 Switch from RC Clock to External Clock
if (Usb_wake_up_detected())
{
Usb_ack_wake_up();
Enable_external_clock();
while (!External_clock_ready());
// if (UDINT.WAKEUPI == 1)
// UDINT.WAKEUPI = 0;
// CKSEL0.EXTE = 1;
// while (CLKSTA.EXTON != 1);
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Select_external_clock();
Enable_pll();
Disable_RC_clock();
while (!Pll_ready());
Usb_unfreeze_clock();
//
//
//
//
//
CLKSEL0.CLKS = 1;
PLLCSR.PLLE = 1;
CLKSEL0.RCE = 0;
while (PLLCSR.PLOCK != 1);
USBCON.FRZCLK = 0;
}
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6.11
Register Description
6.11.1 CLKSEL0 – Clock Selection Register 0
Bit
Read/Write
Initial Value
7
RCSUT1
R/W
0
6
RCSUT0
R/W
0
5
EXSUT1
R/W
0
4
EXSUT0
R/W
0
3
2
RCE
EXTE
R/W
R/W
See Bit Description
1
R
0
CLKS
R/W
CLKSEL0
• Bit 7-6 – RCSUT[1:0]: SUT for RC oscillator
These two bits are the SUT value for the RC Oscillator. If the RC oscillator is selected by fuse bits, the SUT fuse
are copied into these bits. A firmware change will not have any effect because this additional start-up time is
only used after a reset and not after a clock switch.
• Bit 5-4 – EXSUT[1:0]: SUT for External Clock/ Low Power Crystal Oscillator
These two bits are the SUT value for the External Clock / Low Power Crystal Oscillator. If the External Clock /
Low Power Crystal Oscillator is selected by fuse bits, the SUT fuses are copied into these bits. The firmware
can modify these bits by writing a new value. This value will be used at the next start of the External Clock / Low
Power Crystal Oscillator.
• Bit 3 – RCE: Enable RC Oscillator
The RCE bit must be written to logic one to enable the RC Oscillator. The RCE bit must be written to logic zero
to disable the RC Oscillator.
• Bit 2 – EXTE: Enable External Clock / Low Power Crystal Oscillator
The OSCE bit must be written to logic one to enable External Clock / Low Power Crystal Oscillator. The OSCE
bit must be written to logic zero to disable the External Clock / Low Power Crystal Oscillator.
• Bit 0 – CLKS: Clock Selector
The CLKS bit must be written to logic one to select the External Clock / Low Power Crystal Oscillator as CPU
clock. The CLKS bit must be written to logic zero to select the RC Oscillator as CPU clock. After a reset, the
CLKS bit is set by hardware if the External Clock / Low Power Crystal Oscillator is selected by the fuse bits configuration.
The firmware has to check if the clock is correctly started before selected it.
6.11.2 CLKSEL1 – Clock Selection Register 1
Bit
Read/Write
Initial Value
7
RCCKSEL3
R/W
0
6
RCCKSEL2
R/W
0
5
RCCKSEL1
R/W
1
4
RCCKSEL0
R/W
0
3
EXCKSEL3
R/W
0
2
EXCKSEL2
R/W
0
1
EXCKSEL1
R/W
0
0
EXCKSEL0
R/W
0
CLKSEL1
• Bit 7-4 – RCCKSEL[3:0]: CKSEL for RC oscillator
Clock configuration for the RC Oscillator. After a reset, this part of the register is loaded with the 0010b value
that corresponds to the RC oscillator. Modifying this value by firmware before switching to RC oscillator is prohibited because the RC clock will not start.
• Bit 3-0 – EXCKSEL[3:0]: CKSEL for External Clock / Low Power Crystal Oscillator
Clock configuration for the External Clock / Low Power Crystal Oscillator. After a reset, if the External Clock /
Low Power Crystal Oscillator is selected by fuse bits, this part of the register is loaded with the fuse configuration. Firmware can modify it to change the start-up time after the clock switch.
See Table 6-1 on page 28 for EXCKSEL[3:0] configuration. Only Low Power Crystal Oscillator, Calibrated Internal RC Oscillator, and External Clock modes are allowed.
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6.11.3 CLKSTA – Clock Status Register
Bit
Read/Write
Initial Value
7
R
0
6
R
0
5
R
0
4
R
0
3
R
0
2
R
1
RCON
R
0
EXTON
R
CLKSTA
• Bit 7-2 - Reserved bits
These bits are reserved and will always read as zero.
• Bit 1 – RCON: RC Oscillator On
This bit is set by hardware to one if the RC Oscillator is running.
This bit is set by hardware to zero if the RC Oscillator is stopped.
• Bit 0 – EXTON: External Clock / Low Power Crystal Oscillator On
This bit is set by hardware to one if the External Clock / Low Power Crystal Oscillator is running.
This bit is set by hardware to zero if the External Clock / Low Power Crystal Oscillator is stopped.
6.11.4 CLKPR – Clock Prescaler Register
Bit
Read/Write
Initial Value
7
CLKPCE
R/W
0
6
–
R
0
5
–
R
0
4
–
R
0
3
2
CLKPS3
CLKPS2
R/W
R/W
See Bit Description
1
CLKPS1
R/W
0
CLKPS0
R/W
CLKPR
• Bit 7 – CLKPCE: Clock Prescaler Change Enable
The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE bit is only
updated when the other bits in CLKPR are simultaneously written to zero. CLKPCE is cleared by hardware four
cycles after it is written or when CLKPS bits are written. Rewriting the CLKPCE bit within this time-out period
does neither extend the time-out period, nor clear the CLKPCE bit.
• Bits 3..0 – CLKPS[3..0]: Clock Prescaler Select Bits 3 - 0
These bits define the division factor between the selected clock source and the internal system clock. These bits
can be written run-time to vary the clock frequency to suit the application requirements. As the divider divides
the master clock input to the MCU, the speed of all synchronous peripherals is reduced when a division factor is
used. The division factors are given in the table below.
The CKDIV8 Fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed, the CLKPS bits
will be reset to “0000”. If CKDIV8 is programmed, CLKPS bits are reset to “0011”, giving a division factor of 8 at
start up. This feature should be used if the selected clock source has a higher frequency than the maximum
frequency of the device at the present operating conditions. Note that any value can be written to the CLKPS
bits regardless of the CKDIV8 Fuse setting. The Application software must ensure that a sufficient division factor
is chosen if the selected clock source has a higher frequency than the maximum frequency of the device at the
present operating conditions. The device is shipped with the CKDIV8 Fuse programmed.
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Table 6-10.
Clock Prescaler Select
CLKPS3
CLKPS2
CLKPS1
CLKPS0
Clock Division Factor
0
0
0
0
1
0
0
0
1
2
0
0
1
0
4
0
0
1
1
8
0
1
0
0
16
0
1
0
1
32
0
1
1
0
64
0
1
1
1
128
1
0
0
0
256
1
0
0
1
Reserved
1
0
1
0
Reserved
1
0
1
1
Reserved
1
1
0
0
Reserved
1
1
0
1
Reserved
1
1
1
0
Reserved
1
1
1
1
Reserved
6.11.5 PLL Control and Status Register – PLLCSR
Bit
$29 ($29)
Read/Write
Initial Value
7
6
5
R
0
R
0
R
0
4
PINDIV
R/W
0
3
2
R
0
R
0
1
PLLE
R/W
0
0
PLOCK
R
0
PLLCSR
• Bit 7:5 – Res: Reserved Bits
These bits are reserved and always read as zero.
• Bit 4 – PINDIV PLL Input Prescaler (1:1, 1:2)
These bits allow to configure the PLL input prescaler to generate the 8MHz input clock for the PLL from either a
8 or 16MHz input.
When using a 8MHz clock source, this bit must be set to 0 before enabling PLL (1:1).
When using a 16MHz clock source, this bit must be set to 1 before enabling PLL (1:2).
• Bit 3:2 – Res: Reserved Bits
These bits are reserved and always read as zero.
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• Bit 1 – PLLE: PLL Enable
When the PLLE is set, the PLL is started. Note that the Calibrated 8MHz Internal RC oscillator is automatically
enabled when the PLLE bit is set and with PINMUX (see PLLFRQ register) is set. The PLL must be disabled
before entering Power down mode in order to stop Internal RC Oscillator and avoid extra-consumption.
• Bit 0 – PLOCK: PLL Lock Detector
When the PLOCK bit is set, the PLL is locked to the reference clock. After the PLL is enabled, it takes about
several ms for the PLL to lock. To clear PLOCK, clear PLLE.
6.11.6 PLL Frequency Control Register – PLLFRQ
Bit
$32
Read/Write
Initial Value
7
PINMUX
R/W
0
6
PLLUSB
R/W
0
5
PLLTM1
R/W
0
4
PLLTM0
R/W
0
3
2
1
0
PDIV3
R/W
0
PDIV2
R/W
1
PDIV1
R/W
0
PDIV0
R/W
0
PLLFRQ
• Bit 7– PINMUX: PLL Input Multiplexer
This bit selects the clock input of the PLL:
̶
PINMUX = 0: the PLL input is connected to the PLL Prescaler, that has the Primary System Clock
as source
̶
PINMUX = 1: the PLL input is directly connected to the Internal Calibrated 8MHz RC Oscillator. This
mode allows to work in USB Low Speed mode with no crystal or using a crystal with a value
different of 8/16MHz.
• Bit 6– PLLUSB: PLL Postcaler for USB Peripheral
This bit select the division factor between the PLL output frequency and the USB module input frequency:
̶
PLLUSB = 0: no division, direct connection (if PLL Output = 48MHz)
̶
PLLUSB = 1: PLL Output frequency is divided by two and sent to USB module
(if PLL Output = 96MHz)
• Bit 5:4 – PLLTM1:0: PLL Postcaler for High Speed Timer
These bits codes for the division factor between the PLL Output Frequency and the High Speed Timer input
frequency.
Note that the division factor 1.5 will introduce some jitter in the clock, but keeping the error null since the average duty cycle is 50%. See Figures 6-7 for more details.
PLLTM1
PLLTM0
PLL Postcaler Factor for High-Speed Timer
0
0
0 (Disconnected)
0
1
1
1
0
1.5
1
1
2
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Figure 6-7.
PLL Postcaler operation with division factor = 1.5
Fi
2
Fi x --3
• Bit 3:0 – PDIV3:0 PLL Lock Frequency
These bits configure the PLL internal VCO clock reference according to the required output frequency value.
PDIV3
PDIV2
PDIV1
PDIV0
PLL Output Frequency
0
0
0
0
Not allowed
0
0
0
1
Not allowed
0
0
1
0
Not allowed
0
0
1
1
40MHz
0
1
0
0
48MHz
0
1
0
1
56MHz
0
1
1
0
Not allowed
0
1
1
1
72MHz
1
0
0
0
80MHz
1
0
0
1
88MHz
1
0
1
0
96MHz
1
0
1
1
Not allowed
1
1
0
0
Not allowed
1
1
0
1
Not allowed
1
1
1
0
Not allowed
1
1
1
1
Not allowed
The optimal PLL configuration at 5V is: PLL output frequency = 96MHz, divided by 1.5 to generate the 64MHz
High Speed Timer clock, and divided by 2 to generate the 48MHz USB clock.
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7.
Power Management and Sleep Modes
Sleep modes enable the application to shut down unused modules in the MCU, thereby saving power. The AVR
provides various sleep modes allowing the user to tailor the power consumption to the application’s
requirements.
To enter any of the five sleep modes, the SE bit in SMCR must be written to logic one and a SLEEP instruction
must be executed. The SM2, SM1, and SM0 bits in the SMCR Register select which sleep mode (Idle, ADC
Noise Reduction, Power-down, Power-save, or Standby) will be activated by the SLEEP instruction.
See Table 7-1 on page 44 for a summary.
If an enabled interrupt occurs while the MCU is in a sleep mode, the MCU wakes up. The MCU is then halted for
four cycles in addition to the start-up time, executes the interrupt routine, and resumes execution from the
instruction following SLEEP. The contents of the Register File and SRAM are unaltered when the device wakes
up from sleep. If a reset occurs during sleep mode, the MCU wakes up and executes from the Reset Vector.
Figure 6-1 on page 27 presents the different clock systems in the ATmega16U4/ATmega32U4, and their
distribution. The figure is helpful in selecting an appropriate sleep mode.
7.1
Idle Mode
When the SM2..0 bits are written to 000, the SLEEP instruction makes the MCU enter Idle mode, stopping the
CPU but allowing the USB, SPI, USART, Analog Comparator, ADC, 2-wire Serial Interface, Timer/Counters,
Watchdog, and the interrupt system to continue operating. This sleep mode basically halts clkCPU and clkFLASH,
while allowing the other clocks to run.
Idle mode enables the MCU to wake up from external triggered interrupts as well as internal ones like the Timer
Overflow and USART Transmit Complete interrupts. If wake-up from the Analog Comparator interrupt is not
required, the Analog Comparator can be powered down by setting the ACD bit in the Analog Comparator
Control and Status Register – ACSR. This will reduce power consumption in Idle mode. If the ADC is enabled, a
conversion starts automatically when this mode is entered.
7.2
ADC Noise Reduction Mode
When the SM2..0 bits are written to 001, the SLEEP instruction makes the MCU enter ADC Noise Reduction
mode, stopping the CPU but allowing the ADC, the external interrupts, 2-wire Serial Interface address match
and the Watchdog to continue operating (if enabled). This sleep mode basically halts clkI/O, clkCPU, and
clkFLASH, while allowing the other clocks to run (including clkUSB).
This improves the noise environment for the ADC, enabling higher resolution measurements. If the ADC is
enabled, a conversion starts automatically when this mode is entered. Apart form the ADC Conversion
Complete interrupt, only an External Reset, a Watchdog System Reset, a Watchdog interrupt, a Brown-out
Reset, a 2-wire serial interface interrupt, an SPM/EEPROM ready interrupt, an external level interrupt on INT6,
an external interrupt on INT3:0 or a pin change interrupt can wake up the MCU from ADC Noise Reduction
mode.
7.3
Power-down Mode
When the SM2..0 bits are written to 010, the SLEEP instruction makes the MCU enter Power-down mode. In
this mode, the external Oscillator is stopped, while the external interrupts, the 2-wire Serial Interface, and the
Watchdog continue operating (if enabled). Only an External Reset, a Watchdog Reset, a Brown-out Reset, 2wire Serial Interface address match, an external level interrupt on INT6, an external interrupt on INT3:0, a pin
change interrupt or an asynchronous USB interrupt sources (VBUSTI, WAKEUPI), can wake up the MCU. This
sleep mode basically halts all generated clocks, allowing operation of asynchronous modules only.
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Note that if a level triggered interrupt is used for wake-up from Power-down mode, the changed level must be
held for some time to wake up the MCU. Refer to “External Interrupts” on page 88 for details.
When waking up from Power-down mode, there is a delay from the wake-up condition occurs until the wake-up
becomes effective. This allows the clock to restart and become stable after having been stopped. The wake-up
period is defined by the same CKSEL Fuses that define the Reset Time-out period, as described in “Clock
Sources” on page 28.
7.4
Power-save Mode
When the SM2..0 bits are written to 011, the SLEEP instruction makes the MCU enter Power-save mode. For
compatibility reasons with AT90USB64/128 this mode is still present but since Timer 2 Asynchronous operation
is not present here, this mode is identical to Power-down.
7.5
Standby Mode
When the SM2..0 bits are 110 and an external crystal/resonator clock option is selected, the SLEEP instruction
makes the MCU enter Standby mode. This mode is identical to Power-down with the exception that the
Oscillator is kept running. From Standby mode, the device wakes up in six clock cycles.
Extended Standby Mode
When the SM2..0 bits are 111 and an external crystal/resonator clock option is selected, the SLEEP instruction
makes the MCU enter Extended Standby mode. For compatibility reasons with AT90USB64/128 this mode is
still present but since Timer 2 Asynchronous operation is not present here, this mode is identical to Standbymode.
Active Clock Domains and Wake-up Sources in the Different Sleep Modes
Wake-up Sources
ADC
WDT Interrupt
Other I/O
X
X
X
X
X
X
Power-down
X(2)
X
X
X
Power-save
X(2)
X
X
X
ADCNRM
X
X
Interrupts(3)
SPM/
EEPROM Ready
X
X
Interrupts
TWI Address
Match
X
X
Main Clock
Source
Enabled
X
X
clkADC
X
(2)
Idle
clkIO
X
Sleep Mode
clkFLASH
INT6, INT3:0 and
Pin Change
Oscillators
USB Asynchronous
Active Clock
Domains
USB Synchronous
Table 7-1.
clkCPU
7.6
X
X
X
X
Standby(1)
X
X(2)
X
X
X
Extended
Standby
X
X(2)
X
X
X
Notes:
1.
2.
3.
Only recommended with external crystal or resonator selected as clock source.
For INT6, only level interrupt.
Asynchronous USB interrupts are VBUSTI and WAKEUPI.
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7.7
Power Reduction Register
The Power Reduction Register, PRR, provides a method to stop the clock to individual peripherals to reduce
power consumption. The current state of the peripheral is frozen and the I/O registers can not be read or written.
Resources used by the peripheral when stopping the clock will remain occupied, hence the peripheral should in
most cases be disabled before stopping the clock. Waking up a module, which is done by clearing the bit in
PRR, puts the module in the same state as before shutdown.
Module shutdown can be used in Idle mode and Active mode to significantly reduce the overall power
consumption. In all other sleep modes, the clock is already stopped.
7.8
Minimizing Power Consumption
There are several issues to consider when trying to minimize the power consumption in an AVR controlled
system. In general, sleep modes should be used as much as possible, and the sleep mode should be selected
so that as few as possible of the device’s functions are operating. All functions not needed should be disabled.
In particular, the following modules may need special consideration when trying to achieve the lowest possible
power consumption.
7.8.1
Analog to Digital Converter
If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be disabled before
entering any sleep mode. When the ADC is turned off and on again, the next conversion will be an extended
conversion. Refer to “Analog to Digital Converter - ADC” on page 297 for details on ADC operation.
7.8.2
Analog Comparator
When entering Idle mode, the Analog Comparator should be disabled if not used. When entering ADC Noise
Reduction mode, the Analog Comparator should be disabled. In other sleep modes, the Analog Comparator is
automatically disabled. However, if the Analog Comparator is set up to use the Internal Voltage Reference as
input, the Analog Comparator should be disabled in all sleep modes. Otherwise, the Internal Voltage Reference
will be enabled, independent of sleep mode. Refer to “Analog Comparator” on page 293 for details on how to
configure the Analog Comparator.
7.8.3
Brown-out Detector
If the Brown-out Detector is not needed by the application, this module should be turned off. If the Brown-out
Detector is enabled by the BODLEVEL Fuses, it will be enabled in all sleep modes, and hence, always consume
power. In the deeper sleep modes, this will contribute significantly to the total current consumption. Refer to
“Brown-out Detection” on page 52 for details on how to configure the Brown-out Detector.
7.8.4
Internal Voltage Reference
The Internal Voltage Reference will be enabled when needed by the Brown-out Detection, the Analog
Comparator or the ADC. If these modules are disabled as described in the sections above, the internal voltage
reference will be disabled and it will not be consuming power. When turned on again, the user must allow the
reference to start up before the output is used. If the reference is kept on in sleep mode, the output can be used
immediately. Refer to “Internal Voltage Reference” on page 54 for details on the start-up time.
7.8.5
Watchdog Timer
If the Watchdog Timer is not needed in the application, the module should be turned off. If the Watchdog Timer
is enabled, it will be enabled in all sleep modes, and hence, always consume power. In the deeper sleep modes,
this will contribute significantly to the total current consumption. Refer to “Interrupts” on page 63 for details on
how to configure the Watchdog Timer.
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7.8.6
Port Pins
When entering a sleep mode, all port pins should be configured to use minimum power. The most important is
then to ensure that no pins drive resistive loads. In sleep modes where both the I/O clock (clkI/O) and the ADC
clock (clkADC) are stopped, the input buffers of the device will be disabled. This ensures that no power is
consumed by the input logic when not needed. In some cases, the input logic is needed for detecting wake-up
conditions, and it will then be enabled. Refer to the section “Digital Input Enable and Sleep Modes” on page 71
for details on which pins are enabled. If the input buffer is enabled and the input signal is left floating or have an
analog signal level close to VCC/2, the input buffer will use excessive power.
For analog input pins, the digital input buffer should be disabled at all times. An analog signal level close to
VCC/2 on an input pin can cause significant current even in active mode. Digital input buffers can be disabled by
writing to the Digital Input Disable Registers (DIDR1 and DIDR0). Refer to “Digital Input Disable Register 1 –
DIDR1” on page 296 and “Digital Input Disable Register 1 – DIDR1” on page 296 for details.
7.8.7
On-chip Debug System
If the On-chip debug system is enabled by the OCDEN Fuse and the chip enters sleep mode, the main clock
source is enabled, and hence, always consumes power. In the deeper sleep modes, this will contribute
significantly to the total current consumption.
There are three alternative ways to disable the OCD system:

Disable the OCDEN Fuse

Disable the JTAGEN Fuse

Write one to the JTD bit in MCUCR
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7.9
Register Description
7.9.1
Sleep Mode Control Register – SMCR
The Sleep Mode Control Register contains control bits for power management.
Bit
Read/Write
Initial Value
7
–
R
0
6
–
R
0
5
–
R
0
4
–
R
0
3
SM2
R/W
0
2
SM1
R/W
0
1
SM0
R/W
0
0
SE
R/W
0
SMCR
• Bits 3, 2, 1 – SM2..0: Sleep Mode Select Bits 2, 1, and 0
These bits select between the six available sleep modes as shown in Table 7-2.
Table 7-2.
Note:
Sleep Mode Select
SM2
SM1
SM0
0
0
0
Idle
0
0
1
ADC Noise Reduction
0
1
0
Power-down
0
1
1
Power-save
1
0
0
Reserved
1
0
1
Reserved
1
1
0
Standby(1)
1
1
1
Extended Standby(1)
1.
Sleep Mode
Standby modes are only recommended for use with external crystals or resonators.
• Bit 0 – SE: Sleep Enable
The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP instruction is
executed. To avoid the MCU entering the sleep mode unless it is the programmer’s purpose, it is recommended
to write the Sleep Enable (SE) bit to one just before the execution of the SLEEP instruction and to clear it
immediately after waking up.
7.9.2
Power Reduction Register 0 - PRR0
Bit
Read/Write
Initial Value
7
PRTWI
R/W
0
6
–
R
0
5
PRTIM0
R/W
0
4
–
R
0
3
PRTIM1
R/W
0
2
PRSPI
R/W
0
1
–
R
0
0
PRADC
R/W
0
PRR0
• Bit 7 - PRTWI: Power Reduction TWI
Writing a logic one to this bit shuts down the TWI by stopping the clock to the module. When waking up the TWI
again, the TWI should be re initialized to ensure proper operation.
• Bit 6 - Res: Reserved bit
This bits is reserved and will always read as zero.
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• Bit 5 - PRTIM0: Power Reduction Timer/Counter0
Writing a logic one to this bit shuts down the Timer/Counter0 module. When the Timer/Counter0 is enabled,
operation will continue like before the shutdown.
• Bit 4 - Res: Reserved bit
This bit is reserved and will always read as zero.
• Bit 3 - PRTIM1: Power Reduction Timer/Counter1
Writing a logic one to this bit shuts down the Timer/Counter1 module. When the Timer/Counter1 is enabled,
operation will continue like before the shutdown.
• Bit 2 - PRSPI: Power Reduction Serial Peripheral Interface
Writing a logic one to this bit shuts down the Serial Peripheral Interface by stopping the clock to the module.
When waking up the SPI again, the SPI should be re initialized to ensure proper operation.
• Bit 1 - Res: Reserved bit
These bits are reserved and will always read as zero.
• Bit 0 - PRADC: Power Reduction ADC
Writing a logic one to this bit shuts down the ADC. The ADC must be disabled before shut down. The analog
comparator cannot use the ADC input MUX when the ADC is shut down.
7.9.3
Power Reduction Register 1 - PRR1
Bit
Read/Writ
e
Initial Value
7
PRUSB
R/W
6
–
R
5
–
R
4
PRTIM4
R
3
PRTIM3
R/W
2
–
R
1
–
R
0
PRUSART1
R/W
0
0
0
0
0
0
0
0
PRR1
• Bit 7 - PRUSB: Power Reduction USB
Writing a logic one to this bit shuts down the USB by stopping the clock to the module. When waking up the USB
again, the USB should be re initialized to ensure proper operation.
• Bit 6..5 - Res: Reserved bits
These bits are reserved and will always read as zero.
• Bit 4- PRTIM4: Power Reduction Timer/Counter4
Writing a logic one to this bit shuts down the Timer/Counter4 module. When the Timer/Counter4 is enabled,
operation will continue like before the shutdown.
• Bit 3 - PRTIM3: Power Reduction Timer/Counter3
Writing a logic one to this bit shuts down the Timer/Counter3 module. When the Timer/Counter3 is enabled,
operation will continue like before the shutdown.
• Bit 2..1 - Res: Reserved bits
These bits are reserved and will always read as zero.
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• Bit 0 - PRUSART1: Power Reduction USART1
Writing a logic one to this bit shuts down the USART1 by stopping the clock to the module. When waking up the
USART1 again, the USART1 should be re initialized to ensure proper operation.
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8.
System Control and Reset
8.1
Resetting the AVR
During reset, all I/O Registers are set to their initial values, and the program starts execution from the Reset
Vector. The instruction placed at the Reset Vector must be a JMP – Absolute Jump – instruction to the reset
handling routine. If the program never enables an interrupt source, the Interrupt Vectors are not used, and
regular program code can be placed at these locations. This is also the case if the Reset Vector is in the
Application section while the Interrupt Vectors are in the Boot section or vice versa. The circuit diagram in Figure
8-1 on page 51 shows the reset logic. Table 29-3 on page 386 defines the electrical parameters of the reset
circuitry.
The I/O ports of the AVR are immediately reset to their initial state when a reset source goes active. This does
not require any clock source to be running.
After all reset sources have gone inactive, a delay counter is invoked, stretching the internal reset. This allows
the power to reach a stable level before normal operation starts. The time-out period of the delay counter is
defined by the user through the SUT and CKSEL Fuses. The different selections for the delay period are
presented in “Clock Sources” on page 28.
8.2
Reset Sources
The ATmega16U4/ATmega32U4 has five sources of reset:

Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset threshold
(VPOT).

External Reset. The MCU is reset when a low level is present on the RESET pin for longer than the
minimum pulse length.

Watchdog Reset. The MCU is reset when the Watchdog Timer period expires and the Watchdog is
enabled.

Brown-out Reset. The MCU is reset when the supply voltage VCC is below the Brown-out Reset threshold
(VBOT) and the Brown-out Detector is enabled.

JTAG AVR Reset. The MCU is reset as long as there is a logic one in the Reset Register, one of the scan
chains of the JTAG system. Refer to the section “IEEE 1149.1 (JTAG) Boundary-scan” on page 325 for
details.

USB End of Reset. The MCU is reset (excluding the USB controller that remains enabled and attached)
on the detection of a USB End of Reset condition on the bus, if this feature is enabled by the user.
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Figure 8-1.
Reset Logic
DATA BUS
PORF
BORF
EXTRF
WDRF
JTRF
USBRF
MCU Status
Register (MCUSR)
Power-on Reset
Circuit
Brown-out
Reset Circuit
BODLEVEL [2..0]
Pull-up Resistor
SPIKE
FILTER
JTAG Reset
Register
USB Reset
Detection
Watchdog
Oscillator
Clock
Generator
CK
Delay Counters
TIMEOUT
CKSEL[3:0]
SUT[1:0]
8.3
Power-on Reset
A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level is defined in
Table 8-1 on page 53. The POR is activated whenever VCC is below the detection level. The POR circuit can be
used to trigger the start-up Reset, as well as to detect a failure in supply voltage.
A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the Power-on Reset
threshold voltage invokes the delay counter, which determines how long the device is kept in RESET after VCC
rise. The RESET signal is activated again, without any delay, when VCC decreases below the detection level.
Figure 8-2.
MCU Start-up, RESET Tied to VCC
VCC
RESET
TIME-OUT
VPOR
VPOT
VRST
tTOUT
INTERNAL
RESET
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Figure 8-3.
MCU Start-up, RESET Extended Externally
VCC
VPOR
VPOT
RESET
TIME-OUT
VRST
tTOUT
INTERNAL
RESET
8.4
External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the minimum pulse
width (see Table 29-3 on page 386) will generate a reset, even if the clock is not running. Shorter pulses are not
guaranteed to generate a reset. When the applied signal reaches the Reset Threshold Voltage – VRST – on its
positive edge, the delay counter starts the MCU after the Time-out period – tTOUT – has expired.
Figure 8-4.
External Reset During Operation
CC
8.5
Brown-out Detection
ATmega16U4/ATmega32U4 has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC level
during operation by comparing it to a fixed trigger level. The trigger level for the BOD can be selected by the
BODLEVEL Fuses. The trigger level has a hysteresis to ensure spike free Brown-out Detection. The hysteresis
on the detection level should be interpreted as VBOT+ = VBOT + VHYST/2 and VBOT- = VBOT - VHYST/2.
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Table 8-1.
BODLEVEL Fuse Coding
BODLEVEL 2..0 Fuses
Min. VBOT
111
Table 8-2.
Typ. VBOT
Max. VBOT
Units
BOD Disabled
110
1.8
2.0
2.2
101
2.0
2.2
2.4
100
2.2
2.4
2.6
011
2.4
2.6
2.8
010
3.2
3.4
3.6
001
3.3
3.5
3.7
000
4.0
4.3
4.5
V
BOD characteristics
Symbol
Parameter
Min.
VHYST
Brown-out Detector Hysteresis
tBOD
Min Pulse Width on Brown-out Reset
Typ.
50
Max.
Units
mV
ns
When the BOD is enabled, and VCC decreases to a value below the trigger level (VBOT- in Figure 8-5), the
Brown-out Reset is immediately activated. When VCC increases above the trigger level (VBOT+ in Figure 8-5), the
delay counter starts the MCU after the Time-out period tTOUT has expired.
The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for longer than tBOD
given in Table 29-3 on page 386.
Figure 8-5.
Brown-out Reset During Operation
VCC
VBOT-
VBOT+
RESET
TIME-OUT
tTOUT
INTERNAL
RESET
8.6
Watchdog Reset
When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On the falling edge
of this pulse, the delay timer starts counting the Time-out period tTOUT. For details on operation of the Watchdog
Timer, see “Watchdog Timer” on page 55.
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Figure 8-6.
Watchdog Reset During Operation
CC
CK
8.7
USB Reset
When the USB controller is enabled and configured with the USB Reset CPU feature enabled and if a valid USB
Reset signalling is detected on the bus, the CPU core is reset but the USB controller remains enabled and
attached. This feature may be used to enhance device reliability.
Figure 8-7.
USB Reset During Operation
(USB Lines)
CC
8.8
t USBRSTMIN
End of Reset
DP
USB Traffic
USB Traffic
DM
Internal Voltage Reference
ATmega16U4/ATmega32U4 features an internal bandgap reference. This reference is used for Brown-out
Detection, and it can be used as an input to the Analog Comparator or the ADC.
8.8.1
Voltage Reference Enable Signals and Start-up Time
The voltage reference has a start-up time that may influence the way it should be used. The start-up time is
given in Table 8-3 on page 55. To save power, the reference is not always turned on. The reference is on during
the following situations:
1. When the BOD is enabled (by programming the BODLEVEL [2..0] Fuse).
2.
When the bandgap reference is connected to the Analog Comparator (by setting the ACBG bit in ACSR).
3.
When the ADC is enabled.
Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user must always allow
the reference to start up before the output from the Analog Comparator or ADC is used. To reduce power
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consumption in Power-down mode, the user can avoid the three conditions above to ensure that the reference
is turned off before entering Power-down mode.
Table 8-3.
Symbol
Parameter
Condition
VCC=2.7
VBG
Bandgap reference voltage
tBG
Bandgap reference start-up time
IBG
Bandgap reference current consumption
TA=25°C
Min.
Typ.
Max.
Units
1.0
1.1
1.2
V
40
70
µs
VCC=2.7
TA=25°C
VCC=2.7
10
TA=25°C
µA
Watchdog Timer
ATmega16U4/ATmega32U4 has an Enhanced Watchdog Timer (WDT). The main features are:
• Clocked from separate On-chip Oscillator
• Three Operating modes
– Interrupt
– System Reset
– Interrupt and System Reset
• Selectable Time-out period from 16ms to 8s
• Possible Hardware fuse Watchdog always on (WDTON) for fail-safe mode
Figure 8-8.
Watchdog Timer
128kHz
OSCILLATOR
WATCHDOG
RESET
WDE
OSC/2K
OSC/4K
OSC/8K
OSC/16K
OSC/32K
OSC/64K
OSC/128K
OSC/256K
OSC/512K
OSC/1024K
8.9
Internal Voltage Reference Characteristics
WDP0
WDP1
WDP2
WDP3
MCU RESET
WDIF
WDIE
INTERRUPT
The Watchdog Timer (WDT) is a timer counting cycles of a separate on-chip 128kHz oscillator. The WDT gives
an interrupt or a system reset when the counter reaches a given time-out value. In normal operation mode, it is
required that the system uses the WDR - Watchdog Timer Reset - instruction to restart the counter before the
time-out value is reached. If the system doesn't restart the counter, an interrupt or system reset will be issued.
In Interrupt mode, the WDT gives an interrupt when the timer expires. This interrupt can be used to wake the
device from sleep-modes, and also as a general system timer. One example is to limit the maximum time
allowed for certain operations, giving an interrupt when the operation has run longer than expected. In System
Reset mode, the WDT gives a reset when the timer expires. This is typically used to prevent system hang-up in
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case of runaway code. The third mode, Interrupt and System Reset mode, combines the other two modes by
first giving an interrupt and then switch to System Reset mode. This mode will for instance allow a safe
shutdown by saving critical parameters before a system reset.
The Watchdog always on (WDTON) fuse, if programmed, will force the Watchdog Timer to System Reset mode.
With the fuse programmed the System Reset mode bit (WDE) and Interrupt mode bit (WDIE) are locked to 1
and 0 respectively. To further ensure program security, alterations to the Watchdog set-up must follow timed
sequences. The sequence for clearing WDE and changing time-out configuration is as follows:
1. In the same operation, write a logic one to the Watchdog change enable bit (WDCE) and WDE. A logic
one must be written to WDE regardless of the previous value of the WDE bit.
2.
Within the next four clock cycles, write the WDE and Watchdog prescaler bits (WDP) as desired, but with
the WDCE bit cleared. This must be done in one operation.
The following code example shows one assembly and one C function for turning off the Watchdog Timer. The
example assumes that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts will
occur during the execution of these functions.
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Assembly Code Example(1)
WDT_off:
; Turn off global interrupt
cli
; Reset Watchdog Timer
wdr
; Clear WDRF in MCUSR
in
r16, MCUSR
andi r16, (0xff & (0<<WDRF))
out
MCUSR, r16
; Write logical one to WDCE and WDE
; Keep old prescaler setting to prevent
unintentional time-out
in
r16, WDTCSR
ori
r16, (1<<WDCE) | (1<<WDE)
out
WDTCSR, r16
; Turn off WDT
ldi
r16, (0<<WDE)
out
WDTCSR, r16
; Turn on global interrupt
sei
ret
C Code Example(1)
void WDT_off(void)
{
__disable_interrupt();
__watchdog_reset();
/* Clear WDRF in MCUSR */
MCUSR &= ~(1<<WDRF);
/* Write logical one to WDCE and WDE */
/* Keep old prescaler setting to prevent
unintentional time-out */
WDTCSR |= (1<<WDCE) | (1<<WDE);
/* Turn off WDT */
WDTCSR = 0x00;
__enable_interrupt();
}
Note:
1.
The example code assumes that the part specific header file is included.
Note: If the Watchdog is accidentally enabled, for example by a runaway pointer or brown-out condition, the
device will be reset and the Watchdog Timer will stay enabled. If the code is not set up to handle the Watchdog,
this might lead to an eternal loop of time-out resets. To avoid this situation, the application software should
always clear the Watchdog System Reset Flag (WDRF) and the WDE control bit in the initialization routine,
even if the Watchdog is not in use.
The following code example shows one assembly and one C function for changing the time-out value of the
Watchdog Timer.
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Assembly Code Example(1)
WDT_Prescaler_Change:
; Turn off global interrupt
cli
; Reset Watchdog Timer
wdr
; Start timed sequence
in
r16, WDTCSR
ori
r16, (1<<WDCE) | (1<<WDE)
out
WDTCSR, r16
; -- Got four cycles to set the new values from
here ; Set new prescaler(time-out) value = 64K cycles
(~0.5 s)
ldi
r16, (1<<WDE) | (1<<WDP2) | (1<<WDP0)
out
WDTCSR, r16
; -- Finished setting new values, used 2 cycles ; Turn on global interrupt
sei
ret
C Code Example(1)
void WDT_Prescaler_Change(void)
{
__disable_interrupt();
__watchdog_reset();
/* Start timed sequence */
WDTCSR |= (1<<WDCE) | (1<<WDE);
/* Set new prescaler(time-out) value = 64K cycles
(~0.5 s) */
WDTCSR = (1<<WDE) | (1<<WDP2) | (1<<WDP0);
__enable_interrupt();
}
Note:
1.
The example code assumes that the part specific header file is included.
Note: The Watchdog Timer should be reset before any change of the WDP bits, since a change in the WDP bits
can result in a time-out when switching to a shorter time-out period.
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8.10
Register Description
8.11
MCU Status Register – MCUSR
The MCU Status Register provides information on which reset source caused an MCU reset.
Bit
Read/Write
Initial Value
7
–
R
0
6
–
R
0
5
USBRF
R
0
4
3
JTRF
WDRF
R/W
R/W
See Bit Description
2
BORF
R/W
1
EXTRF
R/W
0
PORF
R/W
MCUSR
• Bit 7..6 - Reserved
These bits are reserved and should be read as 0. Do not set these bits.
• Bit 5– USBRF: USB Reset Flag
This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by the JTAG
instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic zero to the flag.
• Bit 4 – JTRF: JTAG Reset Flag
This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by the JTAG
instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic zero to the flag.
• Bit 3 – WDRF: Watchdog Reset Flag
This bit is set if a Watchdog Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the
flag.
• Bit 2 – BORF: Brown-out Reset Flag
This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the
flag.
• Bit 1 – EXTRF: External Reset Flag
This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the
flag.
• Bit 0 – PORF: Power-on Reset Flag
This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to the flag.
To make use of the Reset Flags to identify a reset condition, the user should read and then Reset the MCUSR
as early as possible in the program. If the register is cleared before another reset occurs, the source of the reset
can be found by examining the Reset Flags.
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8.11.1 Watchdog Timer Control Register - WDTCSR
Bit
Read/Write
Initial Value
7
WDIF
R/W
0
6
WDIE
R/W
0
5
WDP3
R/W
0
4
WDCE
R/W
0
3
WDE
R/W
X
2
WDP2
R/W
0
1
WDP1
R/W
0
0
WDP0
R/W
0
WDTCSR
• Bit 7 - WDIF: Watchdog Interrupt Flag
This bit is set when a time-out occurs in the Watchdog Timer and the Watchdog Timer is configured for interrupt.
WDIF is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, WDIF is
cleared by writing a logic one to the flag. When the I-bit in SREG and WDIE are set, the Watchdog Time-out
Interrupt is executed.
• Bit 6 - WDIE: Watchdog Interrupt Enable
When this bit is written to one and the I-bit in the Status Register is set, the Watchdog Interrupt is enabled. If
WDE is cleared in combination with this setting, the Watchdog Timer is in Interrupt Mode, and the
corresponding interrupt is executed if time-out in the Watchdog Timer occurs.
If WDE is set, the Watchdog Timer is in Interrupt and System Reset Mode. The first time-out in the Watchdog
Timer will set WDIF. Executing the corresponding interrupt vector will clear WDIE and WDIF automatically by
hardware (the Watchdog goes to System Reset Mode). This is useful for keeping the Watchdog Timer security
while using the interrupt. To stay in Interrupt and System Reset Mode, WDIE must be set after each interrupt.
This should however not be done within the interrupt service routine itself, as this might compromise the safetyfunction of the Watchdog System Reset mode. If the interrupt is not executed before the next time-out, a System
Reset will be applied.
Table 8-4.
Watchdog Timer Configuration
WDTON
WDE
WDIE
Mode
Action on Time-out
0
0
0
Stopped
None
0
0
1
Interrupt Mode
Interrupt
0
1
0
System Reset Mode
Reset
0
1
1
Interrupt and System Reset
Mode
Interrupt, then go to System
Reset Mode
1
x
x
System Reset Mode
Reset
• Bit 4 - WDCE: Watchdog Change Enable
This bit is used in timed sequences for changing WDE and prescaler bits. To clear the WDE bit, and/or change
the prescaler bits, WDCE must be set.
Once written to one, hardware will clear WDCE after four clock cycles.
• Bit 3 - WDE: Watchdog System Reset Enable
WDE is overridden by WDRF in MCUSR. This means that WDE is always set when WDRF is set. To clear
WDE, WDRF must be cleared first. This feature ensures multiple resets during conditions causing failure, and a
safe start-up after the failure.
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• Bit 5, 2..0 - WDP3..0: Watchdog Timer Prescaler 3, 2, 1, and 0
The WDP3..0 bits determine the Watchdog Timer prescaling when the Watchdog Timer is running. The different
prescaling values and their corresponding time-out periods are shown in the following table.
Table 8-5.
Watchdog Timer Prescale Select
WDP3
WDP2
WDP1
WDP0
Number of WDT Oscillator
Cycles
Typical Time-out at
VCC = 5.0V
0
0
0
0
2K (2048) cycles
16ms
0
0
0
1
4K (4096) cycles
32ms
0
0
1
0
8K (8192) cycles
64ms
0
0
1
1
16K (16384) cycles
0.125s
0
1
0
0
32K (32768) cycles
0.25s
0
1
0
1
64K (65536) cycles
0.5s
0
1
1
0
128K (131072) cycles
1.0s
0
1
1
1
256K (262144) cycles
2.0s
1
0
0
0
512K (524288) cycles
4.0s
1
0
0
1
1024K (1048576) cycles
8.0s
1
0
1
0
1
0
1
1
1
1
0
0
1
1
0
1
1
1
1
0
1
1
1
1
Reserved
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9.
Interrupts
This chapter describes the specifics of the interrupt handling as performed in ATmega16U4/ATmega32U4. For
a general explanation of the AVR interrupt handling, refer to “Reset and Interrupt Handling” on page 15.
9.1
Interrupt Vectors in ATmega16U4/ATmega32U4
Table 9-1.
Reset and Interrupt Vectors(cont’d)
Vector
No.
Program
Address(2)
Source
Interrupt Definition
1
$0000(1)
RESET
External Pin, Power-on Reset, Brown-out Reset, Watchdog
Reset, and JTAG AVR Reset
2
$0002
INT0
External Interrupt Request 0
3
$0004
INT1
External Interrupt Request 1
4
$0006
INT2
External Interrupt Request 2
5
$0008
INT3
External Interrupt Request 3
6
$000A
Reserved
Reserved
7
$000C
Reserved
Reserved
8
$000E
INT6
External Interrupt Request 6
9
$0010
Reserved
Reserved
10
$0012
PCINT0
Pin Change Interrupt Request 0
11
$0014
USB General
USB General Interrupt request
12
$0016
USB Endpoint
USB Endpoint Interrupt request
13
$0018
WDT
Watchdog Time-out Interrupt
14
$001A
Reserved
Reserved
15
$001C
Reserved
Reserved
16
$001E
Reserved
Reserved
17
$0020
TIMER1 CAPT
Timer/Counter1 Capture Event
18
$0022
TIMER1 COMPA
Timer/Counter1 Compare Match A
19
$0024
TIMER1 COMPB
Timer/Counter1 Compare Match B
20
$0026
TIMER1 COMPC
Timer/Counter1 Compare Match C
21
$0028
TIMER1 OVF
Timer/Counter1 Overflow
22
$002A
TIMER0 COMPA
Timer/Counter0 Compare Match A
23
$002C
TIMER0 COMPB
Timer/Counter0 Compare match B
24
$002E
TIMER0 OVF
Timer/Counter0 Overflow
25
$0030
SPI (STC)
SPI Serial Transfer Complete
26
$0032
USART1 RX
USART1 Rx Complete
27
$0034
USART1 UDRE
USART1 Data Register Empty
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Table 9-1.
Reset and Interrupt Vectors(cont’d)
Vector
No.
Program
Address(2)
28
Source
Interrupt Definition
$0036
USART1TX
USART1 Tx Complete
29
$0038
ANALOG COMP
Analog Comparator
30
$003A
ADC
ADC Conversion Complete
31
$003C
EE READY
EEPROM Ready
32
$003E
TIMER3 CAPT
Timer/Counter3 Capture Event
33
$0040
TIMER3 COMPA
Timer/Counter3 Compare Match A
34
$0042
TIMER3 COMPB
Timer/Counter3 Compare Match B
35
$0044
TIMER3 COMPC
Timer/Counter3 Compare Match C
36
$0046
TIMER3 OVF
Timer/Counter3 Overflow
37
$0048
TWI
2-wire Serial Interface
38
$004A
SPM READY
Store Program Memory Ready
39
$004C
TIMER4 COMPA
Timer/Counter4 Compare Match A
40
$004E
TIMER4 COMPB
Timer/Counter4 Compare Match B
41
$0050
TIMER4 COMPD
Timer/Counter4 Compare Match D
42
$0052
TIMER4 OVF
Timer/Counter4 Overflow
43
$0054
TIMER4 FPF
Timer/Counter4 Fault Protection Interrupt
Notes:
1.
2.
When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader address at reset, see
“Memory Programming” on page 353.
When the IVSEL bit in MCUCR is set, Interrupt Vectors will be moved to the start of the Boot Flash Section.
The address of each Interrupt Vector will then be the address in this table added to the start address of the
Boot Flash Section.
The table shows reset and Interrupt Vectors placement for the various combinations of BOOTRST and IVSEL
settings. If the program never enables an interrupt source, the Interrupt Vectors are not used, and regular
program code can be placed at these locations. This is also the case if the Reset Vector is in the Application
section while the Interrupt Vectors are in the Boot section or vice versa.
Table 9-2.
BOOTRST
IVSEL
1
Note:
9.1.1
Reset and Interrupt Vectors Placement
Reset Address
Interrupt Vectors Start Address
0
0x0000
0x0002
1
1
0x0000
Boot Reset Address + 0x0002
0
0
Boot Reset Address
0x0002
0
1
Boot Reset Address
Boot Reset Address + 0x0002
1.
The Boot Reset Address is shown in Table 27-4 on page 340. For the BOOTRST Fuse “1” means
unprogrammed while “0” means programmed.
Moving Interrupts Between Application and Boot Space
The General Interrupt Control Register controls the placement of the Interrupt Vector table.
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9.2
Register Description
9.2.1
MCU Control Register – MCUCR
Bit
Read/Write
Initial Value
7
JTD
R/W
0
6
–
R
0
5
–
R
0
4
PUD
R/W
0
3
–
R
0
2
–
R
0
1
IVSEL
R/W
0
0
IVCE
R/W
0
MCUCR
• Bit 1 – IVSEL: Interrupt Vector Select
When the IVSEL bit is cleared (zero), the Interrupt Vectors are placed at the start of the Flash memory. When
this bit is set (one), the Interrupt Vectors are moved to the beginning of the Boot Loader section of the Flash.
The actual address of the start of the Boot Flash Section is determined by the BOOTSZ Fuses. Refer to the
section “Memory Programming” on page 353 for details. To avoid unintentional changes of Interrupt Vector
tables, a special write procedure must be followed to change the IVSEL bit:
1. Write the Interrupt Vector Change Enable (IVCE) bit to one.
2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.
Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the cycle
IVCE is set, and they remain disabled until after the instruction following the write to IVSEL. If IVSEL is not
written, interrupts remain disabled for four cycles. The I-bit in the Status Register is unaffected by the automatic
disabling.
Note:
If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is programmed, interrupts are
disabled while executing from the Application section. If Interrupt Vectors are placed in the Application section and
Boot Lock bit BLB12 is programed, interrupts are disabled while executing from the Boot Loader section. Refer to
the section “Memory Programming” on page 353 for details on Boot Lock bits.
• Bit 0 – IVCE: Interrupt Vector Change Enable
The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware four
cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as explained in the
IVSEL description above. See Code Example below.
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Assembly Code Example
Move_interrupts:
; Enable change of Interrupt Vectors
ldi
r16, (1<<IVCE)
out
MCUCR, r16
; Move interrupts to Boot Flash section
ldi
r16, (1<<IVSEL)
out
MCUCR, r16
ret
C Code Example
void Move_interrupts(void)
{
/* Enable change of Interrupt Vectors */
MCUCR = (1<<IVCE);
/* Move interrupts to Boot Flash section */
MCUCR = (1<<IVSEL);
}
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10.
I/O-Ports
10.1
Introduction
All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This means that
the direction of one port pin can be changed without unintentionally changing the direction of any other pin with
the SBI and CBI instructions. The same applies when changing drive value (if configured as output) or
enabling/disabling of pull-up resistors (if configured as input). Each output buffer has symmetrical drive
characteristics with both high sink and source capability. The pin driver is strong enough to drive LED displays
directly. All port pins have individually selectable pull-up resistors with a supply-voltage invariant resistance. All
I/O pins have protection diodes to both VCC and Ground as indicated in Figure 10-1. Refer to “Electrical
Characteristics” on page 383 for a complete list of parameters.
Figure 10-1.
I/O Pin Equivalent Schematic
All registers and bit references in this section are written in general form. A lower case “x” represents the
numbering letter for the port, and a lower case “n” represents the bit number. However, when using the register
or bit defines in a program, the precise form must be used. For example, PORTB3 for bit no. 3 in Port B, here
documented generally as PORTxn. The physical I/O Registers and bit locations are listed in “” on page 83.
Three I/O memory address locations are allocated for each port, one each for the Data Register – PORTx, Data
Direction Register – DDRx, and the Port Input Pins – PINx. The Port Input Pins I/O location is read only, while
the Data Register and the Data Direction Register are read/write. However, writing a logic one to a bit in the
PINx Register, will result in a toggle in the corresponding bit in the Data Register. In addition, the Pull-up Disable
– PUD bit in MCUCR disables the pull-up function for all pins in all ports when set.
Using the I/O port as General Digital I/O is described in “Ports as General Digital I/O” on page 67. Most port pins
are multiplexed with alternate functions for the peripheral features on the device. How each alternate function
interferes with the port pin is described in “Alternate Port Functions” on page 72. Refer to the individual module
sections for a full description of the alternate functions.
Note that enabling the alternate function of some of the port pins does not affect the use of the other pins in the
port as general digital I/O.
10.2
Ports as General Digital I/O
The ports are bi-directional I/O ports with optional internal pull-ups. Figure 10-2 shows a functional description of
one I/O-port pin, here generically called Pxn.
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Figure 10-2.
General Digital I/O(1)
PUD
Q
D
DDxn
Q CLR
WDx
RESET
DATA BUS
RDx
1
Q
Pxn
D
0
PORTxn
Q CLR
RESET
WRx
WPx
RRx
SLEEP
SYNCHRONIZER
D
Q
L
Q
D
RPx
Q
PINxn
Q
clk I/O
PUD:
SLEEP:
clkI/O:
Note:
1.
PULLUP DISABLE
SLEEP CONTROL
I/O CLOCK
WDx:
RDx:
WRx:
RRx:
RPx:
WPx:
WRITE DDRx
READ DDRx
WRITE PORTx
READ PORTx REGISTER
READ PORTx PIN
WRITE PINx REGISTER
WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O, SLEEP, and PUD
are common to all ports.
10.2.1 Configuring the Pin
Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in “” on page 83, the DDxn
bits are accessed at the DDRx I/O address, the PORTxn bits at the PORTx I/O address, and the PINxn bits at
the PINx I/O address.
The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one, Pxn is
configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input pin.
If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated. To
switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to be configured as an output
pin. The port pins are tri-stated when reset condition becomes active, even if no clocks are running.
If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven high (one). If
PORTxn is written logic zero when the pin is configured as an output pin, the port pin is driven low (zero).
10.2.2 Toggling the Pin
Writing a logic one to PINxn toggles the value of PORTxn, independent on the value of DDRxn. Note that the
SBI instruction can be used to toggle one single bit in a port.
10.2.3 Switching Between Input and Output
When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn} = 0b11), an
intermediate state with either pull-up enabled {DDxn, PORTxn} = 0b01) or output low ({DDxn, PORTxn} = 0b10)
occurs. Normally, the pull-up enabled state is fully acceptable, as a high-impedance environment will not notice
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the difference between a strong high driver and a pull-up. If this is not the case, the PUD bit in the MCUCR
Register can be set to disable all pull-ups in all ports.
Switching between input with pull-up and output low generates the same problem. The user must use either the
tri-state ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn} = 0b11) as an intermediate step.
Table 10-1 summarizes the control signals for the pin value
Table 10-1.
Port Pin Configurations
DDxn
PORTxn
PUD
(in MCUCR)
I/O
Pull-up
0
0
X
Input
No
Tri-state (Hi-Z)
0
1
0
Input
Yes
Pxn will source current if ext. pulled low
0
1
1
Input
No
Tri-state (Hi-Z)
1
0
X
Output
No
Output Low (Sink)
1
1
X
Output
No
Output High (Source)
Comment
10.2.4 Reading the Pin Value
Independent of the setting of Data Direction bit DDxn, the port pin can be read through the PINxn Register bit.
As shown in Figure 10-2 on page 68, the PINxn Register bit and the preceding latch constitute a synchronizer.
This is needed to avoid metastability if the physical pin changes value near the edge of the internal clock, but it
also introduces a delay. Figure 10-3 on page 69 shows a timing diagram of the synchronization when reading an
externally applied pin value. The maximum and minimum propagation delays are denoted tpd,max and tpd,min
respectively.
Figure 10-3.
Synchronization when Reading an Externally Applied Pin Value
SYSTEM CLK
INSTRUCTIONS
XXX
XXX
in r17, PINx
SYNC LATCH
PINxn
r17
0x00
0xFF
t pd, max
t pd, min
Consider the clock period starting shortly after the first falling edge of the system clock. The latch is closed when
the clock is low, and goes transparent when the clock is high, as indicated by the shaded region of the “SYNC
LATCH” signal. The signal value is latched when the system clock goes low. It is clocked into the PINxn
Register at the succeeding positive clock edge. As indicated by the two arrows tpd max. and tpd min., a single
signal transition on the pin will be delayed between ½ and 1½ system clock period depending upon the time of
assertion.
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When reading back a software assigned pin value, a nop instruction must be inserted as indicated in Figure 104. The out instruction sets the “SYNC LATCH” signal at the positive edge of the clock. In this case, the delay tpd
through the synchronizer is one system clock period.
Figure 10-4.
Synchronization when Reading a Software Assigned Pin Value
SYSTEM CLK
r16
INSTRUCTIONS
0xFF
out PORTx, r16
nop
in r17, PINx
SYNC LATCH
PINxn
r17
0x00
0xFF
t pd
The following code example shows how to set port B pins 0 and 1 high, 2 and 3 low, and define the port pins
from 4 to 7 as input with pull-ups assigned to port pins 6 and 7. The resulting pin values are read back again, but
as previously discussed, a nop instruction is included to be able to read back the value recently assigned to
some of the pins.
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Assembly Code Example(1)
...
; Define pull-ups and set outputs high
; Define directions for port pins
ldi
r16,(1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0)
ldi
r17,(1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0)
out
PORTB,r16
out
DDRB,r17
; Insert nop for synchronization
nop
; Read port pins
in
r16,PINB
...
C Code Example
unsigned char i;
...
/* Define pull-ups and set outputs high */
/* Define directions for port pins */
PORTB = (1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0);
DDRB = (1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0);
/* Insert nop for synchronization*/
__no_operation();
/* Read port pins */
i = PINB;
...
Note:
1.
For the assembly program, two temporary registers are used to minimize the time from pull-ups are set on pins
0, 1, 6, and 7, until the direction bits are correctly set, defining bit 2 and 3 as low and redefining bits 0 and 1 as
strong high drivers.
10.2.5 Digital Input Enable and Sleep Modes
As shown in Figure 10-2, the digital input signal can be clamped to ground at the input of the Schmidt-trigger.
The signal denoted SLEEP in the figure, is set by the MCU Sleep Controller in Power-down mode, Power-save
mode, and Standby mode to avoid high power consumption if some input signals are left floating, or have an
analog signal level close to VCC/2.
SLEEP is overridden for port pins enabled as external interrupt pins. If the external interrupt request is not
enabled, SLEEP is active also for these pins. SLEEP is also overridden by various other alternate functions as
described in “Alternate Port Functions” on page 72.
If a logic high level (“one”) is present on an asynchronous external interrupt pin configured as “Interrupt on
Rising Edge, Falling Edge, or Any Logic Change on Pin” while the external interrupt is not enabled, the
corresponding External Interrupt Flag will be set when resuming from the above mentioned Sleep mode, as the
clamping in these sleep mode produces the requested logic change.
10.2.6 Unconnected Pins
If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though most of
the digital inputs are disabled in the deep sleep modes as described above, floating inputs should be avoided to
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reduce current consumption in all other modes where the digital inputs are enabled (Reset, Active mode, and
Idle mode).
The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up. In this case,
the pull-up will be disabled during reset. If low power consumption during reset is important, it is recommended
to use an external pull-up or pull-down. Connecting unused pins directly to VCC or GND is not recommended,
since this may cause excessive currents if the pin is accidentally configured as an output.
Alternate Port Functions
Most port pins have alternate functions in addition to being general digital I/Os. Figure 10-5 shows how the port
pin control signals from the simplified Figure 10-2 on page 68 can be overridden by alternate functions. The
overriding signals may not be present in all port pins, but the figure serves as a generic description applicable to
all port pins in the AVR microcontroller family.
Figure 10-5.
Alternate Port Functions(1)
PUOExn
PUOVxn
1
PUD
0
DDOExn
DDOVxn
1
Q
D
DDxn
0
Q CLR
WDx
PVOExn
RESET
RDx
PVOVxn
1
DATA BUS
10.3
1
Pxn
Q
0
D
0
PORTxn
PTOExn
Q CLR
DIEOExn
WPx
DIEOVxn
RESET
WRx
1
0
RRx
SLEEP
SYNCHRONIZER
D
SET
Q
RPx
Q
D
PINxn
L
CLR
Q
CLR
Q
clk I/O
DIxn
AIOxn
PUOExn:
PUOVxn:
DDOExn:
DDOVxn:
PVOExn:
PVOVxn:
DIEOExn:
DIEOVxn:
SLEEP:
PTOExn:
Note:
1.
Pxn PULL-UP OVERRIDE ENABLE
Pxn PULL-UP OVERRIDE VALUE
Pxn DATA DIRECTION OVERRIDE ENABLE
Pxn DATA DIRECTION OVERRIDE VALUE
Pxn PORT VALUE OVERRIDE ENABLE
Pxn PORT VALUE OVERRIDE VALUE
Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE
Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE
SLEEP CONTROL
Pxn, PORT TOGGLE OVERRIDE ENABLE
PUD:
WDx:
RDx:
RRx:
WRx:
RPx:
WPx:
clkI/O:
DIxn:
AIOxn:
PULLUP DISABLE
WRITE DDRx
READ DDRx
READ PORTx REGISTER
WRITE PORTx
READ PORTx PIN
WRITE PINx
I/O CLOCK
DIGITAL INPUT PIN n ON PORTx
ANALOG INPUT/OUTPUT PIN n ON PORTx
WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O, SLEEP, and PUD
are common to all ports. All other signals are unique for each pin.
The table summarizes the function of the overriding signals. The pin and port indexes from Figure 10-5 on page
72 are not shown in the succeeding tables. The overriding signals are generated internally in the modules
having the alternate function.
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Table 10-2.
Generic Description of Overriding Signals for Alternate Functions
Signal Name
Full Name
Description
PUOE
Pull-up Override
Enable
If this signal is set, the pull-up enable is controlled by the PUOV
signal. If this signal is cleared, the pull-up is enabled when {DDxn,
PORTxn, PUD} = 0b010.
PUOV
Pull-up Override
Value
If PUOE is set, the pull-up is enabled/disabled when PUOV is
set/cleared, regardless of the setting of the DDxn, PORTxn, and
PUD Register bits.
DDOE
Data Direction
Override Enable
If this signal is set, the Output Driver Enable is controlled by the
DDOV signal. If this signal is cleared, the Output driver is enabled
by the DDxn Register bit.
DDOV
Data Direction
Override Value
If DDOE is set, the Output Driver is enabled/disabled when DDOV
is set/cleared, regardless of the setting of the DDxn Register bit.
PVOE
Port Value
Override Enable
If this signal is set and the Output Driver is enabled, the port value
is controlled by the PVOV signal. If PVOE is cleared, and the
Output Driver is enabled, the port Value is controlled by the
PORTxn Register bit.
PVOV
Port Value
Override Value
If PVOE is set, the port value is set to PVOV, regardless of the
setting of the PORTxn Register bit.
PTOE
Port Toggle
Override Enable
If PTOE is set, the PORTxn Register bit is inverted.
DIEOE
Digital Input
Enable Override
Enable
If this bit is set, the Digital Input Enable is controlled by the DIEOV
signal. If this signal is cleared, the Digital Input Enable is
determined by MCU state (Normal mode, sleep mode).
DIEOV
Digital Input
Enable Override
Value
If DIEOE is set, the Digital Input is enabled/disabled when DIEOV
is set/cleared, regardless of the MCU state (Normal mode, sleep
mode).
DI
Digital Input
This is the Digital Input to alternate functions. In the figure, the
signal is connected to the output of the schmitt trigger but before
the synchronizer. Unless the Digital Input is used as a clock
source, the module with the alternate function will use its own
synchronizer.
AIO
Analog
Input/Output
This is the Analog Input/output to/from alternate functions. The
signal is connected directly to the pad, and can be used bidirectionally.
The following subsections shortly describe the alternate functions for each port, and relate the overriding signals
to the alternate function. Refer to the alternate function description for further details.
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10.3.1 Alternate Functions of Port B
The Port B pins with alternate functions are shown below.
Table 10-3.
Port Pin
Port B Pins Alternate Functions
Alternate Functions
PB7
OC0A/OC1C/PCINT7/RTS (Output Compare and PWM Output A for Timer/Counter0,
Output Compare and PWM Output C for Timer/Counter1 or Pin Change Interrupt 7 or UART
flow control RTS signal)
PB6
OC1B/PCINT6/OC.4B/ADC13 (Output Compare and PWM Output B for Timer/Counter1 or
Pin Change Interrupt 6 or Timer 4 Output Compare B / PWM output or Analog to Digital
Converter channel 13)
PB5
OC1A/PCINT5/OC.4B/ADC12 (Output Compare and PWM Output A for Timer/Counter1 or
Pin Change Interrupt 5 or Timer 4 Complementary Output Compare B / PWM output or
Analog to Digital Converter channel 12)
PB4
PCINT4/ADC11 (Pin Change Interrupt 4 or Analog to Digital Converter channel 11)
PB3
PDO/MISO/PCINT3 (Programming Data Output or SPI Bus Master Input/Slave Output or
Pin Change Interrupt 3)
PB2
PDI/MOSI/PCINT2 (Programming Data Input or SPI Bus Master Output/Slave Input or Pin
Change Interrupt 2)
PB1
SCK/PCINT1 (SPI Bus Serial Clock or Pin Change Interrupt 1)
PB0
SS/PCINT0 (SPI Slave Select input or Pin Change Interrupt 0)
The alternate pin configuration is as follows:
• OC0A/OC1C/PCINT7/RTS, Bit 7
OC0A, Output Compare Match A output: The PB7 pin can serve as an external output for the Timer/Counter0
Output Compare. The pin has to be configured as an output (DDB7 set “one”) to serve this function. The OC0A
pin is also the output pin for the PWM mode timer function.
OC1C, Output Compare Match C output: The PB7 pin can serve as an external output for the Timer/Counter1
Output Compare C. The pin has to be configured as an output (DDB7 set “one”) to serve this function. The
OC1C pin is also the output pin for the PWM mode timer function.
PCINT7, Pin Change Interrupt source 7: The PB7 pin can serve as an external interrupt source.
RTS: RTS flow control signal used by enhanced UART.
• OC1B/PCINT6/OC.4B/ADC12, Bit 6
OC1B, Output Compare Match B output: The PB6 pin can serve as an external output for the Timer/Counter1
Output Compare B. The pin has to be configured as an output (DDB6 set “one”) to serve this function. The
OC1B pin is also the output pin for the PWM mode timer function.
PCINT6, Pin Change Interrupt source 6: The PB7 pin can serve as an external interrupt source.
OC.4B: Timer 4 Output Compare B. This pin can be used to generate a high-speed PWM signal from Timer 4
module. The pin has to be configured as an output (DDB6 set “one”) to serve this function.
ADC13: Analog to Digital Converter, channel 13.
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• OC1A/PCINT5/OC.4B/ADC12, Bit 5
OC1A, Output Compare Match A output: The PB5 pin can serve as an external output for the Timer/Counter1
Output Compare A. The pin has to be configured as an output (DDB5 set (one)) to serve this function. The
OC1A pin is also the output pin for the PWM mode timer function.
PCINT5, Pin Change Interrupt source 5: The PB7 pin can serve as an external interrupt source.
OC.4B: Timer 4 Output Compare B. This pin can be used to generate a high-speed PWM signal from Timer 4
module, complementary to OC.4B (PB5) signal. The pin has to be configured as an output (DDB5 set (one)) to
serve this function.
ADC12: Analog to Digital Converter, channel 12.
• PCINT4/ADC11, Bit 4
PCINT4, Pin Change Interrupt source 4: The PB7 pin can serve as an external interrupt source.
ADC11, Analog to Digital Converter channel 11.
• PDO/MISO/PCINT3 – Port B, Bit 3
PDO, SPI Serial Programming Data Output. During Serial Program Downloading, this pin is used as data output
line for the ATmega16U4/ATmega32U4.
MISO: Master Data input, Slave Data output pin for SPI channel. When the SPI is enabled as a master, this pin
is configured as an input regardless of the setting of DDB3. When the SPI is enabled as a slave, the data
direction of this pin is controlled by DDB3. When the pin is forced to be an input, the pull-up can still be
controlled by the PORTB3 bit.
PCINT3, Pin Change Interrupt source 3: The PB7 pin can serve as an external interrupt source.
• PDI/MOSI/PCINT2 – Port B, Bit 2
PDI, SPI Serial Programming Data Input. During Serial Program Downloading, this pin is used as data input line
for the ATmega16U4/ATmega32U4.
MOSI: SPI Master Data output, Slave Data input for SPI channel. When the SPI is enabled as a slave, this pin is
configured as an input regardless of the setting of DDB2. When the SPI is enabled as a master, the data
direction of this pin is controlled by DDB2. When the pin is forced to be an input, the pull-up can still be
controlled by the PORTB2 bit.
PCINT2, Pin Change Interrupt source 2: The PB7 pin can serve as an external interrupt source.
• SCK/PCINT1 – Port B, Bit 1
SCK: Master Clock output, Slave Clock input pin for SPI channel. When the SPI is enabled as a slave, this pin is
configured as an input regardless of the setting of DDB1. When the SPI0 is enabled as a master, the data
direction of this pin is controlled by DDB1. When the pin is forced to be an input, the pull-up can still be
controlled by the PORTB1 bit.
PCINT1, Pin Change Interrupt source 1: The PB7 pin can serve as an external interrupt source.
• SS/PCINT0 – Port B, Bit 0
SS: Slave Port Select input. When the SPI is enabled as a slave, this pin is configured as an input regardless of
the setting of DDB0. As a slave, the SPI is activated when this pin is driven low. When the SPI is enabled as a
master, the data direction of this pin is controlled by DDB0. When the pin is forced to be an input, the pull-up can
still be controlled by the PORTB0 bit.
Table 10-4 and Table 10-5 on page 76 relate the alternate functions of Port B to the overriding signals shown in
Figure 10-5 on page 72. SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the MISO signal, while MOSI is
divided into SPI MSTR OUTPUT and SPI SLAVE INPUT.
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PCINT0, Pin Change Interrupt source 0: The PB7 pin can serve as an external interrupt source
Table 10-4.
Overriding Signals for Alternate Functions in PB7..PB4
Signal
Name
PB7/PCINT7/OC0A/O
C1C/RTS
PB6/PCINT6/OC1B/
OC.4B/ADC13
PB5/PCINT5/OC1A/
OC.4B/ADC12
PB4/PCINT4/ADC
11
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
0
0
0
0
DDOV
0
0
0
0
PVOE
OC0/OC1C ENABLE
OC1B ENABLE
OC1A ENABLE
0
PVOV
OC0/OC1C
OC1B
OC1A
0
DIEOE
PCINT7 • PCIE0
PCINT6 • PCIE0
PCINT5 • PCIE0
PCINT4 • PCIE0
DIEOV
1
1
1
1
DI
PCINT7 INPUT
PCINT6 INPUT
PCINT5 INPUT
PCINT4 INPUT
AIO
–
–
–
–
Table 10-5.
Overriding Signals for Alternate Functions in PB3..PB0
Signal
Name
PB3/PD0/PCINT3/
MISO
PB2/PDI/PCINT2/
MOSI
PB1/PCINT1/
SCK
PB0/PCINT0/
SS
PUOE
SPE • MSTR
SPE • MSTR
SPE • MSTR
SPE • MSTR
PUOV
PORTB3 • PUD
PORTB2 • PUD
PORTB1 • PUD
PORTB0 • PUD
DDOE
SPE • MSTR
SPE • MSTR
SPE • MSTR
SPE • MSTR
DDOV
0
0
0
0
PVOE
SPE • MSTR
SPE • MSTR
SPE • MSTR
0
PVOV
SPI SLAVE OUTPUT
SPI MSTR OUTPUT
SCK OUTPUT
0
DIEOE
PCINT3 • PCIE0
PCINT2 • PCIE0
PCINT1 • PCIE0
PCINT0 • PCIE0
DIEOV
1
1
1
1
SPI MSTR INPUT
SPI SLAVE INPUT
SCK INPUT
SPI SS
PCINT3 INPUT
PCINT2 INPUT
PCINT1 INPUT
PCINT0 INPUT
–
–
–
–
DI
AIO
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10.3.2 Alternate Functions of Port C
The Port C pins with alternate functions are shown below.
Table 10-6.
Port C Pins Alternate Functions
Port Pin
Alternate Function
PC7
ICP3/CLKO/OC4A(Input Capture Timer 3 or CLK0 (Divided System
Clock) or Output Compare and direct PWM output A for Timer 4)
PC6
OC.3A/OC4A (Output Compare and PWM output A for Timer/Counter3
or Output Compare and complementary PWM output A for Timer 4)
PC5
PC4
PC3
PC2
Not present on pin-out.
PC1
PC0
• ICP3/CLKO/OC.4A – Port C, Bit 7
ICP3: If Timer 3 is correctly configured, this pin can serve as Input Capture feature.
CLKO: When the corresponding fuse is enabled, this pin outputs the internal microcontroller working frequency.
If the clock prescaler is used, this will affect this output frequency.
OC.4A: Timer 4 Output Compare A. This pin can be used to generate a high-speed PWM signal from Timer 4
module. The pin has to be configured as an output (DDC7 set “one”) to serve this function.
• OC.3A/OC.4A – Port C, Bit 6
OC.3A: Timer 3 Output Compare A. This pin can be used to generate a PWM signal from Timer 3 module.
OC.4A: Timer 4 Output Compare A. This pin can be used to generate a high-speed PWM signal from Timer 4
module, complementary to OC.4A (PC7) signal. The pin has to be configured as an output (DDC6 set “one”) to
serve this function.
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The following table shows the alternate functions of Port C to the overriding signals shown in Figure 10-5 on
page 72.
Table 10-7.
Overriding Signals for Alternate Functions in PC7..PC6
Signal Name
PC7/ICP3/CLKO/OC.4A
PC6/OC.3A/OC.4A
PUOE
SRE • (XMM<1)
SRE • (XMM<2)|OC3A enable
PUOV
0
0
DDOE
SRE • (XMM<1)
SRE • (XMM<2)
DDOV
1
1
PVOE
SRE • (XMM<1)
SRE • (XMM<2)
PVOV
A15
if (SRE.XMM<2) then A14 else OC3A
DIEOE
0
0
DIEOV
0
0
DI
ICP3 input
–
AIO
–
–
10.3.3 Alternate Functions of Port D
The Port D pins with alternate functions are shown below.
Table 10-8.
Port Pin
Port D Pins Alternate Functions
Alternate Function
PD7
T0/OC.4D/ADC10 (Timer/Counter0 Clock Input or Timer 4 Output Compare D / PWM
output or Analog to Digital Converter channel 10)
PD6
T1/OC.4D/ADC9 (Timer/Counter1 Clock Input or Timer 4 Output Complementary
Compare D / PWM output or Analog to Digital Converter channel 9)
PD5
XCK1/CTS (USART1 External Clock Input/Output or UART flow control CTS signal)
PD4
ICP1/ADC8 (Timer/Counter1 Input Capture Trigger or Analog to Digital Converter
channel 8)
PD3
INT3/TXD1 (External Interrupt3 Input or USART1 Transmit Pin)
PD2
INT2/RXD1 (External Interrupt2 Input or USART1 Receive Pin)
PD1
INT1/SDA (External Interrupt1 Input or TWI Serial DAta)
PD0
INT0/SCL/OC0B (External Interrupt0 Input or TWI Serial CLock or Output Compare for
Timer/Counter0)
The alternate pin configuration is as follows:
• T0/OC.4D/ADC10 – Port D, Bit 7
T0, Timer/Counter0 counter source.
OC.4D: Timer 4 Output Compare D. This pin can be used to generate a high-speed PWM signal from Timer 4
module. The pin has to be configured as an output (DDD7 set “one”) to serve this function.
ADC10: Analog to Digital Converter, Channel 10.
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• T1/OC.4D/ADC9 – Port D, Bit 6
T1, Timer/Counter1 counter source.
OC.4D: Timer 4 Output Compare D. This pin can be used to generate a high-speed PWM signal from Timer 4
module, complementary to OC.4D (PD7) signal. The pin has to be configured as an output (DDD6 set “one”) to
serve this function.
ADC9: Analog to Digital Converter, Channel 9.
• XCK1/CTS – Port D, Bit 5
XCK1, USART1 External clock. The Data Direction Register (DDD5) controls whether the clock is output (DDD5
set) or input (DDD5 cleared). The XCK1 pin is active only when the USART1 operates in Synchronous mode.
CTS: Clear-To-Send flow control signal used by enhanced UART module.
• ICP1/ADC8 – Port D, Bit 4
ICP1 – Input Capture Pin 1: The PD4 pin can act as an input capture pin for Timer/Counter1.
ADC8: Analog to Digital Converter, Channel 8.
• INT3/TXD1 – Port D, Bit 3
INT3, External Interrupt source 3: The PD3 pin can serve as an external interrupt source to the MCU.
TXD1, Transmit Data (Data output pin for the USART1). When the USART1 Transmitter is enabled, this pin is
configured as an output regardless of the value of DDD3.
• INT2/RXD1 – Port D, Bit 2
INT2, External Interrupt source 2. The PD2 pin can serve as an External Interrupt source to the MCU.
RXD1, Receive Data (Data input pin for the USART1). When the USART1 receiver is enabled this pin is
configured as an input regardless of the value of DDD2. When the USART forces this pin to be an input, the
pull-up can still be controlled by the PORTD2 bit.
• INT1/SDA – Port D, Bit 1
INT1, External Interrupt source 1. The PD1 pin can serve as an external interrupt source to the MCU.
SDA, 2-wire Serial Interface Data: When the TWEN bit in TWCR is set (one) to enable the 2-wire Serial
Interface, pin PD1 is disconnected from the port and becomes the Serial Data I/O pin for the 2-wire Serial
Interface. In this mode, there is a spike filter on the pin to suppress spikes shorter than 50ns on the input signal,
and the pin is driven by an open drain driver with slew-rate limitation.
• INT0/SCL/OC0B – Port D, Bit 0
INT0, External Interrupt source 0. The PD0 pin can serve as an external interrupt source to the MCU.
SCL, 2-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to enable the 2-wire Serial
Interface, pin PD0 is disconnected from the port and becomes the Serial Clock I/O pin for the 2-wire Serial
Interface. In this mode, there is a spike filter on the pin to suppress spikes shorter than 50ns on the input signal,
and the pin is driven by an open drain driver with slew-rate limitation.
OC.0B: Timer 0 Output Compare B. This pin can be used to generate a PWM signal from the Timer 0 module.
The two following tables relate the alternate functions of Port D to the overriding signals shown in Figure 10-5 on
page 72.
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Table 10-9.
Overriding Signals for Alternate Functions PD7..PD4
Signal Name
PD7/T0/OC4D/ADC10
PD6/T1/OC4D/ADC9
PD5/XCK1/CTS
PD4/ICP1/ADC8
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
0
0
XCK1 OUTPUT ENABLE
0
DDOV
0
0
1
0
PVOE
0
0
XCK1 OUTPUT ENABLE
0
PVOV
0
0
XCK1 OUTPUT
0
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
T0 INPUT
T1 INPUT
XCK1 INPUT
ICP1 INPUT
AIO
–
–
–
–
Table 10-10.
Overriding Signals for Alternate Functions in PD3..PD0(1)
Signal Name
PD3/INT3/TXD1
PD2/INT2/RXD1
PD1/INT1/SDA
PD0/INT0/SCL/OC0B
PUOE
TXEN1
RXEN1
TWEN
TWEN
PUOV
0
PORTD2 • PUD
PORTD1 • PUD
PORTD0 • PUD
DDOE
TXEN1
RXEN1
TWEN
TWEN
DDOV
1
0
SDA_OUT
SCL_OUT
PVOE
TXEN1
0
TWEN ENABLE
TWEN | OC0B ENABLE
PVOV
TXD1
0
0
OC0B
DIEOE
INT3 ENABLE
INT2 ENABLE
INT1 ENABLE
INT0 ENABLE
DIEOV
1
1
1
1
DI
INT3 INPUT
INT2 INPUT/RXD1
INT1 INPUT
INT0 INPUT
AIO
–
–
SDA INPUT
SCL INPUT
Note:
1. When enabled, the 2-wire Serial Interface enables Slew-Rate controls on the output pins PD0 and PD1. This is not
shown in this table. In addition, spike filters are connected between the AIO outputs shown in the port figure and the
digital logic of the TWI module.
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10.3.4 Alternate Functions of Port E
The Port E pins with alternate functions are shown below.
Table 10-11.
Port Pin
Port E Pins Alternate Functions
Alternate Function
PE7
Not present on pin-out.
PE6
INT6/AIN0 (External Interrupt 6 Input or Analog Comparator Positive Input)
PE5
PE4
Not present on pin-out.
PE3
PE2
PE1
PE0
HWB (Hardware bootloader activation)
Not present on pin-out.
• INT6/AIN0 – Port E, Bit 6
INT6, External Interrupt source 6: The PE6 pin can serve as an external interrupt source.
AIN0 – Analog Comparator Negative input. This pin is directly connected to the negative input of the Analog
Comparator.
• HWB – Port E, Bit 2
HWB allows to execute the bootloader section after reset when tied to ground during external reset pulse. The
HWB mode of this pin is active only when the HWBE fuse is enable. During normal operation (excluded Reset),
this pin acts as a general purpose I/O.
Table 10-12.
Overriding Signals for Alternate Functions PE6, PE2
Signal Name
PE6/INT6/AIN0
PE2/HWB
PUOE
0
0
PUOV
0
0
DDOE
0
0
DDOV
0
1
PVOE
0
0
PVOV
0
0
DIEOE
INT6 ENABLE
0
DIEOV
1
0
DI
INT6 INPUT
HWB
AIO
AIN0 INPUT
-
10.3.5 Alternate Functions of Port F
The Port F has an alternate function as analog input for the ADC as shown in Table 10-13 on page 82. If some
Port F pins are configured as outputs, it is essential that these do not switch when a conversion is in progress.
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This might corrupt the result of the conversion. If the JTAG interface is enabled, the pull-up resistors on pins
PF7(TDI), PF5(TMS), and PF4(TCK) will be activated even if a Reset occurs.
Table 10-13.
Port Pin
Port F Pins Alternate Functions
Alternate Function
PF7
ADC7/TDI (ADC input channel 7 or JTAG Test Data Input)
PF6
ADC6/TDO (ADC input channel 6 or JTAG Test Data Output)
PF5
ADC5/TMS (ADC input channel 5 or JTAG Test Mode Select)
PF4
ADC4/TCK (ADC input channel 4 or JTAG Test ClocK)
PF3
PF2
Not present on pin-out
PF1
ADC1 (ADC input channel 1)
PF0
ADC0 (ADC input channel 0)
• TDI, ADC7 – Port F, Bit 7
ADC7, Analog to Digital Converter, Channel 7.
TDI, JTAG Test Data In: Serial input data to be shifted in to the Instruction Register or Data Register (scan
chains). When the JTAG interface is enabled, this pin can not be used as an I/O pin.
• TDO, ADC6 – Port F, Bit 6
ADC6, Analog to Digital Converter, Channel 6.
TDO, JTAG Test Data Out: Serial output data from Instruction Register or Data Register. When the JTAG
interface is enabled, this pin can not be used as an I/O pin.
The TDO pin is tri-stated unless TAP states that shift out data are entered.
• TMS, ADC5 – Port F, Bit 5
ADC5, Analog to Digital Converter, Channel 5.
TMS, JTAG Test Mode Select: This pin is used for navigating through the TAP-controller state machine. When
the JTAG interface is enabled, this pin can not be used as an I/O pin.
• TCK, ADC4 – Port F, Bit 4
ADC4, Analog to Digital Converter, Channel 4.
TCK, JTAG Test Clock: JTAG operation is synchronous to TCK. When the JTAG interface is enabled, this pin
can not be used as an I/O pin.
• ADC3 – ADC0 – Port F, Bit 1..0
Analog to Digital Converter, Channel 1.0
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Table 10-14.
Overriding Signals for Alternate Functions in PF7..PF4
Signal Name
PF7/ADC7/TDI
PF6/ADC6/TDO
PF5/ADC5/TMS
PF4/ADC4/TCK
PUOE
JTAGEN
JTAGEN
JTAGEN
JTAGEN
PUOV
1
0
1
1
DDOE
JTAGEN
JTAGEN
JTAGEN
JTAGEN
DDOV
0
SHIFT_IR +
SHIFT_DR
0
0
PVOE
0
JTAGEN
0
0
PVOV
0
TDO
0
0
DIEOE
JTAGEN
JTAGEN
JTAGEN
JTAGEN
DIEOV
0
0
0
0
DI
–
–
–
–
AIO
TDI/ADC7 INPUT
ADC6 INPUT
TMS/ADC5 INPUT
TCK/ADC4 INPUT
Table 10-15.
Overriding Signals for Alternate Functions in PF1..PF0
Signal Name
PF1/ADC1
PF0/ADC0
PUOE
0
0
PUOV
0
0
DDOE
0
0
DDOV
0
0
PVOE
0
0
PVOV
0
0
DIEOE
0
0
DIEOV
0
0
DI
–
–
AIO
ADC1 INPUT
ADC0 INPUT
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10.4
Register Description for I/O-Ports
10.4.1 MCU Control Register – MCUCR
Bit
Read/Writ
e
Initial Value
7
JTD
R/W
6
–
R
5
–
R
4
PUD
R/W
3
–
R
2
–
R
1
IVSEL
R/W
0
IVCE
R/W
0
0
0
0
0
0
0
0
MCUCR
• Bit 4 – PUD: Pull-up Disable
When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and PORTxn Registers
are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01). See “Configuring the Pin” on page 68 for more
details about this feature.
10.4.2 Port B Data Register – PORTB
Bit
Read/Write
Initial Value
7
PORTB7
R/W
0
6
PORTB6
R/W
0
5
PORTB5
R/W
0
4
PORTB4
R/W
0
3
PORTB3
R/W
0
2
PORTB2
R/W
0
1
PORTB1
R/W
0
0
PORTB0
R/W
0
4
DDB4
R/W
0
3
DDB3
R/W
0
2
DDB2
R/W
0
1
DDB1
R/W
0
0
DDB0
R/W
0
4
PINB4
R/W
N/A
3
PINB3
R/W
N/A
2
PINB2
R/W
N/A
1
PINB1
R/W
N/A
0
PINB0
R/W
N/A
4
R/W
0
3
R/W
0
2
R/W
0
1
R/W
0
0
R/W
0
3
R/W
0
2
R/W
0
1
R/W
0
0
R/W
0
PORTB
10.4.3 Port B Data Direction Register – DDRB
Bit
Read/Write
Initial Value
7
DDB7
R/W
0
6
DDB6
R/W
0
5
DDB5
R/W
0
DDRB
10.4.4 Port B Input Pins Address – PINB
Bit
Read/Write
Initial Value
7
PINB7
R/W
N/A
6
PINB6
R/W
N/A
5
PINB5
R/W
N/A
PINB
10.4.5 Port C Data Register – PORTC
Bit
Read/Write
Initial Value
7
PORTC7
R/W
0
6
PORTC6
R/W
0
5
R/W
0
PORTC
10.4.6 Port C Data Direction Register – DDRC
Bit
Read/Write
Initial Value
7
DDC7
R/W
0
6
DDC6
R/W
0
5
R/W
0
4
R/W
0
DDRC
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10.4.7 Port C Input Pins Address – PINC
Bit
Read/Write
Initial Value
7
PINC7
R/W
N/A
6
PINC6
R/W
N/A
5
R/W
N/A
4
R/W
N/A
3
R/W
N/A
2
R/W
N/A
1
R/W
N/A
0
R/W
N/A
4
PORTD4
R/W
0
3
PORTD3
R/W
0
2
PORTD2
R/W
0
1
PORTD1
R/W
0
0
PORTD0
R/W
0
4
DDD4
R/W
0
3
DDD3
R/W
0
2
DDD2
R/W
0
1
DDD1
R/W
0
0
DDD0
R/W
0
4
PIND4
R/W
N/A
3
PIND3
R/W
N/A
2
PIND2
R/W
N/A
1
PIND1
R/W
N/A
0
PIND0
R/W
N/A
4
R/W
0
3
R/W
0
2
PORTE2
R/W
0
1
R/W
0
0
R/W
0
4
R/W
0
3
R/W
0
2
DDE2
R/W
0
1
R/W
0
0
R/W
0
4
R/W
N/A
3
R/W
N/A
2
PINE2
R/W
N/A
1
R/W
N/A
0
R/W
N/A
PINC
10.4.8 Port D Data Register – PORTD
Bit
Read/Write
Initial Value
7
PORTD7
R/W
0
6
PORTD6
R/W
0
5
PORTD5
R/W
0
PORTD
10.4.9 Port D Data Direction Register – DDRD
Bit
Read/Write
Initial Value
7
DDD7
R/W
0
6
DDD6
R/W
0
5
DDD5
R/W
0
DDRD
10.4.10 Port D Input Pins Address – PIND
Bit
Read/Write
Initial Value
7
PIND7
R/W
N/A
6
PIND6
R/W
N/A
5
PIND5
R/W
N/A
PIND
10.4.11 Port E Data Register – PORTE
Bit
Read/Write
Initial Value
7
R/W
0
6
PORTE6
R/W
0
5
R/W
0
PORTE
10.4.12 Port E Data Direction Register – DDRE
Bit
Read/Write
Initial Value
7
R/W
0
6
DDE6
R/W
0
5
R/W
0
DDRE
10.4.13 Port E Input Pins Address – PINE
Bit
Read/Write
Initial Value
7
R/W
N/A
6
PINE6
R/W
N/A
5
R/W
N/A
PINE
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10.4.14 Port F Data Register – PORTF
Bit
Read/Write
Initial Value
7
PORTF7
R/W
0
6
PORTF6
R/W
0
5
PORTF5
R/W
0
4
PORTF4
R/W
0
3
R/W
0
2
R/W
0
1
PORTF1
R/W
0
0
PORTF0
R/W
0
4
DDF4
R/W
0
3
R/W
0
2
R/W
0
1
DDF1
R/W
0
0
DDF0
R/W
0
4
PINF4
R/W
N/A
3
R/W
N/A
2
R/W
N/A
1
PINF1
R/W
N/A
0
PINF0
R/W
N/A
PORTF
10.4.15 Port F Data Direction Register – DDRF
Bit
Read/Write
Initial Value
7
DDF7
R/W
0
6
DDF6
R/W
0
5
DDF5
R/W
0
DDRF
10.4.16 Port F Input Pins Address – PINF
Bit
Read/Write
Initial Value
7
PINF7
R/W
N/A
6
PINF6
R/W
N/A
5
PINF5
R/W
N/A
PINF
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11.
External Interrupts
The External Interrupts are triggered by the INT6, INT3:0 pin or any of the PCINT7..0 pins. Observe that, if
enabled, the interrupts will trigger even if the INT[6;3:0] or PCINT7..0 pins are configured as outputs. This
feature provides a way of generating a software interrupt.
The Pin change interrupt PCI0 will trigger if any enabled PCINT7:0 pin toggles. PCMSK0 Register control which
pins contribute to the pin change interrupts. Pin change interrupts on PCINT7 ..0 are detected asynchronously.
This implies that these interrupts can be used for waking the part also from sleep modes other than Idle mode.
The External Interrupts can be triggered by a falling or rising edge or a low level. This is set up as indicated in
the specification for the External Interrupt Control Registers – EICRA (INT3:0) and EICRB (INT6). When the
external interrupt is enabled and is configured as level triggered, the interrupt will trigger as long as the pin is
held low. Note that recognition of falling or rising edge interrupts on INT6 requires the presence of an I/O clock,
described in “System Clock and Clock Options” on page 27. Low level interrupts and the edge interrupt on
INT3:0 are detected asynchronously. This implies that these interrupts can be used for waking the part also
from sleep modes other than Idle mode. The I/O clock is halted in all sleep modes except Idle mode.
Note that if a level triggered interrupt is used for wake-up from Power-down, the required level must be held long
enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears before the end
of the Start-up Time, the MCU will still wake up, but no interrupt will be generated. The start-up time is defined
by the SUT and CKSEL Fuses as described in “System Clock and Clock Options” on page 27.
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11.1
Register Description
11.1.1 External Interrupt Control Register A – EICRA
The External Interrupt Control Register A contains control bits for interrupt sense control.
Bit
Read/Write
Initial Value
7
ISC31
R/W
0
6
ISC30
R/W
0
5
ISC21
R/W
0
4
ISC20
R/W
0
3
ISC11
R/W
0
2
ISC10
R/W
0
1
ISC01
R/W
0
0
ISC00
R/W
0
EICRA
• Bits 7..0 – ISC31, ISC30 – ISC00, ISC00: External Interrupt 3 - 0 Sense Control Bits
The External Interrupts 3 - 0 are activated by the external pins INT3:0 if the SREG I-flag and the corresponding
interrupt mask in the EIMSK is set. The level and edges on the external pins that activate the interrupts are
defined in the below table. Edges on INT3..INT0 are registered asynchronously. Pulses on INT3:0 pins wider
than the minimum pulse width given in the below table will generate an interrupt. Shorter pulses are not
guaranteed to generate an interrupt. If low level interrupt is selected, the low level must be held until the
completion of the currently executing instruction to generate an interrupt. If enabled, a level triggered interrupt
will generate an interrupt request as long as the pin is held low. When changing the ISCn bit, an interrupt can
occur. Therefore, it is recommended to first disable INTn by clearing its Interrupt Enable bit in the EIMSK
Register. Then, the ISCn bit can be changed. Finally, the INTn interrupt flag should be cleared by writing a
logical one to its Interrupt Flag bit (INTFn) in the EIFR Register before the interrupt is re-enabled.
ISCn1
ISCn0
0
0
The low level of INTn generates an interrupt request.
0
1
Any edge of INTn generates asynchronously an interrupt request.
1
0
The falling edge of INTn generates asynchronously an interrupt request.
1
1
The rising edge of INTn generates asynchronously an interrupt request.
Note:
1.
Symbol
tINT
Description
n = 3, 2, 1, or 0.
When changing the ISCn1/ISCn0 bits, the interrupt must be disabled by clearing its Interrupt Enable bit in the
EIMSK Register. Otherwise an interrupt can occur when the bits are changed.
Parameter
Condition
Min.
Minimum pulse width for asynchronous
external interrupt
Typ.
Max.
Units
50
ns
11.1.2 External Interrupt Control Register B – EICRB
Bit
Read/Write
Initial Value
7
R/W
0
6
R/W
0
5
ISC61
R/W
0
4
ISC60
R/W
0
3
R/W
0
2
R/W
0
1
R/W
0
0
R/W
0
EICRB
• Bit 7..6 – Reserved Bits
These bits are reserved bits in the ATmega16U4/ATmega32U4 and always read as zero.
• Bits 5, 4 – ISC61, ISC60: External Interrupt 6 Sense Control Bits
The External Interrupt 6 is activated by the external pin INT6 if the SREG I-flag and the corresponding interrupt
mask in the EIMSK is set. The level and edges on the external pin that activate the interrupt are defined in the
following table. The value on the INT6 pin are sampled before detecting edges. If edge or toggle interrupt is
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selected, pulses that last longer than one clock period will generate an interrupt. Shorter pulses are not
guaranteed to generate an interrupt. Observe that CPU clock frequency can be lower than the XTAL frequency
if the XTAL divider is enabled. If low level interrupt is selected, the low level must be held until the completion of
the currently executing instruction to generate an interrupt. If enabled, a level triggered interrupt will generate an
interrupt request as long as the pin is held low.
ISC61
ISC60
0
0
The low level of INT6 generates an interrupt request.
0
1
Any logical change on INT6 generates an interrupt request
1
0
The falling edge between two samples of INT6 generates an interrupt request.
1
1
The rising edge between two samples of INT6 generates an interrupt request.
Note:
1.
Description
When changing the ISC61/ISC60 bits, the interrupt must be disabled by clearing its Interrupt Enable bit in the
EIMSK Register. Otherwise an interrupt can occur when the bits are changed.
• Bit 3..0 – Reserved Bits
These bits are reserved bits and always read as zero.
11.1.3 External Interrupt Mask Register – EIMSK
Bit
Read/Write
Initial Value
7
R/W
0
6
INT6
R/W
0
5
R/W
0
4
R/W
0
3
INT3
R/W
0
2
INT2
R/W
0
1
INT1
R/W
0
0
IINT0
R/W
0
EIMSK
• Bits 7..0 – INT6, INT3 – INT0: External Interrupt Request 6, 3 - 0 Enable
When an INT[6;3:0] bit is written to one and the I-bit in the Status Register (SREG) is set (one), the
corresponding external pin interrupt is enabled. The Interrupt Sense Control bits in the External Interrupt Control
Registers – EICRA and EICRB – defines whether the external interrupt is activated on rising or falling edge or
level sensed. Activity on any of these pins will trigger an interrupt request even if the pin is enabled as an output.
This provides a way of generating a software interrupt.
11.1.4 External Interrupt Flag Register – EIFR
Bit
Read/Write
Initial Value
7
R/W
0
6
INTF6
R/W
0
5
R/W
0
4
R/W
0
3
INTF3
R/W
0
2
INTF2
R/W
0
1
INTF1
R/W
0
0
IINTF0
R/W
0
EIFR
• Bits 7..0 – INTF6, INTF3 - INTF0: External Interrupt Flags 6, 3 - 0
When an edge or logic change on the INT[6;3:0] pin triggers an interrupt request, INTF7:0 becomes set (one). If
the I-bit in SREG and the corresponding interrupt enable bit, INT[6;3:0] in EIMSK, are set (one), the MCU will
jump to the interrupt vector. The flag is cleared when the interrupt routine is executed. Alternatively, the flag can
be cleared by writing a logical one to it. These flags are always cleared when INT[6;3:0] are configured as level
interrupt. Note that when entering sleep mode with the INT3:0 interrupts disabled, the input buffers on these
pins will be disabled. This may cause a logic change in internal signals which will set the INTF3:0 flags. See
“Digital Input Enable and Sleep Modes” on page 71 for more information.
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11.1.5 Pin Change Interrupt Control Register - PCICR
Bit
7
6
Read/Write
Initial Value
R
0
R
0
5
–
R
0
4
–
R
0
3
–
R
0
2
–
R
0
1
–
R
0
0
PCIE0
R/W
0
PCICR
• Bit 0 – PCIE0: Pin Change Interrupt Enable 0
When the PCIE0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin change interrupt 0 is
enabled. Any change on any enabled PCINT7..0 pin will cause an interrupt. The corresponding interrupt of Pin
Change Interrupt Request is executed from the PCI0 Interrupt Vector. PCINT7..0 pins are enabled individually
by the PCMSK0 Register.
11.1.6 Pin Change Interrupt Flag Register – PCIFR
Bit
7
6
Read/Write
Initial Value
R
0
R
0
5
–
R
0
4
–
R
0
3
–
R
0
2
–
R
0
1
–
R
0
0
PCIF0
R/W
0
PCIFR
• Bit 0 – PCIF0: Pin Change Interrupt Flag 0
When a logic change on any PCINT7..0 pin triggers an interrupt request, PCIF0 becomes set (one). If the I-bit in
SREG and the PCIE0 bit in EIMSK are set (one), the MCU will jump to the corresponding Interrupt Vector. The
flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical
one to it.
11.1.7 Pin Change Mask Register 0 – PCMSK0
Bit
Read/Write
Initial Value
7
PCINT7
R/W
0
6
PCINT6
R/W
0
5
PCINT5
R/W
0
4
PCINT4
R/W
0
3
PCINT3
R/W
0
2
PCINT2
R/W
0
1
PCINT1
R/W
0
0
PCINT0
R/W
0
PCMSK0
• Bit 7..0 – PCINT7..0: Pin Change Enable Mask 7..0
Each PCINT7..0 bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If PCINT7..0
is set and the PCIE0 bit in PCICR is set, pin change interrupt is enabled on the corresponding I/O pin. If
PCINT7..0 is cleared, pin change interrupt on the corresponding I/O pin is disabled.
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12.
Timer/Counter0, Timer/Counter1, and Timer/Counter3 Prescalers
Timer/Counter0, 1, and 3 share the same prescaler module, but the Timer/Counters can have different
prescaler settings. The description below applies to all Timer/Counters. Tn is used as a general name, n = 0, 1,
or 3.
12.1
Internal Clock Source
The Timer/Counter can be clocked directly by the system clock (by setting the CSn2:0 = 1). This provides the
fastest operation, with a maximum Timer/Counter clock frequency equal to system clock frequency (fCLK_I/O).
Alternatively, one of four taps from the prescaler can be used as a clock source. The prescaled clock has a frequency of either fCLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or fCLK_I/O/1024.
12.2
Prescaler Reset
The prescaler is free running, i.e., operates independently of the Clock Select logic of the Timer/Counter, and it
is shared by the Timer/Counter Tn. Since the prescaler is not affected by the Timer/Counter’s clock select, the
state of the prescaler will have implications for situations where a prescaled clock is used. One example of prescaling artifacts occurs when the timer is enabled and clocked by the prescaler (6 > CSn2:0 > 1). The number of
system clock cycles from when the timer is enabled to the first count occurs can be from 1 to N+1 system clock
cycles, where N equals the prescaler divisor (8, 64, 256, or 1024).
It is possible to use the prescaler reset for synchronizing the Timer/Counter to program execution. However,
care must be taken if the other Timer/Counter that shares the same prescaler also uses prescaling. A prescaler
reset will affect the prescaler period for all Timer/Counters it is connected to.
12.3
External Clock Source
An external clock source applied to the Tn pin can be used as Timer/Counter clock (clkTn). The Tn pin is sampled once every system clock cycle by the pin synchronization logic. The synchronized (sampled) signal is then
passed through the edge detector. Figure 12-1 shows a functional equivalent block diagram of the Tn synchronization and edge detector logic. The registers are clocked at the positive edge of the internal system clock
(clkI/O). The latch is transparent in the high period of the internal system clock.
The edge detector generates one clkTn pulse for each positive (CSn2:0 = 7) or negative (CSn2:0 = 6) edge it
detects.
Figure 12-1.
Tn/T0 Pin Sampling
Tn
D
Q
D
Q
D
Tn_sync
(To Clock
Select Logic)
Q
LE
clk I/O
Synchronization
Edge Detector
The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles from an edge
has been applied to the Tn pin to the counter is updated.
Enabling and disabling of the clock input must be done when Tn has been stable for at least one system clock
cycle, otherwise it is a risk that a false Timer/Counter clock pulse is generated.
Each half period of the external clock applied must be longer than one system clock cycle to ensure correct
sampling. The external clock must be guaranteed to have less than half the system clock frequency (fExtClk <
fclk_I/O/2) given a 50/50% duty cycle. Since the edge detector uses sampling, the maximum frequency of an
external clock it can detect is half the sampling frequency (Nyquist sampling theorem). However, due to variation of the system clock frequency and duty cycle caused by Oscillator source (crystal, resonator, and
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capacitors) tolerances, it is recommended that maximum frequency of an external clock source is less than
fclk_I/O/2.5.
An external clock source can not be prescaled.
Figure 12-2.
Prescaler for Synchronous Timer/Counters
clk I/O
Clear
PSR10
Tn
Synchronization
Tn
Synchronization
CSn0
CSn0
CSn1
CSn1
CSn2
CSn2
TIMER/COUNTERn CLOCK SOURCE
clkTn
Note:
TIMER/COUNTERn CLOCK SOURCE
clkTn
T3 input is not available on the ATmega16U4/ATmega32U4 products. “Tn” only refers to either T0 or T1
inputs.
12.4
Register Description
12.4.1 General Timer/Counter Control Register – GTCCR
Bit
Read/Write
Initial Value
7
TSM
R/W
0
6
–
R
0
5
–
R
0
4
–
R
0
3
–
R
0
2
–
R
0
1
PSRASY
R/W
0
0
PSRSYNC
R/W
0
GTCCR
• Bit 7 – TSM: Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the value that is
written to the PSRASY and PSRSYNC bits is kept, hence keeping the corresponding prescaler reset signals
asserted. This ensures that the corresponding Timer/Counters are halted and can be configured to the same
value without the risk of one of them advancing during configuration. When the TSM bit is written to zero, the
PSRASY and PSRSYNC bits are cleared by hardware, and the Timer/Counters start counting simultaneously.
• Bit 0 – PSRSYNC: Prescaler Reset for Synchronous Timer/Counters
When this bit is one, Timer/Counter0 and Timer/Counter1 and Timer/Counter3 prescaler will be Reset. This bit
is normally cleared immediately by hardware, except if the TSM bit is set. Note that Timer/Counter0,
Timer/Counter1 and Timer/Counter3 share the same prescaler and a reset of this prescaler will affect all timers.
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13.
8-bit Timer/Counter0 with PWM
Timer/Counter0 is a general purpose 8-bit Timer/Counter module, with two independent Output Compare Units,
and with PWM support. It allows accurate program execution timing (event management) and wave generation.
The main features are:
•
•
•
•
•
•
•
13.1
Two Independent Output Compare Units
Double Buffered Output Compare Registers
Clear Timer on Compare Match (Auto Reload)
Glitch Free, Phase Correct Pulse Width Modulator (PWM)
Variable PWM Period
Frequency Generator
Three Independent Interrupt Sources (TOV0, OCF0A, and OCF0B)
Overview
A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 13-1. For the actual placement of I/O
pins, refer to “Pinout” on page 3. CPU accessible I/O Registers, including I/O bits and I/O pins, are shown in
bold. The device-specific I/O Register and bit locations are listed in the “8-bit Timer/Counter Register
Description” on page 104.
Figure 13-1.
8-bit Timer/Counter Block Diagram
Count
Clear
Direction
TOVn
(Int.Req.)
Control Logic
clkTn
Clock Select
Edge
Detector
TOP
Tn
BOTTOM
( From Prescaler )
Timer/Counter
TCNTn
=
=0
OCnA
(Int.Req.)
Waveform
Generation
=
OCnA
DATA BUS
OCRnA
Fixed
TOP
Value
OCnB
(Int.Req.)
Waveform
Generation
=
OCnB
OCRnB
TCCRnA
TCCRnB
13.1.1 Registers
The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) are 8-bit registers. Interrupt
request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag Register (TIFR0).
All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK0). TIFR0 and TIMSK0 are
not shown in the figure.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the T0 pin.
The Clock Select logic block controls which clock source and edge the Timer/Counter uses to increment (or
decrement) its value. The Timer/Counter is inactive when no clock source is selected. The output from the Clock
Select logic is referred to as the timer clock (clkT0).
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The double buffered Output Compare Registers (OCR0A and OCR0B) are compared with the Timer/Counter
value at all times. The result of the compare can be used by the Waveform Generator to generate a PWM or
variable frequency output on the Output Compare pins (OC0A and OC0B). See “Output Compare Unit” on
page 96. for details. The Compare Match event will also set the Compare Flag (OCF0A or OCF0B) which can
be used to generate an Output Compare interrupt request.
13.1.2 Definitions
Many register and bit references in this section are written in general form. A lower case “n” replaces the
Timer/Counter number, in this case 0. A lower case “x” replaces the Output Compare Unit, in this case Compare
Unit A or Compare Unit B. However, when using the register or bit defined in a program, the precise form must
be used, i.e., TCNT0 for accessing Timer/Counter0 counter value and so on.
The definitions in the table are also used extensively throughout the document.
13.2
BOTTOM
The counter reaches the BOTTOM when it becomes 0x00.
MAX
The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
TOP
The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX) or the value
stored in the OCR0A Register. The assignment is dependent on the mode of operation.
Timer/Counter Clock Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock source is selected by
the Clock Select logic which is controlled by the Clock Select (CS02:0) bits located in the Timer/Counter Control
Register (TCCR0B). For details on clock sources and prescaler, see “Timer/Counter0, Timer/Counter1, and
Timer/Counter3 Prescalers” on page 92.
13.3
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure 13-2 shows a
block diagram of the counter and its surroundings.
Figure 13-2.
Counter Unit Block Diagram
TOVn
(Int.Req.)
DATA BUS
Clock Select
count
TCNTn
clear
Control Logic
Edge
Detector
clkTn
Tn
direction
( From Prescaler )
bottom
top
Signal description (internal signals):
count: Increment or decrement TCNT0 by 1.
direction: Select between increment and decrement.
clear: Clear TCNT0 (set all bits to zero).
clkTn: Timer/Counter clock, referred to as clkT0 in the following.
top: Signalize that TCNT0 has reached maximum value.
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bottom: Signalize that TCNT0 has reached minimum value (zero).
Depending of the mode of operation used, the counter is cleared, incremented, or decremented at each timer
clock (clkT0). clkT0 can be generated from an external or internal clock source, selected by the Clock Select bits
(CS02:0). When no clock source is selected (CS02:0 = 0) the timer is stopped. However, the TCNT0 value can
be accessed by the CPU, regardless of whether clkT0 is present or not. A CPU write overrides (has priority over)
all counter clear or count operations.
The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in the
Timer/Counter Control Register (TCCR0A) and the WGM02 bit located in the Timer/Counter Control Register B
(TCCR0B). There are close connections between how the counter behaves (counts) and how waveforms are
generated on the Output Compare outputs OC0A and OC0B. For more details about advanced counting
sequences and waveform generation, see “Modes of Operation” on page 98.
The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by the WGM02:0
bits. TOV0 can be used for generating a CPU interrupt.
13.4
Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Registers (OCR0A and
OCR0B). Whenever TCNT0 equals OCR0A or OCR0B, the comparator signals a match. A match will set the
Output Compare Flag (OCF0A or OCF0B) at the next timer clock cycle. If the corresponding interrupt is
enabled, the Output Compare Flag generates an Output Compare interrupt. The Output Compare Flag is
automatically cleared when the interrupt is executed. Alternatively, the flag can be cleared by software by writing
a logical one to its I/O bit location. The Waveform Generator uses the match signal to generate an output
according to operating mode set by the WGM02:0 bits and Compare Output mode (COM0x1:0) bits. The max.
and bottom signals are used by the Waveform Generator for handling the special cases of the extreme values in
some modes of operation (“Modes of Operation” on page 98).
Figure 13-3 on page 96 shows a block diagram of the Output Compare unit.
Figure 13-3.
Output Compare Unit, Block Diagram
DATA BUS
OCRnx
TCNTn
= (8-bit Comparator )
OCFnx (Int.Req.)
top
bottom
Waveform Generator
OCnx
FOCn
WGMn1:0
COMnX1:0
The OCR0x Registers are double buffered when using any of the Pulse Width Modulation (PWM) modes. For
the normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The
double buffering synchronizes the update of the OCR0x Compare Registers to either top or bottom of the
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counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses,
thereby making the output glitch-free.
The OCR0x Register access may seem complex, but this is not case. When the double buffering is enabled, the
CPU has access to the OCR0x Buffer Register, and if double buffering is disabled the CPU will access the
OCR0x directly.
13.4.1 Force Output Compare
In non-PWM waveform generation modes, the match output of the comparator can be forced by writing a one to
the Force Output Compare (FOC0x) bit. Forcing Compare Match will not set the OCF0x Flag or reload/clear the
timer, but the OC0x pin will be updated as if a real Compare Match had occurred (the COM0x1:0 bits settings
define whether the OC0x pin is set, cleared or toggled).
13.4.2 Compare Match Blocking by TCNT0 Write
All CPU write operations to the TCNT0 Register will block any Compare Match that occur in the next timer clock
cycle, even when the timer is stopped. This feature allows OCR0x to be initialized to the same value as TCNT0
without triggering an interrupt when the Timer/Counter clock is enabled.
13.4.3 Using the Output Compare Unit
Since writing TCNT0 in any mode of operation will block all Compare Matches for one timer clock cycle, there
are risks involved when changing TCNT0 when using the Output Compare Unit, independently of whether the
Timer/Counter is running or not. If the value written to TCNT0 equals the OCR0x value, the Compare Match will
be missed, resulting in incorrect waveform generation. Similarly, do not write the TCNT0 value equal to
BOTTOM when the counter is down-counting.
The setup of the OC0x should be performed before setting the Data Direction Register for the port pin to output.
The easiest way of setting the OC0x value is to use the Force Output Compare (FOC0x) strobe bits in Normal
mode. The OC0x Registers keep their values even when changing between Waveform Generation modes.
Be aware that the COM0x1:0 bits are not double buffered together with the compare value. Changing the
COM0x1:0 bits will take effect immediately.
13.5
Compare Match Output Unit
The Compare Output mode (COM0x1:0) bits have two functions. The Waveform Generator uses the COM0x1:0
bits for defining the Output Compare (OC0x) state at the next Compare Match. Also, the COM0x1:0 bits control
the OC0x pin output source. Figure 13-4 shows a simplified schematic of the logic affected by the COM0x1:0 bit
setting. The I/O Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the general I/O
Port Control Registers (DDR and PORT) that are affected by the COM0x1:0 bits are shown. When referring to
the OC0x state, the reference is for the internal OC0x Register, not the OC0x pin. If a system reset occur, the
OC0x Register is reset to “0”.
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Figure 13-4.
Compare Match Output Unit, Schematic
COMnx1
COMnx0
FOCn
Waveform
Generator
D
Q
1
OCnx
DATA BUS
D
0
OCnx
Pin
Q
PORT
D
Q
DDR
clk I/O
The general I/O port function is overridden by the Output Compare (OC0x) from the Waveform Generator if
either of the COM0x1:0 bits are set. However, the OC0x pin direction (input or output) is still controlled by the
Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OC0x pin (DDR_OC0x)
must be set as output before the OC0x value is visible on the pin. The port override function is independent of
the Waveform Generation mode.
The design of the Output Compare pin logic allows initialization of the OC0x state before the output is enabled.
Note that some COM0x1:0 bit settings are reserved for certain modes of operation. See “8-bit Timer/Counter
Register Description” on page 104.
13.5.1 Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM0x1:0 bits differently in Normal, CTC, and PWM modes. For all modes,
setting the COM0x1:0 = 0 tells the Waveform Generator that no action on the OC0x Register is to be performed
on the next Compare Match. For compare output actions in the non-PWM modes refer to Table 13-1 on
page 104. For fast PWM mode, refer to Table 13-2 on page 104, and for phase correct PWM refer to Table 133 on page 105.
A change of the COM0x1:0 bits state will have effect at the first Compare Match after the bits are written. For
non-PWM modes, the action can be forced to have immediate effect by using the FOC0x strobe bits.
13.6
Modes of Operation
The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined by the
combination of the Waveform Generation mode (WGM02:0) and Compare Output mode (COM0x1:0) bits. The
Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do.
The COM0x1:0 bits control whether the PWM output generated should be inverted or not (inverted or noninverted PWM). For non-PWM modes the COM0x1:0 bits control whether the output should be set, cleared, or
toggled at a Compare Match (See “Compare Match Output Unit” on page 97.).
For detailed timing information see “Timer/Counter Timing Diagrams” on page 102.
13.6.1 Normal Mode
The simplest mode of operation is the Normal mode (WGM02:0 = 0). In this mode the counting direction is
always up (incrementing), and no counter clear is performed. The counter simply overruns when it passes its
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maximum 8-bit value (TOP = 0xFF) and then restarts from the bottom (0x00). In normal operation the
Timer/Counter Overflow Flag (TOV0) will be set in the same timer clock cycle as the TCNT0 becomes zero. The
TOV0 Flag in this case behaves like a ninth bit, except that it is only set, not cleared. However, combined with
the timer overflow interrupt that automatically clears the TOV0 Flag, the timer resolution can be increased by
software. There are no special cases to consider in the Normal mode, a new counter value can be written
anytime.
The Output Compare Unit can be used to generate interrupts at some given time. Using the Output Compare to
generate waveforms in Normal mode is not recommended, since this will occupy too much of the CPU time.
13.6.2 Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM02:0 = 2), the OCR0A Register is used to manipulate the
counter resolution. In CTC mode the counter is cleared to zero when the counter value (TCNT0) matches the
OCR0A. The OCR0A defines the top value for the counter, hence also its resolution. This mode allows greater
control of the Compare Match output frequency. It also simplifies the operation of counting external events.
The timing diagram for the CTC mode is shown in Figure 13-5 on page 99. The counter value (TCNT0)
increases until a Compare Match occurs between TCNT0 and OCR0A, and then counter (TCNT0) is cleared.
Figure 13-5.
CTC Mode, Timing Diagram
OCnx Interrupt Flag Set
TCNTn
OCn
(Toggle)
Period
(COMnx1:0 = 1)
1
2
3
4
An interrupt can be generated each time the counter value reaches the TOP value by using the OCF0A Flag. If
the interrupt is enabled, the interrupt handler routine can be used for updating the TOP value. However,
changing TOP to a value close to BOTTOM when the counter is running with none or a low prescaler value must
be done with care since the CTC mode does not have the double buffering feature. If the new value written to
OCR0A is lower than the current value of TCNT0, the counter will miss the Compare Match. The counter will
then have to count to its maximum value (0xFF) and wrap around starting at 0x00 before the Compare Match
can occur.
For generating a waveform output in CTC mode, the OC0A output can be set to toggle its logical level on each
Compare Match by setting the Compare Output mode bits to toggle mode (COM0A1:0 = 1). The OC0A value
will not be visible on the port pin unless the data direction for the pin is set to output. The waveform generated
will have a maximum frequency of fOC0 = fclk_I/O/2 when OCR0A is set to zero (0x00). The waveform frequency is
defined by the following equation:
f clk_I/O
f OCnx = -------------------------------------------------2  N   1 + OCRnx 
The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOV0 Flag is set in the same timer clock cycle that the counter counts
from MAX to 0x00.
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13.6.3 Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM02:0 = 3 or 7) provides a high frequency PWM
waveform generation option. The fast PWM differs from the other PWM option by its single-slope operation. The
counter counts from BOTTOM to TOP then restarts from BOTTOM. TOP is defined as 0xFF when WGM2:0 = 3,
and OCR0A when WGM2:0 = 7. In non-inverting Compare Output mode, the Output Compare (OC0x) is cleared
on the Compare Match between TCNT0 and OCR0x, and set at BOTTOM. In inverting Compare Output mode,
the output is set on Compare Match and cleared at BOTTOM. Due to the single-slope operation, the operating
frequency of the fast PWM mode can be twice as high as the phase correct PWM mode that use dual-slope
operation. This high frequency makes the fast PWM mode well suited for power regulation, rectification, and
DAC applications. High frequency allows physically small sized external components (coils, capacitors), and
therefore reduces total system cost.
In fast PWM mode, the counter is incremented until the counter value matches the TOP value. The counter is
then cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is shown in Figure
13-6. The TCNT0 value is in the timing diagram shown as a histogram for illustrating the single-slope operation.
The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT0
slopes represent Compare Matches between OCR0x and TCNT0.
Figure 13-6.
Fast PWM Mode, Timing Diagram
OCRnx Interrupt Flag Set
OCRnx Update and
TOVn Interrupt Flag Set
TCNTn
OCnx
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
Period
1
2
3
4
5
6
7
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches TOP. If the interrupt is enabled,
the interrupt handler routine can be used for updating the compare value.
In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins. Setting the
COM0x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by
setting the COM0x1:0 to three: Setting the COM0A1:0 bits to one allows the OC0A pin to toggle on Compare
Matches if the WGM02 bit is set. This option is not available for the OC0B pin (See Table 13-2 on page 104).
The actual OC0x value will only be visible on the port pin if the data direction for the port pin is set as output. The
PWM waveform is generated by setting (or clearing) the OC0x Register at the Compare Match between OCR0x
and TCNT0, and clearing (or setting) the OC0x Register at the timer clock cycle the counter is cleared (changes
from TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
f clk_I/O
f OCnxPWM = -----------------N  256
The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).
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The extreme values for the OCR0A Register represents special cases when generating a PWM waveform
output in the fast PWM mode. If the OCR0A is set equal to BOTTOM, the output will be a narrow spike for each
MAX+1 timer clock cycle. Setting the OCR0A equal to MAX will result in a constantly high or low output
(depending on the polarity of the output set by the COM0A1:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC0x to
toggle its logical level on each Compare Match (COM0x1:0 = 1). The waveform generated will have a maximum
frequency of fOC0 = fclk_I/O/2 when OCR0A is set to zero. This feature is similar to the OC0A toggle in CTC mode,
except the double buffer feature of the Output Compare unit is enabled in the fast PWM mode.
13.6.4 Phase Correct PWM Mode
The phase correct PWM mode (WGM02:0 = 1 or 5) provides a high resolution phase correct PWM waveform
generation option. The phase correct PWM mode is based on a dual-slope operation. The counter counts
repeatedly from BOTTOM to TOP and then from TOP to BOTTOM. TOP is defined as 0xFF when WGM2:0 = 1,
and OCR0A when WGM2:0 = 5. In non-inverting Compare Output mode, the Output Compare (OC0x) is cleared
on the Compare Match between TCNT0 and OCR0x while up counting, and set on the Compare Match while
down-counting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation has
lower maximum operation frequency than single slope operation. However, due to the symmetric feature of the
dual-slope PWM modes, these modes are preferred for motor control applications.
In phase correct PWM mode the counter is incremented until the counter value matches TOP. When the
counter reaches TOP, it changes the count direction. The TCNT0 value will be equal to TOP for one timer clock
cycle. The timing diagram for the phase correct PWM mode is shown on Figure 13-7. The TCNT0 value is in the
timing diagram shown as a histogram for illustrating the dual-slope operation. The diagram includes noninverted and inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent Compare
Matches between OCR0x and TCNT0.
Figure 13-7.
Phase Correct PWM Mode, Timing Diagram
OCnx Interrupt Flag Set
OCRnx Update
TOVn Interrupt Flag Set
TCNTn
OCnx
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
Period
1
2
3
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches BOTTOM. The Interrupt Flag
can be used to generate an interrupt each time the counter reaches the BOTTOM value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins. Setting
the COM0x1:0 bits to two will produce a non-inverted PWM. An inverted PWM output can be generated by
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setting the COM0x1:0 to three: Setting the COM0A0 bits to one allows the OC0A pin to toggle on Compare
Matches if the WGM02 bit is set. This option is not available for the OC0B pin. The actual OC0x value will only
be visible on the port pin if the data direction for the port pin is set as output. The PWM waveform is generated
by clearing (or setting) the OC0x Register at the Compare Match between OCR0x and TCNT0 when the counter
increments, and setting (or clearing) the OC0x Register at Compare Match between OCR0x and TCNT0 when
the counter decrements. The PWM frequency for the output when using phase correct PWM can be calculated
by the following equation:
f clk_I/O
f OCnxPCPWM = -----------------N  510
The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A Register represent special cases when generating a PWM waveform output
in the phase correct PWM mode. If the OCR0A is set equal to BOTTOM, the output will be continuously low and
if set equal to MAX the output will be continuously high for non-inverted PWM mode. For inverted PWM the
output will have the opposite logic values.
At the very start of period 2 in Figure 13-7 on page 101 OCnx has a transition from high to low even though
there is no Compare Match. The point of this transition is to guarantee symmetry around BOTTOM. There are
two cases that give a transition without Compare Match.
13.7

OCR0A changes its value from MAX, like in Figure 13-7 on page 101. When the OCR0A value is MAX the
OCn pin value is the same as the result of a down-counting Compare Match. To ensure symmetry around
BOTTOM the OCn value at MAX must correspond to the result of an up-counting Compare Match.

The timer starts counting from a value higher than the one in OCR0A, and for that reason misses the
Compare Match and hence the OCn change that would have happened on the way up.
Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkT0) is therefore shown as a clock enable
signal in the following figures. The figures include information on when Interrupt Flags are set. Figure 13-8
contains timing data for basic Timer/Counter operation. The figure shows the count sequence close to the MAX
value in all modes other than phase correct PWM mode.
Figure 13-8.
Timer/Counter Timing Diagram, no Prescaling
clkI/O
clkTn
(clkI/O /1)
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
TOVn
Figure 13-9 on page 103 shows the same timing data, but with the prescaler enabled.
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Figure 13-9.
Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
TOVn
Figure 13-10 shows the setting of OCF0B in all modes and OCF0A in all modes except CTC mode and PWM
mode, where OCR0A is TOP.
Figure 13-10. Timer/Counter Timing Diagram, Setting of OCF0x, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
OCRnx - 1
OCRnx
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
OCFnx
Figure 13-11 shows the setting of OCF0A and the clearing of TCNT0 in CTC mode and fast PWM mode where
OCR0A is TOP.
Figure 13-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
(CTC)
OCRnx
TOP - 1
TOP
BOTTOM
BOTTOM + 1
TOP
OCFnx
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13.8
8-bit Timer/Counter Register Description
13.8.1 Timer/Counter Control Register A – TCCR0A
Bit
Read/Write
Initial Value
7
COM0A1
R/W
0
6
COM0A0
R/W
0
5
COM0B1
R/W
0
4
COM0B0
R/W
0
3
–
R
0
2
–
R
0
1
WGM01
R/W
0
0
WGM00
R/W
0
TCCR0A
• Bits 7:6 – COM01A:0: Compare Match Output A Mode
These bits control the Output Compare pin (OC0A) behavior. If one or both of the COM0A1:0 bits are set, the
OC0A output overrides the normal port functionality of the I/O pin it is connected to. However, note that the Data
Direction Register (DDR) bit corresponding to the OC0A pin must be set in order to enable the output driver.
When OC0A is connected to the pin, the function of the COM0A1:0 bits depends on the WGM02:0 bit setting.
The table shows the COM0A1:0 bit functionality when the WGM02:0 bits are set to a normal or CTC mode (nonPWM).
Table 13-1.
Compare Output Mode, non-PWM Mode
COM0A1
COM0A0
Description
0
0
Normal port operation, OC0A disconnected
0
1
Toggle OC0A on Compare Match
1
0
Clear OC0A on Compare Match
1
1
Set OC0A on Compare Match
The table shows the COM0A1:0 bit functionality when the WGM01:0 bits are set to fast PWM mode
Table 13-2.
Compare Output Mode, Fast PWM Mode(1)
COM0A1
COM0A0
0
0
Normal port operation, OC0A disconnected
0
1
WGM02 = 0: Normal Port Operation, OC0A Disconnected
WGM02 = 1: Toggle OC0A on Compare Match
1
0
Clear OC0A on Compare Match, set OC0A at TOP
1
1
Set OC0A on Compare Match, clear OC0A at TOP
Note:
1.
Description
A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the Compare Match is
ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page 100 for more details.
The table shows the COM0A1:0 bit functionality when the WGM02:0 bits are set to phase correct PWM mode.
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Table 13-3.
Compare Output Mode, Phase Correct PWM Mode(1)
COM0A1
COM0A0
0
0
Normal port operation, OC0A disconnected
0
1
WGM02 = 0: Normal Port Operation, OC0A Disconnected
WGM02 = 1: Toggle OC0A on Compare Match
1
0
Clear OC0A on Compare Match when up-counting. Set OC0A on Compare
Match when down-counting.
1
1
Set OC0A on Compare Match when up-counting. Clear OC0A on Compare
Match when down-counting.
Note:
1.
Description
A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the Compare Match is
ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page 101 for more details.
• Bits 5:4 – COM0B1:0: Compare Match Output B Mode
These bits control the Output Compare pin (OC0B) behavior. If one or both of the COM0B1:0 bits are set, the
OC0B output overrides the normal port functionality of the I/O pin it is connected to. However, note that the Data
Direction Register (DDR) bit corresponding to the OC0B pin must be set in order to enable the output driver.
When OC0B is connected to the pin, the function of the COM0B1:0 bits depends on the WGM02:0 bit setting.
The table shows the COM0A1:0 bit functionality when the WGM02:0 bits are set to a normal or CTC mode (nonPWM).
Table 13-4.
Compare Output Mode, non-PWM Mode
COM01
COM00
Description
0
0
Normal port operation, OC0B disconnected
0
1
Toggle OC0B on Compare Match
1
0
Clear OC0B on Compare Match
1
1
Set OC0B on Compare Match
The table shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to fast PWM mode
Table 13-5.
Compare Output Mode, Fast PWM Mode(1)
COM01
COM00
0
0
Normal port operation, OC0B disconnected
0
1
Reserved
1
0
Clear OC0B on Compare Match, set OC0B at TOP
1
1
Set OC0B on Compare Match, clear OC0B at TOP
Note:
1.
Description
A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Compare Match is
ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page 100 for more details.
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The table shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to phase correct PWM mode.
Compare Output Mode, Phase Correct PWM Mode(1)
Table 13-6.
COM0A1
COM0A0
0
0
Normal port operation, OC0B disconnected
0
1
Reserved
1
0
Clear OC0B on Compare Match when up-counting. Set OC0B on Compare
Match when down-counting.
1
1
Set OC0B on Compare Match when up-counting. Clear OC0B on Compare
Match when down-counting.
Note:
1.
Description
A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Compare Match is
ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page 101 for more details.
• Bits 3, 2 – Res: Reserved Bits
These bits are reserved bits in the ATmega16U4/ATmega32U4 and will always read as zero.
• Bits 1:0 – WGM01:0: Waveform Generation Mode
Combined with the WGM02 bit found in the TCCR0B Register, these bits control the counting sequence of the
counter, the source for maximum (TOP) counter value, and what type of waveform generation to be used as
shown in the table. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear
Timer on Compare Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes (see “Modes of
Operation” on page 98).
Table 13-7.
Waveform Generation Mode Bit Description
Timer/Counter Mode of
Operation
TOP
Update of
OCRx at
TOV Flag
Set on(1)(2)
0
Normal
0xFF
Immediate
MAX
0
1
PWM, Phase Correct
0xFF
TOP
BOTTOM
0
1
0
CTC
OCRA
Immediate
MAX
3
0
1
1
Fast PWM
0xFF
TOP
MAX
4
1
0
0
Reserved
–
–
–
5
1
0
1
PWM, Phase Correct
OCRA
TOP
BOTTOM
6
1
1
0
Reserved
–
–
–
7
1
1
1
Fast PWM
OCRA
TOP
TOP
Mode
WGM2
WGM1
WGM0
0
0
0
1
0
2
Notes:
1.
2.
MAX
= 0xFF
BOTTOM = 0x00
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13.8.2 Timer/Counter Control Register B – TCCR0B
Bit
Read/Write
Initial Value
7
FOC0A
W
0
6
FOC0B
W
0
5
–
R
0
4
–
R
0
3
WGM02
R/W
0
2
CS02
R/W
0
1
CS01
R/W
0
0
CS00
R/W
0
TCCR0B
• Bit 7 – FOC0A: Force Output Compare A
The FOC0A bit is only active when the WGM bits specify a non-PWM mode.
However, for ensuring compatibility with future devices, this bit must be set to zero when TCCR0B is written
when operating in PWM mode. When writing a logical one to the FOC0A bit, an immediate Compare Match is
forced on the Waveform Generation unit. The OC0A output is changed according to its COM0A1:0 bits setting.
Note that the FOC0A bit is implemented as a strobe. Therefore it is the value present in the COM0A1:0 bits that
determines the effect of the forced compare.
A FOC0A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0A as TOP.
The FOC0A bit is always read as zero.
• Bit 6 – FOC0B: Force Output Compare B
The FOC0B bit is only active when the WGM bits specify a non-PWM mode.
However, for ensuring compatibility with future devices, this bit must be set to zero when TCCR0B is written
when operating in PWM mode. When writing a logical one to the FOC0B bit, an immediate Compare Match is
forced on the Waveform Generation unit. The OC0B output is changed according to its COM0B1:0 bits setting.
Note that the FOC0B bit is implemented as a strobe. Therefore it is the value present in the COM0B1:0 bits that
determines the effect of the forced compare.
A FOC0B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0B as TOP.
The FOC0B bit is always read as zero.
• Bits 5:4 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
• Bit 3 – WGM02: Waveform Generation Mode
See the description in the “Timer/Counter Control Register A – TCCR0A” on page 104.
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• Bits 2:0 – CS02:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter.
Table 13-8.
Clock Select Bit Description
CS02
CS01
CS00
Description
0
0
0
No clock source (Timer/Counter stopped)
0
0
1
clkI/O/(No prescaling)
0
1
0
clkI/O/8 (From prescaler)
0
1
1
clkI/O/64 (From prescaler)
1
0
0
clkI/O/256 (From prescaler)
1
0
1
clkI/O/1024 (From prescaler)
1
1
0
External clock source on T0 pin. Clock on falling edge.
1
1
1
External clock source on T0 pin. Clock on rising edge.
If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the counter even if the
pin is configured as an output. This feature allows software control of the counting.
13.8.3 Timer/Counter Register – TCNT0
Bit
Read/Write
Initial Value
7
6
TCNT0[7:0]
R/W
R/W
0
0
5
4
3
2
1
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
TCNT0
The Timer/Counter Register gives direct access, both for read and write operations, to the Timer/Counter unit
8 -bit counter. Writing to the TCNT0 Register blocks (removes) the Compare Match on the following timer clock.
Modifying the counter (TCNT0) while the counter is running, introduces a risk of missing a Compare Match
between TCNT0 and the OCR0x Registers.
13.8.4 Output Compare Register A – OCR0A
Bit
7
6
5
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
4
3
OCR0A[7:0]
R/W
R/W
0
0
2
1
0
R/W
0
R/W
0
R/W
0
OCR0A
The Output Compare Register A contains an 8-bit value that is continuously compared with the counter value
(TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a waveform output on
the OC0A pin.
13.8.5 Output Compare Register B – OCR0B
Bit
7
6
5
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
4
3
OCR0B[7:0]
R/W
R/W
0
0
2
1
0
R/W
0
R/W
0
R/W
0
OCR0B
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The Output Compare Register B contains an 8-bit value that is continuously compared with the counter value
(TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a waveform output on
the OC0B pin.
13.8.6 Timer/Counter Interrupt Mask Register – TIMSK0
Bit
Read/Write
Initial Value
7
–
R
0
6
–
R
0
5
–
R
0
4
–
R
0
3
–
R
0
2
OCIE0B
R/W
0
1
OCIE0A
R/W
0
0
TOIE0
R/W
0
TIMSK0
• Bits 7..3, 0 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
• Bit 2 – OCIE0B: Timer/Counter Output Compare Match B Interrupt Enable
When the OCIE0B bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter Compare
Match B interrupt is enabled. The corresponding interrupt is executed if a Compare Match in Timer/Counter
occurs, i.e., when the OCF0B bit is set in the Timer/Counter Interrupt Flag Register – TIFR0.
• Bit 1 – OCIE0A: Timer/Counter0 Output Compare Match A Interrupt Enable
When the OCIE0A bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter0 Compare
Match A interrupt is enabled. The corresponding interrupt is executed if a Compare Match in Timer/Counter0
occurs, i.e., when the OCF0A bit is set in the Timer/Counter 0 Interrupt Flag Register – TIFR0.
• Bit 0 – TOIE0: Timer/Counter0 Overflow Interrupt Enable
When the TOIE0 bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter0 Overflow
interrupt is enabled. The corresponding interrupt is executed if an overflow in Timer/Counter0 occurs, i.e., when
the TOV0 bit is set in the Timer/Counter 0 Interrupt Flag Register – TIFR0.
13.8.7 Timer/Counter 0 Interrupt Flag Register – TIFR0
Bit
Read/Write
Initial Value
7
–
R
0
6
–
R
0
5
–
R
0
4
–
R
0
3
–
R
0
2
OCF0B
R/W
0
1
OCF0A
R/W
0
0
TOV0
R/W
0
TIFR0
• Bits 7..3, 0 – Res: Reserved Bits
These bits are reserved bits in the ATmega16U4/ATmega32U4 and will always read as zero.
• Bit 2 – OCF0B: Timer/Counter 0 Output Compare B Match Flag
The OCF0B bit is set when a Compare Match occurs between the Timer/Counter and the data in OCR0B –
Output Compare Register0 B. OCF0B is cleared by hardware when executing the corresponding interrupt
handling vector. Alternatively, OCF0B is cleared by writing a logic one to the flag. When the I-bit in SREG,
OCIE0B (Timer/Counter Compare B Match Interrupt Enable), and OCF0B are set, the Timer/Counter Compare
Match Interrupt is executed.
• Bit 1 – OCF0A: Timer/Counter 0 Output Compare A Match Flag
The OCF0A bit is set when a Compare Match occurs between the Timer/Counter0 and the data in OCR0A –
Output Compare Register0. OCF0A is cleared by hardware when executing the corresponding interrupt
handling vector. Alternatively, OCF0A is cleared by writing a logic one to the flag. When the I-bit in SREG,
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OCIE0A (Timer/Counter0 Compare Match Interrupt Enable), and OCF0A are set, the Timer/Counter0 Compare
Match Interrupt is executed.
• Bit 0 – TOV0: Timer/Counter0 Overflow Flag
The bit TOV0 is set when an overflow occurs in Timer/Counter0. TOV0 is cleared by hardware when executing
the corresponding interrupt handling vector. Alternatively, TOV0 is cleared by writing a logic one to the flag.
When the SREG I-bit, TOIE0 (Timer/Counter0 Overflow Interrupt Enable), and TOV0 are set, the
Timer/Counter0 Overflow interrupt is executed.
The setting of this flag is dependent of the WGM02:0 bit setting.
Refer to “Waveform Generation Mode Bit Description” on page 106.
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14.
16-bit Timers/Counters (Timer/Counter1 and Timer/Counter3)
The 16-bit Timer/Counter unit allows accurate program execution timing (event management), wave generation,
and signal timing measurement. The main features are:
•
•
•
•
•
•
•
•
•
•
•
14.1
True 16-bit Design (i.e., Allows 16-bit PWM)
Three independent Output Compare Units
Double Buffered Output Compare Registers
One Input Capture Unit
Input Capture Noise Canceler
Clear Timer on Compare Match (Auto Reload)
Glitch-free, Phase Correct Pulse Width Modulator (PWM)
Variable PWM Period
Frequency Generator
External Event Counter
Ten independent interrupt sources (TOV1, OCF1A, OCF1B, OCF1C, ICF1, TOV3, OCF3A, OCF3B, OCF3C, and
ICF3)
Overview
Most register and bit references in this section are written in general form. A lower case “n” replaces the
Timer/Counter number, and a) lower case “x” replaces the Output Compare unit channel. However, when using
the register or bit defines in a program, the precise form must be used, i.e., TCNT1 for accessing
Timer/Counter1 counter value and so on.
A simplified block diagram of the 16-bit Timer/Counter is shown in Figure 14-1 on page 112. For the actual
placement of I/O pins, see “Pinout” on page 3. CPU accessible I/O Registers, including I/O bits and I/O pins, are
shown in bold. The device-specific I/O Register and bit locations are listed in the “16-bit Timers/Counters
(Timer/Counter1 and Timer/Counter3)” on page 111.
The Power Reduction Timer/Counter1 bit, PRTIM1, in “Power Reduction Register 0 - PRR0” on page 47 must
be written to zero to enable Timer/Counter1 module.
The Power Reduction Timer/Counter3 bit, PRTIM3, in “Power Reduction Register 1 - PRR1” on page 48 must
be written to zero to enable Timer/Counter3 module.
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Figure 14-1.
16-bit Timer/Counter Block Diagram(1)
Count
Clear
Direction
TOVn
(Int.Req.)
Control Logic
Clock Select
TCLK
(2)
Edge
Detector
TOP
Tn
BOTTOM
( From Prescaler )
Timer/Counter
TCNTn
=
=0
OCFnA
(Int.Req.)
Waveform
Generation
=
OCnA
OCRnA
OCFnB
(Int.Req.)
Fixed
TOP
Values
Waveform
Generation
DATABUS
=
OCnB
OCRnB
OCFnC
(Int.Req.)
Waveform
Generation
=
OCnC
OCRnC
( From Analog
Comparator Ouput )
ICFn (Int.Req.)
Edge
Detector
ICRn
Noise
Canceler
ICPn
TCCRnA
Note:
1.
2.
TCCRnB
TCCRnC
Refer to “Pinout” on page 3, Table 10-3 on page 74, and Table 10-6 on page 77 for Timer/Counter1 and 3 and
3 pin placement and description.
Tn only refers to T1 since T3 input is not available on the product.
14.1.1 Registers
The Timer/Counter (TCNTn), Output Compare Registers (OCRnA/B/C), and Input Capture Register (ICRn) are
all 16-bit registers. Special procedures must be followed when accessing the 16-bit registers. These procedures
are described in the section “Accessing 16-bit Registers” on page 113. The Timer/Counter Control Registers
(TCCRnA/B/C) are 8-bit registers and have no CPU access restrictions. Interrupt requests (shorten as Int.Req.)
signals are all visible in the Timer Interrupt Flag Register (TIFRn). All interrupts are individually masked with the
Timer Interrupt Mask Register (TIMSKn). TIFRn and TIMSKn are not shown in the figure since these registers
are shared by other timer units.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the Tn pin.
The Clock Select logic block controls which clock source and edge the Timer/Counter uses to increment (or
decrement) its value. The Timer/Counter is inactive when no clock source is selected. The output from the clock
select logic is referred to as the timer clock (clkTn).
The double buffered Output Compare Registers (OCRnA/B/C) are compared with the Timer/Counter value at all
time. The result of the compare can be used by the Waveform Generator to generate a PWM or variable
frequency output on the Output Compare pin (OCnA/B/C). See “Output Compare Units” on page 119. The
compare match event will also set the Compare Match Flag (OCFnA/B/C) which can be used to generate an
Output Compare interrupt request.
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The Input Capture Register can capture the Timer/Counter value at a given external (edge triggered) event on
either the Input Capture pin (ICPn) or on the Analog Comparator pins (See “Analog Comparator” on page 293.)
The Input Capture unit includes a digital filtering unit (Noise Canceler) for reducing the chance of capturing
noise spikes.
The TOP value, or maximum Timer/Counter value, can in some modes of operation be defined by either the
OCRnA Register, the ICRn Register, or by a set of fixed values. When using OCRnA as TOP value in a PWM
mode, the OCRnA Register can not be used for generating a PWM output. However, the TOP value will in this
case be double buffered allowing the TOP value to be changed in run time. If a fixed TOP value is required, the
ICRn Register can be used as an alternative, freeing the OCRnA to be used as PWM output.
14.1.2 Definitions
The following definitions are used extensively throughout the document:
14.2
BOTTOM
The counter reaches the BOTTOM when it becomes 0x0000.
MAX
The counter reaches its MAXimum when it becomes 0xFFFF (decimal 65535).
TOP
The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be one of the fixed values: 0x00FF,
0x01FF, or 0x03FF, or to the value stored in the OCRnA or ICRn Register. The
assignment is dependent of the mode of operation.
Accessing 16-bit Registers
The TCNTn, OCRnA/B/C, and ICRn are 16-bit registers that can be accessed by the AVR CPU via the 8-bit
data bus. The 16-bit register must be byte accessed using two read or write operations. Each 16-bit timer has a
single 8-bit register for temporary storing of the high byte of the 16-bit access. The same Temporary Register is
shared between all 16-bit registers within each 16-bit timer. Accessing the low byte triggers the 16-bit read or
write operation. When the low byte of a 16-bit register is written by the CPU, the high byte stored in the
Temporary Register, and the low byte written are both copied into the 16-bit register in the same clock cycle.
When the low byte of a 16-bit register is read by the CPU, the high byte of the 16-bit register is copied into the
Temporary Register in the same clock cycle as the low byte is read.
Not all 16-bit accesses uses the Temporary Register for the high byte. Reading the OCRnA/B/C 16-bit registers
does not involve using the Temporary Register.
To do a 16-bit write, the high byte must be written before the low byte. For a 16-bit read, the low byte must be
read before the high byte.
The following code examples show how to access the 16-bit timer registers assuming that no interrupts updates
the temporary register. The same principle can be used directly for accessing the OCRnA/B/C and ICRn
Registers. Note that when using “C”, the compiler handles the 16-bit access.
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Assembly Code Examples(1)
...
; Set TCNTn to 0x01FF
ldi
r17,0x01
ldi
r16,0xFF
out
TCNTnH,r17
out
TCNTnL,r16
; Read TCNTn into r17:r16
in
r16,TCNTnL
in
r17,TCNTnH
...
C Code Examples(1)
unsigned int i;
...
/* Set TCNTn to 0x01FF */
TCNTn = 0x1FF;
/* Read TCNTn into i */
i = TCNTn;
...
Note:
1.
See “Code Examples” on page 8.
The assembly code example returns the TCNTn value in the r17:r16 register pair.
It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt occurs between the
two instructions accessing the 16-bit register, and the interrupt code updates the temporary register by
accessing the same or any other of the 16-bit Timer Registers, then the result of the access outside the interrupt
will be corrupted. Therefore, when both the main code and the interrupt code update the temporary register, the
main code must disable the interrupts during the 16-bit access.
The following code examples show how to do an atomic read of the TCNTn Register contents. Reading any of
the OCRnA/B/C or ICRn Registers can be done by using the same principle.
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Assembly Code Example(1)
TIM16_ReadTCNTn:
; Save global interrupt flag
in
r18,SREG
; Disable interrupts
cli
; Read TCNTn into r17:r16
in
r16,TCNTnL
in
r17,TCNTnH
; Restore global interrupt flag
out
SREG,r18
ret
C Code Example(1)
unsigned int TIM16_ReadTCNTn( void )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
__disable_interrupt();
/* Read TCNTn into i */
i = TCNTn;
/* Restore global interrupt flag */
SREG = sreg;
return i;
}
Note:
1.
See “Code Examples” on page 8.
The assembly code example returns the TCNTn value in the r17:r16 register pair.
The following code examples show how to do an atomic write of the TCNTn Register contents. Writing any of
the OCRnA/B/C or ICRn Registers can be done by using the same principle.
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Assembly Code Example(1)
TIM16_WriteTCNTn:
; Save global interrupt flag
in
r18,SREG
; Disable interrupts
cli
; Set TCNTn to r17:r16
out
TCNTnH,r17
out
TCNTnL,r16
; Restore global interrupt flag
out
SREG,r18
ret
C Code Example(1)
void TIM16_WriteTCNTn( unsigned int i )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
__disable_interrupt();
/* Set TCNTn to i */
TCNTn = i;
/* Restore global interrupt flag */
SREG = sreg;
}
Note:
1.
See “Code Examples” on page 8.
The assembly code example requires that the r17:r16 register pair contains the value to be written to TCNTn.
14.2.1 Reusing the Temporary High Byte Register
If writing to more than one 16-bit register where the high byte is the same for all registers written, then the high
byte only needs to be written once. However, note that the same rule of atomic operation described previously
also applies in this case.
14.3
Timer/Counter Clock Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock source is selected by
the Clock Select logic which is controlled by the Clock Select (CSn2:0) bits located in the Timer/Counter control
Register B (TCCRnB). For details on clock sources and prescaler, see “Timer/Counter0, Timer/Counter1, and
Timer/Counter3 Prescalers” on page 92.
14.4
Counter Unit
The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit. Figure 14-2
shows a block diagram of the counter and its surroundings.
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Figure 14-2.
Counter Unit Block Diagram
DATA BUS
(8-bit)
TOVn
(Int.Req.)
TEMP (8-bit)
Clock Select
Count
TCNTnH (8-bit)
TCNTnL (8-bit)
TCNTn (16-bit Counter)
Clear
Direction
Control Logic
clkTn
Edge
Detector
Tn
( From Prescaler )
TOP
BOTTOM
Signal description (internal signals):
Count: Increment or decrement TCNTn by 1.
Direction: Select between increment and decrement.
Clear: Clear TCNTn (set all bits to zero).
clkTn: Timer/Counter clock.
TOP: Signalize that TCNTn has reached maximum value.
BOTTOM: Signalize that TCNTn has reached minimum value (zero).
The 16-bit counter is mapped into two 8-bit I/O memory locations: Counter High (TCNTnH) containing the upper
eight bits of the counter, and Counter Low (TCNTnL) containing the lower eight bits. The TCNTnH Register can
only be indirectly accessed by the CPU. When the CPU does an access to the TCNTnH I/O location, the CPU
accesses the high byte temporary register (TEMP). The temporary register is updated with the TCNTnH value
when the TCNTnL is read, and TCNTnH is updated with the temporary register value when TCNTnL is written.
This allows the CPU to read or write the entire 16-bit counter value within one clock cycle via the 8-bit data bus.
It is important to notice that there are special cases of writing to the TCNTn Register when the counter is
counting that will give unpredictable results. The special cases are described in the sections where they are of
importance.
Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each timer
clock (clkTn). The clkTn can be generated from an external or internal clock source, selected by the Clock Select
bits (CSn2:0). When no clock source is selected (CSn2:0 = 0) the timer is stopped. However, the TCNTn value
can be accessed by the CPU, independent of whether clkTn is present or not. A CPU write overrides (has priority
over) all counter clear or count operations.
The counting sequence is determined by the setting of the Waveform Generation mode bits (WGMn3:0) located
in the Timer/Counter Control Registers A and B (TCCRnA and TCCRnB). There are close connections between
how the counter behaves (counts) and how waveforms are generated on the Output Compare outputs OCnx.
For more details about advanced counting sequences and waveform generation, see “Modes of Operation” on
page 122.
The Timer/Counter Overflow Flag (TOVn) is set according to the mode of operation selected by the WGMn3:0
bits. TOVn can be used for generating a CPU interrupt.
14.5
Input Capture Unit
The Timer/Counter incorporates an input capture unit that can capture external events and give them a timestamp indicating time of occurrence. The external signal indicating an event, or multiple events, can be applied
via the ICPn pin or alternatively, for the Timer/Counter1 only, via the Analog Comparator unit. The time-stamps
can then be used to calculate frequency, duty-cycle, and other features of the signal applied. Alternatively the
time-stamps can be used for creating a log of the events.
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The Input Capture unit is illustrated by the block diagram shown in Figure 14-3. The elements of the block
diagram that are not directly a part of the input capture unit are gray shaded. The small “n” in register and bit
names indicates the Timer/Counter number.
Figure 14-3.
Input Capture Unit Block Diagram
DATA BUS
(8-bit)
TEMP (8-bit)
ICRnH (8-bit)
WRITE
ICRnL (8-bit)
TCNTnH (8-bit)
ICRn (16-bit Register)
ACO*
Analog
Comparator
ACIC*
TCNTnL (8-bit)
TCNTn (16-bit Counter)
ICNC
ICES
Noise
Canceler
Edge
Detector
ICFn (Int.Req.)
ICPn
Note:
The Analog Comparator Output (ACO) can only trigger the Timer/Counter1 ICP – not Timer/Counter3, 4, or 5.
When a change of the logic level (an event) occurs on the Input Capture Pin (ICPn), alternatively on the analog
Comparator output (ACO), and this change confirms to the setting of the edge detector, a capture will be
triggered. When a capture is triggered, the 16-bit value of the counter (TCNTn) is written to the Input Capture
Register (ICRn). The Input Capture Flag (ICFn) is set at the same system clock as the TCNTn value is copied
into ICRn Register. If enabled (TICIEn = 1), the input capture flag generates an input capture interrupt. The ICFn
flag is automatically cleared when the interrupt is executed. Alternatively the ICFn flag can be cleared by
software by writing a logical one to its I/O bit location.
Reading the 16-bit value in the Input Capture Register (ICRn) is done by first reading the low byte (ICRnL) and
then the high byte (ICRnH). When the low byte is read the high byte is copied into the high byte Temporary
Register (TEMP). When the CPU reads the ICRnH I/O location it will access the TEMP Register.
The ICRn Register can only be written when using a Waveform Generation mode that utilizes the ICRn Register
for defining the counter’s TOP value. In these cases the Waveform Generation mode (WGMn3:0) bits must be
set before the TOP value can be written to the ICRn Register. When writing the ICRn Register the high byte
must be written to the ICRnH I/O location before the low byte is written to ICRnL.
For more information on how to access the 16-bit registers refer to “Accessing 16-bit Registers” on page 113.
14.5.1 Input Capture Trigger Source
The main trigger source for the input capture unit is the Input Capture Pin (ICPn). Timer/Counter1 can
alternatively use the analog comparator output as trigger source for the input capture unit. The Analog
Comparator is selected as trigger source by setting the analog Comparator Input Capture (ACIC) bit in the
Analog Comparator Control and Status Register (ACSR). Be aware that changing trigger source can trigger a
capture. The input capture flag must therefore be cleared after the change.
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Both the Input Capture Pin (ICPn) and the Analog Comparator output (ACO) inputs are sampled using the same
technique as for the Tn pin (Figure 12-1 on page 92). The edge detector is also identical. However, when the
noise canceler is enabled, additional logic is inserted before the edge detector, which increases the delay by
four system clock cycles. Note that the input of the noise canceler and edge detector is always enabled unless
the Timer/Counter is set in a Waveform Generation mode that uses ICRn to define TOP.
An input capture can be triggered by software by controlling the port of the ICPn pin.
14.5.2 Noise Canceler
The noise canceler improves noise immunity by using a simple digital filtering scheme. The noise canceler input
is monitored over four samples, and all four must be equal for changing the output that in turn is used by the
edge detector.
The noise canceler is enabled by setting the Input Capture Noise Canceler (ICNCn) bit in Timer/Counter Control
Register B (TCCRnB). When enabled the noise canceler introduces additional four system clock cycles of delay
from a change applied to the input, to the update of the ICRn Register. The noise canceler uses the system
clock and is therefore not affected by the prescaler.
14.5.3 Using the Input Capture Unit
The main challenge when using the Input Capture unit is to assign enough processor capacity for handling the
incoming events. The time between two events is critical. If the processor has not read the captured value in the
ICRn Register before the next event occurs, the ICRn will be overwritten with a new value. In this case the result
of the capture will be incorrect.
When using the Input Capture interrupt, the ICRn Register should be read as early in the interrupt handler
routine as possible. Even though the Input Capture interrupt has relatively high priority, the maximum interrupt
response time is dependent on the maximum number of clock cycles it takes to handle any of the other interrupt
requests.
Using the Input Capture unit in any mode of operation when the TOP value (resolution) is actively changed
during operation, is not recommended.
Measurement of an external signal’s duty cycle requires that the trigger edge is changed after each capture.
Changing the edge sensing must be done as early as possible after the ICRn Register has been read. After a
change of the edge, the Input Capture Flag (ICFn) must be cleared by software (writing a logical one to the I/O
bit location). For measuring frequency only, the clearing of the ICFn Flag is not required (if an interrupt handler
is used).
14.6
Output Compare Units
The 16-bit comparator continuously compares TCNTn with the Output Compare Register (OCRnx). If TCNT
equals OCRnx the comparator signals a match. A match will set the Output Compare Flag (OCFnx) at the next
timer clock cycle. If enabled (OCIEnx = 1), the Output Compare Flag generates an Output Compare interrupt.
The OCFnx Flag is automatically cleared when the interrupt is executed. Alternatively the OCFnx Flag can be
cleared by software by writing a logical one to its I/O bit location. The Waveform Generator uses the match
signal to generate an output according to operating mode set by the Waveform Generation mode (WGMn3:0)
bits and Compare Output mode (COMnx1:0) bits. The TOP and BOTTOM signals are used by the Waveform
Generator for handling the special cases of the extreme values in some modes of operation (See “Modes of
Operation” on page 122.)
A special feature of Output Compare unit A allows it to define the Timer/Counter TOP value (i.e., counter
resolution). In addition to the counter resolution, the TOP value defines the period time for waveforms generated
by the Waveform Generator.
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Figure 14-4 shows a block diagram of the Output Compare unit. The small “n” in the register and bit names
indicates the device number (n = n for Timer/Counter n), and the “x” indicates Output Compare unit (A/B/C). The
elements of the block diagram that are not directly a part of the Output Compare unit are gray shaded.
Figure 14-4.
Output Compare Unit, Block Diagram
DATA BUS
(8-bit)
TEMP (8-bit)
OCRnxH Buf. (8-bit)
OCRnxL Buf. (8-bit)
TCNTnH (8-bit)
OCRnx Buffer (16-bit Register)
OCRnxH (8-bit)
TCNTnL (8-bit)
TCNTn (16-bit Counter)
OCRnxL (8-bit)
OCRnx (16-bit Register)
= (16-bit Comparator )
OCFnx (Int.Req.)
TOP
BOTTOM
Waveform Generator
WGMn3:0
OCnx
COMnx1:0
The OCRnx Register is double buffered when using any of the twelve Pulse Width Modulation (PWM) modes.
For the Normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The
double buffering synchronizes the update of the OCRnx Compare Register to either TOP or BOTTOM of the
counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses,
thereby making the output glitch-free.
The OCRnx Register access may seem complex, but this is not case. When the double buffering is enabled, the
CPU has access to the OCRnx Buffer Register, and if double buffering is disabled the CPU will access the
OCRnx directly. The content of the OCR1x (Buffer or Compare) Register is only changed by a write operation
(the Timer/Counter does not update this register automatically as the TCNT1 and ICR1 Register). Therefore
OCR1x is not read via the high byte temporary register (TEMP). However, it is a good practice to read the low
byte first as when accessing other 16-bit registers. Writing the OCRnx Registers must be done via the TEMP
Register since the compare of all 16 bits is done continuously. The high byte (OCRnxH) has to be written first.
When the high byte I/O location is written by the CPU, the TEMP Register will be updated by the value written.
Then when the low byte (OCRnxL) is written to the lower eight bits, the high byte will be copied into the upper 8bits of either the OCRnx buffer or OCRnx Compare Register in the same system clock cycle.
For more information of how to access the 16-bit registers refer to “Accessing 16-bit Registers” on page 113.
14.6.1 Force Output Compare
In non-PWM Waveform Generation modes, the match output of the comparator can be forced by writing a one
to the Force Output Compare (FOCnx) bit. Forcing compare match will not set the OCFnx Flag or reload/clear
the timer, but the OCnx pin will be updated as if a real compare match had occurred (the COMn1:0 bits settings
define whether the OCnx pin is set, cleared or toggled).
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14.6.2 Compare Match Blocking by TCNTn Write
All CPU writes to the TCNTn Register will block any compare match that occurs in the next timer clock cycle,
even when the timer is stopped. This feature allows OCRnx to be initialized to the same value as TCNTn without
triggering an interrupt when the Timer/Counter clock is enabled.
14.6.3 Using the Output Compare Unit
Since writing TCNTn in any mode of operation will block all compare matches for one timer clock cycle, there
are risks involved when changing TCNTn when using any of the Output Compare channels, independent of
whether the Timer/Counter is running or not. If the value written to TCNTn equals the OCRnx value, the
compare match will be missed, resulting in incorrect waveform generation. Do not write the TCNTn equal to
TOP in PWM modes with variable TOP values. The compare match for the TOP will be ignored and the counter
will continue to 0xFFFF. Similarly, do not write the TCNTn value equal to BOTTOM when the counter is downcounting.
The setup of the OCnx should be performed before setting the Data Direction Register for the port pin to output.
The easiest way of setting the OCnx value is to use the Force Output Compare (FOCnx) strobe bits in Normal
mode. The OCnx Register keeps its value even when changing between Waveform Generation modes.
Be aware that the COMnx1:0 bits are not double buffered together with the compare value. Changing the
COMnx1:0 bits will take effect immediately.
Compare Match Output Unit
The Compare Output mode (COMnx1:0) bits have two functions. The Waveform Generator uses the COMnx1:0
bits for defining the Output Compare (OCnx) state at the next compare match. Secondly the COMnx1:0 bits
control the OCnx pin output source. Figure 14-5 shows a simplified schematic of the logic affected by the
COMnx1:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of
the general I/O Port Control Registers (DDR and PORT) that are affected by the COMnx1:0 bits are shown.
When referring to the OCnx state, the reference is for the internal OCnx Register, not the OCnx pin. If a system
reset occur, the OCnx Register is reset to “0”.
Figure 14-5.
Compare Match Output Unit, Schematic
COMnx1
COMnx0
FOCnx
Waveform
Generator
D
Q
1
OCnx
D
DATA BUS
14.7
0
OCnx
Pin
Q
PORT
D
Q
DDR
clk I/O
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The general I/O port function is overridden by the Output Compare (OCnx) from the Waveform Generator if
either of the COMnx1:0 bits are set. However, the OCnx pin direction (input or output) is still controlled by the
Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OCnx pin (DDR_OCnx)
must be set as output before the OCnx value is visible on the pin. The port override function is generally
independent of the Waveform Generation mode, but there are some exceptions. Refer to Table 14-1 on
page 131, Table 14-2 on page 132, and Table 14-3 on page 132 for details.
The design of the Output Compare pin logic allows initialization of the OCnx state before the output is enabled.
Note that some COMnx1:0 bit settings are reserved for certain modes of operation. See “16-bit Timers/Counters
(Timer/Counter1 and Timer/Counter3)” on page 111.
The COMnx1:0 bits have no effect on the Input Capture unit.
14.7.1 Compare Output Mode and Waveform Generation
The Waveform Generator uses the COMnx1:0 bits differently in normal, CTC, and PWM modes. For all modes,
setting the COMnx1:0 = 0 tells the Waveform Generator that no action on the OCnx Register is to be performed
on the next compare match. For compare output actions in the non-PWM modes refer to Table 14-1 on
page 131. For fast PWM mode refer to Table 14-2 on page 132, and for phase correct and phase and frequency
correct PWM refer to Table 14-3 on page 132.
A change of the COMnx1:0 bits state will have effect at the first compare match after the bits are written. For
non-PWM modes, the action can be forced to have immediate effect by using the FOCnx strobe bits.
14.8
Modes of Operation
The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined by the
combination of the Waveform Generation mode (WGMn3:0) and Compare Output mode (COMnx1:0) bits. The
Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do.
The COMnx1:0 bits control whether the PWM output generated should be inverted or not (inverted or noninverted PWM). For non-PWM modes the COMnx1:0 bits control whether the output should be set, cleared or
toggle at a compare match (See “Compare Match Output Unit” on page 121.)
For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 129.
14.8.1 Normal Mode
The simplest mode of operation is the Normal mode (WGMn3:0 = 0). In this mode the counting direction is
always up (incrementing), and no counter clear is performed. The counter simply overruns when it passes its
maximum 16-bit value (MAX = 0xFFFF) and then restarts from the BOTTOM (0x0000). In normal operation the
Timer/Counter Overflow Flag (TOVn) will be set in the same timer clock cycle as the TCNTn becomes zero. The
TOVn Flag in this case behaves like a 17th bit, except that it is only set, not cleared. However, combined with
the timer overflow interrupt that automatically clears the TOVn Flag, the timer resolution can be increased by
software. There are no special cases to consider in the Normal mode, a new counter value can be written
anytime.
The Input Capture unit is easy to use in Normal mode. However, observe that the maximum interval between
the external events must not exceed the resolution of the counter. If the interval between events are too long,
the timer overflow interrupt or the prescaler must be used to extend the resolution for the capture unit.
The Output Compare units can be used to generate interrupts at some given time. Using the Output Compare to
generate waveforms in Normal mode is not recommended, since this will occupy too much of the CPU time.
14.8.2 Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGMn3:0 = 4 or 12), the OCRnA or ICRn Register are used to
manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value (TCNTn)
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matches either the OCRnA (WGMn3:0 = 4) or the ICRn (WGMn3:0 = 12). The OCRnA or ICRn define the top
value for the counter, hence also its resolution. This mode allows greater control of the compare match output
frequency. It also simplifies the operation of counting external events.
The timing diagram for the CTC mode is shown in Figure 14-6. The counter value (TCNTn) increases until a
compare match occurs with either OCRnA or ICRn, and then counter (TCNTn) is cleared.
Figure 14-6.
CTC Mode, Timing Diagram
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
TCNTn
OCnA
(Toggle)
Period
(COMnA1:0 = 1)
1
2
3
4
An interrupt can be generated at each time the counter value reaches the TOP value by either using the OCFnA
or ICFn Flag according to the register used to define the TOP value. If the interrupt is enabled, the interrupt
handler routine can be used for updating the TOP value. However, changing the TOP to a value close to
BOTTOM when the counter is running with none or a low prescaler value must be done with care since the CTC
mode does not have the double buffering feature. If the new value written to OCRnA or ICRn is lower than the
current value of TCNTn, the counter will miss the compare match. The counter will then have to count to its
maximum value (0xFFFF) and wrap around starting at 0x0000 before the compare match can occur. In many
cases this feature is not desirable. An alternative will then be to use the fast PWM mode using OCRnA for
defining TOP (WGMn3:0 = 15) since the OCRnA then will be double buffered.
For generating a waveform output in CTC mode, the OCnA output can be set to toggle its logical level on each
compare match by setting the Compare Output mode bits to toggle mode (COMnA1:0 = 1). The OCnA value will
not be visible on the port pin unless the data direction for the pin is set to output (DDR_OCnA = 1). The
waveform generated will have a maximum frequency of fOCnA = fclk_I/O/2 when OCRnA is set to zero (0x0000).
The waveform frequency is defined by the following equation:
f clk_I/O
f OCnA = --------------------------------------------------2  N   1 + OCRnA 
The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOVn Flag is set in the same timer clock cycle that the counter counts
from MAX to 0x0000.
14.8.3 Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGMn3:0 = 5, 6, 7, 14, or 15) provides a high frequency
PWM waveform generation option. The fast PWM differs from the other PWM options by its single-slope
operation. The counter counts from BOTTOM to TOP then restarts from BOTTOM. In non-inverting Compare
Output mode, the Output Compare (OCnx) is set on the compare match between TCNTn and OCRnx, and
cleared at TOP. In inverting Compare Output mode output is cleared on compare match and set at TOP. Due to
the single-slope operation, the operating frequency of the fast PWM mode can be twice as high as the phase
correct and phase and frequency correct PWM modes that use dual-slope operation. This high frequency
makes the fast PWM mode well suited for power regulation, rectification, and DAC applications. High frequency
allows physically small sized external components (coils, capacitors), hence reduces total system cost.
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The PWM resolution for fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either ICRn or OCRnA. The
minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum resolution is 16-bit
(ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated by using the following equation:
log  TOP + 1 
R FPWM = ----------------------------------log  2 
In fast PWM mode the counter is incremented until the counter value matches either one of the fixed values
0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 5, 6, or 7), the value in ICRn (WGMn3:0 = 14), or the value in OCRnA
(WGMn3:0 = 15). The counter is then cleared at the following timer clock cycle. The timing diagram for the fast
PWM mode is shown in Figure 14-7. The figure shows fast PWM mode when OCRnA or ICRn is used to define
TOP. The TCNTn value is in the timing diagram shown as a histogram for illustrating the single-slope operation.
The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNTn
slopes represent compare matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a
compare match occurs.
Figure 14-7.
Fast PWM Mode, Timing Diagram
OCRnx / TOP Update
and TOVn Interrupt Flag
Set and OCnA Interrupt
Flag Set or ICFn
Interrupt Flag Set
(Interrupt on TOP)
TCNTn
OCnx
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
Period
1
2
3
4
5
6
7
8
The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches TOP. In addition the OCnA or
ICFn Flag is set at the same timer clock cycle as TOVn is set when either OCRnA or ICRn is used for defining
the TOP value. If one of the interrupts are enabled, the interrupt handler routine can be used for updating the
TOP and compare values.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the value
of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a compare match
will never occur between the TCNTn and the OCRnx. Note that when using fixed TOP values the unused bits
are masked to zero when any of the OCRnx Registers are written.
The procedure for updating ICRn differs from updating OCRnA when used for defining the TOP value. The ICRn
Register is not double buffered. This means that if ICRn is changed to a low value when the counter is running
with none or a low prescaler value, there is a risk that the new ICRn value written is lower than the current value
of TCNTn. The result will then be that the counter will miss the compare match at the TOP value. The counter
will then have to count to the MAX value (0xFFFF) and wrap around starting at 0x0000 before the compare
match can occur. The OCRnA Register however, is double buffered. This feature allows the OCRnA I/O location
to be written anytime. When the OCRnA I/O location is written the value written will be put into the OCRnA
Buffer Register. The OCRnA Compare Register will then be updated with the value in the Buffer Register at the
next timer clock cycle the TCNTn matches TOP. The update is done at the same timer clock cycle as the
TCNTn is cleared and the TOVn Flag is set.
Using the ICRn Register for defining TOP works well when using fixed TOP values. By using ICRn, the OCRnA
Register is free to be used for generating a PWM output on OCnA. However, if the base PWM frequency is
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actively changed (by changing the TOP value), using the OCRnA as TOP is clearly a better choice due to its
double buffer feature.
In fast PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Setting the
COMnx1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by
setting the COMnx1:0 to three.
Refer to Table 14-1 on page 131, Table 14-2 on page 132, and Table 14-3 on page 132.
The actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output
(DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the compare
match between OCRnx and TCNTn, and clearing (or setting) the OCnx Register at the timer clock cycle the
counter is cleared (changes from TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
f clk_I/O
f OCnxPWM = ----------------------------------N   1 + TOP 
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCRnx Register represents special cases when generating a PWM waveform
output in the fast PWM mode. If the OCRnx is set equal to BOTTOM (0x0000) the output will be a narrow spike
for each TOP+1 timer clock cycle. Setting the OCRnx equal to TOP will result in a constant high or low output
(depending on the polarity of the output set by the COMnx1:0 bits).
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OCnA to
toggle its logical level on each compare match (COMnA1:0 = 1). This applies only if OCR1A is used to define
the TOP value (WGM13:0 = 15). The waveform generated will have a maximum frequency of fOCnA = fclk_I/O/2
when OCRnA is set to zero (0x0000). This feature is similar to the OCnA toggle in CTC mode, except the double
buffer feature of the Output Compare unit is enabled in the fast PWM mode.
14.8.4 Phase Correct PWM Mode
The phase correct Pulse Width Modulation or phase correct PWM mode (WGMn3:0 = 1, 2, 3, 10, or 11)
provides a high resolution phase correct PWM waveform generation option. The phase correct PWM mode is,
like the phase and frequency correct PWM mode, based on a dual-slope operation. The counter counts
repeatedly from BOTTOM (0x0000) to TOP and then from TOP to BOTTOM. In non-inverting Compare Output
mode, the Output Compare (OCnx) is cleared on the compare match between TCNTn and OCRnx while
upcounting, and set on the compare match while downcounting. In inverting Output Compare mode, the
operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope
operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for
motor control applications.
The PWM resolution for the phase correct PWM mode can be fixed to 8-, 9-, or 10-bit, or defined by either ICRn
or OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum
resolution is 16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated by using the
following equation:
log  TOP + 1 
R PCPWM = ----------------------------------log  2 
In phase correct PWM mode the counter is incremented until the counter value matches either one of the fixed
values 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 1, 2, or 3), the value in ICRn (WGMn3:0 = 10), or the value in
OCRnA (WGMn3:0 = 11). The counter has then reached the TOP and changes the count direction. The TCNTn
value will be equal to TOP for one timer clock cycle. The timing diagram for the phase correct PWM mode is
shown on Figure 14-8 on page 126. The figure shows phase correct PWM mode when OCRnA or ICRn is used
to define TOP. The TCNTn value is in the timing diagram shown as a histogram for illustrating the dual-slope
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operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on
the TCNTn slopes represent compare matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be
set when a compare match occurs.
Figure 14-8.
Phase Correct PWM Mode, Timing Diagram
OCRnx/TOP Update and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
TOVn Interrupt Flag Set
(Interrupt on Bottom)
TCNTn
OCnx
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
Period
1
2
3
4
The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches BOTTOM. When either OCRnA
or ICRn is used for defining the TOP value, the OCnA or ICFn Flag is set accordingly at the same timer clock
cycle as the OCRnx Registers are updated with the double buffer value (at TOP). The Interrupt Flags can be
used to generate an interrupt each time the counter reaches the TOP or BOTTOM value.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the value
of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a compare match
will never occur between the TCNTn and the OCRnx. Note that when using fixed TOP values, the unused bits
are masked to zero when any of the OCRnx Registers are written. As the third period shown in Figure 14-8 on
page 126 illustrates, changing the TOP actively while the Timer/Counter is running in the phase correct mode
can result in an unsymmetrical output. The reason for this can be found in the time of update of the OCRnx
Register. Since the OCRnx update occurs at TOP, the PWM period starts and ends at TOP. This implies that
the length of the falling slope is determined by the previous TOP value, while the length of the rising slope is
determined by the new TOP value. When these two values differ the two slopes of the period will differ in length.
The difference in length gives the unsymmetrical result on the output.
It is recommended to use the phase and frequency correct mode instead of the phase correct mode when
changing the TOP value while the Timer/Counter is running. When using a static TOP value there are practically
no differences between the two modes of operation.
In phase correct PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Setting
the COMnx1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by
setting the COMnx1:0 to three.
Refer to Table 14-1 on page 131, Table 14-2 on page 132, and Table 14-3 on page 132
The actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output
(DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the compare
match between OCRnx and TCNTn when the counter increments, and clearing (or setting) the OCnx Register at
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compare match between OCRnx and TCNTn when the counter decrements. The PWM frequency for the output
when using phase correct PWM can be calculated by the following equation:
f clk_I/O
f OCnxPCPWM = ---------------------------2  N  TOP
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCRnx Register represent special cases when generating a PWM waveform output
in the phase correct PWM mode. If the OCRnx is set equal to BOTTOM the output will be continuously low and
if set equal to TOP the output will be continuously high for non-inverted PWM mode. For inverted PWM the
output will have the opposite logic values. If OCR1A is used to define the TOP value (WGM13:0 = 11) and
COM1A1:0 = 1, the OC1A output will toggle with a 50% duty cycle.
14.8.5 Phase and Frequency Correct PWM Mode
The Phase and Frequency Correct PWM Mode (PWM4x = 1 and WGM40 = 1) provides a high resolution Phase
and Frequency Correct PWM waveform generation option. The Phase and Frequency Correct PWM mode is
based on a dual-slope operation. The counter counts repeatedly from BOTTOM to TOP (defined as OCR4C)
and then from TOP to BOTTOM. In noninverting Compare Output Mode, and in complimentary Compare Output
Mode, the Waveform Output (OCW4x) is cleared on the Compare Match between TCNT4 and OCR4x while
upcounting, and set on the Compare Match while down-counting. In inverting Output Compare mode, the
operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope
operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for
motor control applications.
The main difference between the phase correct, and the phase and frequency correct PWM mode is the time
the OCRnx Register is updated by the OCRnx Buffer Register, (see Figure 14-8 on page 126 and Figure 14-9
on page 128).
The PWM resolution for the phase and frequency correct PWM mode can be defined by either ICRn or OCRnA.
The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum resolution is 16-bit
(ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated using the following equation:
log  TOP + 1 
R PFCPWM = ----------------------------------log  2 
In phase and frequency correct PWM mode the counter is incremented until the counter value matches either
the value in ICRn (WGMn3:0 = 8), or the value in OCRnA (WGMn3:0 = 9). The counter has then reached the
TOP and changes the count direction. The TCNTn value will be equal to TOP for one timer clock cycle. The
timing diagram for the phase correct and frequency correct PWM mode is shown on Figure 14-9. The figure
shows phase and frequency correct PWM mode when OCRnA or ICRn is used to define TOP. The TCNTn
value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The diagram
includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNTn slopes
represent compare matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a compare
match occurs.
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Figure 14-9.
Phase and Frequency Correct PWM Mode, Timing Diagram
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
OCRnx/TOP Updateand
TOVn Interrupt Flag Set
(Interrupt on Bottom)
TCNTn
OCnx
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
Period
1
2
3
4
The Timer/Counter Overflow Flag (TOVn) is set at the same timer clock cycle as the OCRnx Registers are
updated with the double buffer value (at BOTTOM). When either OCRnA or ICRn is used for defining the TOP
value, the OCnA or ICFn Flag set when TCNTn has reached TOP. The Interrupt Flags can then be used to
generate an interrupt each time the counter reaches the TOP or BOTTOM value.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the value
of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a compare match
will never occur between the TCNTn and the OCRnx.
As Figure 14-9 on page 128 shows the output generated is, in contrast to the phase correct mode, symmetrical
in all periods. Since the OCRnx Registers are updated at BOTTOM, the length of the rising and the falling
slopes will always be equal. This gives symmetrical output pulses and is therefore frequency correct.
Using the ICRn Register for defining TOP works well when using fixed TOP values. By using ICRn, the OCRnA
Register is free to be used for generating a PWM output on OCnA. However, if the base PWM frequency is
actively changed by changing the TOP value, using the OCRnA as TOP is clearly a better choice due to its
double buffer feature.
In phase and frequency correct PWM mode, the compare units allow generation of PWM waveforms on the
OCnx pins. Setting the COMnx1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can
be generated by setting the COMnx1:0 to three.
Refer to Table 14-1 on page 131, Table 14-2 on page 132, and Table 14-3 on page 132.
The actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output
(DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the compare
match between OCRnx and TCNTn when the counter increments, and clearing (or setting) the OCnx Register at
compare match between OCRnx and TCNTn when the counter decrements. The PWM frequency for the output
when using phase and frequency correct PWM can be calculated by the following equation:
f clk_I/O
f OCnxPFCPWM = ---------------------------2  N  TOP
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCRnx Register represents special cases when generating a PWM waveform
output in the phase correct PWM mode. If the OCRnx is set equal to BOTTOM the output will be continuously
low and if set equal to TOP the output will be set to high for non-inverted PWM mode. For inverted PWM the
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output will have the opposite logic values. If OCR1A is used to define the TOP value (WGM13:0 = 9) and
COM1A1:0 = 1, the OC1A output will toggle with a 50% duty cycle.
14.9
Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkTn) is therefore shown as a clock enable
signal in the following figures. The figures include information on when Interrupt Flags are set, and when the
OCRnx Register is updated with the OCRnx buffer value (only for modes utilizing double buffering). Figure 1410 shows a timing diagram for the setting of OCFnx.
Figure 14-10. Timer/Counter Timing Diagram, Setting of OCFnx, no Prescaling
clkI/O
clkTn
(clkI/O /1)
TCNTn
OCRnx - 1
OCRnx
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
OCFnx
Figure 14-11 shows the same timing data, but with the prescaler enabled.
Figure 14-11. Timer/Counter Timing Diagram, Setting of OCFnx, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
OCRnx
OCRnx - 1
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
OCFnx
Figure 14-12 shows the count sequence close to TOP in various modes. When using phase and frequency
correct PWM mode the OCRnx Register is updated at BOTTOM. The timing diagrams will be the same, but
TOP should be replaced by BOTTOM, TOP-1 by BOTTOM+1 and so on. The same renaming applies for modes
that set the TOVn Flag at BOTTOM.
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Figure 14-12. Timer/Counter Timing Diagram, no Prescaling
clkI/O
clkTn
(clkI/O /1)
TCNTn
(CTC and FPWM)
TCNTn
(PC and PFC PWM)
TOP - 1
TOP
BOTTOM
BOTTOM + 1
TOP - 1
TOP
TOP - 1
TOP - 2
TOVn (FPWM)
and ICFn (if used
as TOP)
OCRnx
(Update at TOP)
New OCRnx Value
Old OCRnx Value
Figure 14-13 shows the same timing data, but with the prescaler enabled.
Figure 14-13. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O/8)
TCNTn
(CTC and FPWM)
TCNTn
(PC and PFC PWM)
TOP - 1
TOP
BOTTOM
BOTTOM + 1
TOP - 1
TOP
TOP - 1
TOP - 2
TOVn (FPWM)
and ICF n (if used
as TOP)
OCRnx
(Update at TOP)
Old OCRnx Value
New OCRnx Value
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14.10 16-bit Timer/Counter Register Description
14.10.1 Timer/Counter1 Control Register A – TCCR1A
Bit
Read/Write
Initial Value
7
COM1A1
6
COM1A0
5
COM1B1
4
COM1B0
3
COM1C1
2
COM1C0
1
WGM11
0
WGM10
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
2
COM3C0
R/W
0
1
WGM31
R/W
0
0
WGM30
R/W
0
TCCR1
A
14.10.2 Timer/Counter3 Control Register A – TCCR3A
Bit
Read/Write
Initial Value
7
COM3A1
R/W
0
6
COM3A0
R/W
0
5
COM3B1
R/W
0
4
COM3B0
R/W
0
3
COM3C1
R/W
0
TCCR3A
• Bit 7:6 – COMnA1:0: Compare Output Mode for Channel A
• Bit 5:4 – COMnB1:0: Compare Output Mode for Channel B
• Bit 3:2 – COMnC1:0: Compare Output Mode for Channel C
The COMnA1:0, COMnB1:0, and COMnC1:0 control the output compare pins (OCnA, OCnB, and OCnC
respectively) behavior. If one or both of the COMnA1:0 bits are written to one, the OCnA output overrides the
normal port functionality of the I/O pin it is connected to. If one or both of the COMnB1:0 bits are written to one,
the OCnB output overrides the normal port functionality of the I/O pin it is connected to. If one or both of the
COMnC1:0 bits are written to one, the OCnC output overrides the normal port functionality of the I/O pin it is
connected to. However, note that the Data Direction Register (DDR) bit corresponding to the OCnA, OCnB or
OCnC pin must be set in order to enable the output driver.
When the OCnA, OCnB or OCnC is connected to the pin, the function of the COMnx1:0 bits is dependent of the
WGMn3:0 bits setting. The table shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to a
normal or a CTC mode (non-PWM).
Table 14-1.
Compare Output Mode, non-PWM
COMnA1/COMnB1/COMnC1
COMnA0/COMnB0/COMnC0
Description
0
0
Normal port operation, OCnA/OCnB/OCnC
disconnected
0
1
Toggle OCnA/OCnB/OCnC on compare match
1
0
Clear OCnA/OCnB/OCnC on compare match (set
output to low level)
1
1
Set OCnA/OCnB/OCnC on compare match (set output
to high level)
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The table shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to the fast PWM mode.
Table 14-2.
Compare Output Mode, Fast PWM(1)
COMnA1/COMnB1/COMnC0
COMnA0/COMnB0/COMnC0
0
0
Normal port operation, OCnA/OCnB/OCnC
disconnected
0
1
WGM13:0 = 14 or 15: Toggle OC1A on Compare
Match, OC1B and OC1C disconnected (normal port
operation). For all other WGM1 settings, normal port
operation, OC1A/OC1B/OC1C disconnected.
1
0
Clear OCnA/OCnB/OCnC on compare match, set
OCnA/OCnB/OCnC at TOP
1
1
Set OCnA/OCnB/OCnC on compare match, clear
OCnA/OCnB/OCnC at TOP
Note:
1.
Description
A special case occurs when OCRnA/OCRnB/OCRnC equals TOP and COMnA1/COMnB1/COMnC1 is set. In
this case the compare match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on
page 100. for more details.
The table shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to the phase correct and
frequency correct PWM mode.
Table 14-3.
Compare Output Mode, Phase Correct and Phase and Frequency Correct PWM(1)
COMnA1/COMnB/COMnC1
COMnA0/COMnB0/COMnC0
0
0
Normal port operation, OCnA/OCnB/OCnC
disconnected.
0
1
WGM13:0 = 8, 9, 10, or 11: Toggle OC1A on Compare
Match, OC1B and OC1C disconnected (normal port
operation). For all other WGM1 settings, normal port
operation, OC1A/OC1B/OC1C disconnected.
1
0
Clear OCnA/OCnB/OCnC on compare match when upcounting. Set OCnA/OCnB/OCnC on compare match
when down-counting.
1
1
Set OCnA/OCnB/OCnC on compare match when upcounting. Clear OCnA/OCnB/OCnC on compare match
when down-counting.
Note:
1.
Description
A special case occurs when OCRnA/OCRnB/OCRnC equals TOP and COMnA1/COMnB1//COMnC1 is set.
See “Phase Correct PWM Mode” on page 101. for more details.
• Bit 1:0 – WGMn1:0: Waveform Generation Mode
Combined with the WGMn3:2 bits found in the TCCRnB Register, these bits control the counting sequence of
the counter, the source for maximum (TOP) counter value, and what type of waveform generation to be used,
see the table below. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear
Timer on Compare match (CTC) mode, and three types of Pulse Width Modulation (PWM) modes (“Modes of
Operation” on page 98).
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Table 14-4.
Waveform Generation Mode Bit Description
Mode
WGMn3
WGMn2
(CTCn)
WGMn0
(PWMn0)
0
0
0
0
1
0
0
2
0
3
TOVn
Flag Set
on
Timer/Counter Mode of Operation
TOP
Update of
OCRnx at
0
Normal
0xFFFF
Immediate
MAX
0
1
PWM, Phase Correct, 8-bit
0x00FF
TOP
BOTTOM
0
1
0
PWM, Phase Correct, 9-bit
0x01FF
TOP
BOTTOM
0
0
1
1
PWM, Phase Correct, 10-bit
0x03FF
TOP
BOTTOM
4
0
1
0
0
CTC
OCRnA
Immediate
MAX
5
0
1
0
1
Fast PWM, 8-bit
0x00FF
TOP
TOP
6
0
1
1
0
Fast PWM, 9-bit
0x01FF
TOP
TOP
7
0
1
1
1
Fast PWM, 10-bit
0x03FF
TOP
TOP
8
1
0
0
0
PWM, Phase and Frequency Correct
ICRn
BOTTOM
BOTTOM
9
1
0
0
1
PWM, Phase and Frequency Correct
OCRnA
BOTTOM
BOTTOM
10
1
0
1
0
PWM, Phase Correct
ICRn
TOP
BOTTOM
11
1
0
1
1
PWM, Phase Correct
OCRnA
TOP
BOTTOM
12
1
1
0
0
CTC
ICRn
Immediate
MAX
13
1
1
0
1
(Reserved)
–
–
–
14
1
1
1
0
Fast PWM
ICRn
TOP
TOP
15
1
1
1
1
Fast PWM
OCRnA
TOP
TOP
Note:
WGMn1
(PWMn1)
1. The CTCn and PWMn1:0 bit definition names are obsolete. Use the WGMn2:0 definitions. However, the functionality
and location of these bits are compatible with previous versions of the timer.
14.10.3 Timer/Counter1 Control Register B – TCCR1B
Bit
Read/Write
Initial Value
7
ICNC1
R/W
0
6
ICES1
R/W
0
5
–
R
0
4
WGM13
R/W
0
3
WGM12
R/W
0
2
CS12
R/W
0
1
CS11
R/W
0
0
CS10
R/W
0
3
WGM32
R/W
0
2
CS32
R/W
0
1
CS31
R/W
0
0
CS30
R/W
0
TCCR1B
14.10.4 Timer/Counter3 Control Register B – TCCR3B
Bit
Read/Write
Initial Value
7
ICNC3
R/W
0
6
ICES3
R/W
0
5
–
R
0
4
WGM33
R/W
0
TCCR3B
• Bit 7 – ICNCn: Input Capture Noise Canceler
Setting this bit (to one) activates the Input Capture Noise Canceler. When the Noise Canceler is activated, the
input from the Input Capture Pin (ICPn) is filtered. The filter function requires four successive equal valued
samples of the ICPn pin for changing its output. The input capture is therefore delayed by four Oscillator cycles
when the noise canceler is enabled.
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• Bit 6 – ICESn: Input Capture Edge Select
This bit selects which edge on the Input Capture Pin (ICPn) that is used to trigger a capture event. When the
ICESn bit is written to zero, a falling (negative) edge is used as trigger, and when the ICESn bit is written to one,
a rising (positive) edge will trigger the capture.
When a capture is triggered according to the ICESn setting, the counter value is copied into the Input Capture
Register (ICRn). The event will also set the Input Capture Flag (ICFn), and this can be used to cause an Input
Capture Interrupt, if this interrupt is enabled.
When the ICRn is used as TOP value (see description of the WGMn3:0 bits located in the TCCRnA and the
TCCRnB Register), the ICPn is disconnected and consequently the input capture function is disabled.
• Bit 5 – Reserved Bit
This bit is reserved for future use. For ensuring compatibility with future devices, this bit must be written to zero
when TCCRnB is written.
• Bit 4:3 – WGMn3:2: Waveform Generation Mode
See TCCRnA Register description.
• Bit 2:0 – CSn2:0: Clock Select
The three clock select bits select the clock source to be used by the Timer/Counter, see Figure 13-8 on page
102 and Figure 13-9 on page 103.
Table 14-5.
Clock Select Bit Description
CSn2
CSn1
CSn0
Description
0
0
0
No clock source. (Timer/Counter stopped)
0
0
1
clkI/O/1 (No prescaling
0
1
0
clkI/O/8 (From prescaler)
0
1
1
clkI/O/64 (From prescaler)
1
0
0
clkI/O/256 (From prescaler)
1
0
1
clkI/O/1024 (From prescaler)
1
1
0
External clock source on Tn pin. Clock on falling edge
1
1
1
External clock source on Tn pin. Clock on rising edge
If external pin modes are used for the Timer/Countern, transitions on the Tn pin will clock the counter even if the
pin is configured as an output. This feature allows software control of the counting.
14.10.5 Timer/Counter1 Control Register C – TCCR1C
Bit
Read/Write
Initial Value
7
FOC1A
W
0
6
FOC1B
W
0
5
FOC1C
W
0
4
–
R
0
3
–
R
0
2
–
R
0
1
–
R
0
0
–
R
0
TCCR1C
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14.10.6 Timer/Counter3 Control Register C – TCCR3C
Bit
Read/Write
Initial Value
7
FOC3A
W
0
6
–
R
0
5
–
R
0
4
–
R
0
3
–
R
0
2
–
R
0
1
–
R
0
0
–
R
0
TCCR3C
• Bit 7 – FOCnA: Force Output Compare for Channel A
The FOCnA/FOCnB/FOCnC bits are only active when the WGMn3:0 bits specifies a non-PWM mode. When
writing a logical one to the FOCnA/FOCnB/FOCnC bit, an immediate compare match is forced on the waveform
generation unit. The OCnA/OCnB/OCnC output is changed according to its COMnx1:0 bits setting. Note that the
FOCnA/FOCnB/FOCnC bits are implemented as strobes. Therefore it is the value present in the COMnx1:0 bits
that determine the effect of the forced compare.
A FOCnA/FOCnB/FOCnC strobe will not generate any interrupt nor will it clear the timer in Clear Timer on
Compare Match (CTC) mode using OCRnA as TOP.
The FOCnA/FOCnB/FOCnB bits are always read as zero.
• Bit 4:0 – Reserved Bits
These bits are reserved for future use. For ensuring compatibility with future devices, these bits must be written
to zero when TCCRnC is written.
14.10.7 Timer/Counter1 – TCNT1H and TCNT1L
Bit
Read/Write
Initial Value
7
6
TCNT1[15:8]
TCNT1[7:0]
R/W
R/W
0
0
5
4
3
2
1
0
TCNT1H
TCNT1L
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
3
2
1
0
14.10.8 Timer/Counter3 – TCNT3H and TCNT3L
Bit
Read/Write
Initial Value
7
6
TCNT3[15:8]
TCNT3[7:0]
R/W
R/W
0
0
5
4
TCNT3H
TCNT3L
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
The two Timer/Counter I/O locations (TCNTnH and TCNTnL, combined TCNTn) give direct access, both for
read and for write operations, to the Timer/Counter unit 16-bit counter. To ensure that both the high and low
bytes are read and written simultaneously when the CPU accesses these registers, the access is performed
using an 8-bit temporary High Byte Register (TEMP). This temporary register is shared by all the other 16-bit
registers. See “Accessing 16-bit Registers” on page 113.
Modifying the counter (TCNTn) while the counter is running introduces a risk of missing a compare match
between TCNTn and one of the OCRnx Registers.
Writing to the TCNTn Register blocks (removes) the compare match on the following timer clock for all compare
units.
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14.10.9 Output Compare Register 1 A – OCR1AH and OCR1AL
Bit
Read/Write
Initial Value
7
R/W
0
6
R/W
0
5
R/W
0
4
3
OCR1A[15:8]
OCR1A[7:0]
R/W
R/W
0
0
2
1
0
OCR1AH
OCR1AL
R/W
0
R/W
0
R/W
0
2
1
0
14.10.10 Output Compare Register 1 B – OCR1BH and OCR1BL
Bit
Read/Write
Initial Value
7
R/W
0
6
R/W
0
5
R/W
0
4
3
OCR1B[15:8]
OCR1B[7:0]
R/W
R/W
0
0
OCR1BH
OCR1BL
R/W
0
R/W
0
R/W
0
2
1
0
14.10.11 Output Compare Register 1 C – OCR1CH and OCR1CL
Bit
Read/Write
Initial Value
7
R/W
0
6
R/W
0
5
R/W
0
4
3
OCR1C[15:8]
OCR1C[7:0]
R/W
R/W
0
0
OCR1CH
OCR1CL
R/W
0
R/W
0
R/W
0
1
0
14.10.12 Output Compare Register 3 A – OCR3AH and OCR3AL
Bit
Read/Write
Initial Value
7
R/W
0
6
R/W
0
5
R/W
0
4
3
OCR3A[15:8]
OCR3A[7:0]
R/W
R/W
0
0
2
OCR3AH
OCR3AL
R/W
0
R/W
0
R/W
0
2
1
0
14.10.13 Output Compare Register 3 B – OCR3BH and OCR3BL
Bit
Read/Write
Initial Value
7
R/W
0
6
R/W
0
5
R/W
0
4
3
OCR3B[15:8]
OCR3B[7:0]
R/W
R/W
0
0
OCR3BH
OCR3BL
R/W
0
R/W
0
R/W
0
2
1
0
14.10.14 Output Compare Register 3 C – OCR3CH and OCR3CL
Bit
Read/Write
Initial Value
7
R/W
0
6
R/W
0
5
R/W
0
4
3
OCR3C[15:8]
OCR3C[7:0]
R/W
R/W
0
0
OCR3CH
OCR3CL
R/W
0
R/W
0
R/W
0
The Output Compare Registers contain a 16-bit value that is continuously compared with the counter value
(TCNTn). A match can be used to generate an Output Compare interrupt, or to generate a waveform output on
the OCnx pin.
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The Output Compare Registers are 16-bit in size. To ensure that both the high and low bytes are written
simultaneously when the CPU writes to these registers, the access is performed using an 8-bit temporary High
Byte Register (TEMP). This temporary register is shared by all the other 16-bit registers. See “Accessing 16-bit
Registers” on page 113.
14.10.15 Input Capture Register 1 – ICR1H and ICR1L
Bit
Read/Write
Initial Value
7
R/W
0
6
R/W
0
5
R/W
0
4
3
ICR1[15:8]
ICR1[7:0]
R/W
R/W
0
0
2
1
0
ICR1H
ICR1L
R/W
0
R/W
0
R/W
0
2
1
0
14.10.16 Input Capture Register 3 – ICR3H and ICR3L
Bit
Read/Write
Initial Value
7
R/W
0
6
R/W
0
5
R/W
0
4
3
ICR3[15:8]
ICR3[7:0]
R/W
R/W
0
0
ICR3H
ICR3L
R/W
0
R/W
0
R/W
0
The Input Capture is updated with the counter (TCNTn) value each time an event occurs on the ICPn pin (or
optionally on the Analog Comparator output for Timer/Counter1). The Input Capture can be used for defining the
counter TOP value.
The Input Capture Register is 16-bit in size. To ensure that both the high and low bytes are read simultaneously
when the CPU accesses these registers, the access is performed using an 8-bit temporary High Byte Register
(TEMP). This temporary register is shared by all the other 16-bit registers. See “Accessing 16-bit Registers” on
page 113.
14.10.17Timer/Counter1 Interrupt Mask Register – TIMSK1
Bit
Read/Write
Initial Value
7
–
R
0
6
–
R
0
5
ICIE1
R/W
0
4
–
R
0
3
OCIE1C
R/W
0
2
OCIE1B
R/W
0
1
OCIE1A
R/W
0
0
TOIE1
R/W
0
2
OCIE3B
R/W
0
1
OCIE3A
R/W
0
0
TOIE3
R/W
0
TIMSK1
14.10.18Timer/Counter3 Interrupt Mask Register – TIMSK3
Bit
Read/Write
Initial Value
7
–
R
0
6
–
R
0
5
ICIE3
R/W
0
4
–
R
0
3
OCIE3C
R/W
0
TIMSK3
• Bit 5 – ICIEn: Timer/Countern, Input Capture Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Countern Input Capture interrupt is enabled. The corresponding Interrupt Vector (See “Interrupts” on
page 63.) is executed when the ICFn Flag, located in TIFRn, is set.
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• Bit 3 – OCIEnC: Timer/Countern, Output Compare C Match Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Countern Output Compare C Match interrupt is enabled. The corresponding Interrupt Vector (See
“Interrupts” on page 63.) is executed when the OCFnC Flag, located in TIFRn, is set.
• Bit 2 – OCIEnB: Timer/Countern, Output Compare B Match Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Countern Output Compare B Match interrupt is enabled. The corresponding Interrupt Vector (See
“Interrupts” on page 63.) is executed when the OCFnB Flag, located in TIFRn, is set.
• Bit 1 – OCIEnA: Timer/Countern, Output Compare A Match Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Countern Output Compare A Match interrupt is enabled. The corresponding Interrupt Vector (See
“Interrupts” on page 63.) is executed when the OCFnA Flag, located in TIFRn, is set.
• Bit 0 – TOIEn: Timer/Countern, Overflow Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Countern Overflow interrupt is enabled. The corresponding Interrupt Vector (See “Interrupts” on page 63.)
is executed when the TOVn Flag, located in TIFRn, is set.
14.10.19Timer/Counter1 Interrupt Flag Register – TIFR1
Bit
Read/Write
Initial Value
7
–
R
0
6
–
R
0
5
ICF1
R/W
0
4
–
R
0
3
OCF1C
R/W
0
2
OCF1B
R/W
0
1
OCF1A
R/W
0
0
TOV1
R/W
0
2
OCF3B
R/W
0
1
OCF3A
R/W
0
0
TOV3
R/W
0
TIFR1
14.10.20Timer/Counter3 Interrupt Flag Register – TIFR3
Bit
Read/Write
Initial Value
7
–
R
0
6
–
R
0
5
ICF3
R/W
0
4
–
R
0
3
OCF3C
R/W
0
TIFR3
• Bit 5 – ICFn: Timer/Countern, Input Capture Flag
This flag is set when a capture event occurs on the ICPn pin. When the Input Capture Register (ICRn) is set by
the WGMn3:0 to be used as the TOP value, the ICFn Flag is set when the counter reaches the TOP value.
ICFn is automatically cleared when the Input Capture Interrupt Vector is executed. Alternatively, ICFn can be
cleared by writing a logic one to its bit location.
• Bit 3– OCFnC: Timer/Countern, Output Compare C Match Flag
This flag is set in the timer clock cycle after the counter (TCNTn) value matches the Output Compare Register C
(OCRnC).
Note that a Forced Output Compare (FOCnC) strobe will not set the OCFnC Flag.
OCFnC is automatically cleared when the Output Compare Match C Interrupt Vector is executed. Alternatively,
OCFnC can be cleared by writing a logic one to its bit location.
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• Bit 2 – OCFnB: Timer/Counter1, Output Compare B Match Flag
This flag is set in the timer clock cycle after the counter (TCNTn) value matches the Output Compare Register B
(OCRnB).
Note that a Forced Output Compare (FOCnB) strobe will not set the OCFnB Flag.
OCFnB is automatically cleared when the Output Compare Match B Interrupt Vector is executed. Alternatively,
OCFnB can be cleared by writing a logic one to its bit location.
• Bit 1 – OCF1A: Timer/Counter1, Output Compare A Match Flag
This flag is set in the timer clock cycle after the counter (TCNTn value matches the Output Compare Register A
(OCRnA).
Note that a Forced Output Compare (FOCnA) strobe will not set the OCFnA Flag.
OCFnA is automatically cleared when the Output Compare Match A Interrupt Vector is executed. Alternatively,
OCFnA can be cleared by writing a logic one to its bit location.
• Bit 0 – TOVn: Timer/Countern, Overflow Flag
The setting of this flag is dependent of the WGMn3:0 bits setting. In Normal and CTC modes, the TOVn Flag is
set when the timer overflows. Refer to Table 14-4 on page 133 for the TOVn Flag behavior when using another
WGMn3:0 bit setting.
TOVn is automatically cleared when the Timer/Countern Overflow Interrupt Vector is executed. Alternatively,
TOVn can be cleared by writing a logic one to its bit location.
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15.
10-bit High Speed Timer/Counter4
15.1
Features
•
•
•
•
•
•
•
•
•
•
•
15.2
Up to 10-Bit Accuracy
Three Independent Output Compare Units
Clear Timer on Compare Match (Auto Reload)
Glitch Free, Phase and Frequency Correct Pulse Width Modulator (PWM)
Enhanced PWM mode: one optional additional accuracy bit without effect on output frequency
Variable PWM Period
Independent Dead Time Generators for each PWM channels
Synchronous update of PWM registers
Five Independent Interrupt Sources (TOV4, OCF4A, OCF4B, OCF4D, FPF4)
High Speed Asynchronous and Synchronous Clocking Modes
Separate Prescaler Unit
Overview
Timer/Counter4 is a general purpose high speed Timer/Counter module, with three independent Output
Compare Units, and with enhanced PWM support.
The Timer/Counter4 features a high resolution and a high accuracy usage with the lower prescaling
opportunities. It can also support three accurate and high speed Pulse Width Modulators using clock speeds up
to 64MHz. In PWM mode Timer/Counter4 and the output compare registers serve as triple stand-alone PWMs
with non-overlapping, non-inverted and inverted outputs. The enhanced PWM mode allows to get one more
accuracy bit while keeping the frequency identical to normal mode (a PWM 8 bits accuracy in enhanced mode
outputs the same frequency that a PWM 7 bits accuracy in normal mode). Similarly, the high prescaling
opportunities make this unit useful for lower speed functions or exact timing functions with infrequent actions. A
lock feature allows user to update the PWM registers and
A simplified block diagram of the Timer/Counter4 is shown in Figure 15-1 on page 141. For actual placement of
the I/O pins, refer to “Pinout” on page 3. The device-specific I/O register and bit locations are listed in the
“Register Description” on page 164.
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Figure 15-1.
Timer/Counter4 Block Diagram
TOV4
OCF4A
OCF4B
OCF4D
OC4A
OC4A
OC4B
OC4B
FAULT_PROTECTION
DEAD TIME GENERATOR
DEAD TIME GENERATOR
DEAD TIME GENERATOR
OC4D
OC4D
OCW4A
OCW4B
WGM41
WGM40
FPAC4
FPF4
OC4OE1
OC4OE0
FPES4
OC4OE3
OC4OE2
FPNC4
OC4OE4
FPIE4
FPEN4
T/C CONTROL
REGISTER C (TCCR4D)
OC4OE5
PWM4D
COM4D0
FOC4D
COM4D1
COM4B0
COM4B1
COM4A1
T/C CONTROL
REGISTER C (TCCR4C)
COM4A0
CS40
CS41
CS43
CS42
PSR4
PSR4
T/C CONTROL
REGISTER B (TCCR4B)
PSR4
PWM4B
FOC4B
PWM4A
FOC4A
COM4B1
COM4A0
COM4B0
T/C CONTROL
REGISTER A (TCCR4A)
FPF4
FPIE4
OCF4D
TOV4
OCF4A
T/C INT. FLAG
REGISTER (TIFR4)
COM4A1
T/C INT. MASK
REGISTER (TIMSK4)
OCF4B
OCIE4D
OCIE4A
OCIE4B
TOIE4
OCW4D
CLK
TIMER/COUNTER4
(TCNT4)
COUNT
TIMER/COUNTER4 CONTROL LOGIC
CLEAR
DIRECTION
10-BIT COMPARATOR
10-BIT COMPARATOR
10-BIT COMPARATOR
10-BIT OUTPUT
COMPARE REGISTER A
10-BIT OUTPUT
COMPARE REGISTER B
10-BIT OUTPUT
COMPARE REGISTER C
8-BIT OUTPUT COMPARE
REGISTER A (OCR4A)
8-BIT OUTPUT COMPARE
REGISTER B (OCR4B)
8-BIT OUTPUT COMPARE
REGISTER C (OCR4C)
10-BIT COMPARATOR
T/C CONTROL
REGISTER D (TCCR4E)
10-BIT OUTPUT
COMPARE REGISTER D
8-BIT OUTPUT COMPARE
REGISTER D (OCR4D)
2-BIT HIGH BYTE
REGISTER (TC4H)
8-BIT DATABUS
15.2.1 Speed
The maximum speed of the Timer/Counter4 is 64MHz. However, if a supply voltage below 4V is used, it is
recommended to decrease the input frequency, because the Timer/Counter4 is not running fast enough on low
voltage levels.
15.2.2 Accuracy
The Timer/Counter4 is a 10-bit Timer/Counter module that can alternatively be used as an 8-bit Timer/Counter.
The Timer/Counter4 registers are basically 8-bit registers, but on top of that there is a 2-bit High Byte Register
(TC4H) that can be used as a common temporary buffer to access the two MSBs of the 10-bit Timer/Counter4
registers by the AVR CPU via the 8-bit data bus, if the 10-bit accuracy is used. Whereas, if the two MSBs of the
10-bit registers are written to zero the Timer/Counter4 is working as an 8-bit Timer/Counter. When reading the
low byte of any 8-bit register the two MSBs are written to the TC4H register, and when writing the low byte of
any 8-bit register the two MSBs are written from the TC4H register. Special procedures must be followed when
accessing the 10-bit Timer/Counter4 values via the 8-bit data bus. These procedures are described in the
section “Accessing 10-bit Registers” on page 160.
The Enhanced PWM mode allows to add a resolution bit to each Compare register A/B/D, while the output
frequency remains identical to a Normal PWM mode. That means that the TC4H register contains one more bit
that will be the MSB in a 11-bits enhanced PWM operation. See the section “Enhanced Compare/PWM mode”
on page 150 for details about this feature and how to use it.
15.2.3 Registers
The Timer/Counter (TCNT4) and Output Compare Registers (OCR4A, OCR4B, OCR4C and OCR4D) are 8-bit
registers that are used as a data source to be compared with the TCNT4 contents. The OCR4A, OCR4B and
OCR4D registers determine the action on the OC4A, OC4B and OC4D pins and they can also generate the
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compare match interrupts. The OCR4C holds the Timer/Counter TOP value, i.e. the clear on compare match
value. The Timer/Counter4 High Byte Register (TC4H) is a 2-bit register that is used as a common temporary
buffer to access the MSB bits of the Timer/Counter4 registers, if the 10-bit accuracy is used.
Interrupt request (overflow TOV4, compare matches OCF4A, OCF4B, OCF4D and fault protection FPF4)
signals are visible in the Timer Interrupt Flag Register (TIFR4) and Timer/Counter4 Control Register D
(TCCR4D). The interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK4) and the
FPIE4 bit in the Timer/Counter4 Control Register D (TCCR4D).
Control signals are found in the Timer/Counter Control Registers TCCR4A, TCCR4B, TCCR4C, TCCR4D, and
TCCR4E.
15.2.4 Synchronization
In asynchronous clocking mode the Timer/Counter4 and the prescaler allow running the CPU from any clock
source while the prescaler is operating on the fast peripheral clock (PCK) having frequency up to 64MHz. This is
possible because there is a synchronization boundary between the CPU clock domain and the fast peripheral
clock domain. Figure 15-2 on page 143 shows Timer/Counter 4 synchronization register block diagram and
describes synchronization delays in between registers. Note that all clock gating details are not shown in the
figure.
The Timer/Counter4 register values go through the internal synchronization registers, which cause the input
synchronization delay, before affecting the counter operation. The registers TCCR4A, TCCR4B, TCCR4C,
TCCR4D, OCR4A, OCR4B, OCR4C, and OCR4D can be read back right after writing the register. The read
back values are delayed for the Timer/Counter4 (TCNT4) register, Timer/Counter4 High Byte Register (TC4H)
and flags (OCF4A, OCF4B, OCF4D, and TOV4), because of the input and output synchronization.
The system clock frequency must be lower than half of the PCK frequency, because the synchronization
mechanism of the asynchronous Timer/Counter4 needs at least two edges of the PCK when the system clock is
high. If the frequency of the system clock is too high, it is a risk that data or control values are lost.
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Figure 15-2.
Timer/Counter4 Synchronization Register Block Diagram
8-BIT DATABUS
IO-registers
Input synchronization
registers
OCR4A
OCR4A_SI
OCR4B
OCR4B_SI
OCR4C
OCR4C_SI
Timer/Counter4
Output synchronization
registers
TCNT4
TCNT4_SO
TC4H
TC4H_SO
OCR4D
OCR4D_SI
TCCR4A
TCCR4A_SI
TCCR4B
TCCR4B_SI
TCCR4C
TCCR4C_SI
TCCR4D
TCCR4D_SI
TCNT4
TCNT4_SI
TC4H
TC4H_SI
OCF4A
OCF4A_SI
OCF4
OCF4A_SO
TCNT4
OCF4B
OCF4B_SO
OCF4D
OCF4D_SO
OCF4B
OCF4B_SI
OCF4D
OCF4D_SI
TOV4
TOV4_SI
TOV4
TOV4_SO
PLLTM1:0
!= '00'
CK
S
A
S
PCK
(clk
)
TMR
SYNC
MODE
A
1/2 CK Delay
ASYNC
MODE
~1/2 CK Delay
1 CK Delay
1 CK Delay
1/2 CK Delay
1 PCK Delay
1 PCK Delay
~1 CK Delay
15.2.5 Definitions
Many register and bit references in this section are written in general form. A lower case “n” replaces the
Timer/Counter number, in this case 0. A lower case “x” replaces the Output Compare Unit, in this case Compare
Unit A, B, C or D. However, when using the register or bit defines in a program, the precise form must be used,
i.e., TCNT4 for accessing Timer/Counter4 counter value and so on.
The definitions in the table are used extensively throughout the document.
BOTTOM
The counter reaches the BOTTOM when it becomes 0.
MAX
The counter reaches its MAXimum value when it becomes 0x3FF (decimal 1023).
TOP
The counter reaches the TOP value (stored in the OCR1C) when it becomes equal to the
highest value in the count sequence. The TOP has a value 0x0FF as default after reset.
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15.3
Counter Unit
The main part of the Timer/Counter4 is the programmable bi-directional counter unit. Figure 15-3 shows a block
diagram of the counter and its surroundings.
Figure 15-3.
Counter Unit Block Diagram
DATA BUS
TOV4
clkT4
Timer/Counter4 Count Enable
( From Prescaler )
count
TCNT4
clear
Control Logic
direction
PLLTM1:0
PCK
CK
bottom
top
Signal description (internal signals):
count: TCNT4 increment or decrement enable.
direction: Select between increment and decrement.
clear: Clear TCNT4 (set all bits to zero).
clkTn: Timer/Counter clock, referred to as clkT4 in the following.
top: Signalize that TCNT4 has reached maximum value.
bottom: Signalize that TCNT4 has reached minimum value (zero).
Depending of the mode of operation used, the counter is cleared, incremented, or decremented at each timer
clock (clkT4). The timer clock is generated from an synchronous system clock or an asynchronous PLL clock
using the Clock Select bits (CS4<3:0>) and the PLL Postscaler for High Speed Timer bits (PLLTM1:0). When no
clock source is selected (CS4<3:0> = 0) the timer is stopped. However, the TCNT4 value can be accessed by
the CPU, regardless of whether clkT1 is present or not. A CPU write overrides (has priority over) all counter clear
or count operations.
The counting sequence of the Timer/Counter4 is determined by the setting of the WGM10 and PWM4x bits
located in the Timer/Counter4 Control Registers (TCCR4A, TCCR4C, and TCCR4D). For more details about
advanced counting sequences and waveform generation, see “Modes of Operation” on page 151. The
Timer/Counter Overflow Flag (TOV4) is set according to the mode of operation selected by the PWM4x and
WGM40 bits. The Overflow Flag can be used for generating a CPU interrupt.
15.3.1 Counter Initialization for Asynchronous Mode
To change Timer/Counter4 to the asynchronous mode follow the procedure below:
1. Enable PLL.
2. Wait 100µs for PLL to stabilize.
3. Poll the PLOCK bit until it is set.
4. Configure the PLLTM1:0 bits in the PLLFRQ register to enable the asynchronous mode (different from
0:0 value).
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15.4
Output Compare Unit
The comparator continuously compares TCNT4 with the Output Compare Registers (OCR4A, OCR4B, OCR4C,
and OCR4D). Whenever TCNT4 equals to the Output Compare Register, the comparator signals a match. A
match will set the Output Compare Flag (OCF4A, OCF4B, or OCF4D) at the next timer clock cycle. If the
corresponding interrupt is enabled, the Output Compare Flag generates an Output Compare interrupt. The
Output Compare Flag is automatically cleared when the interrupt is executed. Alternatively, the flag can be
cleared by software by writing a logical one to its I/O bit location. The Waveform Generator uses the match
signal to generate an output according to operating mode set by the PWM4x, WGM40, and Compare Output
mode (COM4x1:0) bits. The top and bottom signals are used by the Waveform Generator for handling the
special cases of the extreme values in some modes of operation (See “Modes of Operation” on page 151.)
Figure 15-4 shows a block diagram of the Output Compare unit.
Figure 15-4.
Output Compare Unit, Block Diagram
8-BIT DATA BUS
TCNTn
TCnH
OCRnx
10-BIT OCRnx
10-BIT TCNTn
= (10-bit Comparator )
OCFnx (Int.Req.)
TOP
BOTTOM
PWMnx
Waveform Generator
FOCn
WGMn0
COMnX1:0
OCWnx
The OCR4x Registers are double buffered when using any of the Pulse Width Modulation (PWM) modes. For
the normal mode of operation, the double buffering is disabled. The double buffering synchronizes the update of
the OCR4x Compare Registers to either top or bottom of the counting sequence. The synchronization prevents
the occurrence of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free. See Figure
15-5 on page 146 for an example. During the time between the write and the update operation, a read from
OCR4A, OCR4B, OCR4C, or OCR4D will read the contents of the temporary location. This means that the most
recently written value always will read out of OCR4A, OCR4B, OCR4C, or OCR4D.
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Figure 15-5.
Effects of Unsynchronized OCR Latching
Compare Value changes
Counter Value
Compare Value
Output Compare
Waveform OCWnx
Synchronized WFnx Latch
Compare Value changes
Counter Value
Compare Value
Unsynchronized WFnx Latch
Glitch
Output Compare
Wafeform OCWnx
15.4.1 Force Output Compare
In non-PWM waveform generation modes, the match output of the comparator can be forced by writing a one to
the Force Output Compare (FOC4x) bit. Forcing Compare Match will not set the OCF4x Flag or reload/clear the
timer, but the Waveform Output (OCW4x) will be updated as if a real Compare Match had occurred (the
COM4x1:0 bits settings define whether the Waveform Output (OCW4x) is set, cleared or toggled).
15.4.2 Compare Match Blocking by TCNT4 Write
All CPU write operations to the TCNT4 Register will block any Compare Match that occur in the next timer clock
cycle, even when the timer is stopped. This feature allows OCR4x to be initialized to the same value as TCNT4
without triggering an interrupt when the Timer/Counter clock is enabled.
15.4.3 Using the Output Compare Unit
Since writing TCNT4 in any mode of operation will block all Compare Matches for one timer clock cycle, there
are risks involved when changing TCNT4 when using the Output Compare Unit, independently of whether the
Timer/Counter is running or not. If the value written to TCNT4 equals the OCR4x value, the Compare Match will
be missed, resulting in incorrect waveform generation. Similarly, do not write the TCNT4 value equal to
BOTTOM when the counter is down-counting.
The setup of the Waveform Output (OCW4x) should be performed before setting the Data Direction Register for
the port pin to output. The easiest way of setting the OCW4x value is to use the Force Output Compare (FOC4x)
strobe bits in Normal mode. The OC4x keeps its value even when changing between Waveform Generation
modes.
Be aware that the COM4x1:0 bits are not double buffered together with the compare value. Changing the
COM4x1:0 bits will take effect immediately.
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Dead Time Generator
The Dead Time Generator is provided for the Timer/Counter4 PWM output pairs to allow driving external power
control switches safely. The Dead Time Generator is a separate block that can be used to insert dead times
(non-overlapping times) for the Timer/Counter4 complementary output pairs OC4x and OC4x when the PWM
mode is enabled and the COM4x1:0 bits are set to “01”. The sharing of tasks is as follows: the Waveform
Generator generates the Waveform Output (OCW4x) and the Dead Time Generator generates the nonoverlapping PWM output pair from the Waveform Output. Three Dead Time Generators are provided, one for
each PWM output. The non-overlap time is adjustable and the PWM output and it’s complementary output are
adjusted separately, and independently for both PWM outputs.
Figure 15-6.
Output Compare Unit, Block Diagram
top
bottom
Waveform Generator
OCWnx
PWMnx WGMn0 COMnx
OCnx
OCnx
pin
OCnx
OCnx
pin
Dead Time Generator
FOCn
CK OR PCK
CLOCK
DTPSn
DTnH
DTnL
The Dead Time Generation is based on the 4-bit down counters that count the dead time, as shown in Figure
15-7. There is a dedicated prescaler in front of the Dead Time Generator that can divide the Timer/Counter4
clock (PCK or CK) by 1, 2, 4, or 8. This provides for large range of dead times that can be generated. The
prescaler is controlled by two control bits DTPS41..40. The block has also a rising and falling edge detector that
is used to start the dead time counting period. Depending on the edge, one of the transitions on the rising
edges, OC4x or OC4x is delayed until the counter has counted to zero. The comparator is used to compare the
counter with zero and stop the dead time insertion when zero has been reached. The counter is loaded with a 4bit DT4H or DT4L value from DT4 I/O register, depending on the edge of the Waveform Output (OCW4x) when
the dead time insertion is started. The Output Compare Output are delayed by one timer clock cycle at minimum
from the Waveform Output when the Dead Time is adjusted to zero. The outputs OC4x and OC4x are inverted,
if the PWM Inversion Mode bit PWM4X is set. This will also cause both outputs to be high during the dead time.
Figure 15-7.
Dead Time Generator
PWMnX
COMPARATOR
OCnx
CK OR PCK
CLOCK
DEAD TIME
PRE-SCALER
CLOCK CONTROL
4-BIT COUNTER
DTnL
DTnH
OCnx
DTPSn
15.5
PWMnX
TCCRnB REGISTER
DTn I/O REGISTER
OCWnx
DATA BUS (8-bit)
The length of the counting period is user adjustable by selecting the dead time prescaler setting by using the
DTPS41:40 control bits, and selecting then the dead time value in I/O register DT4. The DT4 register consists of
two 4-bit fields, DT4H and DT4L that control the dead time periods of the PWM output and its' complementary
output separately in terms of the number of prescaled dead time generator clock cycles. Thus the rising edge of
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OC4x and OC4x can have different dead time periods as the tnon-overlap / rising edge is adjusted by the 4-bit DT4H
value and the tnon-overlap / falling edge is adjusted by the 4-bit DT4L value.
Figure 15-8.
The Complementary Output Pair, COM4x1:0 = 1
OCWnx
OCnx
OCnx
(COMnx = 1)
t non-overlap / rising edge
15.6
t non-overlap / falling edge
Compare Match Output Unit
The Compare Output Mode (COM4x1:0) bits have two functions. The Waveform Generator uses the COM4x1:0
bits for defining the inverted or non-inverted Waveform Output (OCW4x) at the next Compare Match. Also, the
COM4x1:0 bits control the OC4x and OC4x pin output source. Figure 15-9 shows a simplified schematic of the
logic affected by the COM4x1:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are shown in
bold. Only the parts of the general I/O Port Control Registers (DDR and PORT) that are affected by the
COM4x1:0 bits are shown.
In Normal Mode (non-PWM) the Dead Time Generator is disabled and it is working like a synchronizer: the
Output Compare (OC4x) is delayed from the Waveform Output (OCW4x) by one timer clock cycle. Whereas in
Fast PWM Mode and in Phase and Frequency Correct PWM Mode when the COM4x1:0 bits are set to “01” both
the non-inverted and the inverted Output Compare output are generated, and an user programmable Dead
Time delay is inserted for these complementary output pairs (OC4x and OC4x). The functionality in PWM
modes is similar to Normal mode when any other COM4x1:0 bit setup is used. When referring to the OC4x
state, the reference is for the Output Compare output (OC4x) from the Dead Time Generator, not the OC4x pin.
If a system reset occur, the OC4x is reset to “0”.
The general I/O port function is overridden by the Output Compare (OC4x / OC4x) from the Dead Time
Generator if either of the COM4x1:0 bits are set. However, the OC4x pin direction (input or output) is still
controlled by the Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OC4x
and OC4x pins (DDR_OC4x and DDR_OC4x) must be set as output before the OC4x and OC4x values are
visible on the pin. The port override function is independent of the Output Compare mode.
The design of the Output Compare Pin Configuration logic allows initialization of the OC4x state before the
output is enabled. Note that some COM4x1:0 bit settings are reserved for certain modes of operation. For
Output Compare Pin Configurations refer to Table 15-1 on page 152, Table 15-2 on page 154, Table 15-3 on
page 155, and Table 15-4 on page 157.
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Figure 15-9.
Compare Match Output Unit, Schematic
WGM41
clk I/O
OC4OE1:0
COM4A1:0
Output Compare
Pin Configuration
D Q
PORTC6
0
D Q
PORTC7
1
1
D Q
DDRC6
OC4A
PIN
0
OCW4A
clk Tn
Dead Time Q
Generator A Q
OC4A
1
OC4A
0
D Q
DDRC7
WGM41
OC4OE3:2
COM4B1:0
OC4A
PIN
Output Compare
Pin Configuration
DATA BUS
D Q
PORTB5
2
1
0
D Q
DDRB5
D Q
PORTB6
1
OC4B
PIN
0
OCW4B
clk Tn
Dead Time Q
Generator B Q
OC4B
1
OC4B
1
0
0
D Q
DDRB6
WGM41
OC4OE5:4
COM4D1:0
OC4B
PIN
Output Compare
Pin Configuration
D Q
PORTD6
2
1
0
D Q
DDRD6
D Q
PORTD7
1
OC4D
PIN
0
OCW4D
clk Tn
Dead Time Q
Generator D Q
OC4
OC4D
1
0
1
0
OC4D
PIN
D Q
DDRD7
15.6.1 Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM4x1:0 bits differently in Normal mode and PWM modes. For all modes,
setting the COM4x1:0 = 0 tells the Waveform Generator that no action on the OCW4x Output is to be performed
on the next Compare Match. For compare output actions in the non-PWM modes refer to Table 15-5 on
page 164. For fast PWM mode, refer to Table 15-6 on page 164, and for the Phase and Frequency Correct
PWM refer to Table 15-7 on page 165. A change of the COM4x1:0 bits state will have effect at the first Compare
Match after the bits are written. For non-PWM modes, the action can be forced to have immediate effect by
using the FOC4x strobe bits.
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15.6.2 Enhanced Compare/PWM mode
When the bit ENHC4 of TCCR4E register is set, the Enhanced Compare/PWM mode is enabled. This mode
allows user to add an accuracy bit to Output Compare Register OCR4A, OCR4B, and OCR4D. Like explained
previously, a compare condition appears when one of the three Output Compare Registers (OCR4A/B/D)
matches the value of TCNT4 (10-bits resolution). In basic PWM Mode, the corresponding enabled output
toggles on the Compare Match. The Enhanced Compare/PWM mode introduces a bit that determines on which
internal clock edge the Compare Match condition is actually signalled. That means that the corresponding
outputs will toggle on the standard clock edge (like in Normal mode) if the LSB of OCR4A/B/D is ‘0’, or on the
opposite (next) edge if the LSB is ‘1’.
User will notice that between Normal and Enhanced PWM modes, the output frequency will be identical, while
the PWM resolution will be better in second case.
Writing to the Output Compare registers OCR4A/B/D or reading them will be identical in both modes. In
Enhanced mode, user must just consider that the TC4H register can be up to 3-bits wide (and have the same
behavior than during 2-bits operation). That will concern OCR4A, OCR4B and OCR4D registers accesses only.
Indeed, the OCR4C register must not include the additional accuracy bit, and remains in the resolution that
determines the output signal period.
Figure 15-10. How Register Access Works in Enhanced Mode
(TC4H)
10
9
(OCR4A/B/D)
7
8
6
5
4
3
2
1
0
User Interface Side
Timer Logic Side
9
8
7
6
5
4
3
2
1
0
(LSB)
True
OCR4A/B/D
Output Compare Module A/B/D
Waveform Generation
TCNT4<9:0>
OCR4C<9:0>
Configuration
bits
Enhanced
Mode
ENHC4
Pin Toggle
Figure 15-10 shows that the true OCR4A/B/D value corresponds to the value loaded by the user shifted on the
right in order to transfer the least significant bit directly to the Waveform generation module.
The maximum available resolution is 11-bits, but any other resolution can be specified. For example, a 8-bits
resolution will allow to obtain the same frequency than a Normal PWM mode with 7-bits resolution.
Example:
̶
PLL Postcaler output = 64MHz, No Prescaler on Timer/Counter4.
̶
Setting OCR4C = 0x7F determines a full 7-bits theoretical resolution, and so a 500kHz output
frequency.
̶
Setting OCR4A = 0x85 (= b’10000101’) signifies that the true value of “Compare A” register is 0x42
(b’01000010’) and that the Enhanced bit is set. That means that the duty cycle obtained (51.95%)
will be the intermediate value between duty cycles that can be obtained by 0x42 and 0x43 Compare
values (51.56%, 52.34%).
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15.7
Synchronous update
To avoid unasynchronous and incoherent values in a cycle, if a synchronous update of one of several values is
necessary, all values can be updated at the same time at the end of the PWM cycle by the Timer controller. The
new set of values is calculated by software and the effective update can be initiated by software.
Figure 15-11. Lock Feature and Synchronous update
TLOCK4=1
Regulation Loop
Calculation
Cycle with
Set i
TLOCK4=0
Writing to Timer
Registers Set j
Cycle with
Set i
Cycle with
Set i
Request for an
Update
Cycle with
Set i
Cycle with
Set j
In normal operation, each write to a Compare register is effective at the end of the current cycle. But some
cases require that two or more Compare registers are updated synchronously, and that may not be always
possible, mostly at high speed PWM frequencies. That may result in some PWM periods with incoherent values.
When using the Lock feature (TLOCK4=1), the values written to the Compare registers are not effective and
temporarily buffered. When releasing the TLOCK4 bit, the update is initiated and the new whole set of values
will be loaded at the end of the current PWM cycle.
Refer to “TCCR4E – Timer/Counter4 Control Register E” on page 171.
15.8
Modes of Operation
The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined by the
combination of the Waveform Generation mode (bits PWM4x and WGM40) and Compare Output mode
(COM4x1:0) bits. The Compare Output mode bits do not affect the counting sequence, while the Waveform
Generation mode bits do. The COM4x1:0 bits control whether the PWM output generated should be inverted,
non-inverted or complementary. For non-PWM modes the COM4x1:0 bits control whether the output should be
set, cleared, or toggled at a Compare Match.
15.8.1 Normal Mode
The simplest mode of operation is the Normal mode (PWM4x = 0), the counter counts from BOTTOM to TOP
(defined as OCR4C) then restarts from BOTTOM. The OCR4C defines the TOP value for the counter, hence
also its resolution, and allows control of the Compare Match output frequency. In toggle Compare Output Mode
the Waveform Output (OCW4x) is toggled at Compare Match between TCNT4 and OCR4x. In non-inverting
Compare Output Mode the Waveform Output is cleared on the Compare Match. In inverting Compare Output
Mode the Waveform Output is set on Compare Match.
The timing diagram for the Normal mode is shown in Figure 15-12. The counter value (TCNT4) that is shown as
a histogram in the timing diagram is incremented until the counter value matches the TOP value. The counter is
then cleared at the following clock cycle The diagram includes the Waveform Output (OCW4x) in toggle
Compare Mode. The small horizontal line marks on the TCNT4 slopes represent Compare Matches between
OCR4x and TCNT4.
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Figure 15-12. Normal Mode, Timing Diagram
TOVn Interrupt Flag Set
OCnx Interrupt Flag Set
TCNTn
OCWnx
(COMnx=1)
1
Period
2
3
4
The Timer/Counter Overflow Flag (TOV4) is set in the same clock cycle as the TCNT4 becomes zero. The
TOV4 Flag in this case behaves like a 11th bit, except that it is only set, not cleared. However, combined with
the timer overflow interrupt, that automatically clears the TOV4 Flag, the timer resolution can be increased by
software. There are no special cases to consider in the Normal mode, a new counter value can be written
anytime.
The Output Compare Unit can be used to generate interrupts at some given time. Using the Output Compare to
generate waveforms in Normal mode is not recommended, since this will occupy too much of the CPU time. For
generating a waveform, the OCW4x output can be set to toggle its logical level on each Compare Match by
setting the Compare Output mode bits to toggle mode (COM4x1:0 = 1). The OC4x value will not be visible on
the port pin unless the data direction for the pin is set to output. The waveform generated will have a maximum
frequency of fOC4x = fclkT4/4 when OCR4C is set to zero. The waveform frequency is defined by the following
equation:
f clkT4
f OC4x = ------------------------------------------2   1 + OCR 4 C 
Resolution shows how many bit is required to express the value in the OCR4C register. It is calculated by
following equation:
ResolutionPWM = log2(OCR4C + 1).
The Output Compare Pin configurations in Normal Mode are described in the table below.
Table 15-1.
Output Compare Pin Configurations in Normal Mode
COM4x1
COM4x0
OC4x Pin
OC4x Pin
0
0
Disconnected
Disconnected
0
1
Disconnected
OC4x
1
0
Disconnected
OC4x
1
1
Disconnected
OC4x
15.8.2 Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (PWM4x = 1 and WGM40 = 0) provides a high frequency
PWM waveform generation option. The fast PWM differs from the other PWM option by its single-slope
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operation. The counter counts from BOTTOM to TOP (defined as OCR4C) then restarts from BOTTOM. In noninverting Compare Output mode the Waveform Output (OCW4x) is cleared on the Compare Match between
TCNT4 and OCR4x and set at BOTTOM. In inverting Compare Output mode, the Waveform Output is set on
Compare Match and cleared at BOTTOM. In complementary Compare Output mode the Waveform Output is
cleared on the Compare Match and set at BOTTOM.
Due to the single-slope operation, the operating frequency of the fast PWM mode can be twice as high as the
Phase and Frequency Correct PWM mode that use dual-slope operation. This high frequency makes the fast
PWM mode well suited for power regulation, rectification, and DAC applications. High frequency allows
physically small sized external components (coils, capacitors), and therefore reduces total system cost.
The timing diagram for the fast PWM mode is shown in Figure 15-13. The counter is incremented until the
counter value matches the TOP value. The counter is then cleared at the following timer clock cycle. The
TCNT4 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The
diagram includes the Waveform Output in non-inverted and inverted Compare Output modes. The small
horizontal line marks on the TCNT4 slopes represent Compare Matches between OCR4x and TCNT4.
Figure 15-13. Fast PWM Mode, Timing Diagram
OCRnx Interrupt Flag Set
OCRnx Update and
TOVn Interrupt Flag Set
TCNTn
OCWnx
(COMnx1:0 = 2)
OCWnx
(COMnx1:0 = 3)
Period
1
2
3
4
5
6
7
The Timer/Counter Overflow Flag (TOV4) is set each time the counter reaches TOP. If the interrupt is enabled,
the interrupt handler routine can be used for updating the compare value. In fast PWM mode, the compare unit
allows generation of PWM waveforms on the OC4x pins. Setting the COM4x1:0 bits to two will produce a noninverted PWM and setting the COM4x1:0 to three will produce an inverted PWM output. Setting the COM4x1:0
bits to one will enable complementary Compare Output mode and produce both the non-inverted (OC4x) and
inverted output (OC4x). The actual value will only be visible on the port pin if the data direction for the port pin is
set as output. The PWM waveform is generated by setting (or clearing) the Waveform Output (OCW4x) at the
Compare Match between OCR4x and TCNT4, and clearing (or setting) the Waveform Output at the timer clock
cycle the counter is cleared (changes from TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
f clkT4
f OCnxPWM = ------------N
The N variable represents the number of steps in single-slope operation. The value of N equals either to the
TOP value.
The extreme values for the OCR4C Register represents special cases when generating a PWM waveform
output in the fast PWM mode. If the OCR4C is set equal to BOTTOM, the output will be a narrow spike for each
MAX+1 timer clock cycle. Setting the OCR4C equal to MAX will result in a constantly high or low output
(depending on the polarity of the output set by the COM4x1:0 bits.)
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A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting the
Waveform Output (OCW4x) to toggle its logical level on each Compare Match (COM4x1:0 = 1). The waveform
generated will have a maximum frequency of fOC4 = fclkT4/4 when OCR4C is set to three.
The general I/O port function is overridden by the Output Compare value (OC4x / OC4x) from the Dead Time
Generator, if either of the COM4x1:0 bits are set and the Data Direction Register bits for the OC4X and OC4X
pins are set as an output. If the COM4x1:0 bits are cleared, the actual value from the port register will be visible
on the port pin. The Output Compare Pin configurations are described in the table below.
Table 15-2.
Output Compare Pin Configurations in Fast PWM Mode
COM4x1
COM4x0
OC4x Pin
OC4x Pin
0
0
Disconnected
Disconnected
0
1
OC4x
OC4x
1
0
Disconnected
OC4x
1
1
Disconnected
OC4x
15.8.3 Phase and Frequency Correct PWM Mode
The Phase and Frequency Correct PWM Mode (PWM4x = 1 and WGM40 = 1) provides a high resolution Phase
and Frequency Correct PWM waveform generation option. The Phase and Frequency Correct PWM mode is
based on a dual-slope operation. The counter counts repeatedly from BOTTOM to TOP (defined as OCR4C)
and then from TOP to BOTTOM. In non-inverting Compare Output Mode, and in complimentary Compare
Output Mode, the Waveform Output (OCW4x) is cleared on the Compare Match between TCNT4 and OCR4x
while upcounting, and set on the Compare Match while down-counting. In inverting Output Compare mode, the
operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope
operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for
motor control applications.
The timing diagram for the Phase and Frequency Correct PWM mode is shown on Figure 15-14 in which the
TCNT4 value is shown as a histogram for illustrating the dual-slope operation. The counter is incremented until
the counter value matches TOP. When the counter reaches TOP, it changes the count direction. The TCNT4
value will be equal to TOP for one timer clock cycle. The diagram includes the Waveform Output (OCW4x) in
non-inverted and inverted Compare Output Mode. The small horizontal line marks on the TCNT4 slopes
represent Compare Matches between OCR4x and TCNT4.
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Figure 15-14. Phase and Frequency Correct PWM Mode, Timing Diagram
OCnx Interrupt Flag Set
OCRnx Update
TOVn Interrupt Flag Set
TCNTn
OCWnx
(COMnx = 2)
OCWnx
(COMnx = 3)
Period
1
2
3
The Timer/Counter Overflow Flag (TOV4) is set each time the counter reaches BOTTOM. The Interrupt Flag
can be used to generate an interrupt each time the counter reaches the BOTTOM value.
In the Phase and Frequency Correct PWM mode, the compare unit allows generation of PWM waveforms on
the OC4x pins. Setting the COM4x1:0 bits to two will produce a non-inverted PWM and setting the COM4x1:0 to
three will produce an inverted PWM output. Setting the COM4A1:0 bits to one will enable complementary
Compare Output mode and produce both the non-inverted (OC4x) and inverted output (OC4x). The actual
values will only be visible on the port pin if the data direction for the port pin is set as output. The PWM
waveform is generated by clearing (or setting) the Waveform Output (OCW4x) at the Compare Match between
OCR4x and TCNT4 when the counter increments, and setting (or clearing) the Waveform Output at Compare
Match when the counter decrements. The PWM frequency for the output when using the Phase and Frequency
Correct PWM can be calculated by the following equation:
f clkT4
f OCnxPCPWM = ------------N
The N variable represents the number of steps in dual-slope operation. The value of N equals to the TOP value.
The extreme values for the OCR4C Register represent special cases when generating a PWM waveform output
in the Phase and Frequency Correct PWM mode. If the OCR4C is set equal to BOTTOM, the output will be
continuously low and if set equal to MAX the output will be continuously high for non-inverted PWM mode. For
inverted PWM the output will have the opposite logic values.
The general I/O port function is overridden by the Output Compare value (OC4x / OC4x) from the Dead Time
Generator, if either of the COM4x1:0 bits are set and the Data Direction Register bits for the OC4X and OC4X
pins are set as an output. If the COM4x1:0 bits are cleared, the actual value from the port register will be visible
on the port pin. The configurations of the Output Compare Pins are described in the table below.
Table 15-3.
Output Compare pin configurations in Phase and Frequency Correct PWM Mode
COM4x1
COM4x0
OC4x Pin
OC4x Pin
0
0
Disconnected
Disconnected
0
1
OC4x
OC4x
1
0
Disconnected
OC4x
1
1
Disconnected
OC4x
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15.8.4 PWM6 Mode
The PWM6 Mode (PWM4A = 1, WGM41 = 1, and WGM40 = x) provide PWM waveform generation option e.g.
for controlling Brushless DC (BLDC) motors. In the PWM6 Mode the OCR4A Register controls all six Output
Compare waveforms as the same Waveform Output (OCW4A) from the Waveform Generator is used for
generating all waveforms. The PWM6 Mode also provides an Output Compare Override Enable Register
(OC4OE) that can be used with an instant response for disabling or enabling the Output Compare pins. If the
Output Compare Override Enable bit is cleared, the actual value from the port register will be visible on the port
pin.
The PWM6 Mode provides two counter operation modes, a single-slope operation and a dual-slope operation. If
the single-slope operation is selected (the WGM40 bit is set to 0), the counter counts from BOTTOM to TOP
(defined as OCR4C) then restart from BOTTOM like in Fast PWM Mode. The PWM waveform is generated by
setting (or clearing) the Waveform Output (OCW4A) at the Compare Match between OCR4A and TCNT4, and
clearing (or setting) the Waveform Output at the timer clock cycle the counter is cleared (changes from TOP to
BOTTOM). The Timer/Counter Overflow Flag (TOV4) is set each time the counter reaches the TOP and, if the
interrupt is enabled, the interrupt handler routine can be used for updating the compare value.
Whereas, if the dual-slope operation is selected (the WGM40 bit is set to 1), the counter counts repeatedly from
BOTTOM to TOP (defined as OCR4C) and then from TOP to BOTTOM like in Phase and Frequency Correct
PWM Mode. The PWM waveform is generated by setting (or clearing) the Waveform Output (OCW4A) at the
Compare Match between OCR4A and TCNT4 when the counter increments, and clearing (or setting) the
Waveform Output at the he Compare Match between OCR4A and TCNT4 when the counter decrements. The
Timer/Counter Overflow Flag (TOV4) is set each time the counter reaches the BOTTOM and, if the interrupt is
enabled, the interrupt handler routine can be used for updating the compare value.
The timing diagram for the PWM6 Mode in single-slope operation (WGM41 = 0) when the COM4A1:0 bits are
set to “10” is shown in Figure 15-15 on page 157. The counter is incremented until the counter value matches
the TOP value. The counter is then cleared at the following timer clock cycle. The TCNT4 value is in the timing
diagram shown as a histogram for illustrating the single-slope operation. The timing diagram includes Output
Compare pins OC4A and OC4A, and the corresponding Output Compare Override Enable bits
(OC4OE1..OC4OE0).
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Figure 15-15. PWM6 Mode, Single-slope Operation, Timing Diagram
TCNT4
OCW4A
OC4OE0
OC4A Pin
OC4OE1
OC4A Pin
OC4OE2
OC4B Pin
OC4OE3
OC4B Pin
OC4OE4
OC4D Pin
OC4OE5
OC4D Pin
The general I/O port function is overridden by the Output Compare value (OC4x / OC4x) from the Dead Time
Generator if either of the COM4x1:0 bits are set. The Output Compare pins can also be overridden by the
Output Compare Override Enable bits OC4OE5..OC4OE0. If an Override Enable bit is cleared, the actual value
from the port register will be visible on the port pin and, if the Override Enable bit is set, the Output Compare pin
is allowed to be connected on the port pin. The Output Compare Pin configurations are described in the table.
Table 15-4.
Output Compare Pin configurations in PWM6 Mode
COM4A1
COM4A0
OC4A Pin (PC6)
OC4A Pin (PC7)
0
0
Disconnected
Disconnected
0
1
OC4A • OC4OE0
OC4A • OC4OE1
1
0
OC4A • OC4OE0
OC4A • OC4OE1
1
1
OC4A • OC4OE0
OC4A • OC4OE1
COM4B1
COM4B0
OC4B Pin (PB5)
OC4B Pin (PB6)
0
0
Disconnected
Disconnected
0
1
OC4A • OC4OE2
OC4A • OC4OE3
1
0
OC4A • OC4OE2
OC4A • OC4OE3
1
1
OC4A • OC4OE2
OC4A • OC4OE3
COM4D1
COM4D0
OC4D Pin (PD6)
OC4D Pin (PD7)
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Table 15-4.
15.9
Output Compare Pin configurations in PWM6 Mode
COM4A1
COM4A0
OC4A Pin (PC6)
OC4A Pin (PC7)
0
0
Disconnected
Disconnected
0
1
OC4A • OC4OE4
OC4A • OC4OE5
1
0
OC4A • OC4OE4
OC4A • OC4OE5
1
1
OC4A • OC4OE4
OC4A • OC4OE5
Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkT4) is therefore shown as a clock enable
signal in the following figures. The figures include information on when Interrupt Flags are set.
Figure 15-16 contains timing data for basic Timer/Counter operation. The figure shows the count sequence
close to the MAX value in all modes other than Phase and Frequency Correct PWM Mode. Figure 15-17 shows
the same timing data, but with the prescaler enabled, in all modes other than Phase and Frequency Correct
PWM Mode. Figure 15-18 on page 159 shows the setting of OCF4A, OCF4B, and OCF4D in all modes, and
Figure 15-19 on page 159 shows the setting of TOV4 in Phase and Frequency Correct PWM Mode.
Figure 15-16. Timer/Counter Timing Diagram, no Prescaling
clkPCK
clkTn
(clkPCK /1)
TCNTn
TOP - 1
TOP
BOTTOM
BOTTOM + 1
TOVn
Figure 15-17. Timer/Counter Timing Diagram, with Prescaler (fclkT4/8)
clkPCK
clkTn
(clkPCK /8)
TCNTn
TOP - 1
TOP
BOTTOM
BOTTOM + 1
TOVn
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Figure 15-18. Timer/Counter Timing Diagram, Setting of OCF1x, with Prescaler (fclkT4/8)
clkPCK
clkTn
(clkPCK /8)
OCRnx - 1
TCNTn
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
OCRnx
OCFnx
Figure 15-19. Timer/Counter Timing Diagram, with Prescaler (fclkT4/8)
clkPCK
clkTn
(clkPCK /8)
TCNTn
BOTTOM + 1
BOTTOM + 1
BOTTOM
BOTTOM + 1
TOVn
15.10 Fault Protection Unit
The Timer/Counter4 incorporates a Fault Protection unit that can disable the PWM output pins, if an external
event is triggered. The external signal indicating an event can be applied via the external interrupt INT0 pin or
alternatively, via the analog-comparator unit. The Fault Protection unit is illustrated by the block diagram shown
in Figure 15-20. The elements of the block diagram that are not directly a part of the Fault Protection unit are
gray shaded.
Figure 15-20. Fault Protection Unit Block Diagram
FAULT_PROTECTION (Int. Req.)
ACO*
Analog
Comparator
INT0
FPAC4
FPNC4
Noise
Canceler
FPES4 FPEN4
Edge
Detector
Timer/Counter4
When the Fault Protection mode is enabled by the Fault Protection Enable (FPEN4) bit and a change of the
logic level (an event) occurs on the external interrupt pin (INT0), alternatively on the Analog Comparator output
(ACO), and this change confirms to the setting of the edge detector, a Fault Protection mode will be triggered.
When a Fault Protection is triggered, the COM4x bits are cleared, Output Comparators are disconnected from
the PWM output pins and the PORTB register bits are connected on the PWM output pins. The Fault Protection
Enable (FPEN4) is automatically cleared at the same system clock as the COM4nx bits are cleared. If the Fault
Protection Interrupt Enable bit (FPIE4) is set, a Fault Protection interrupt is generated and the FPEN4 bit is
cleared. Alternatively the FPEN4 bit can be polled by software to figure out when the Timer/Counter has entered
to Fault Protection mode.
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15.10.1 Fault Protection Trigger Source
The main trigger source for the Fault Protection unit is the external interrupt pin (INT0). Alternatively the Analog
Comparator output can be used as trigger source for the Fault Protection unit. The Analog Comparator is
selected as trigger source by setting the Fault Protection Analog Comparator (FPAC4) bit in the Timer/Counter4
Control Register (TCCR4D). Be aware that changing trigger source can trigger a Fault Protection mode.
Therefore it is recommended to clear the FPF4 flag after changing trigger source, setting edge detector or
enabling the Fault Protection.
Both the external interrupt pin (INT0) and the Analog Comparator output (ACO) inputs are sampled using the
same technique as for the T0 pin (Figure 12-1 on page 92). The edge detector is also identical. However, when
the noise canceler is enabled, additional logic is inserted before the edge detector, which increases the delay by
four system clock cycles. An Input Capture can also be triggered by software by controlling the port of the INT0
pin.
15.10.2 Noise Canceler
The noise canceler improves noise immunity by using a simple digital filtering scheme. The noise canceler input
is monitored over four samples, and all four must be equal for changing the output that in turn is used by the
edge detector.
The noise canceler is enabled by setting the Fault Protection Noise Canceler (FPNC4) bit in Timer/Counter4
Control Register D (TCCR4D). When enabled the noise canceler introduces additional four system clock cycles
of delay from a change applied to the input. The noise canceler uses the system clock and is therefore not
affected by the prescaler.
15.11 Accessing 10-bit Registers
If 10-bit values are written to the TCNTn and OCRnA/B/C/D registers, the 10-bit registers can be byte accessed
by the AVR CPU via the 8-bit data bus using two read or write operations. The 10-bit registers have a common
2-bit Timer/Counter4 High Byte Register (TC4H) that is used for temporary storing of the two MSBs of the 10-bit
access. The same TC4H register is shared between all 10-bit registers. Accessing the low byte triggers the 10bit read or write operation. When the low byte of a 10-bit register is written by the CPU, the high byte stored in
the TC4H register, and the low byte written are both copied into the 10-bit register in the same clock cycle.
When the low byte of a 10-bit register is read by the CPU, the high byte of the 10-bit register is copied into the
TC4H register in the same clock cycle as the low byte is read.
To do a 10-bit write, the high byte must be written to the TC4H register before the low byte is written. For a 10bit read, the low byte must be read before the high byte.
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The following code examples show how to access the 10-bit timer registers assuming that no interrupts updates
the TC4H register. The same principle can be used directly for accessing the OCRnA/B/C/C/D registers.
Assembly Code Example
...
; Set TCNTn to 0x01FF
ldi
r17,0x01
ldi
r16,0xFF
out
TCnH,r17
out
TCNTn,r16
; Read TCNTn into r17:r16
in
r16,TCNTn
in
r17,TCnH
...
C Code Example
unsigned int i;
...
/* Set TCNTn to 0x01FF */
TCnH = 0x01;
TCNTn = 0xFF;
/* Read TCNTn into i */
i = TCNTn;
i |= ((unsigned int)TCnH << 8);
...
Note:
1. The example code assumes that the part specific header file is included.
For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be
replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”,
“SBRC”, “SBR”, and “CBR”.
The assembly code example returns the TCNTn value in the r17:r16 register pair.
It is important to notice that accessing 10-bit registers are atomic operations. If an interrupt occurs between the
two instructions accessing the 10-bit register, and the interrupt code updates the TC4H register by accessing
the same or any other of the 10-bit timer registers, then the result of the access outside the interrupt will be
corrupted. Therefore, when both the main code and the interrupt code update the TC4H register, the main code
must disable the interrupts during the 16-bit access.
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The following code examples show how to do an atomic read of the TCNTn register contents. Reading any of
the OCRnA/B/C/D registers can be done by using the same principle.
Assembly Code Example
TIM1_ReadTCNTn:
; Save global interrupt flag
in
r18,SREG
; Disable interrupts
cli
; Read TCNTn into r17:r16
in
r16,TCNTn
in
r17,TCnH
; Restore global interrupt flag
out
SREG,r18
ret
C Code Example
unsigned int TIM1_ReadTCNTn( void )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
_CLI();
/* Read TCNTn into i */
i = TCNTn;
i |= ((unsigned int)TCnH << 8);
/* Restore global interrupt flag
SREG = sreg;
return i;
}
Note:
1. The example code assumes that the part specific header file is included.
For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be
replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”,
“SBRC”, “SBR”, and “CBR”.
The assembly code example returns the TCNTn value in the r17:r16 register pair.
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The following code examples show how to do an atomic write of the TCNTn register contents. Writing any of the
OCRnA/B/C/D registers can be done by using the same principle.
Assembly Code Example
TIM1_WriteTCNTn:
; Save global interrupt flag
in
r18,SREG
; Disable interrupts
cli
; Set TCNTn to r17:r16
out
TCnH,r17
out
TCNTn,r16
; Restore global interrupt flag
out
SREG,r18
ret
C Code Example
void TIM1_WriteTCNTn( unsigned int i )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
_CLI();
/* Set TCNTn to i */
TCnH = (i >> 8);
TCNTn = (unsigned char)i;
/* Restore global interrupt flag */
SREG = sreg;
}
Note:
1. The example code assumes that the part specific header file is included.
For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be
replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”,
“SBRC”, “SBR”, and “CBR”.
The assembly code example requires that the r17:r16 register pair contains the value to be written to TCNTn.
15.11.1 Reusing the Temporary High Byte Register
If writing to more than one 10-bit register where the high byte is the same for all registers written, then the high
byte only needs to be written once. However, note that the same rule of atomic operation described previously
also applies in this case.
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15.12 Register Description
15.12.1 TCCR4A – Timer/Counter4 Control Register A
Bit
Read/Write
Initial value
7
6
5
4
3
2
1
0
COM4A1
COM4A0
COM4B1
COM4B0
FOC4A
FOC4B
PWM4A
PWM4B
R/W
0
R/W
0
R/W
0
R/W
0
W
0
W
0
R/W
0
R/W
0
TCCR4A
• Bits 7, 6 - COM4A1, COM4A0: Comparator A Output Mode, Bits 1 and 0
These bits control the behavior of the Waveform Output (OCW4A) and the connection of the Output Compare
pin (OC4A). If one or both of the COM4A1:0 bits are set, the OC4A output overrides the normal port functionality
of the I/O pin it is connected to. The complementary OC4B output is connected only in PWM modes when the
COM4A1:0 bits are set to “01”. Note that the Data Direction Register (DDR) bit corresponding to the OC4A and
OC4A pins must be set in order to enable the output driver.
The function of the COM4A1:0 bits depends on the PWM4A, WGM40 and WGM41 bit settings. The table shows
the COM4A1:0 bit functionality when the PWM4A bit is set to Normal Mode (non-PWM).
Table 15-5.
COM4A1..0
Compare Output Mode, Normal Mode (non-PWM)
OCW4A Behavior
OC4A Pin
OC4A Pin
Disconnected
Disconnected
00
Normal port operation
01
Toggle on Compare Match
Connected
Disconnected
10
Clear on Compare Match
Connected
Disconnected
11
Set on Compare Match
Connected
Disconnected
The table shows the COM4A1:0 bit functionality when the PWM4A, WGM40 and WGM41 bits are set to fast
PWM mode.
Table 15-6.
COM4A1..0
Compare Output Mode, Fast PWM Mode
OCW4A Behavior
OC4A
OC4A
Disconnected
Disconnected
00
Normal port operation
01
Cleared on Compare Match.
Set when TCNT4 = 0x000.
Connected
Connected
10
Cleared on Compare Match.
Set when TCNT4 = 0x000.
Connected
Disconnected
11
Set on Compare Match.
Cleared when TCNT4 = 0x000.
Connected
Disconnected
The table shows the COM4A1:0 bit functionality when the PWM4A, WGM40, and WGM41 bits are set to Phase
and Frequency Correct PWM Mode.
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Table 15-7.
COM1A1..0
Compare Output Mode, Phase and Frequency Correct PWM Mode
OCW1A Behavior
OC4A Pin
OC4A Pin
Disconnected
Disconnected
00
Normal port operation.
01
Cleared on Compare Match when up-counting.
Set on Compare Match when down-counting.
Connected
Connected
10
Cleared on Compare Match when up-counting.
Set on Compare Match when down-counting.
Connected
Disconnected
11
Set on Compare Match when up-counting.
Cleared on Compare Match when down-counting.
Connected
Disconnected
The table shows the COM4A1:0 bit functionality when the PWM4A, WGM40, and WGM41 bits are set to singleslope PWM6 Mode. In the PWM6 Mode the same Waveform Output (OCW4A) is used for generating all
waveforms and the Output Compare values OC4A and OC4A are connected on OC4x and OC4x pins as
described below.
Table 15-8.
COM4A1..0
Compare Output Mode, Single-Slope PWM6 Mode
OCW4A Behavior
OC4x Pin
OC4x Pin
Disconnected
Disconnected
00
Normal port operation
01
Cleared on Compare Match.
Set when TCNT4 = 0x000.
OC4A
OC4A
10
Cleared on Compare Match.
Set when TCNT4 = 0x000.
OC4A
OC4A
11
Set on Compare Match.
Cleared when TCNT4 = 0x000.
OC4A
OC4A
The table shows the COM4A1:0 bit functionality when the PWM4A, WGM40, and WGM41 bits are set to dualslope PWM6 Mode.
Table 15-9.
COM4A1..0
Compare Output Mode, Dual-Slope PWM6 Mode
OCW4A Behavior
OC4x Pin
OC4x Pin
Disconnected
Disconnected
00
Normal port operation
01
Cleared on Compare Match when up-counting.
Set on Compare Match when down-counting.
OC4A
OC4A
10
Cleared on Compare Match when up-counting.
Set on Compare Match when down-counting.
OC4A
OC4A
11
Set on Compare Match when up-counting.
Cleared on Compare Match when down-counting.
OC4A
OC4A
• Bits 5,4 - COM4B1, COM4B0: Comparator B Output Mode, Bits 1 and 0
These bits control the behavior of the Waveform Output (OCW4B) and the connection of the Output Compare
pin (OC4B). If one or both of the COM4B1:0 bits are set, the OC4B output overrides the normal port functionality
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of the I/O pin it is connected to. The complementary OC4B output is connected only in PWM modes when the
COM4B1:0 bits are set to “01”. Note that the Data Direction Register (DDR) bit corresponding to the OC4B pin
must be set in order to enable the output driver.
The function of the COM4B1:0 bits depends on the PWM4B and WGM40 bit settings.
The table shows the COM4B1:0 bit functionality when the PWM4B bit is set to Normal Mode (non-PWM).
Table 15-10.
COM4B1..0
Compare Output Mode, Normal Mode (non-PWM)
OCW4B Behavior
OC4B Pin
OC4B Pin
Disconnected
Disconnected
00
Normal port operation
01
Toggle on Compare Match
Connected
Disconnected
10
Clear on Compare Match
Connected
Disconnected
11
Set on Compare Match
Connected
Disconnected
The table shows the COM4B1:0 bit functionality when the PWM4B and WGM40 bits are set to Fast PWM Mode.
Table 15-11.
COM4B1..0
Compare Output Mode, Fast PWM Mode
OCW4B Behavior
OC4B Pin
OC4B Pin
Disconnected
Disconnected
00
Normal port operation
01
Cleared on Compare Match.
Set when TCNT4 = 0x000.
Connected
Connected
10
Cleared on Compare Match.
Set when TCNT4 = 0x000.
Connected
Disconnected
11
Set on Compare Match.
Cleared when TCNT4 = 0x000.
Connected
Disconnected
The table shows the COM4B1:0 bit functionality when the PWM4B and WGM40 bits are set to Phase and
Frequency Correct PWM Mode.
Table 15-12.
COM4B1..0
Compare Output Mode, Phase and Frequency Correct PWM Mode
OCW4B Behavior
OC4B Pin
OC4B Pin
Disconnected
Disconnected
00
Normal port operation
01
Cleared on Compare Match when up-counting.
Set on Compare Match when down-counting.
Connected
Connected
10
Cleared on Compare Match when up-counting.
Set on Compare Match when down-counting.
Connected
Disconnected
11
Set on Compare Match when up-counting.
Cleared on Compare Match when down-counting.
Connected
Disconnected
• Bit 3 - FOC4A: Force Output Compare Match 4A
The FOC4A bit is only active when the PWM4A bit specify a non-PWM mode.
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Writing a logical one to this bit forces a change in the Waveform Output (OCW4A) and the Output Compare pin
(OC4A) according to the values already set in COM4A1 and COM4A0. If COM4A1 and COM4A0 written in the
same cycle as FOC4A, the new settings will be used. The Force Output Compare bit can be used to change the
output pin value regardless of the timer value. The automatic action programmed in COM4A1 and COM4A0
takes place as if a compare match had occurred, but no interrupt is generated. The FOC4A bit is always read as
zero.
• Bit 2 - FOC4B: Force Output Compare Match 4B
The FOC4B bit is only active when the PWM4B bit specify a non-PWM mode.
Writing a logical one to this bit forces a change in the Waveform Output (OCW4B) and the Output Compare pin
(OC4B) according to the values already set in COM4B1 and COM4B0. If COM4B1 and COM4B0 written in the
same cycle as FOC4B, the new settings will be used. The Force Output Compare bit can be used to change the
output pin value regardless of the timer value. The automatic action programmed in COM4B1 and COM4B0
takes place as if a compare match had occurred, but no interrupt is generated.
The FOC4B bit is always read as zero.
• Bit 1 - PWM4A: Pulse Width Modulator A Enable
When set (one) this bit enables PWM mode based on comparator OCR4A.
• Bit 0 - PWM4B: Pulse Width Modulator B Enable
When set (one) this bit enables PWM mode based on comparator OCR4B.
15.12.2 TCCR4B – Timer/Counter4 Control Register B
Bit
Read/Write
Initial value
7
PWM4X
R/W
0
6
PSR4
R/W
0
5
DTPS41
R/W
0
4
DTPS40
R/W
0
3
CS43
R/W
0
2
CS42
R/W
0
1
CS41
R/W
0
0
CS40
R/W
0
TCCR4B
• Bit 7 - PWM4X: PWM Inversion Mode
When this bit is set (one), the PWM Inversion Mode is selected and the Dead Time Generator outputs, OC4x
and OC4x are inverted.
• Bit 6 - PSR4: Prescaler Reset Timer/Counter4
When this bit is set (one), the Timer/Counter4 prescaler (TCNT4 is unaffected) will be reset. The bit will be
cleared by hardware after the operation is performed. Writing a zero to this bit will have no effect. This bit will
always read as zero.
• Bits 5,4 - DTPS41, DTPS40: Dead Time Prescaler Bits
The Timer/Counter4 Control Register B is a 8-bit read/write register.
The dedicated Dead Time prescaler in front of the Dead Time Generator can divide the Timer/Counter4 clock
(PCK or CK) by 1, 2, 4, or 8 providing a large range of dead times that can be generated. The Dead Time
prescaler is controlled by two bits DTPS41 and DTPS40 from the Dead Time Prescaler register. These bits
define the division factor of the Dead Time prescaler. The division factors are given in the table below.
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Table 15-13.
Division factors of the Dead Time prescaler
DTPS41
DTPS40
Prescaler divides the T/C4 clock by
0
0
1x (no division)
0
1
2x
1
0
4x
1
1
8x
• Bits 3..0 - CS43, CS42, CS41, CS40: Clock Select Bits 3, 2, 1, and 0
The Clock Select bits 3, 2, 1, and 0 define the prescaling source of Timer/Counter4.
Table 15-14.
Timer/Counter4 Prescaler Select
CS43
CS42
CS41
CS40
Asynchronous Clocking Mode
Synchronous Clocking Mode
0
0
0
0
T/C4 stopped
T/C4 stopped
0
0
0
1
PCK
CK
0
0
1
0
PCK/2
CK/2
0
0
1
1
PCK/4
CK/4
0
1
0
0
PCK/8
CK/8
0
1
0
1
PCK/16
CK/16
0
1
1
0
PCK/32
CK/32
0
1
1
1
PCK/64
CK/64
1
0
0
0
PCK/128
CK/128
1
0
0
1
PCK/256
CK/256
1
0
1
0
PCK/512
CK/512
1
0
1
1
PCK/1024
CK/1024
1
1
0
0
PCK/2048
CK/2048
1
1
0
1
PCK/4096
CK/4096
1
1
1
0
PCK/8192
CK/8192
1
1
1
1
PCK/16384
CK/16384
The Stop condition provides a Timer Enable/Disable function.
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15.12.3 TCCR4C – Timer/Counter4 Control Register C
Bit
Read/Write
Initial value
7
6
5
4
COM4A1S COM4A0S COM4B1S COMAB0S
R/W
R/W
R/W
R/W
0
0
0
0
3
COM4D1
R/W
0
2
COM4D0
R/W
0
1
FOC4D
R/W
0
0
PWM4D
R/W
0
TCCR4C
• Bits 7,6 - COM4A1S, COM4A0S: Comparator A Output Mode, Bits 1 and 0
These bits are the shadow bits of the COM4A1 and COM4A0 bits that are described in the section “TCCR4A –
Timer/Counter4 Control Register A” on page 164.
• Bits 5,4 - COM4B1S, COM4B0S: Comparator B Output Mode, Bits 1 and 0
These bits are the shadow bits of the COM4A1 and COM4A0 bits that are described in the section “TCCR4A –
Timer/Counter4 Control Register A” on page 164.
• Bits 3,2 - COM4D1, COM4D0: Comparator D Output Mode, Bits 1 and 0
These bits control the behavior of the Waveform Output (OCW4D) and the connection of the Output Compare
pin (OC4D). If one or both of the COM4D1:0 bits are set, the OC4D output overrides the normal port
functionality of the I/O pin it is connected to. The complementary OC4D output is connected only in PWM modes
when the COM4D1:0 bits are set to “01”. Note that the Data Direction Register (DDR) bit corresponding to the
OC4D pin must be set in order to enable the output driver.
The function of the COM4D1:0 bits depends on the PWM4D and WGM40 bit settings.
The table shows the COM4D1:0 bit functionality when the PWM4D bit is set to a Normal Mode (non-PWM).
Table 15-15.
COM4D1..0
Compare Output Mode, Normal Mode (non-PWM)
OCW4D Behavior
OC4D Pin
OC4D Pin
Disconnected
Disconnected
00
Normal port operation
01
Toggle on Compare Match
Connected
Disconnected
10
Clear on Compare Match
Connected
Disconnected
11
Set on Compare Match
Connected
Disconnected
The table shows the COM4D1:0 bit functionality when the PWM4D and WGM40 bits are set to Fast PWM Mode.
Table 15-16.
COM4D1..0
Compare Output Mode, Fast PWM Mode
OCW4D Behavior
OC4D Pin
OC4D Pin
Disconnected
Disconnected
00
Normal port operation
01
Cleared on Compare Match
Set when TCNT4 = 0x000
Connected
Connected
10
Cleared on Compare Match
Set when TCNT4 = 0x000
Connected
Disconnected
11
Set on Compare Match
Clear when TCNT4 = 0x000
Connected
Disconnected
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The table shows the COM4D1:0 bit functionality when the PWM4D and WGM40 bits are set to Phase and
Frequency Correct PWM Mode
Table 15-17.
COM4D1..0
Compare Output Mode, Phase and Frequency Correct PWM Mode
OCW4D Behavior
OC4D Pin
OC4D Pin
Disconnected
Disconnected
00
Normal port operation
01
Cleared on Compare Match when up-counting
Set on Compare Match when down-counting
Connected
Connected
10
Cleared on Compare Match when up-counting
Set on Compare Match when down-counting
Connected
Disconnected
11
Set on Compare Match when up-counting
Cleared on Compare Match when down-counting
Connected
Disconnected
• Bit 1 - FOC4D: Force Output Compare Match 4D
The FOC4D bit is only active when the PWM4D bit specify a non-PWM mode.
Writing a logical one to this bit forces a change in the Waveform Output (OCW4D) and the Output Compare pin
(OC4D) according to the values already set in COM4D1 and COM4D0. If COM4D1 and COM4D0 written in the
same cycle as FOC4D, the new settings will be used. The Force Output Compare bit can be used to change the
output pin value regardless of the timer value. The automatic action programmed in COM4D1 and COM4D0
takes place as if a compare match had occurred, but no interrupt is generated. The FOC4D bit is always read as
zero.
• Bit 0 - PWM4D: Pulse Width Modulator D Enable
When set (one) this bit enables PWM mode based on comparator OCR4D.
15.12.4 TCCR4D – Timer/Counter4 Control Register D
Bit
Read/Write
Initial value
7
FPIE4
R/W
0
6
FPEN4
R/W
0
5
FPNC4
R/W
0
4
FPES4
R/W
0
3
FPAC4
R/W
0
2
FPF4
R/W
0
1
WGM41
R/W
0
0
WGM40
R/W
0
TCCR4D
• Bit 7 - FPIE4: Fault Protection Interrupt Enable
Setting this bit (to one) enables the Fault Protection Interrupt.
• Bit 6– FPEN4: Fault Protection Mode Enable
Setting this bit (to one) activates the Fault Protection Mode.
• Bit 5 – FPNC4: Fault Protection Noise Canceler
Setting this bit activates the Fault Protection Noise Canceler. When the noise canceler is activated, the input
from the Fault Protection Pin (INT0) is filtered. The filter function requires four successive equal valued samples
of the INT0 pin for changing its output. The Fault Protection is therefore delayed by four Oscillator cycles when
the noise canceler is enabled.
• Bit 4 – FPES4: Fault Protection Edge Select
This bit selects which edge on the Fault Protection pin (INT0) is used to trigger a fault event. When the FPES4
bit is written to zero, a falling (negative) edge is used as trigger, and when the FPES4 bit is written to one, a
rising (positive) edge will trigger the fault.
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• Bit 3 - FPAC4: Fault Protection Analog Comparator Enable
When written logic one, this bit enables the Fault Protection function in Timer/Counter4 to be triggered by the
Analog Comparator. The comparator output is in this case directly connected to the Fault Protection front-end
logic, making the comparator utilize the noise canceler and edge select features of the Timer/Counter4 Fault
Protection interrupt. When written logic zero, no connection between the Analog Comparator and the Fault
Protection function exists. To make the comparator trigger the Timer/Counter4 Fault Protection interrupt, the
FPIE4 bit in the Timer/Counter4 Control Register D (TCCR4D) must be set.
• Bit 2- FPF4: Fault Protection Interrupt Flag
When the FPIE4 bit is set (one), the Fault Protection Interrupt is enabled. Activity on the pin will cause an
interrupt request even, if the Fault Protection pin is configured as an output. The corresponding interrupt of Fault
Protection Interrupt Request is executed from the Fault Protection Interrupt Vector. The bit FPF4 is cleared by
hardware when executing the corresponding interrupt handling vector. Alternatively, FPF4 is cleared after a
synchronization clock cycle by writing a logical one to the flag. When the SREG I-bit, FPIE4 and FPF4 are set,
the Fault Interrupt is executed.
• Bits 1:0 - WGM41, WGM40: Waveform Generation Mode Bits
This bit associated with the PWM4x bits control the counting sequence of the counter, the source for type of
waveform generation to be used, see the table below.
The Modes of operation supported by the Timer/Counter4 are: Normal mode (counter), Fast PWM Mode, Phase
and Frequency Correct PWM and PWM6 Modes.
Table 15-18.
Waveform Generation Mode Bit Description
Timer/Counter Mode of Operation
TOP
Update of
OCR4x at
TOV4 Flag
Set on
xx
Normal
OCR4C
Immediate
TOP
1
00
Fast PWM
OCR4C
TOP
TOP
1
01
Phase and Frequency Correct PWM
OCR4C
BOTTOM
BOTTOM
1
10
PWM6 / Single-slope
OCR4C
TOP
TOP
1
11
PWM6 / Dual-slope
OCR4C
BOTTOM
BOTTOM
PWM4x
WGM41..40
0
15.12.5 TCCR4E – Timer/Counter4 Control Register E
Bit
Read/Write
Initial value
7
TLOCK4
R
0
6
ENHC4
R
0
5
OC4OE5
R/W
0
4
OC4OE4
R/W
0
3
OC4OE3
R/W
0
2
OC4OE2
R/W
0
1
OC4OE1
R/W
0
0
OC4OE0
R/W
0
TCCR4E
• Bit 7 - TLOCK4: Register Update Lock
This bit controls the Compare registers update. When this bit is set, writing to the Compare registers will not
affect the output, however the values are stored and will be updated to the Compare registers when the
TLOCK4 bit will be cleared.
Refer to “Synchronous update” on page 151 for more details.
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• Bit 6- ENHC4: Enhanced Compare/PWM Mode
When this bit is set, the Waveform Generation Module works in enhanced mode: the compare registers
OCR4A/B/D can welcome one more accuracy bit, while the LSB determines on which clock edge the Compare
condition is signalled and the output pin level is updated.
• Bits 5:0 – OC4OE5:OC4OE0: Output Compare Override Enable Bits
These bits are the Output Compare Override Enable bits that are used to connect or disconnect the Output
Compare Pins in PWM6 Modes with an instant response on the corresponding Output Compare Pins. The
actual value from the port register will be visible on the port pin, when the Output Compare Override Enable Bit
is cleared. The table shows the Output Compare Override Enable Bits and their corresponding Output Compare
pins.
OC4OE0
OC4OE1
OC4OE2
OC4OE3
OC4OE4
OC4OE5
OC4A (PC6)
OC4A (PC7)
OC4B (PB5)
OC4B (PB6)
OC4D (PD6)
OC4D (PD7)
15.12.6 TCNT4 – Timer/Counter4
Bit
4
Read/Write
Initial value
7
MSB
R/W
0
6
5
4
3
2
1
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
0
LSB
R/W
0
TCNT4
This 8-bit register contains the value of Timer/Counter4.
The Timer/Counter4 is realized as a 10-bit up/down counter with read and write access. Due to synchronization
of the CPU, Timer/Counter4 data written into Timer/Counter4 is delayed by one and half CPU clock cycles in
synchronous mode and at most one CPU clock cycles for asynchronous mode. When a 10-bit accuracy is
preferred, special procedures must be followed for accessing the 10-bit TCNT4 register via the 8-bit AVR data
bus. These procedures are described in section “Accessing 10-bit Registers” on page 160. Alternatively the
Timer/Counter4 can be used as an 8-bit Timer/Counter. Note that the Timer/Counter4 always starts counting up
after writing the TCNT4 register.
15.12.7 TC4H – Timer/Counter4 High Byte
Bit
Read/Write
Initial value
7
R
0
6
R
0
5
R
0
4
R
0
3
R
0
2
TC410
R
0
1
TC49
R/W
0
0
TC48
R/W
0
TC4H
The temporary Timer/Counter4 register is an 2-bit read/write register.
• Bits 7:3- Res: Reserved Bits
These bits are reserved bits and always reads as zero.
• Bits 2- TC410: Additional MSB bits for 11-bit accesses in Enhanced PWM mode
If 10-bit accuracy is used, the Timer/Counter4 High Byte Register (TC4H) is used for temporary storing the MSB
bits (TC49, TC48) of the 10-bit accesses. The same TC4H register is shared between all 10-bit registers within
the Timer/Counter4. Note that special procedures must be followed when accessing the 10-bit TCNT4 register
via the 8-bit AVR data bus. These procedures are described in section “Accessing 10-bit Registers” on
page 160.
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• Bits 1:0 - TC49, TC48: Two MSB bits of the 10-bit accesses
If 10-bit accuracy is used, the Timer/Counter4 High Byte Register (TC4H) is used for temporary storing the MSB
bits (TC49, TC48) of the 10-bit accesses. The same TC4H register is shared between all 10-bit registers within
the Timer/Counter4. Note that special procedures must be followed when accessing the 10-bit TCNT4 register
via the 8-bit AVR data bus. These procedures are described in section “Accessing 10-bit Registers” on
page 160.
15.12.8 OCR4A – Timer/Counter4 Output Compare Register A
Bit
Read/Write
Initial value
7
MSB
R/W
0
6
5
4
3
2
1
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
0
LSB
R/W
0
OCR4A
The output compare register A is an 8-bit read/write register.
The Timer/Counter Output Compare Register A contains data to be continuously compared with
Timer/Counter4. Actions on compare matches are specified in TCCR4A. A compare match does only occur if
Timer/Counter4 counts to the OCR4A value. A software write that sets TCNT4 and OCR4A to the same value
does not generate a compare match.
A compare match will set the compare interrupt flag OCF4A after a synchronization delay following the compare
event.
Note that, if 10-bit accuracy is used special procedures must be followed when accessing the internal 10-bit
Output Compare Registers via the 8-bit AVR data bus. These procedures are described in section “Accessing
10-bit Registers” on page 160.
15.12.9 OCR4B – Timer/Counter4 Output Compare Register B
Bit
Read/Write
Initial value
7
MSB
R/W
0
6
5
4
3
2
1
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
0
LSB
R/W
0
OCR4B
The output compare register B is an 8-bit read/write register.
The Timer/Counter Output Compare Register B contains data to be continuously compared with
Timer/Counter4. Actions on compare matches are specified in TCCR4. A compare match does only occur if
Timer/Counter4 counts to the OCR4B value. A software write that sets TCNT4 and OCR4B to the same value
does not generate a compare match.
A compare match will set the compare interrupt flag OCF4B after a synchronization delay following the compare
event.
Note that, if 10-bit accuracy is used special procedures must be followed when accessing the internal 10-bit
Output Compare Registers via the 8-bit AVR data bus. These procedures are described in section “Accessing
10-bit Registers” on page 160.
15.12.10OCR4C – Timer/Counter4 Output Compare Register C
Bit
Read/Write
Initial value
7
MSB
R/W
1
6
5
4
3
2
1
R/W
1
R/W
1
R/W
1
R/W
1
R/W
1
R/W
1
0
LSB
R/W
1
OCR44C
The output compare register C is an 8-bit read/write register.
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The Timer/Counter Output Compare Register C contains data to be continuously compared with
Timer/Counter4, and a compare match will clear TCNT4. This register has the same function in Normal mode
and PWM modes.
Note that, if a smaller value than three is written to the Output Compare Register C, the value is automatically
replaced by three as it is a minimum value allowed to be written to this register.
Note that, if 10-bit accuracy is used special procedures must be followed when accessing the internal 10-bit
Output Compare Registers via the 8-bit AVR data bus. These procedures are described in section “Accessing
10-bit Registers” on page 160.
15.12.11OCR4D – Timer/Counter4 Output Compare Register D
The output compare register D is an 8-bit read/write register.
Bit
Read/Write
Initial value
7
MSB
R/W
0
6
5
4
3
2
1
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
0
LSB
R/W
0
OCR4D
The Timer/Counter Output Compare Register D contains data to be continuously compared with
Timer/Counter4. Actions on compare matches are specified in TCCR4A. A compare match does only occur if
Timer/Counter4 counts to the OCR4D value. A software write that sets TCNT4 and OCR4D to the same value
does not generate a compare match.
A compare match will set the compare interrupt flag OCF4D after a synchronization delay following the compare
event.
Note that, if 10-bit accuracy is used special procedures must be followed when accessing the internal 10-bit
Output Compare Registers via the 8-bit AVR data bus. These procedures are described in section “Accessing
10-bit Registers” on page 160.
15.12.12TIMSK4 – Timer/Counter4 Interrupt Mask Register
Bit
Read/Write
Initial value
7
OCIE4D
R/W
0
6
OCIE4A
R/W
0
5
OCIE4B
R/W
0
4
3
R/W
0
R/W
0
2
TOIE4
R/W
0
1
0
R/W
0
R/W
0
TIMSK4
• Bit 7- OCIE4D: Timer/Counter4 Output Compare Interrupt Enable
When the OCIE4D bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter4 Compare
Match D interrupt is enabled. The corresponding interrupt at vector $010 is executed if a compare match D
occurs. The Compare Flag in Timer/Counter4 is set (one) in the Timer/Counter Interrupt Flag Register.
• Bit 6 - OCIE4A: Timer/Counter4 Output Compare Interrupt Enable
When the OCIE4A bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter4 Compare
Match A interrupt is enabled. The corresponding interrupt at vector $003 is executed if a compare match A
occurs. The Compare Flag in Timer/Counter4 is set (one) in the Timer/Counter Interrupt Flag Register.
• Bit 5 - OCIE4B: Timer/Counter4 Output Compare Interrupt Enable
When the OCIE4B bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter4 Compare
Match B interrupt is enabled. The corresponding interrupt at vector $009 is executed if a compare match B
occurs. The Compare Flag in Timer/Counter4 is set (one) in the Timer/Counter Interrupt Flag Register.
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• Bit 2 - TOIE4: Timer/Counter4 Overflow Interrupt Enable
When the TOIE4 bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter4 Overflow
interrupt is enabled. The corresponding interrupt (at vector $004) is executed if an overflow in Timer/Counter4
occurs. The Overflow Flag (Timer4) is set (one) in the Timer/Counter Interrupt Flag Register - TIFR4.
15.12.13TIFR4 – Timer/Counter4 Interrupt Flag Register
Bit
Read/Write
Initial value
7
OCF4D
R/W
0
6
OCF4A
R/W
0
5
OCF4B
R/W
0
4
3
R/W
0
R/W
0
2
TOV4
R/W
0
1
0
R/W
0
R/W
0
TIFR4
• Bit 7- OCF4D: Output Compare Flag 4D
The OCF4D bit is set (one) when compare match occurs between Timer/Counter4 and the data value in
OCR4D - Output Compare Register 4D. OCF4D is cleared by hardware when executing the corresponding
interrupt handling vector. Alternatively, OCF4D is cleared, after synchronization clock cycle, by writing a logic
one to the flag. When the I-bit in SREG, OCIE4D, and OCF4D are set (one), the Timer/Counter4 D compare
match interrupt is executed.
• Bit 6 - OCF4A: Output Compare Flag 4A
The OCF4A bit is set (one) when compare match occurs between Timer/Counter4 and the data value in OCR4A
- Output Compare Register 4A. OCF4A is cleared by hardware when executing the corresponding interrupt
handling vector. Alternatively, OCF4A is cleared, after synchronization clock cycle, by writing a logic one to the
flag. When the I-bit in SREG, OCIE4A, and OCF4A are set (one), the Timer/Counter4 A compare match
interrupt is executed.
• Bit 5 - OCF4B: Output Compare Flag 4B
The OCF4B bit is set (one) when compare match occurs between Timer/Counter4 and the data value in OCR4B
- Output Compare Register 4B. OCF4B is cleared by hardware when executing the corresponding interrupt
handling vector. Alternatively, OCF4B is cleared, after synchronization clock cycle, by writing a logic one to the
flag. When the I-bit in SREG, OCIE4B, and OCF4B are set (one), the Timer/Counter4 B compare match
interrupt is executed.
• Bit 2 - TOV4: Timer/Counter4 Overflow Flag
In Normal Mode and Fast PWM Mode the TOV4 bit is set (one) each time the counter reaches TOP at the same
clock cycle when the counter is reset to BOTTOM. In Phase and Frequency Correct PWM Mode the TOV4 bit is
set (one) each time the counter reaches BOTTOM at the same clock cycle when zero is clocked to the counter.
The bit TOV4 is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively,
TOV4 is cleared, after synchronization clock cycle, by writing a logical one to the flag. When the SREG I-bit, and
TOIE4 (Timer/Counter4 Overflow Interrupt Enable), and TOV4 are set (one), the Timer/Counter4 Overflow
interrupt is executed.
15.12.14DT4 – Timer/Counter4 Dead Time Value
Bit
Read/Write
Initial value
7
DT4H3
R/W
0
6
DT4H2
R/W
0
5
DT4H1
R/W
0
4
DT4H0
R/W
0
3
DT4L3
R/W
0
2
DT4L2
R/W
0
1
DT4L1
R/W
0
0
DT4L0
R/W
0
DT4
The dead time value register is an 8-bit read/write register.
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The dead time delay of all Timer/Counter4 channels are adjusted by the dead time value register, DT4. The
register consists of two fields, DT4H3..0 and DT4L3..0, one for each complementary output. Therefore a
different dead time delay can be adjusted for the rising edge of OC4x and the rising edge of OC4x.
• Bits 7:4- DT4H3:DT4H0: Dead Time Value for OC4x Output
The dead time value for the OC1x output. The dead time delay is set as a number of the prescaled timer/counter
clocks. The minimum dead time is zero and the maximum dead time is the prescaled time/counter clock period
multiplied by 15.
• Bits 3:0- DT4L3:DT4L0: Dead Time Value for OC4x Output
The dead time value for the OC4x output. The dead time delay is set as a number of the prescaled timer/counter
clocks. The minimum dead time is zero and the maximum dead time is the prescaled time/counter clock period
multiplied by 15.
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16.
Output Compare Modulator (OCM1C0A)
16.1
Overview
The Output Compare Modulator (OCM) allows generation of waveforms modulated with a carrier frequency.
The modulator uses the outputs from the Output Compare Unit C of the 16-bit Timer/Counter1 and the Output
Compare Unit of the 8-bit Timer/Counter0. For more details about these Timer/Counters see “Timer/Counter0,
Timer/Counter1, and Timer/Counter3 Prescalers” on page 92.
Figure 16-1.
Output Compare Modulator, Block Diagram
Timer/Counter 1
OC1C
Pin
Timer/Counter 0
OC1C /
OC0A / PB7
OC0A
When the modulator is enabled, the two output compare channels are modulated together as shown in the block
diagram (Figure 16-1).
16.2
Description
The Output Compare unit 1C and Output Compare unit 2 shares the PB7 port pin for output. The outputs of the
Output Compare units (OC1C and OC0A) overrides the normal PORTB7 Register when one of them is enabled
(i.e., when COMnx1:0 is not equal to zero). When both OC1C and OC0A are enabled at the same time, the
modulator is automatically enabled.
The functional equivalent schematic of the modulator is shown on Figure 16-2. The schematic includes part of
the Timer/Counter units and the port B pin 7 output driver circuit.
Figure 16-2.
Output Compare Modulator, Schematic
COMA01
COMA00
Vcc
COM1C1
COM1C0
( From Waveform Generator )
Modulator
0
D
1
Q
1
OC1C
Pin
0
( From Waveform Generator )
D
Q
OC1C /
OC0A/ PB7
OC0A
D
Q
D
PORTB7
Q
DDRB7
DATABUS
When the modulator is enabled the type of modulation (logical AND or OR) can be selected by the PORTB7
Register. When PORTB7=0, logical AND will be performed and when PORTB7=1, logical OR will be performed
(see Figure 16-3 on page 178).
Note that the DDRB7 controls the direction of the port independent of the COMnx1:0 bit setting.
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16.2.1 Timing Example
Figure 16-3 illustrates the modulator in action. In this example the Timer/Counter1 is set to operate in fast PWM
mode (non-inverted) and Timer/Counter0 uses CTC waveform mode with toggle Compare Output mode
(COMnx1:0 = 1).
Figure 16-3.
Output Compare Modulator, Timing Diagram
clk I/O
OC1C
(FPWM Mode)
OC0A
(CTC Mode)
PB7
(PORTB7 = 0)
PB7
(PORTB7 = 1)
(Period)
1
2
3
In this example, Timer/Counter0 provides the carrier, while the modulating signal is generated by the Output
Compare unit C of the Timer/Counter1.
The resolution of the PWM signal (OC1C) is reduced by the modulation. The reduction factor is equal to the
number of system clock cycles of one period of the carrier (OC0A). In this example the resolution is reduced by
a factor of two. The reason for the reduction is illustrated in Figure 16-3 at the second and third period of the
PB7 output when PORTB7 equals zero. The period 2 high time is one cycle longer than the period 3 high time,
but the result on the PB7 output is equal in both periods.
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Serial Peripheral Interface – SPI
The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the
ATmega16U4/ATmega32U4 and peripheral devices or between several AVR devices.
The SPI includes the following features:
•
•
•
•
•
•
•
•
Full-duplex, Three-wire Synchronous Data Transfer
Master or Slave Operation
LSB First or MSB First Data Transfer
Seven Programmable Bit Rates
End of Transmission Interrupt Flag
Write Collision Flag Protection
Wake-up from Idle Mode
Double Speed (CK/2) Master SPI Mode
USART can also be used in Master SPI mode, see “USART in SPI Mode” on page 214.
The Power Reduction SPI bit, PRSPI, in “Power Reduction Register 0 - PRR0” on page 47 must be written to
zero to enable SPI module.
Figure 17-1.
SPI Block Diagram(1)
DIVIDER
/2/4/8/16/32/64/128
SPI2X
SPI2X
17.
Note:
1.
Refer to “Pinout” on page 3, and Table 10-3 on page 74 for SPI pin placement.
The interconnection between Master and Slave CPUs with SPI is shown in Figure 17-2 on page 180. The
system consists of two shift Registers, and a Master clock generator. The SPI Master initiates the
communication cycle when pulling low the Slave Select SS pin of the desired Slave. Master and Slave prepare
the data to be sent in their respective shift Registers, and the Master generates the required clock pulses on the
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SCK line to interchange data. Data is always shifted from Master to Slave on the Master Out – Slave In, MOSI,
line, and from Slave to Master on the Master In – Slave Out, MISO, line. After each data packet, the Master will
synchronize the Slave by pulling high the Slave Select, SS, line.
When configured as a Master, the SPI interface has no automatic control of the SS line. This must be handled
by user software before communication can start. When this is done, writing a byte to the SPI Data Register
starts the SPI clock generator, and the hardware shifts the eight bits into the Slave. After shifting one byte, the
SPI clock generator stops, setting the end of Transmission Flag (SPIF). If the SPI Interrupt Enable bit (SPIE) in
the SPCR Register is set, an interrupt is requested. The Master may continue to shift the next byte by writing it
into SPDR, or signal the end of packet by pulling high the Slave Select, SS line. The last incoming byte will be
kept in the Buffer Register for later use.
When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated as long as the SS pin is
driven high. In this state, software may update the contents of the SPI Data Register, SPDR, but the data will not
be shifted out by incoming clock pulses on the SCK pin until the SS pin is driven low. As one byte has been
completely shifted, the end of Transmission Flag, SPIF is set. If the SPI Interrupt Enable bit, SPIE, in the SPCR
Register is set, an interrupt is requested. The Slave may continue to place new data to be sent into SPDR
before reading the incoming data. The last incoming byte will be kept in the Buffer Register for later use.
Figure 17-2.
SPI Master-slave Interconnection
SHIFT
ENABLE
The system is single buffered in the transmit direction and double buffered in the receive direction. This means
that bytes to be transmitted cannot be written to the SPI Data Register before the entire shift cycle is completed.
When receiving data, however, a received character must be read from the SPI Data Register before the next
character has been completely shifted in. Otherwise, the first byte is lost.
In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To ensure correct sampling
of the clock signal, the frequency of the SPI clock should never exceed fosc/4.
When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden according to
the table below. For more details on automatic port overrides, refer to “Alternate Port Functions” on page 72.
Table 17-1.
Pin
SPI Pin Overrides(1)
Direction, Master SPI
Direction, Slave SPI
MOSI
User Defined
Input
MISO
Input
User Defined
SCK
User Defined
Input
SS
User Defined
Input
Note:
1.
See “Alternate Functions of Port B” on page 74 for a detailed description of how to define the direction of the
user defined SPI pins.
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The following code examples show how to initialize the SPI as a Master and how to perform a simple
transmission. DDR_SPI in the examples must be replaced by the actual Data Direction Register controlling the
SPI pins. DD_MOSI, DD_MISO and DD_SCK must be replaced by the actual data direction bits for these pins.
E.g. if MOSI is placed on pin PB5, replace DD_MOSI with DDB5 and DDR_SPI with DDRB.
Assembly Code Example(1)
SPI_MasterInit:
; Set MOSI and SCK output, all others input
ldi
r17,(1<<DD_MOSI)|(1<<DD_SCK)
out
DDR_SPI,r17
; Enable SPI, Master, set clock rate fck/16
ldi
r17,(1<<SPE)|(1<<MSTR)|(1<<SPR0)
out
SPCR,r17
ret
SPI_MasterTransmit:
; Start transmission of data (r16)
out
SPDR,r16
Wait_Transmit:
; Wait for transmission complete
sbis
SPSR,SPIF
rjmp
Wait_Transmit
ret
C Code Example(1)
void SPI_MasterInit(void)
{
/* Set MOSI and SCK output, all others input */
DDR_SPI = (1<<DD_MOSI)|(1<<DD_SCK);
/* Enable SPI, Master, set clock rate fck/16 */
SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);
}
void SPI_MasterTransmit(char cData)
{
/* Start transmission */
SPDR = cData;
/* Wait for transmission complete */
while(!(SPSR & (1<<SPIF)))
;
}
Note:
1.
See “Code Examples” on page 8.
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The following code examples show how to initialize the SPI as a Slave and how to perform a simple reception.
Assembly Code Example(1)
SPI_SlaveInit:
; Set MISO output,
ldi
out
; Enable SPI
ldi
out
ret
all others input
r17,(1<<DD_MISO)
DDR_SPI,r17
r17,(1<<SPE)
SPCR,r17
SPI_SlaveReceive:
; Wait for reception complete
sbis
SPSR,SPIF
rjmp
SPI_SlaveReceive
; Read received data and return
in
r16,SPDR
ret
C Code Example(1)
void SPI_SlaveInit(void)
{
/* Set MISO output, all others input */
DDR_SPI = (1<<DD_MISO);
/* Enable SPI */
SPCR = (1<<SPE);
}
char SPI_SlaveReceive(void)
{
/* Wait for reception complete */
while(!(SPSR & (1<<SPIF)))
;
/* Return Data Register */
return SPDR;
}
Note:
17.1
1.
See “Code Examples” on page 8.
SS Pin Functionality
17.1.1 Slave Mode
When the SPI is configured as a Slave, the Slave Select (SS) pin is always input. When SS is held low, the SPI
is activated, and MISO becomes an output if configured so by the user. All other pins are inputs. When SS is
driven high, all pins are inputs, and the SPI is passive, which means that it will not receive incoming data. Note
that the SPI logic will be reset once the SS pin is driven high.
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The SS pin is useful for packet/byte synchronization to keep the slave bit counter synchronous with the master
clock generator. When the SS pin is driven high, the SPI slave will immediately reset the send and receive logic,
and drop any partially received data in the Shift Register.
17.1.2 Master Mode
When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine the direction of the SS
pin.
If SS is configured as an output, the pin is a general output pin which does not affect the SPI system. Typically,
the pin will be driving the SS pin of the SPI Slave.
If SS is configured as an input, it must be held high to ensure Master SPI operation. If the SS pin is driven
low by peripheral circuitry when the SPI is configured as a Master with the SS pin defined as an input, the SPI
system interprets this as another master selecting the SPI as a slave and starting to send data to it. To avoid
bus contention, the SPI system takes the following actions:
1. The MSTR bit in SPCR is cleared and the SPI system becomes a Slave. As a result of the SPI becoming a Slave, the MOSI and SCK pins become inputs.
2.
The SPIF Flag in SPSR is set, and if the SPI interrupt is enabled, and the I-bit in SREG is set, the interrupt
routine will be executed.
Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a possibility that SS is
driven low, the interrupt should always check that the MSTR bit is still set. If the MSTR bit has been cleared by
a slave select, it must be set by the user to re-enable SPI Master mode.
17.1.3 Data Modes
There are four combinations of SCK phase and polarity with respect to serial data, which are determined by
control bits CPHA and CPOL. The SPI data transfer formats are shown in Figure 17-3 and Figure 17-4 on page
184. Data bits are shifted out and latched in on opposite edges of the SCK signal, ensuring sufficient time for
data signals to stabilize. This is clearly seen by summarizing Table 17-3 and Table 17-4 on page 185, as done
below:
Table 17-2.
CPOL and CPHA Functionality
Leading Edge
Trailing eDge
SPI Mode
CPOL=0, CPHA=0
Sample (Rising)
Setup (Falling)
0
CPOL=0, CPHA=1
Setup (Rising)
Sample (Falling)
1
CPOL=1, CPHA=0
Sample (Falling)
Setup (Rising)
2
CPOL=1, CPHA=1
Setup (Falling)
Sample (Rising)
3
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Figure 17-3.
SPI Transfer Format with CPHA = 0
SCK (CPOL = 0)
mode 0
SCK (CPOL = 1)
mode 2
SAMPLE I
MOSI/MISO
CHANGE 0
MOSI PIN
CHANGE 0
MISO PIN
SS
MSB first (DORD = 0) MSB
LSB first (DORD = 1) LSB
Figure 17-4.
Bit 6
Bit 1
Bit 5
Bit 2
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
Bit 1
Bit 6
LSB
MSB
SPI Transfer Format with CPHA = 1
SCK (CPOL = 0)
mode 1
SCK (CPOL = 1)
mode 3
SAMPLE I
MOSI/MISO
CHANGE 0
MOSI PIN
CHANGE 0
MISO PIN
SS
MSB first (DORD = 0)
LSB first (DORD = 1)
MSB
LSB
Bit 6
Bit 1
Bit 5
Bit 2
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
Bit 1
Bit 6
LSB
MSB
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17.2
Register Description
17.2.1 SPI Control Register – SPCR
Bit
Read/Write
Initial Value
7
SPIE
R/W
0
6
SPE
R/W
0
5
DORD
R/W
0
4
MSTR
R/W
0
3
CPOL
R/W
0
2
CPHA
R/W
0
1
SPR1
R/W
0
0
SPR0
R/W
0
SPCR
• Bit 7 – SPIE: SPI Interrupt Enable
This bit causes the SPI interrupt to be executed if SPIF bit in the SPSR Register is set and the if the Global
Interrupt Enable bit in SREG is set.
• Bit 6 – SPE: SPI Enable
When the SPE bit is written to one, the SPI is enabled. This bit must be set to enable any SPI operations.
• Bit 5 – DORD: Data Order
When the DORD bit is written to one, the LSB of the data word is transmitted first.
When the DORD bit is written to zero, the MSB of the data word is transmitted first.
• Bit 4 – MSTR: Master/Slave Select
This bit selects Master SPI mode when written to one, and Slave SPI mode when written logic zero. If SS is
configured as an input and is driven low while MSTR is set, MSTR will be cleared, and SPIF in SPSR will
become set. The user will then have to set MSTR to re-enable SPI Master mode.
• Bit 3 – CPOL: Clock Polarity
When this bit is written to one, SCK is high when idle. When CPOL is written to zero, SCK is low when idle.
Refer to Figure 17-3 and Figure 17-4 for an example. The CPOL functionality is summarized below:
Table 17-3.
CPOL Functionality
CPOL
Leading Edge
Trailing Edge
0
Rising
Falling
1
Falling
Rising
• Bit 2 – CPHA: Clock Phase
The settings of the Clock Phase bit (CPHA) determine if data is sampled on the leading (first) or trailing (last)
edge of SCK. Refer to Figure 17-3 on page 184 and Figure 17-4 on page 184 for an example. The CPOL
functionality is summarized below:
Table 17-4.
CPHA Functionality
CPHA
Leading Edge
Trailing Edge
0
Sample
Setup
1
Setup
Sample
• Bits 1, 0 – SPR1, SPR0: SPI Clock Rate Select 1 and 0
These two bits control the SCK rate of the device configured as a Master. SPR1 and SPR0 have no effect on
the Slave. The relationship between SCK and the Oscillator Clock frequency fosc is shown below.
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Table 17-5.
Relationship Between SCK and the Oscillator Frequency
SPI2X
SPR1
SPR0
0
0
0
0
0
1
0
1
0
0
1
1
1
0
0
1
0
1
1
1
0
1
1
1
SCK Frequency
fosc/4
fosc/16
fosc/64
fosc/128
fosc/2
fosc/8
fosc/32
fosc/64
17.2.2 SPI Status Register – SPSR
Bit
Read/Write
Initial Value
7
SPIF
R
0
6
WCOL
R
0
5
–
R
0
4
–
R
0
3
–
R
0
2
–
R
0
1
–
R
0
0
SPI2X
R/W
0
SPSR
• Bit 7 – SPIF: SPI Interrupt Flag
When a serial transfer is complete, the SPIF Flag is set. An interrupt is generated if SPIE in SPCR is set and
global interrupts are enabled. If SS is an input and is driven low when the SPI is in Master mode, this will also
set the SPIF Flag. SPIF is cleared by hardware when executing the corresponding interrupt handling vector.
Alternatively, the SPIF bit is cleared by first reading the SPI Status Register with SPIF set, then accessing the
SPI Data Register (SPDR).
• Bit 6 – WCOL: Write COLlision Flag
The WCOL bit is set if the SPI Data Register (SPDR) is written during a data transfer. The WCOL bit (and the
SPIF bit) are cleared by first reading the SPI Status Register with WCOL set, and then accessing the SPI Data
Register.
• Bit 5..1 – Res: Reserved Bits
These bits are reserved and will always read as zero.
• Bit 0 – SPI2X: Double SPI Speed Bit
When this bit is written logic one the SPI speed (SCK Frequency) will be doubled when the SPI is in Master
mode (see Table 17-2 on page 183). This means that the minimum SCK period will be two CPU clock periods.
When the SPI is configured as Slave, the SPI is only guaranteed to work at fosc/4 or lower.
The SPI interface on the device is also used for program memory and EEPROM downloading or uploading. See
page 367 for serial programming and verification.
17.2.3 SPI Data Register – SPDR
Bit
Read/Write
Initial Value
7
MSB
R/W
X
6
5
4
3
2
1
R/W
X
R/W
X
R/W
X
R/W
X
R/W
X
R/W
X
0
LSB
R/W
X
SPDR
Undefined
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The SPI Data Register is a read/write register used for data transfer between the Register File and the SPI Shift
Register. Writing to the register initiates data transmission. Reading the register causes the Shift Register
Receive buffer to be read.
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18.
USART
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a highly flexible
serial communication device. The main features are:
•
•
•
•
•
•
•
•
•
•
•
•
•
Full Duplex Operation (Independent Serial Receive and Transmit Registers)
Asynchronous or Synchronous Operation
Flow control CTS/RTS signals hardware management
Master or Slave Clocked Synchronous Operation
High Resolution Baud Rate Generator
Supports Serial Frames with 5, 6, 7, 8, or 9 Data Bits and 1 or 2 Stop Bits
Odd or Even Parity Generation and Parity Check Supported by Hardware
Data OverRun Detection
Framing Error Detection
Noise Filtering Includes False Start Bit Detection and Digital Low Pass Filter
Three Separate Interrupts on TX Complete, TX Data Register Empty, and RX Complete
Multi-processor Communication Mode
Double Speed Asynchronous Communication Mode
.
18.1
Overview
A simplified block diagram of the USART Transmitter is shown in Figure 18-1 on page 189. CPU accessible I/O
Registers and I/O pins are shown in bold.
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Figure 18-1.
USART Block Diagram(1)
Clock Generator
UBRR[H:L]
OSC
BAUD RATE GENERATOR
SYNC LOGIC
PIN
CONTROL
XCK
Transmitter
TX
CONTROL
UDR (Transmit)
DATA BUS
PARITY
GENERATOR
1.
TxD
Receiver
UCSRA
Note:
PIN
CONTROL
TRANSMIT SHIFT REGISTER
CLOCK
RECOVERY
RX
CONTROL
RECEIVE SHIFT REGISTER
DATA
RECOVERY
PIN
CONTROL
UDR (Receive)
PARITY
CHECKER
UCSRB
RxD
UCSRC
See “Pinout” on page 3, Table 10-8 on page 78 and for USART pin placement.
The dashed boxes in the block diagram separate the three main parts of the USART (listed from the top): Clock
Generator, Transmitter and Receiver. Control Registers are shared by all units. The Clock Generation logic
consists of synchronization logic for external clock input used by synchronous slave operation, and the baud
rate generator. The XCKn (Transfer Clock) pin is only used by synchronous transfer mode. The Transmitter
consists of a single write buffer, a serial Shift Register, Parity Generator, and Control logic for handling different
serial frame formats. The write buffer allows a continuous transfer of data without any delay between frames.
The Receiver is the most complex part of the USART module due to its clock and data recovery units. The
recovery units are used for asynchronous data reception. In addition to the recovery units, the Receiver includes
a Parity Checker, Control logic, a Shift Register and a two level receive buffer (UDRn). The Receiver supports
the same frame formats as the Transmitter, and can detect Frame Error, Data OverRun and Parity Errors.
18.2
Clock Generation
The Clock Generation logic generates the base clock for the Transmitter and Receiver. The USARTn supports
four modes of clock operation: Normal asynchronous, Double Speed asynchronous, Master synchronous and
Slave synchronous mode. The UMSELn bit in USART Control and Status Register C (UCSRnC) selects
between asynchronous and synchronous operation. Double Speed (asynchronous mode only) is controlled by
the U2Xn found in the UCSRnA Register. When using synchronous mode (UMSELn = 1), the Data Direction
Register for the XCKn pin (DDR_XCKn) controls whether the clock source is internal (Master mode) or external
(Slave mode). The XCKn pin is only active when using synchronous mode.
Figure 18-2 shows a block diagram of the clock generation logic.
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Figure 18-2.
Clock Generation Logic, Block Diagram
UBRR
U2X
fosc
Prescaling
Down-Counter
UBRR+1
/2
/4
/2
0
1
0
OSC
DDR_XCK
xcki
XCK
Pin
Sync
Register
Edge
Detector
DDR_XCK
0
UMSEL
1
xcko
UCPOL
txclk
1
1
0
rxclk
Signal description:
txclk
Transmitter clock (Internal Signal)
rxclk
Receiver base clock (Internal Signal)
xcki
Input from XCK pin (internal Signal). Used for synchronous slave operation
xcko
Clock output to XCK pin (Internal Signal). Used for synchronous master operation
fOSC
XTAL pin frequency (System Clock)
18.2.1 Internal Clock Generation – The Baud Rate Generator
Internal clock generation is used for the asynchronous and the synchronous master modes of operation. The
description in this section refers to Figure 18-2 on page 190.
The USART Baud Rate Register (UBRRn) and the down-counter connected to it function as a programmable
prescaler or baud rate generator. The down-counter, running at system clock (fosc), is loaded with the UBRRn
value each time the counter has counted down to zero or when the UBRRLn Register is written. A clock is
generated each time the counter reaches zero. This clock is the baud rate generator clock output
(= fosc/(UBRRn+1)). The Transmitter divides the baud rate generator clock output by 2, 8, or 16 depending on
mode. The baud rate generator output is used directly by the Receiver’s clock and data recovery units.
However, the recovery units use a state machine that uses 2, 8, or 16 states depending on mode set by the
state of the UMSELn, U2Xn, and DDR_XCKn bits.
The following table contains equations for calculating the baud rate (in bits per second) and for calculating the
UBRRn value for each mode of operation using an internally generated clock source.
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Table 18-1.
Equations for Calculating Baud Rate Register Setting
Operating Mode
Equation for Calculating Baud Rate(1)
Equation for Calculating UBRR Value
f OSC
BAUD = -----------------------------------------16  UBRRn + 1 
f OSC
UBRRn = ------------------------ – 1
16 BAUD
Asynchronous Double
Speed mode (U2Xn = 1)
f OSC
BAUD = --------------------------------------8  UBRRn + 1 
f OSC
UBRRn = -------------------- – 1
8 BAUD
Synchronous Master
mode
f OSC
BAUD = --------------------------------------2  UBRRn + 1 
Asynchronous Normal
mode (U2Xn = 0)
Note:
1.
f OSC
UBRRn = -------------------- – 1
2 BAUD
The baud rate is defined to be the transfer rate in bit per second (bps)
BAUD
Baud rate (in bits per second, bps)
fOSC
System Oscillator clock frequency
UBRRn
Contents of the UBRRHn and UBRRLn Registers, (0-4095)
Some examples of UBRRn values for some system clock frequencies are found in Table 18-6 on page 208.
18.2.2 Double Speed Operation (U2Xn)
The transfer rate can be doubled by setting the U2Xn bit in UCSRnA. Setting this bit only has effect for the
asynchronous operation. Set this bit to zero when using synchronous operation.
Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling the transfer rate
for asynchronous communication. Note however that the Receiver will in this case only use half the number of
samples (reduced from 16 to 8) for data sampling and clock recovery, and therefore a more accurate baud rate
setting and system clock are required when this mode is used. For the Transmitter, there are no downsides.
18.2.3 External Clock
External clocking is used by the synchronous slave modes of operation. The description in this section refers to
Figure 18-2 on page 190 for details.
External clock input from the XCKn pin is sampled by a synchronization register to minimize the chance of metastability. The output from the synchronization register must then pass through an edge detector before it can be
used by the Transmitter and Receiver. This process introduces a two CPU clock period delay and therefore the
maximum external XCKn clock frequency is limited by the following equation:
f OSC
f XCK  ----------4
Note that fosc depends on the stability of the system clock source. It is therefore recommended to add some
margin to avoid possible loss of data due to frequency variations.
18.2.4 Synchronous Clock Operation
When synchronous mode is used (UMSELn = 1), the XCKn pin will be used as either clock input (Slave) or clock
output (Master). The dependency between the clock edges and data sampling or data change is the same. The
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basic principle is that data input (on RxDn) is sampled at the opposite XCKn clock edge of the edge the data
output (TxDn) is changed.
Figure 18-3.
Synchronous Mode XCKn Timing
UCPOL = 1
XCK
RxD / TxD
Sample
XCK
UCPOL = 0
RxD / TxD
Sample
The UCPOLn bit UCRSC selects which XCKn clock edge is used for data sampling and which is used for data
change. As the above figure shows, when UCPOLn is zero the data will be changed at rising XCKn edge and
sampled at falling XCKn edge. If UCPOLn is set, the data will be changed at falling XCKn edge and sampled at
rising XCKn edge.
18.3
Frame Formats
A serial frame is defined to be one character of data bits with synchronization bits (start and stop bits), and
optionally a parity bit for error checking. The USART accepts all 30 combinations of the following as valid frame
formats:

1 start bit

5, 6, 7, 8, or 9 data bits

no, even or odd parity bit

1 or 2 stop bits
A frame starts with the start bit followed by the least significant data bit. Then the next data bits, up to a total of
nine, are succeeding, ending with the most significant bit. If enabled, the parity bit is inserted after the data bits,
before the stop bits. When a complete frame is transmitted, it can be directly followed by a new frame, or the
communication line can be set to an idle (high) state. The following figure illustrates the possible combinations
of the frame formats. Bits inside brackets are optional.
Figure 18-4.
Frame Formats
FRAME
(IDLE)
St
0
1
2
3
4
[5]
[6]
[7]
[8]
[P]
Sp1 [Sp2]
St
Start bit, always low
(n)
Data bits (0 to 8)
P
Parity bit. Can be odd or even
Sp
Stop bit, always high
IDLE
No transfers on the communication line (RxDn or TxDn). An IDLE line must be high
(St / IDLE)
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The frame format used by the USART is set by the UCSZn2:0, UPMn1:0, and USBSn bits in UCSRnB and
UCSRnC. The Receiver and Transmitter use the same setting. Note that changing the setting of any of these
bits will corrupt all ongoing communication for both the Receiver and Transmitter.
The USART Character SiZe (UCSZn2:0) bits select the number of data bits in the frame. The USART Parity
mode (UPMn1:0) bits enable and set the type of parity bit. The selection between one or two stop bits is done by
the USART Stop Bit Select (USBSn) bit. The Receiver ignores the second stop bit. An FE (Frame Error) will
therefore only be detected in the cases where the first stop bit is zero.
18.3.1 Parity Bit Calculation
The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is used, the result of the
exclusive or is inverted. The relation between the parity bit and data bits is as follows:
P even = d n – 1    d 3  d 2  d 1  d 0  0
P odd = d n – 1    d 3  d 2  d 1  d 0  1
Peven
Start bit, always low
Podd
Parity bit using odd parity
dn
Data bit n of the character
If used, the parity bit is located between the last data bit and first stop bit of a serial frame.
18.4
USART Initialization
The USART has to be initialized before any communication can take place. The initialization process normally
consists of setting the baud rate, setting frame format and enabling the Transmitter or the Receiver depending
on the usage. For interrupt driven USART operation, the Global Interrupt Flag should be cleared (and interrupts
globally disabled) when doing the initialization.
Before doing a re-initialization with changed baud rate or frame format, be sure that there are no ongoing
transmissions during the period the registers are changed. The TXCn Flag can be used to check that the
Transmitter has completed all transfers, and the RXC Flag can be used to check that there are no unread data
in the receive buffer. Note that the TXCn Flag must be cleared before each transmission (before UDRn is
written) if it is used for this purpose.
The following simple USART initialization code examples show one assembly and one C function that are equal
in functionality. The examples assume asynchronous operation using polling (no interrupts enabled) and a fixed
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frame format. The baud rate is given as a function parameter. For the assembly code, the baud rate parameter
is assumed to be stored in the r17:r16 Registers.
Assembly Code Example(1)
USART_Init:
; Set baud rate
out
UBRRHn, r17
out
UBRRLn, r16
; Enable receiver and transmitter
ldi
r16, (1<<RXENn)|(1<<TXENn)
out
UCSRnB,r16
; Set frame format: 8data, 2stop bit
ldi
r16, (1<<USBSn)|(3<<UCSZn0)
out
UCSRnC,r16
ret
C Code Example(1)
void USART_Init( unsigned int baud )
{
/* Set baud rate */
UBRRHn = (unsigned char)(baud>>8);
UBRRLn = (unsigned char)baud;
/* Enable receiver and transmitter */
UCSRnB = (1<<RXENn)|(1<<TXENn);
/* Set frame format: 8data, 2stop bit */
UCSRnC = (1<<USBSn)|(3<<UCSZn0);
}
Note:
1.
See “Code Examples” on page 8.
More advanced initialization routines can be made that include frame format as parameters, disable interrupts
and so on. However, many applications use a fixed setting of the baud and control registers, and for these types
of applications the initialization code can be placed directly in the main routine, or be combined with initialization
code for other I/O modules.
18.5
Data Transmission – The USART Transmitter
The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRnB Register. When
the Transmitter is enabled, the normal port operation of the TxDn pin is overridden by the USART and given the
function as the Transmitter’s serial output. The baud rate, mode of operation and frame format must be set up
once before doing any transmissions. If synchronous operation is used, the clock on the XCKn pin will be
overridden and used as transmission clock.
18.5.1 Sending Frames with 5 to 8 Data Bit
A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The CPU can load
the transmit buffer by writing to the UDRn I/O location. The buffered data in the transmit buffer will be moved to
the Shift Register when the Shift Register is ready to send a new frame. The Shift Register is loaded with new
data if it is in idle state (no ongoing transmission) or immediately after the last stop bit of the previous frame is
transmitted. When the Shift Register is loaded with new data, it will transfer one complete frame at the rate
given by the Baud Register, U2Xn bit or by XCKn depending on mode of operation.
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The following code examples show a simple USART transmit function based on polling of the Data Register
Empty (UDREn) Flag. When using frames with less than eight bits, the most significant bits written to the UDRn
are ignored. The USART has to be initialized before the function can be used. For the assembly code, the data
to be sent is assumed to be stored in Register R16.
Assembly Code Example(1)
USART_Transmit:
; Wait for empty transmit buffer
sbis
UCSRnA,UDREn
rjmp
USART_Transmit
; Put data (r16) into buffer, sends the data
out
UDRn,r16
ret
C Code Example(1)
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn)) )
;
/* Put data into buffer, sends the data */
UDRn = data;
}
Note:
1.
See “Code Examples” on page 8.
The function simply waits for the transmit buffer to be empty by checking the UDREn Flag, before loading it with
new data to be transmitted. If the Data Register Empty interrupt is utilized, the interrupt routine writes the data
into the buffer.
18.5.2 Sending Frames with 9 Data Bit
If 9-bit characters are used (UCSZn = 7), the ninth bit must be written to the TXB8 bit in UCSRnB before the low
byte of the character is written to UDRn. The following code examples show a transmit function that handles 9bit characters. For the assembly code, the data to be sent is assumed to be stored in registers R17:R16.
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Assembly Code Example(1)(2)
USART_Transmit:
; Wait for empty transmit buffer
sbis
UCSRnA,UDREn
rjmp
USART_Transmit
; Copy 9th bit from r17 to TXB8
cbi
UCSRnB,TXB8
sbrc
r17,0
sbi
UCSRnB,TXB8
; Put LSB data (r16) into buffer, sends the data
out
UDRn,r16
ret
C Code Example(1)(2)
void USART_Transmit( unsigned int data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn))) )
;
/* Copy 9th bit to TXB8 */
UCSRnB &= ~(1<<TXB8);
if ( data & 0x0100 )
UCSRnB |= (1<<TXB8);
/* Put data into buffer, sends the data */
UDRn = data;
}
Notes:
1.
2.
These transmit functions are written to be general functions. They can be optimized if the contents of the
UCSRnB is static. For example, only the TXB8 bit of the UCSRnB Register is used after initialization.
See “Code Examples” on page 8.
The 9th bit can be used for indicating an address frame when using multi processor communication mode or for
other protocol handling as for example synchronization.
18.5.3 Transmitter Flags and Interrupts
The USART Transmitter has two flags that indicate its state: USART Data Register Empty (UDREn) and
Transmit Complete (TXCn). Both flags can be used for generating interrupts.
The Data Register Empty (UDREn) Flag indicates whether the transmit buffer is ready to receive new data. This
bit is set when the transmit buffer is empty, and cleared when the transmit buffer contains data to be transmitted
that has not yet been moved into the Shift Register. For compatibility with future devices, always write this bit to
zero when writing the UCSRnA Register.
When the Data Register Empty Interrupt Enable (UDRIEn) bit in UCSRnB is written to one, the USART Data
Register Empty Interrupt will be executed as long as UDREn is set (provided that global interrupts are enabled).
UDREn is cleared by writing UDRn. When interrupt-driven data transmission is used, the Data Register Empty
interrupt routine must either write new data to UDRn in order to clear UDREn or disable the Data Register
Empty interrupt, otherwise a new interrupt will occur once the interrupt routine terminates.
The Transmit Complete (TXCn) Flag bit is set one when the entire frame in the Transmit Shift Register has been
shifted out and there are no new data currently present in the transmit buffer. The TXCn Flag bit is automatically
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cleared when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location.
The TXCn Flag is useful in half-duplex communication interfaces (like the RS-485 standard), where a
transmitting application must enter receive mode and free the communication bus immediately after completing
the transmission.
When the Transmit Compete Interrupt Enable (TXCIEn) bit in UCSRnB is set, the USART Transmit Complete
Interrupt will be executed when the TXCn Flag becomes set (provided that global interrupts are enabled). When
the transmit complete interrupt is used, the interrupt handling routine does not have to clear the TXCn Flag, this
is done automatically when the interrupt is executed.
18.5.4 Parity Generator
The Parity Generator calculates the parity bit for the serial frame data. When parity bit is enabled (UPMn1 = 1),
the transmitter control logic inserts the parity bit between the last data bit and the first stop bit of the frame that is
sent.
18.5.5 Disabling the Transmitter
The disabling of the Transmitter (setting the TXEN to zero) will not become effective until ongoing and pending
transmissions are completed, i.e., when the Transmit Shift Register and Transmit Buffer Register do not contain
data to be transmitted. When disabled, the Transmitter will no longer override the TxDn pin.
18.6
Data Reception – The USART Receiver
The USART Receiver is enabled by writing the Receive Enable (RXENn) bit in the
UCSRnB Register to one. When the Receiver is enabled, the normal pin operation of the RxDn pin is overridden
by the USART and given the function as the Receiver’s serial input. The baud rate, mode of operation and
frame format must be set up once before any serial reception can be done. If synchronous operation is used,
the clock on the XCKn pin will be used as transfer clock.
18.6.1 Receiving Frames with 5 to 8 Data Bits
The Receiver starts data reception when it detects a valid start bit. Each bit that follows the start bit will be
sampled at the baud rate or XCKn clock, and shifted into the Receive Shift Register until the first stop bit of a
frame is received. A second stop bit will be ignored by the Receiver. When the first stop bit is received, i.e., a
complete serial frame is present in the Receive Shift Register, the contents of the Shift Register will be moved
into the receive buffer. The receive buffer can then be read by reading the UDRn I/O location.
The following code example shows a simple USART receive function based on polling of the Receive Complete
(RXCn) Flag. When using frames with less than eight bits the most significant bits of the data read from the
UDRn will be masked to zero. The USART has to be initialized before the function can be used.
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Assembly Code Example(1)
USART_Receive:
; Wait for data to be received
sbis
UCSRnA, RXCn
rjmp
USART_Receive
; Get and return received data from buffer
in
r16, UDRn
ret
C Code Example(1)
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSRnA & (1<<RXCn)) )
;
/* Get and return received data from buffer */
return UDRn;
}
Note:
1.
See “Code Examples” on page 8.
The function simply waits for data to be present in the receive buffer by checking the RXCn Flag, before reading
the buffer and returning the value.
18.6.2 Receiving Frames with 9 Data Bits
If 9-bit characters are used (UCSZn=7) the ninth bit must be read from the RXB8n bit in UCSRnB before
reading the low bits from the UDRn. This rule applies to the FEn, DORn, and UPEn Status Flags as well. Read
status from UCSRnA, then data from UDRn. Reading the UDRn I/O location will change the state of the receive
buffer FIFO and consequently the TXB8n, FEn, DORn, and UPEn bits, which all are stored in the FIFO, will
change.
The following code example shows a simple USART receive function that handles both nine bit characters and
the status bits.
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Assembly Code Example(1)
USART_Receive:
; Wait for data to be received
sbis
UCSRnA, RXCn
rjmp
USART_Receive
; Get status and 9th bit, then data from buffer
in
r18, UCSRnA
in
r17, UCSRnB
in
r16, UDRn
; If error, return -1
andi
r18,(1<<FEn)|(1<<DORn)|(1<<UPEn)
breq
USART_ReceiveNoError
ldi
r17, HIGH(-1)
ldi
r16, LOW(-1)
USART_ReceiveNoError:
; Filter the 9th bit, then return
lsr
r17
andi
r17, 0x01
ret
C Code Example(1)
unsigned int USART_Receive( void )
{
unsigned char status, resh, resl;
/* Wait for data to be received */
while ( !(UCSRnA & (1<<RXCn)) )
;
/* Get status and 9th bit, then data */
/* from buffer */
status = UCSRnA;
resh = UCSRnB;
resl = UDRn;
/* If error, return -1 */
if ( status & (1<<FEn)|(1<<DORn)|(1<<UPEn) )
return -1;
/* Filter the 9th bit, then return */
resh = (resh >> 1) & 0x01;
return ((resh << 8) | resl);
}
Note:
1.
See “Code Examples” on page 8.
The receive function example reads all the I/O Registers into the Register File before any computation is done.
This gives an optimal receive buffer utilization since the buffer location read will be free to accept new data as
early as possible.
18.6.3 Receive Compete Flag and Interrupt
The USART Receiver has one flag that indicates the Receiver state.
The Receive Complete (RXCn) Flag indicates if there are unread data present in the receive buffer. This flag is
one when unread data exist in the receive buffer, and zero when the receive buffer is empty (i.e., does not
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contain any unread data). If the Receiver is disabled (RXENn = 0), the receive buffer will be flushed and
consequently the RXCn bit will become zero.
When the Receive Complete Interrupt Enable (RXCIEn) in UCSRnB is set, the USART Receive Complete
interrupt will be executed as long as the RXCn Flag is set (provided that global interrupts are enabled). When
interrupt-driven data reception is used, the receive complete routine must read the received data from UDRn in
order to clear the RXCn Flag, otherwise a new interrupt will occur once the interrupt routine terminates.
18.6.4 Receiver Error Flags
The USART Receiver has three Error Flags: Frame Error (FEn), Data OverRun (DORn), and Parity Error
(UPEn). All can be accessed by reading UCSRnA. Common for the Error Flags is that they are located in the
receive buffer together with the frame for which they indicate the error status. Due to the buffering of the Error
Flags, the UCSRnA must be read before the receive buffer (UDRn), since reading the UDRn I/O location
changes the buffer read location. Another equality for the Error Flags is that they can not be altered by software
doing a write to the flag location. However, all flags must be set to zero when the UCSRnA is written for upward
compatibility of future USART implementations. None of the Error Flags can generate interrupts.
The Frame Error (FEn) Flag indicates the state of the first stop bit of the next readable frame stored in the
receive buffer. The FEn Flag is zero when the stop bit was correctly read (as one), and the FEn Flag will be one
when the stop bit was incorrect (zero). This flag can be used for detecting out-of-sync conditions, detecting
break conditions and protocol handling. The FEn Flag is not affected by the setting of the USBSn bit in UCSRnC
since the Receiver ignores all, except for the first, stop bits. For compatibility with future devices, always set this
bit to zero when writing to UCSRnA.
The Data OverRun (DORn) Flag indicates data loss due to a receiver buffer full condition. A Data OverRun
occurs when the receive buffer is full (two characters), it is a new character waiting in the Receive Shift Register,
and a new start bit is detected. If the DORn Flag is set there was one or more serial frame lost between the
frame last read from UDRn, and the next frame read from UDRn. For compatibility with future devices, always
write this bit to zero when writing to UCSRnA. The DORn Flag is cleared when the frame received was
successfully moved from the Shift Register to the receive buffer.
The Parity Error (UPEn) Flag indicates that the next frame in the receive buffer had a Parity Error when
received. If Parity Check is not enabled the UPEn bit will always be read zero. For compatibility with future
devices, always set this bit to zero when writing to UCSRnA. For more details see “Parity Bit Calculation” on
page 193 and “Parity Checker” on page 200.
18.6.5 Parity Checker
The Parity Checker is active when the high USART Parity mode (UPMn1) bit is set. Type of Parity Check to be
performed (odd or even) is selected by the UPMn0 bit. When enabled, the Parity Checker calculates the parity
of the data bits in incoming frames and compares the result with the parity bit from the serial frame. The result of
the check is stored in the receive buffer together with the received data and stop bits. The Parity Error (UPEn)
Flag can then be read by software to check if the frame had a Parity Error.
The UPEn bit is set if the next character that can be read from the receive buffer had a Parity Error when
received and the Parity Checking was enabled at that point (UPMn1 = 1). This bit is valid until the receive buffer
(UDRn) is read.
18.6.6 Disabling the Receiver
In contrast to the Transmitter, disabling of the Receiver will be immediate. Data from ongoing receptions will
therefore be lost. When disabled (i.e., the RXENn is set to zero) the Receiver will no longer override the normal
function of the RxDn port pin. The Receiver buffer FIFO will be flushed when the Receiver is disabled.
Remaining data in the buffer will be lost
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18.6.7 Flushing the Receive Buffer
The receiver buffer FIFO will be flushed when the Receiver is disabled, i.e., the buffer will be emptied of its
contents. Unread data will be lost. If the buffer has to be flushed during normal operation, due to for instance an
error condition, read the UDRn I/O location until the RXCn Flag is cleared. The following code example shows
how to flush the receive buffer.
Assembly Code Example(1)
USART_Flush:
sbis
ret
in
rjmp
UCSRnA, RXCn
r16, UDRn
USART_Flush
C Code Example(1)
void USART_Flush( void )
{
unsigned char dummy;
while ( UCSRnA & (1<<RXCn) ) dummy = UDRn;
}
Note:
18.7
1.
See “Code Examples” on page 8.
Asynchronous Data Reception
The USART includes a clock recovery and a data recovery unit for handling asynchronous data reception. The
clock recovery logic is used for synchronizing the internally generated baud rate clock to the incoming
asynchronous serial frames at the RxDn pin. The data recovery logic samples and low pass filters each
incoming bit, thereby improving the noise immunity of the Receiver. The asynchronous reception operational
range depends on the accuracy of the internal baud rate clock, the rate of the incoming frames, and the frame
size in number of bits.
18.7.1 Asynchronous Clock Recovery
The clock recovery logic synchronizes internal clock to the incoming serial frames. Figure 18-5 on page 201
illustrates the sampling process of the start bit of an incoming frame. The sample rate is 16 times the baud rate
for Normal mode, and eight times the baud rate for Double Speed mode. The horizontal arrows illustrate the
synchronization variation due to the sampling process. Note the larger time variation when using the Double
Speed mode (U2Xn = 1) of operation. Samples denoted zero are samples done when the RxDn line is idle (i.e.,
no communication activity).
Figure 18-5.
Start Bit Sampling
RxD
IDLE
START
BIT 0
Sample
(U2X = 0)
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
2
3
Sample
(U2X = 1)
0
1
2
3
4
5
6
7
8
1
2
When the clock recovery logic detects a high (idle) to low (start) transition on the RxDn line, the start bit
detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in Figure 18-5. The clock
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recovery logic then uses samples 8, 9, and 10 for Normal mode, and samples 4, 5, and 6 for Double Speed
mode (indicated with sample numbers inside boxes on the figure), to decide if a valid start bit is received. If two
or more of these three samples have logical high levels (the majority wins), the start bit is rejected as a noise
spike and the Receiver starts looking for the next high to low-transition. If however, a valid start bit is detected,
the clock recovery logic is synchronized and the data recovery can begin. The synchronization process is
repeated for each start bit.
18.7.2 Asynchronous Data Recovery
When the receiver clock is synchronized to the start bit, the data recovery can begin. The data recovery unit
uses a state machine that has 16 states for each bit in Normal mode and eight states for each bit in Double
Speed mode. Figure 18-6 shows the sampling of the data bits and the parity bit. Each of the samples is given a
number that is equal to the state of the recovery unit.
Figure 18-6.
Sampling of Data and Parity Bit
RxD
BIT n
Sample
(U2X = 0)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
Sample
(U2X = 1)
1
2
3
4
5
6
7
8
1
The decision of the logic level of the received bit is taken by doing a majority voting of the logic value to the three
samples in the center of the received bit. The center samples are emphasized on the figure by having the
sample number inside boxes. The majority voting process is done as follows: If two or all three samples have
high levels, the received bit is registered to be a logic 1. If two or all three samples have low levels, the received
bit is registered to be a logic 0. This majority voting process acts as a low pass filter for the incoming signal on
the RxDn pin. The recovery process is then repeated until a complete frame is received. Including the first stop
bit. Note that the Receiver only uses the first stop bit of a frame.
Figure 18-7 on page 202 shows the sampling of the stop bit and the earliest possible beginning of the start bit of
the next frame.
Figure 18-7.
Stop Bit Sampling and Next Start Bit Sampling
RxD
STOP 1
(A)
(B)
(C)
Sample
(U2X = 0)
1
2
3
4
5
6
7
8
9
10
0/1
0/1
0/1
Sample
(U2X = 1)
1
2
3
4
5
6
0/1
The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop bit is
registered to have a logic 0 value, the Frame Error (FEn) Flag will be set.
A new high to low transition indicating the start bit of a new frame can come right after the last of the bits used
for majority voting. For Normal Speed mode, the first low level sample can be at point marked (A) in Figure 187. For Double Speed mode the first low level must be delayed to (B). (C) marks a stop bit of full length. The early
start bit detection influences the operational range of the Receiver.
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18.7.3 Asynchronous Operational Range
The operational range of the Receiver is dependent on the mismatch between the received bit rate and the
internally generated baud rate. If the Transmitter is sending frames at too fast or too slow bit rates, or the
internally generated baud rate of the Receiver does not have a similar (see Table 18-6 on page 208) base
frequency, the Receiver will not be able to synchronize the frames to the start bit.
The following equations can be used to calculate the ratio of the incoming data rate and internal receiver baud
rate.
 D + 1 S
R slow = ------------------------------------------S – 1 + D  S + SF
 D + 2 S
R fast = ---------------------------------- D + 1 S + S M
D
Sum of character size and parity size (D = 5 to 10 bit)
S
Samples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed mode
SF
First sample number used for majority voting. SF = 8 for normal speed and SF = 4 for Double Speed mode
SM
Middle sample number used for majority voting. SM = 9 for normal speed and SM = 5 for Double Speed mode
Rslow
is the ratio of the slowest incoming data rate that can be accepted in relation to the receiver baud rate
Rfast
is the ratio of the fastest incoming data rate that can be accepted in relation to the receiver baud rate
The following tables list the maximum receiver baud rate error that can be tolerated. Note that Normal Speed
mode has higher toleration of baud rate variations.
Table 18-2.
Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2Xn = 0)
D
# (Data+Parity Bit)
Rslow [%]
Rfast [%]
Max. total error [%]
Recommended max. receiver
error [%]
5
93.20
106.67
+6.67/-6.8
±3.0
6
94.12
105.79
+5.79/-5.88
±2.5
7
94.81
105.11
+5.11/-5.19
±2.0
8
95.36
104.58
+4.58/-4.54
±2.0
9
95.81
104.14
+4.14/-4.19
±1.5
10
96.17
103.78
+3.78/-3.83
±1.5
Table 18-3.
Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2Xn = 1)
D
# (Data+Parity Bit)
Rslow [%]
Rfast [%]
Max. total error [%]
Recommended max. receiver
error [%]
5
94.12
105.66
+5.66/-5.88
±2.5
6
94.92
104.92
+4.92/-5.08
±2.0
7
95.52
104,35
+4.35/-4.48
±1.5
8
96.00
103.90
+3.90/-4.00
±1.5
9
96.39
103.53
+3.53/-3.61
±1.5
10
96.70
103.23
+3.23/-3.30
±1.0
The recommendations of the maximum receiver baud rate error was made under the assumption that the
Receiver and Transmitter equally divides the maximum total error.
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There are two possible sources for the receivers baud rate error. The Receiver’s system clock (XTAL) will
always have some minor instability over the supply voltage range and the temperature range. When using a
crystal to generate the system clock, this is rarely a problem, but for a resonator the system clock may differ
more than 2% depending of the resonators tolerance. The second source for the error is more controllable. The
baud rate generator can not always do an exact division of the system frequency to get the baud rate wanted. In
this case an UBRR value that gives an acceptable low error can be used if possible.
18.8
Multi-processor Communication Mode
Setting the Multi-processor Communication mode (MPCMn) bit in UCSRnA enables a filtering function of
incoming frames received by the USART Receiver. Frames that do not contain address information will be
ignored and not put into the receive buffer. This effectively reduces the number of incoming frames that has to
be handled by the CPU, in a system with multiple MCUs that communicate via the same serial bus. The
Transmitter is unaffected by the MPCMn setting, but has to be used differently when it is a part of a system
utilizing the Multi-processor Communication mode.
If the Receiver is set up to receive frames that contain 5 to 8 data bits, then the first stop bit indicates if the frame
contains data or address information. If the Receiver is set up for frames with nine data bits, then the ninth bit
(RXB8n) is used for identifying address and data frames. When the frame type bit (the first stop or the ninth bit)
is one, the frame contains an address. When the frame type bit is zero the frame is a data frame.
The Multi-processor Communication mode enables several slave MCUs to receive data from a master MCU.
This is done by first decoding an address frame to find out which MCU has been addressed. If a particular slave
MCU has been addressed, it will receive the following data frames as normal, while the other slave MCUs will
ignore the received frames until another address frame is received.
18.8.1 Using MPCMn
For an MCU to act as a master MCU, it can use a 9-bit character frame format (UCSZn = 7). The ninth bit
(TXB8n) must be set when an address frame (TXB8n = 1) or cleared when a data frame (TXB = 0) is being
transmitted. The slave MCUs must in this case be set to use a 9-bit character frame format.
The following procedure should be used to exchange data in Multi-processor Communication mode:
1. All Slave MCUs are in Multi-processor Communication mode (MPCMn in UCSRnA is set).
2.
The Master MCU sends an address frame, and all slaves receive and read this frame. In the Slave MCUs,
the RXCn Flag in UCSRnA will be set as normal.
3.
Each Slave MCU reads the UDRn Register and determines if it has been selected. If so, it clears the
MPCMn bit in UCSRnA, otherwise it waits for the next address byte and keeps the MPCMn setting.
4.
The addressed MCU will receive all data frames until a new address frame is received. The other Slave
MCUs, which still have the MPCMn bit set, will ignore the data frames.
5.
When the last data frame is received by the addressed MCU, the addressed MCU sets the MPCMn bit
and waits for a new address frame from master. The process then repeats from 2.
Using any of the 5- to 8-bit character frame formats is possible, but impractical since the Receiver must change
between using n and n+1 character frame formats. This makes full-duplex operation difficult since the
Transmitter and Receiver uses the same character size setting. If 5- to 8-bit character frames are used, the
Transmitter must be set to use two stop bit (USBSn = 1) since the first stop bit is used for indicating the frame
type.
Do not use Read-Modify-Write instructions (SBI and CBI) to set or clear the MPCMn bit. The MPCMn bit shares
the same I/O location as the TXCn Flag and this might accidentally be cleared when using SBI or CBI
instructions.
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18.9
Hardware Flow Control
The hardware flow control can be enabled by software.
CTS: (Clear to Send)
RTS: (Request to Send)
HOST
ATmega16U4/ATm
TXD
TXD
RXD
RXD
CTS
CTS
RTS
RTS
18.9.1 Receiver Flow Control
The reception flow can be controlled by hardware using the RTS pin. The aim of the flow control is to inform the
external transmitter when the internal receive Fifo is full. Thus the transmitter can stop sending characters. RTS
usage and so associated flow control is enabled using RTSEN bit in UCSRnD. Figure 18-8. shows a reception
example.
Figure 18-8.
Reception Flow Control Waveform Example
FIFO
Index
0
1
2 1
0
1
CPU Read
C1 C2
RXD
C3
RTS
Figure 18-9.
RTS behavior
RXD
Start
Byte0
Stop
Start
Byte1
Stop
Start
Byte2
1 additional byte may be sent
if the transmitter misses the RTS trig
RTS
Read from CPU
RTS will rise at 2/3 of the last received stop bit if the receive fifo is full.
To ensure reliable transmissions, even after a RTS rise, an extra-data can still be received and stored in the
Receive Shift Register.
18.9.2 Transmission Flow Control
The transmission flow can be controlled by hardware using the CTS pin controlled by the external receiver. The
aim of the flow control is to stop transmission when the receiver is full of data (CTS = 1). CTS usage and so
associated flow control is enabled using CTSEN bit in UCSRnD. The CTS pin is sampled at each CPU write and
at the middle of the last stop bit that is currently being sent.
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Figure 18-10. CTS behavior
Write from CPU
TXD
Start
Byte0
sample
Stop
Start
Byte1
sample
Stop
Start
Byte2
sample
CTS
18.10 Examples of Baud Rate Setting
For standard crystal and resonator frequencies, the most commonly used baud rates for asynchronous
operation can be generated by using the UBRR settings in Table 18-4 on page 206 to Table 18-11 on page 212.
UBRR values which yield an actual baud rate differing less than 0.5% from the target baud rate, are bold in the
table. Higher error ratings are acceptable, but the Receiver will have less noise resistance when the error
ratings are high, especially for large serial frames (see “Asynchronous Operational Range” on page 203). The
error values are calculated using the following equation:
BaudRate Closest Match
Error[%] =  -------------------------------------------------------- – 1  100%


BaudRate
Table 18-4.
Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
fosc = 1.0000MHz
fosc = 1.8432MHz
fosc = 2.0000MHz
Baud
Rate
[bps]
UBRR
2400
25
0.2%
51
0.2%
47
0.0%
95
0.0%
51
0.2%
103
0.2%
4800
12
0.2%
25
0.2%
23
0.0%
47
0.0%
25
0.2%
51
0.2%
9600
6
-7.0%
12
0.2%
11
0.0%
23
0.0%
12
0.2%
25
0.2%
14.4k
3
8.5%
8
-3.5%
7
0.0%
15
0.0%
8
-3.5%
16
2.1%
19.2k
2
8.5%
6
-7.0%
5
0.0%
11
0.0%
6
-7.0%
12
0.2%
28.8k
1
8.5%
3
8.5%
3
0.0%
7
0.0%
3
8.5%
8
-3.5%
38.4k
1
-18.6%
2
8.5%
2
0.0%
5
0.0%
2
8.5%
6
-7.0%
57.6k
0
8.5%
1
8.5%
1
0.0%
3
0.0%
1
8.5%
3
8.5%
76.8k
–
–
1
-18.6%
1
-25.0%
2
0.0%
1
-18.6%
2
8.5%
115.2k
–
–
0
8.5%
0
0.0%
1
0.0%
0
8.5%
1
8.5%
230.4k
–
–
–
–
–
–
0
0.0%
–
–
–
–
250k
–
–
–
–
–
–
–
–
–
–
0
0.0%
Max.(1)
Note:
U2Xn = 0
Error
U2Xn = 1
UBRR
62.5kbps
Error
125kbps
U2Xn = 0
UBRR
Error
115.2kbps
U2Xn = 1
UBRR
Error
230.4kbps
U2Xn = 0
UBRR
Error
125kbps
U2Xn = 1
UBRR
Error
250kbps
1. UBRR = 0, Error = 0.0%
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Table 18-5.
Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
fosc = 3.6864MHz
Baud
Rate
[bps]
U2Xn = 0
UBRR
fosc = 4.0000MHz
U2Xn = 1
Error
UBRR
U2Xn = 0
Error
UBRR
fosc = 7.3728MHz
U2Xn = 1
Error
UBRR
U2Xn = 0
Error
UBRR
U2Xn = 1
Error
UBRR
Error
2400
95
0.0%
191
0.0%
103
0.2%
207
0.2%
191
0.0%
383
0.0%
4800
47
0.0%
95
0.0%
51
0.2%
103
0.2%
95
0.0%
191
0.0%
9600
23
0.0%
47
0.0%
25
0.2%
51
0.2%
47
0.0%
95
0.0%
14.4k
15
0.0%
31
0.0%
16
2.1%
34
-0.8%
31
0.0%
63
0.0%
19.2k
11
0.0%
23
0.0%
12
0.2%
25
0.2%
23
0.0%
47
0.0%
28.8k
7
0.0%
15
0.0%
8
-3.5%
16
2.1%
15
0.0%
31
0.0%
38.4k
5
0.0%
11
0.0%
6
-7.0%
12
0.2%
11
0.0%
23
0.0%
57.6k
3
0.0%
7
0.0%
3
8.5%
8
-3.5%
7
0.0%
15
0.0%
76.8k
2
0.0%
5
0.0%
2
8.5%
6
-7.0%
5
0.0%
11
0.0%
115.2k
1
0.0%
3
0.0%
1
8.5%
3
8.5%
3
0.0%
7
0.0%
230.4k
0
0.0%
1
0.0%
0
8.5%
1
8.5%
1
0.0%
3
0.0%
250k
0
-7.8%
1
-7.8%
0
0.0%
1
0.0%
1
-7.8%
3
-7.8%
0.5M
–
–
0
-7.8%
–
–
0
0.0%
0
-7.8%
1
-7.8%
1M
–
–
–
–
–
–
–
–
–
–
0
-7.8%
Max.(1)
Note:
230.4kbps
460.8kbps
250kbps
0.5Mbps
460.8kbps
921.6kbps
1. UBRR = 0, Error = 0.0%
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Table 18-6.
Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
fosc = 11.0592MHz
fosc = 8.0000MHz
fosc = 14.7456MHz
Baud
Rate
[bps]
UBRR
2400
207
0.2%
416
-0.1%
287
0.0%
575
0.0%
383
0.0%
767
0.0%
4800
103
0.2%
207
0.2%
143
0.0%
287
0.0%
191
0.0%
383
0.0%
9600
51
0.2%
103
0.2%
71
0.0%
143
0.0%
95
0.0%
191
0.0%
14.4k
34
-0.8%
68
0.6%
47
0.0%
95
0.0%
63
0.0%
127
0.0%
19.2k
25
0.2%
51
0.2%
35
0.0%
71
0.0%
47
0.0%
95
0.0%
28.8k
16
2.1%
34
-0.8%
23
0.0%
47
0.0%
31
0.0%
63
0.0%
38.4k
12
0.2%
25
0.2%
17
0.0%
35
0.0%
23
0.0%
47
0.0%
57.6k
8
-3.5%
16
2.1%
11
0.0%
23
0.0%
15
0.0%
31
0.0%
76.8k
6
-7.0%
12
0.2%
8
0.0%
17
0.0%
11
0.0%
23
0.0%
115.2k
3
8.5%
8
-3.5%
5
0.0%
11
0.0%
7
0.0%
15
0.0%
230.4k
1
8.5%
3
8.5%
2
0.0%
5
0.0%
3
0.0%
7
0.0%
250k
1
0.0%
3
0.0%
2
-7.8%
5
-7.8%
3
-7.8%
6
5.3%
0.5M
0
0.0%
1
0.0%
–
–
2
-7.8%
1
-7.8%
3
-7.8%
1M
–
–
0
0.0%
–
–
–
–
0
-7.8%
1
-7.8%
Max.(1)
Note:
U2Xn = 0
Error
U2Xn = 1
UBRR
0.5Mbps
Error
1Mbps
U2Xn = 0
UBRR
Error
691.2kbps
U2Xn = 1
UBRR
Error
1.3824Mbps
U2Xn = 0
UBRR
Error
921.6kbps
U2Xn = 1
UBRR
Error
1.8432Mbps
1. UBRR = 0, Error = 0.0%
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18.11 USART Register Description
18.11.1 USART I/O Data Register n– UDRn
Bit
Read/Write
Initial Value
7
RXB[7:0]
TXB[7:0]
R/W
0
6
5
4
3
2
1
0
UDRn (Read)
UDRn (Write)
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
The USART Transmit Data Buffer Register and USART Receive Data Buffer Registers share the same I/O
address referred to as USART Data Register or UDRn. The Transmit Data Buffer Register (TXB) will be the
destination for data written to the UDRn Register location. Reading the UDRn Register location will return the
contents of the Receive Data Buffer Register (RXB).
For 5-, 6-, or 7-bit characters the upper unused bits will be ignored by the Transmitter and set to zero by the
Receiver.
The transmit buffer can only be written when the UDREn Flag in the UCSRnA Register is set. Data written to
UDRn when the UDREn Flag is not set, will be ignored by the USART Transmitter. When data is written to the
transmit buffer, and the Transmitter is enabled, the Transmitter will load the data into the Transmit Shift Register
when the Shift Register is empty. Then the data will be serially transmitted on the TxDn pin.
The receive buffer consists of a two level FIFO. The FIFO will change its state whenever the receive buffer is
accessed. Due to this behavior of the receive buffer, do not use Read-Modify-Write instructions (SBI and CBI)
on this location. Be careful when using bit test instructions (SBIC and SBIS), since these also will change the
state of the FIFO.
18.11.2 USART Control and Status Register A – UCSRnA
Bit
Read/Write
Initial Value
7
RXCn
R
0
6
TXCn
R/W
0
5
UDREn
R
1
4
FEn
R
0
3
DORn
R
0
2
UPEn
R
0
1
U2Xn
R/W
0
0
MPCMn
R/W
0
UCSRnA
• Bit 7 – RXCn: USART Receive Complete
This flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is empty
(i.e., does not contain any unread data). If the Receiver is disabled, the receive buffer will be flushed and
consequently the RXCn bit will become zero. The RXCn Flag can be used to generate a Receive Complete
interrupt (see description of the RXCIEn bit).
• Bit 6 – TXCn: USART Transmit Complete
This flag bit is set when the entire frame in the Transmit Shift Register has been shifted out and there are no
new data currently present in the transmit buffer (UDRn). The TXCn Flag bit is automatically cleared when a
transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. The TXCn Flag
can generate a Transmit Complete interrupt (see description of the TXCIEn bit).
• Bit 5 – UDREn: USART Data Register Empty
The UDREn Flag indicates if the transmit buffer (UDRn) is ready to receive new data. If UDREn is one, the
buffer is empty, and therefore ready to be written. The UDREn Flag can generate a Data Register Empty
interrupt (see description of the UDRIEn bit).
UDREn is set after a reset to indicate that the Transmitter is ready.
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• Bit 4 – FEn: Frame Error
This bit is set if the next character in the receive buffer had a Frame Error when received. I.e., when the first stop
bit of the next character in the receive buffer is zero. This bit is valid until the receive buffer (UDRn) is read. The
FEn bit is zero when the stop bit of received data is one. Always set this bit to zero when writing to UCSRnA.
• Bit 3 – DORn: Data OverRun
This bit is set if a Data OverRun condition is detected. A Data OverRun occurs when the receive buffer is full
(two characters), it is a new character waiting in the Receive Shift Register, and a new start bit is detected. This
bit is valid until the receive buffer (UDRn) is read. Always set this bit to zero when writing to UCSRnA.
• Bit 2 – UPEn: USART Parity Error
This bit is set if the next character in the receive buffer had a Parity Error when received and the Parity Checking
was enabled at that point (UPMn1 = 1). This bit is valid until the receive buffer (UDRn) is read. Always set this
bit to zero when writing to UCSRnA.
• Bit 1 – U2Xn: Double the USART Transmission Speed
This bit only has effect for the asynchronous operation. Write this bit to zero when using synchronous operation.
Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively doubling the transfer
rate for asynchronous communication.
• Bit 0 – MPCMn: Multi-processor Communication Mode
This bit enables the Multi-processor Communication mode. When the MPCMn bit is written to one, all the
incoming frames received by the USART Receiver that do not contain address information will be ignored. The
Transmitter is unaffected by the MPCMn setting. For more detailed information see “Multi-processor
Communication Mode” on page 204.
18.11.3 USART Control and Status Register n B – UCSRnB
Bit
Read/Write
Initial Value
7
RXCIEn
R/W
0
6
TXCIEn
R/W
0
5
UDRIEn
R/W
0
4
RXENn
R/W
0
3
TXENn
R/W
0
2
UCSZn2
R/W
0
1
RXB8n
R
0
0
TXB8n
R/W
0
UCSRnB
• Bit 7 – RXCIEn: RX Complete Interrupt Enable n
Writing this bit to one enables interrupt on the RXCn Flag. A USART Receive Complete interrupt will be
generated only if the RXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the
RXCn bit in UCSRnA is set.
• Bit 6 – TXCIEn: TX Complete Interrupt Enable n
Writing this bit to one enables interrupt on the TXCn Flag. A USART Transmit Complete interrupt will be
generated only if the TXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the
TXCn bit in UCSRnA is set.
• Bit 5 – UDRIEn: USART Data Register Empty Interrupt Enable n
Writing this bit to one enables interrupt on the UDREn Flag. A Data Register Empty interrupt will be generated
only if the UDRIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the UDREn bit in
UCSRnA is set.
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• Bit 4 – RXENn: Receiver Enable n
Writing this bit to one enables the USART Receiver. The Receiver will override normal port operation for the
RxDn pin when enabled. Disabling the Receiver will flush the receive buffer invalidating the FEn, DORn, and
UPEn Flags.
• Bit 3 – TXENn: Transmitter Enable n
Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port operation for
the TxDn pin when enabled. The disabling of the Transmitter (writing TXENn to zero) will not become effective
until ongoing and pending transmissions are completed, i.e., when the Transmit Shift Register and Transmit
Buffer Register do not contain data to be transmitted. When disabled, the Transmitter will no longer override the
TxDn port.
• Bit 2 – UCSZn2: Character Size n
The UCSZn2 bits combined with the UCSZn1:0 bit in UCSRnC sets the number of data bits (Character SiZe) in
a frame the Receiver and Transmitter use.
• Bit 1 – RXB8n: Receive Data Bit 8 n
RXB8n is the ninth data bit of the received character when operating with serial frames with nine data bits. Must
be read before reading the low bits from UDRn.
• Bit 0 – TXB8n: Transmit Data Bit 8 n
TXB8n is the ninth data bit in the character to be transmitted when operating with serial frames with nine data
bits. Must be written before writing the low bits to UDRn.
18.11.4 USART Control and Status Register n C – UCSRnC
Bit
7
UMSELn1
R/W
0
Read/Write
Initial Value
6
UMSELn0
R/W
0
5
UPMn1
R/W
0
4
UPMn0
R/W
0
3
USBSn
R/W
0
2
UCSZn1
R/W
1
1
UCSZn0
R/W
1
0
UCPOLn
R/W
0
UCSRnC
• Bits 7:6 – UMSELn1:0 USART Mode Select
These bits select the mode of operation of the USARTn as shown below.
Table 18-7.
Note:
UMSELn Bit Settings
UMSELn1
UMSELn0
0
0
Asynchronous USART
0
1
Synchronous USART
1
0
(Reserved)
1
1
Master SPI (MSPIM)(1)
1.
Mode
See “USART in SPI Mode” on page 214 for full description of the Master SPI Mode (MSPIM) operation
• Bits 5:4 – UPMn1:0: Parity Mode
These bits enable and set type of parity generation and check. If enabled, the Transmitter will automatically
generate and send the parity of the transmitted data bits within each frame. The Receiver will generate a parity
value for the incoming data and compare it to the UPMn setting. If a mismatch is detected, the UPEn Flag in
UCSRnA will be set.
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Table 18-8.
UPMn Bit Settings
UPMn1
UPMn0
Parity Mode
0
0
Disabled
0
1
Reserved
1
0
Enabled, Even Parity
1
1
Enabled, Odd Parity
• Bit 3 – USBSn: Stop Bit Select
This bit selects the number of stop bits to be inserted by the Transmitter. The Receiver ignores this setting.
Table 18-9.
USBS Bit Settings
USBSn
Stop Bit(s)
0
1-bit
1
2-bit
• Bit 2:1 – UCSZn1:0: Character Size
The UCSZn1:0 bits combined with the UCSZn2 bit in UCSRnB sets the number of data bits (Character SiZe) in
a frame the Receiver and Transmitter use.
Table 18-10.
UCSZn Bit Settings
UCSZn2
UCSZn1
UCSZn0
Character Size
0
0
0
5-bit
0
0
1
6-bit
0
1
0
7-bit
0
1
1
8-bit
1
0
0
Reserved
1
0
1
Reserved
1
1
0
Reserved
1
1
1
9-bit
• Bit 0 – UCPOLn: Clock Polarity
This bit is used for synchronous mode only. Write this bit to zero when asynchronous mode is used. The
UCPOLn bit sets the relationship between data output change and data input sample, and the synchronous
clock (XCKn).
Table 18-11.
UCPOLn
UCPOLn Bit Settings
Transmitted Data Changed (Output of TxDn Pin)
Received Data Sampled (Input on RxDn Pin)
0
Rising XCKn Edge
Falling XCKn Edge
1
Falling XCKn Edge
Rising XCKn Edge
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18.11.5 USART Control and Status Register n D– UCSRnD
Bit
7
6
5
–
Read/Write
Initial Value
R
0
4
–
R
0
3
–
R
0
–
R
0
2
–
R
0
–
R
0
1
CTSEN
R/W
0
0
RTSEN
R/W
0
UCSRnD
• Bits 7:2 – Reserved bits
These bits are reserved and will be read as ‘0’. Do not set these bits.
• Bits 1 – CTSEN: UART CTS Signal Enable
Set this bit by firmware to enable the transmission flow control signal (CTS). Transmission will be enabled only if
CTS input = 0. Clear this bit to disable the transmission flow control signal. Transmission will occur without
hardware condition. Data Direction Register bit must be correctly clear to enable the pin as an input.
• Bits 0 – RTSEN: UART RTS Signal Enable
Set this bit by firmware to enable the reception flow control signal (RTS). In this case the RTS line will
automatically rise when the FIFO is full. Clear this bit to disable the reception flow control signal. Data Direction
Register bit must be correctly set to enable the pin as an output.
18.11.6 USART Baud Rate Registers – UBRRLn and UBRRHn
Bit
15
Read/Write
–
UBRR[7:0]
7
R
R/W
0
0
Initial Value
14
13
–
6
R
R/W
0
0
12
–
5
R
R/W
0
0
–
4
R
R/W
0
0
11
10
UBRR[11:8]
9
3
R/W
R/W
0
0
1
R/W
R/W
0
0
2
R/W
R/W
0
0
8
UBRRHn
UBRRLn
0
R/W
R/W
0
0
• Bit 15:12 – Reserved Bits
These bits are reserved for future use. For compatibility with future devices, these bit must be written to zero
when UBRRH is written.
• Bit 11:0 – UBRR11:0: USART Baud Rate Register
This is a 12-bit register which contains the USART baud rate. The UBRRH contains the four most significant
bits, and the UBRRL contains the eight least significant bits of the USART baud rate. Ongoing transmissions by
the Transmitter and Receiver will be corrupted if the baud rate is changed. Writing UBRRL will trigger an
immediate update of the baud rate prescaler.
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19.
USART in SPI Mode
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) can be set to a master
SPI compliant mode of operation. The Master SPI Mode (MSPIM) has the following features:
•
•
•
•
•
•
•
•
19.1
Full Duplex, Three-wire Synchronous Data Transfer
Master Operation
Supports all four SPI Modes of Operation (Mode 0, 1, 2, and 3)
LSB First or MSB First Data Transfer (Configurable Data Order)
Queued Operation (Double Buffered)
High Resolution Baud Rate Generator
High Speed Operation (fXCKmax = fCK/2)
Flexible Interrupt Generation
Overview
Setting both UMSELn1:0 bits to one enables the USART in MSPIM logic. In this mode of operation the SPI
master control logic takes direct control over the USART resources. These resources include the transmitter
and receiver shift register and buffers, and the baud rate generator. The parity generator and checker, the data
and clock recovery logic, and the RX and TX control logic is disabled. The USART RX and TX control logic is
replaced by a common SPI transfer control logic. However, the pin control logic and interrupt generation logic is
identical in both modes of operation.
The I/O register locations are the same in both modes. However, some of the functionality of the control
registers changes when using MSPIM.
19.2
Clock Generation
The Clock Generation logic generates the base clock for the Transmitter and Receiver. For USART MSPIM
mode of operation only internal clock generation (i.e. master operation) is supported. The Data Direction
Register for the XCKn pin (DDR_XCKn) must therefore be set to one (i.e. as output) for the USART in MSPIM to
operate correctly. Preferably the DDR_XCKn should be set up before the USART in MSPIM is enabled (i.e.
TXENn and RXENn bit set to one).
The internal clock generation used in MSPIM mode is identical to the USART synchronous master mode. The
baud rate or UBRRn setting can therefore be calculated using the same equations in the following table:
Operating Mode
Synchronous Master
mode
Note:
1.
Equation for Calculating Baud
Rate(1)
Equation for Calculating UBRRn
Value
f OSC
BAUD = --------------------------------------2  UBRRn + 1 
f OSC
UBRRn = -------------------- – 1
2 BAUD
The baud rate is defined to be the transfer rate in bit per second (bps).
BAUD: Baud rate (in bits per second, bps).
fOSC: System Oscillator clock frequency.
UBRRn: Contents of the UBRRnH and UBRRnL Registers, (0-4095).
19.3
SPI Data Modes and Timing
There are four combinations of XCKn (SCK) phase and polarity with respect to serial data, which are
determined by control bits UCPHAn and UCPOLn. The data transfer timing diagrams are shown in Figure 19-1.
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Data bits are shifted out and latched in on opposite edges of the XCKn signal, ensuring sufficient time for data
signals to stabilize. The UCPOLn and UCPHAn functionality is summarized in the following table.
Note that changing the setting of any of these bits will corrupt all ongoing communication for both the Receiver
and Transmitter.
Table 19-1.
UCPOLn and UCPHAn Functionality
UCPOLn
UCPHAn
SPI Mode
Leading Edge
Trailing Edge
0
0
0
Sample (Rising)
Setup (Falling)
0
1
1
Setup (Rising)
Sample (Falling)
1
0
2
Sample (Falling)
Setup (Rising)
1
1
3
Setup (Falling)
Sample (Rising)
Figure 19-1.
UCPHAn and UCPOLn data transfer timing diagrams.
UCPHA=0
UCPHA=1
UCPOL=0
19.4
UCPOL=1
XCK
XCK
Data setup (TXD)
Data setup (TXD)
Data sample (RXD)
Data sample (RXD)
XCK
XCK
Data setup (TXD)
Data setup (TXD)
Data sample (RXD)
Data sample (RXD)
Frame Formats
A serial frame for the MSPIM is defined to be one character of 8 data bits. The USART in MSPIM mode has two
valid frame formats:

8-bit data with MSB first

8-bit data with LSB first
A frame starts with the least or most significant data bit. Then the next data bits, up to a total of eight, are
succeeding, ending with the most or least significant bit accordingly. When a complete frame is transmitted, a
new frame can directly follow it, or the communication line can be set to an idle (high) state.
The UDORDn bit in UCSRnC sets the frame format used by the USART in MSPIM mode. The Receiver and
Transmitter use the same setting. Note that changing the setting of any of these bits will corrupt all ongoing
communication for both the Receiver and Transmitter.
16-bit data transfer can be achieved by writing two data bytes to UDRn. A UART transmit complete interrupt will
then signal that the 16-bit value has been shifted out.
19.4.1 USART MSPIM Initialization
The USART in MSPIM mode has to be initialized before any communication can take place. The initialization
process normally consists of setting the baud rate, setting master mode of operation (by setting DDR_XCKn to
one), setting frame format and enabling the Transmitter and the Receiver. Only the transmitter can operate
independently. For interrupt driven USART operation, the Global Interrupt Flag should be cleared (and thus
interrupts globally disabled) when doing the initialization.
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Note:
To ensure immediate initialization of the XCKn output the baud-rate register (UBRRn) must be zero at the time the
transmitter is enabled. Contrary to the normal mode USART operation the UBRRn must then be written to the
desired value after the transmitter is enabled, but before the first transmission is started. Setting UBRRn to zero
before enabling the transmitter is not necessary if the initialization is done immediately after a reset since UBRRn is
reset to zero.
Before doing a re-initialization with changed baud rate, data mode, or frame format, be sure that there is no
ongoing transmissions during the period the registers are changed. The TXCn Flag can be used to check that
the Transmitter has completed all transfers, and the RXCn Flag can be used to check that there are no unread
data in the receive buffer. Note that the TXCn Flag must be cleared before each transmission (before UDRn is
written) if it is used for this purpose.
The following simple USART initialization code examples show one assembly and one C function that are equal
in functionality. The examples assume polling (no interrupts enabled). The baud rate is given as a function
parameter. For the assembly code, the baud rate parameter is assumed to be stored in the r17:r16 registers.
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Assembly Code Example(1)
USART_Init:
clr r18
out UBRRnH,r18
out UBRRnL,r18
; Setting the XCKn port pin as output, enables
master mode.
sbi XCKn_DDR, XCKn
; Set MSPI mode of operation and SPI data mode 0.
ldi r18,
(1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn)
out UCSRnC,r18
; Enable receiver and transmitter.
ldi r18, (1<<RXENn)|(1<<TXENn)
out UCSRnB,r18
; Set baud rate.
; IMPORTANT: The Baud Rate must be set after the
transmitter is enabled!
out UBRRnH, r17
out UBRRnL, r18
ret
C Code Example(1)
void USART_Init( unsigned int baud )
{
UBRRn = 0;
/* Setting the XCKn port pin as output, enables
master mode. */
XCKn_DDR |= (1<<XCKn);
/* Set MSPI mode of operation and SPI data mode 0.
*/
UCSRnC =
(1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn);
/* Enable receiver and transmitter. */
UCSRnB = (1<<RXENn)|(1<<TXENn);
/* Set baud rate. */
/* IMPORTANT: The Baud Rate must be set after the
transmitter is enabled */
UBRRn = baud;
}
Note:
1.
See “Code Examples” on page 8.
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19.5
Data Transfer
Using the USART in MSPI mode requires the Transmitter to be enabled, i.e. the TXENn bit in the UCSRnB
register is set to one. When the Transmitter is enabled, the normal port operation of the TxDn pin is overridden
and given the function as the Transmitter's serial output. Enabling the receiver is optional and is done by setting
the RXENn bit in the UCSRnB register to one. When the receiver is enabled, the normal pin operation of the
RxDn pin is overridden and given the function as the Receiver's serial input. The XCKn will in both cases be
used as the transfer clock.
After initialization the USART is ready for doing data transfers. A data transfer is initiated by writing to the UDRn
I/O location. This is the case for both sending and receiving data since the transmitter controls the transfer
clock. The data written to UDRn is moved from the transmit buffer to the shift register when the shift register is
ready to send a new frame.
Note:
To keep the input buffer in sync with the number of data bytes transmitted, the UDRn register must be read once for
each byte transmitted. The input buffer operation is identical to normal USART mode, i.e. if an overflow occurs the
character last received will be lost, not the first data in the buffer. This means that if four bytes are transferred, byte
1 first, then byte 2, 3, and 4, and the UDRn is not read before all transfers are completed, then byte 3 to be received
will be lost, and not byte 1.
The following code examples show a simple USART in MSPIM mode transfer function based on polling of the
Data Register Empty (UDREn) Flag and the Receive Complete (RXCn) Flag. The USART has to be initialized
before the function can be used. For the assembly code, the data to be sent is assumed to be stored in Register
R16 and the data received will be available in the same register (R16) after the function returns.
The function simply waits for the transmit buffer to be empty by checking the UDREn Flag, before loading it with
new data to be transmitted. The function then waits for data to be present in the receive buffer by checking the
RXCn Flag, before reading the buffer and returning the value.
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Assembly Code Example(1)
USART_MSPIM_Transfer:
; Wait for empty transmit buffer
sbis UCSRnA, UDREn
rjmp USART_MSPIM_Transfer
; Put data (r16) into buffer, sends the data
out UDRn,r16
; Wait for data to be received
USART_MSPIM_Wait_RXCn:
sbis UCSRnA, RXCn
rjmp USART_MSPIM_Wait_RXCn
; Get and return received data from buffer
in r16, UDRn
ret
C Code Example(1)
unsigned char USART_Receive( void )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn)) );
/* Put data into buffer, sends the data */
UDRn = data;
/* Wait for data to be received */
while ( !(UCSRnA & (1<<RXCn)) );
/* Get and return received data from buffer */
return UDRn;
}
Note:
1.
See “Code Examples” on page 8.
19.5.1 Transmitter and Receiver Flags and Interrupts
The RXCn, TXCn, and UDREn flags and corresponding interrupts in USART in MSPIM mode are identical in
function to the normal USART operation. However, the receiver error status flags (FE, DOR, and PE) are not in
use and is always read as zero.
19.5.2 Disabling the Transmitter or Receiver
The disabling of the transmitter or receiver in USART in MSPIM mode is identical in function to the normal
USART operation.
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19.6
AVR USART MSPIM vs. AVR SPI
The USART in MSPIM mode is fully compatible with the AVR SPI regarding:

Master mode timing diagram

The UCPOLn bit functionality is identical to the SPI CPOL bit

The UCPHAn bit functionality is identical to the SPI CPHA bit

The UDORDn bit functionality is identical to the SPI DORD bit
However, since the USART in MSPIM mode reuses the USART resources, the use of the USART in MSPIM
mode is somewhat different compared to the SPI. In addition to differences of the control register bits, and that
only master operation is supported by the USART in MSPIM mode, the following features differ between the two
modules:

The USART in MSPIM mode includes (double) buffering of the transmitter. The SPI has no buffer.

The USART in MSPIM mode receiver includes an additional buffer level

The SPI WCOL (Write Collision) bit is not included in USART in MSPIM mode

The SPI double speed mode (SPI2X) bit is not included. However, the same effect is achieved by setting
UBRRn accordingly

Interrupt timing is not compatible

Pin control differs due to the master only operation of the USART in MSPIM mode
A comparison of the USART in MSPIM mode and the SPI pins is shown in the table below.
Table 19-2.
Comparison of USART in MSPIM mode and SPI pins
USART_MSPIM
SPI
Comment
TxDn
MOSI
Master Out only
RxDn
MISO
Master In only
XCKn
SCK
(Functionally identical)
(N/A)
SS
Not supported by USART in MSPIM
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19.7
USART MSPIM Register Description
The following section describes the registers used for SPI operation using the USART.
19.7.1 USART MSPIM I/O Data Register - UDRn
The function and bit description of the USART data register (UDRn) in MSPI mode is identical to normal USART
operation. See “USART I/O Data Register n– UDRn” on page 209.
19.7.2 USART MSPIM Control and Status Register n A - UCSRnA
Bit
7
6
5
4
3
2
1
0
RXCn
TXCn
UDREn
-
-
-
-
-
Read/Write
R/W
R/W
R/W
R
R
R
R
R
Initial Value
0
0
0
0
0
1
1
0
UCSRnA
• Bit 7 - RXCn: USART Receive Complete
This flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is empty
(i.e., does not contain any unread data). If the Receiver is disabled, the receive buffer will be flushed and
consequently the RXCn bit will become zero. The RXCn Flag can be used to generate a Receive Complete
interrupt (see description of the RXCIEn bit).
• Bit 6 - TXCn: USART Transmit Complete
This flag bit is set when the entire frame in the Transmit Shift Register has been shifted out and there are no
new data currently present in the transmit buffer (UDRn). The TXCn Flag bit is automatically cleared when a
transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. The TXCn Flag
can generate a Transmit Complete interrupt (see description of the TXCIEn bit).
• Bit 5 - UDREn: USART Data Register Empty
The UDREn Flag indicates if the transmit buffer (UDRn) is ready to receive new data. If UDREn is one, the
buffer is empty, and therefore ready to be written. The UDREn Flag can generate a Data Register Empty
interrupt (see description of the UDRIE bit). UDREn is set after a reset to indicate that the Transmitter is ready.
• Bit 4:0 - Reserved Bits in MSPI mode
When in MSPI mode, these bits are reserved for future use. For compatibility with future devices, these bits
must be written to zero when UCSRnA is written.
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19.7.3 USART MSPIM Control and Status Register n B - UCSRnB
Bit
Read/Write
Initial Value
7
RXCIEn
R/W
0
6
TXCIEn
R/W
0
5
UDRIE
R/W
0
4
RXENn
R/W
0
3
TXENn
R/W
0
2
R
1
1
R
1
0
R
0
UCSRnB
• Bit 7 - RXCIEn: RX Complete Interrupt Enable
Writing this bit to one enables interrupt on the RXCn Flag. A USART Receive Complete interrupt will be
generated only if the RXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the
RXCn bit in UCSRnA is set.
• Bit 6 - TXCIEn: TX Complete Interrupt Enable
Writing this bit to one enables interrupt on the TXCn Flag. A USART Transmit Complete interrupt will be
generated only if the TXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the
TXCn bit in UCSRnA is set.
• Bit 5 - UDRIE: USART Data Register Empty Interrupt Enable
Writing this bit to one enables interrupt on the UDREn Flag. A Data Register Empty interrupt will be generated
only if the UDRIE bit is written to one, the Global Interrupt Flag in SREG is written to one and the UDREn bit in
UCSRnA is set.
• Bit 4 - RXENn: Receiver Enable
Writing this bit to one enables the USART Receiver in MSPIM mode. The Receiver will override normal port
operation for the RxDn pin when enabled. Disabling the Receiver will flush the receive buffer. Only enabling the
receiver in MSPI mode (i.e. setting RXENn=1 and TXENn=0) has no meaning since it is the transmitter that
controls the transfer clock and since only master mode is supported.
• Bit 3 - TXENn: Transmitter Enable
Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port operation for
the TxDn pin when enabled. The disabling of the Transmitter (writing TXENn to zero) will not become effective
until ongoing and pending transmissions are completed, i.e., when the Transmit Shift Register and Transmit
Buffer Register do not contain data to be transmitted. When disabled, the Transmitter will no longer override the
TxDn port.
• Bit 2:0 - Reserved Bits in MSPI mode
When in MSPI mode, these bits are reserved for future use. For compatibility with future devices, these bits
must be written to zero when UCSRnB is written.
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19.7.4 USART MSPIM Control and Status Register n C - UCSRnC
Bit
Read/Write
Initial Value
7
UMSELn1
R/W
0
6
UMSELn0
R/W
0
5
R
0
4
R
0
3
R
0
2
UDORDn
R/W
1
1
UCPHAn
R/W
1
0
UCPOLn
R/W
0
UCSRnC
• Bit 7:6 - UMSELn1:0: USART Mode Select
These bits select the mode of operation of the USART as shown in the table. See “USART Control and Status
Register n C – UCSRnC” on page 211 for full description of the normal USART operation. The MSPIM is
enabled when both UMSELn bits are set to one. The UDORDn, UCPHAn, and UCPOLn can be set in the same
write operation where the MSPIM is enabled.
Table 19-3.
UMSELn Bits Settings
UMSELn1
UMSELn0
Mode
0
0
Asynchronous USART
0
1
Synchronous USART
1
0
(Reserved)
1
1
Master SPI (MSPIM)
• Bit 5:3 - Reserved
When in MSPI mode, these bits are reserved for future use. For compatibility with future devices, these bits
must be written to zero when UCSRnC is written.
• Bit 2 - UDORDn: Data Order
When set to one the LSB of the data word is transmitted first. When set to zero the MSB of the data word is
transmitted first. For details, see “Frame Formats” on page 192.
• Bit 1 - UCPHAn: Clock Phase
The UCPHAn bit setting determine if data is sampled on the leasing edge (first) or tailing (last) edge of XCKn.
For details, see “SPI Data Modes and Timing” on page 214.
• Bit 0 - UCPOLn: Clock Polarity
The UCPOLn bit sets the polarity of the XCKn clock. The combination of the UCPOLn and UCPHAn bit settings
determine the timing of the data transfer. For details, see “SPI Data Modes and Timing” on page 214.
19.7.5 USART MSPIM Baud Rate Registers - UBRRnL and UBRRnH
The function and bit description of the baud rate registers in MSPI mode is identical to normal USART operation.
See “USART Baud Rate Registers – UBRRLn and UBRRHn” on page 213.
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20.
2-wire Serial Interface
20.1
Features
•
•
•
•
•
•
•
•
•
•
20.2
Simple Yet Powerful and Flexible Communication Interface, only two Bus Lines Needed
Both Master and Slave Operation Supported
Device can Operate as Transmitter or Receiver
7-bit Address Space Allows up to 128 Different Slave Addresses
Multi-master Arbitration Support
Up to 400kHz Data Transfer Speed
Slew-rate Limited Output Drivers
Noise Suppression Circuitry Rejects Spikes on Bus Lines
Fully Programmable Slave Address with General Call Support
Address Recognition Causes Wake-up When AVR is in Sleep Mode
2-wire Serial Interface Bus Definition
The 2-wire Serial Interface (TWI) is ideally suited for typical microcontroller applications. The TWI protocol
allows the systems designer to interconnect up to 128 different devices using only two bi-directional bus lines,
one for clock (SCL) and one for data (SDA). The only external hardware needed to implement the bus is a single
pull-up resistor for each of the TWI bus lines. All devices connected to the bus have individual addresses, and
mechanisms for resolving bus contention are inherent in the TWI protocol.
Figure 20-1.
TWI Bus Interconnection
VCC
Device 1
Device 2
Device 3
........
Device n
R1
R2
SDA
SCL
20.2.1 TWI Terminology
The following definitions are frequently encountered in this section.
Term
Description
Master
The device that initiates and terminates a transmission. The Master also generates the
SCL clock.
Slave
The device addressed by a Master
Transmitter
The device placing data on the bus
Receiver
The device reading data from the bus
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The Power Reduction TWI bit, PRTWI bit in “Power Reduction Register 0 - PRR0” on page 47 must be written to
zero to enable the 2-wire Serial Interface.
20.2.2 Electrical Interconnection
As depicted in Figure 20-1 on page 225, both bus lines are connected to the positive supply voltage through
pull-up resistors. The bus drivers of all TWI-compliant devices are open-drain or open-collector. This
implements a wired-AND function which is essential to the operation of the interface. A low level on a TWI bus
line is generated when one or more TWI devices output a zero. A high level is output when all TWI devices trimstate their outputs, allowing the pull-up resistors to pull the line high. Note that all AVR devices connected to the
TWI bus must be powered in order to allow any bus operation.
The number of devices that can be connected to the bus is only limited by the bus capacitance limit of 400pF
and the 7-bit slave address space. A detailed specification of the electrical characteristics of the TWI is given in
“SPI Timing Characteristics” on page 388. Two different sets of specifications are presented there, one relevant
for bus speeds below 100kHz, and one valid for bus speeds up to 400kHz.
20.3
Data Transfer and Frame Format
20.3.1 Transferring Bits
Each data bit transferred on the TWI bus is accompanied by a pulse on the clock line. The level of the data line
must be stable when the clock line is high. The only exception to this rule is for generating start and stop
conditions.
Figure 20-2.
Data Validity
SDA
SCL
Data Stable
Data Stable
Data Change
20.3.2 START and STOP Conditions
The Master initiates and terminates a data transmission. The transmission is initiated when the Master issues a
START condition on the bus, and it is terminated when the Master issues a STOP condition. Between a START
and a STOP condition, the bus is considered busy, and no other master should try to seize control of the bus. A
special case occurs when a new START condition is issued between a START and STOP condition. This is
referred to as a REPEATED START condition, and is used when the Master wishes to initiate a new transfer
without relinquishing control of the bus. After a REPEATED START, the bus is considered busy until the next
STOP. This is identical to the START behavior, and therefore START is used to describe both START and
REPEATED START for the remainder of this datasheet, unless otherwise noted. As depicted below, START
and STOP conditions are signalled by changing the level of the SDA line when the SCL line is high.
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Figure 20-3.
START, REPEATED START and STOP Conditions
SDA
SCL
STOP
START
START
REPEATED START
STOP
20.3.3 Address Packet Format
All address packets transmitted on the TWI bus are nine bits long, consisting of seven address bits, one
READ/WRITE control bit and an acknowledge bit. If the READ/WRITE bit is set, a read operation is to be
performed, otherwise a write operation should be performed. When a Slave recognizes that it is being
addressed, it should acknowledge by pulling SDA low in the ninth SCL (ACK) cycle. If the addressed Slave is
busy, or for some other reason can not service the Master’s request, the SDA line should be left high in the ACK
clock cycle. The Master can then transmit a STOP condition, or a REPEATED START condition to initiate a new
transmission. An address packet consisting of a slave address and a READ or a WRITE bit is called SLA+R or
SLA+W, respectively.
The MSB of the address byte is transmitted first. Slave addresses can freely be allocated by the designer, but
the address 0000 000 is reserved for a general call.
When a general call is issued, all slaves should respond by pulling the SDA line low in the ACK cycle. A general
call is used when a Master wishes to transmit the same message to several slaves in the system. When the
general call address followed by a Write bit is transmitted on the bus, all slaves set up to acknowledge the
general call will pull the SDA line low in the ack cycle. The following data packets will then be received by all the
slaves that acknowledged the general call. Note that transmitting the general call address followed by a Read bit
is meaningless, as this would cause contention if several slaves started transmitting different data.
All addresses of the format 1111 xxx should be reserved for future purposes.
Figure 20-4.
Address Packet Format
Addr MSB
Addr LSB
R/W
ACK
7
8
9
SDA
SCL
1
2
START
20.3.4 Data Packet Format
All data packets transmitted on the TWI bus are nine bits long, consisting of one data byte and an acknowledge
bit. During a data transfer, the Master generates the clock and the START and STOP conditions, while the
Receiver is responsible for acknowledging the reception. An Acknowledge (ACK) is signalled by the Receiver
pulling the SDA line low during the ninth SCL cycle. If the Receiver leaves the SDA line high, a NACK is
signalled. When the Receiver has received the last byte, or for some reason cannot receive any more bytes, it
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should inform the Transmitter by sending a NACK after the final byte. The MSB of the data byte is transmitted
first.
Figure 20-5.
Data Packet Format
Data MSB
Data LSB
ACK
8
9
Aggregate
SDA
SDA from
Transmitter
SDA from
Receiver
SCL from
Master
1
2
SLA+R/W
7
STOP, REPEATED
START or Next
Data Byte
Data Byte
20.3.5 Combining Address and Data Packets into a Transmission
A transmission basically consists of a START condition, a SLA+R/W, one or more data packets and a STOP
condition. An empty message, consisting of a START followed by a STOP condition, is illegal. Note that the
Wired-ANDing of the SCL line can be used to implement handshaking between the Master and the Slave. The
Slave can extend the SCL low period by pulling the SCL line low. This is useful if the clock speed set up by the
Master is too fast for the Slave, or the Slave needs extra time for processing between the data transmissions.
The Slave extending the SCL low period will not affect the SCL high period, which is determined by the Master.
As a consequence, the Slave can reduce the TWI data transfer speed by prolonging the SCL duty cycle.
Figure 20-6 shows a typical data transmission. Note that several data bytes can be transmitted between the
SLA+R/W and the STOP condition, depending on the software protocol implemented by the application
software.
Figure 20-6.
Typical Data Transmission
Addr MSB
Addr LSB
R/W
ACK
Data MSB
7
8
9
1
Data LSB
ACK
8
9
SDA
SCL
1
START
20.4
2
SLA+R/W
2
7
Data Byte
STOP
Multi-master Bus Systems, Arbitration and Synchronization
The TWI protocol allows bus systems with several masters. Special concerns have been taken in order to
ensure that transmissions will proceed as normal, even if two or more masters initiate a transmission at the
same time. Two problems arise in multi-master systems:

An algorithm must be implemented allowing only one of the masters to complete the transmission. All
other masters should cease transmission when they discover that they have lost the selection process.
This selection process is called arbitration. When a contending master discovers that it has lost the
arbitration process, it should immediately switch to Slave mode to check whether it is being addressed by
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the winning master. The fact that multiple masters have started transmission at the same time should not
be detectable to the slaves, i.e. the data being transferred on the bus must not be corrupted.

Different masters may use different SCL frequencies. A scheme must be devised to synchronize the serial
clocks from all masters, in order to let the transmission proceed in a lockstep fashion. This will facilitate
the arbitration process.
The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from all masters will
be wired-ANDed, yielding a combined clock with a high period equal to the one from the Master with the
shortest high period. The low period of the combined clock is equal to the low period of the Master with the
longest low period. Note that all masters listen to the SCL line, effectively starting to count their SCL high and
low time-out periods when the combined SCL line goes high or low, respectively.
Figure 20-7.
SCL Synchronization Between Multiple Masters
TA low
TA high
SCL from
Master A
SCL from
Master B
SCL Bus
Line
TBlow
Masters Start
Counting Low Period
TBhigh
Masters Start
Counting High Period
Arbitration is carried out by all masters continuously monitoring the SDA line after outputting data. If the value
read from the SDA line does not match the value the Master had output, it has lost the arbitration. Note that a
Master can only lose arbitration when it outputs a high SDA value while another Master outputs a low value. The
losing Master should immediately go to Slave mode, checking if it is being addressed by the winning Master.
The SDA line should be left high, but losing masters are allowed to generate a clock signal until the end of the
current data or address packet. Arbitration will continue until only one Master remains, and this may take many
bits. If several masters are trying to address the same Slave, arbitration will continue into the data packet.
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Figure 20-8.
Arbitration Between Two Masters
START
SDA from
Master A
Master A Loses
Arbitration, SDAA SDA
SDA from
Master B
SDA Line
Synchronized
SCL Line
Note that arbitration is not allowed between:

A REPEATED START condition and a data bit

A STOP condition and a data bit

A REPEATED START and a STOP condition
It is the user software’s responsibility to ensure that these illegal arbitration conditions never occur. This implies
that in multi-master systems, all data transfers must use the same composition of SLA+R/W and data packets.
In other words: All transmissions must contain the same number of data packets, otherwise the result of the
arbitration is undefined.
20.5
Overview of the TWI Module
The TWI module is comprised of several submodules, as shown in Figure 20-9 on page 231. All registers drawn
in a thick line are accessible through the AVR data bus.
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Overview of the TWI Module
SCL
Slew-rate
Control
SDA
Spike
Filter
Slew-rate
Control
Spike
Filter
Bus Interface Unit
START / STOP
Control
Spike Suppression
Arbitration detection
Address/Data Shift
Register (TWDR)
Address Match Unit
Address Register
(TWAR)
Address Comparator
Bit Rate Generator
Prescaler
Bit Rate Register
(TWBR)
Ack
Control Unit
Status Register
(TWSR)
Control Register
(TWCR)
State Machine and
Status control
TWI Unit
Figure 20-9.
20.5.1 SCL and SDA Pins
These pins interface the AVR TWI with the rest of the MCU system. The output drivers contain a slew-rate
limiter in order to conform to the TWI specification. The input stages contain a spike suppression unit removing
spikes shorter than 50ns. Note that the internal pull-ups in the AVR pads can be enabled by setting the PORT
bits corresponding to the SCL and SDA pins, as explained in the I/O Port section. The internal pull-ups can in
some systems eliminate the need for external ones.
20.5.2 Bit Rate Generator Unit
This unit controls the period of SCL when operating in a Master mode. The SCL period is controlled by settings
in the TWI Bit Rate Register (TWBR) and the Prescaler bits in the TWI Status Register (TWSR). Slave operation
does not depend on Bit Rate or Prescaler settings, but the CPU clock frequency in the Slave must be at least 16
times higher than the SCL frequency. Note that slaves may prolong the SCL low period, thereby reducing the
average TWI bus clock period. The SCL frequency is generated according to the following equation:

TWBR = Value of the TWI Bit Rate Register
CPU Clock frequency
SCL frequency = ----------------------------------------------------------TWPS
16 + 2(TWBR)  4

Note:
TWPS = Value of the prescaler bits in the TWI Status Register
TWBR should be 10 or higher if the TWI operates in Master mode. If TWBR is lower than 10, the Master may
produce an incorrect output on SDA and SCL for the reminder of the byte. The problem occurs when operating the
TWI in Master mode, sending Start + SLA + R/W to a Slave (a Slave does not need to be connected to the bus for
the condition to happen).
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20.5.3 Bus Interface Unit
This unit contains the Data and Address Shift Register (TWDR), a START/STOP Controller and Arbitration
detection hardware. The TWDR contains the address or data bytes to be transmitted, or the address or data
bytes received. In addition to the 8-bit TWDR, the Bus Interface Unit also contains a register containing the
(N)ACK bit to be transmitted or received. This (N)ACK Register is not directly accessible by the application
software. However, when receiving, it can be set or cleared by manipulating the TWI Control Register (TWCR).
When in Transmitter mode, the value of the received (N)ACK bit can be determined by the value in the TWSR.
The START/STOP Controller is responsible for generation and detection of START, REPEATED START, and
STOP conditions. The START/STOP controller is able to detect START and STOP conditions even when the
AVR MCU is in one of the sleep modes, enabling the MCU to wake up if addressed by a Master.
If the TWI has initiated a transmission as Master, the Arbitration Detection hardware continuously monitors the
transmission trying to determine if arbitration is in process. If the TWI has lost an arbitration, the Control Unit is
informed. Correct action can then be taken and appropriate status codes generated.
20.5.4 Address Match Unit
The Address Match unit checks if received address bytes match the seven-bit address in the TWI Address
Register (TWAR). If the TWI General Call Recognition Enable (TWGCE) bit in the TWAR is written to one, all
incoming address bits will also be compared against the General Call address. Upon an address match, the
Control Unit is informed, allowing correct action to be taken. The TWI may or may not acknowledge its address,
depending on settings in the TWCR. The Address Match unit is able to compare addresses even when the AVR
MCU is in sleep mode, enabling the MCU to wake up if addressed by a Master. If another interrupt (e.g., INT0)
occurs during TWI Power-down address match and wakes up the CPU, the TWI aborts operation and return to
it’s idle state. If this cause any problems, ensure that TWI Address Match is the only enabled interrupt when
entering Power-down.
20.5.5 Control Unit
The Control unit monitors the TWI bus and generates responses corresponding to settings in the TWI Control
Register (TWCR). When an event requiring the attention of the application occurs on the TWI bus, the TWI
Interrupt Flag (TWINT) is asserted. In the next clock cycle, the TWI Status Register (TWSR) is updated with a
status code identifying the event. The TWSR only contains relevant status information when the TWI Interrupt
Flag is asserted. At all other times, the TWSR contains a special status code indicating that no relevant status
information is available. As long as the TWINT Flag is set, the SCL line is held low. This allows the application
software to complete its tasks before allowing the TWI transmission to continue.
The TWINT Flag is set in the following situations:
20.6

After the TWI has transmitted a START/REPEATED START condition

After the TWI has transmitted SLA+R/W

After the TWI has transmitted an address byte

After the TWI has lost arbitration

After the TWI has been addressed by own slave address or general call

After the TWI has received a data byte

After a STOP or REPEATED START has been received while still addressed as a Slave

When a bus error has occurred due to an illegal START or STOP condition.
Using the TWI
The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like reception of a
byte or transmission of a START condition. Because the TWI is interrupt-based, the application software is free
to carry on other operations during a TWI byte transfer. Note that the TWI Interrupt Enable (TWIE) bit in TWCR
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together with the Global Interrupt Enable bit in SREG allow the application to decide whether or not assertion of
the TWINT Flag should generate an interrupt request. If the TWIE bit is cleared, the application must poll the
TWINT Flag in order to detect actions on the TWI bus.
When the TWINT Flag is asserted, the TWI has finished an operation and awaits application response. In this
case, the TWI Status Register (TWSR) contains a value indicating the current state of the TWI bus. The
application software can then decide how the TWI should behave in the next TWI bus cycle by manipulating the
TWCR and TWDR Registers.
Figure 20-10 on page 233 is a simple example of how the application can interface to the TWI hardware. In this
example, a Master wishes to transmit a single data byte to a Slave. This description is quite abstract, a more
detailed explanation follows later in this section. A simple code example implementing the desired behavior is
also presented.
Application
Action
Figure 20-10. Interfacing the Application to the TWI in a Typical Transmission
1. Application
writes to TWCR to
initiate
transmission of
START
TWI
Hardware
Action
TWI bus
3. Check TWSR to see if START was
sent. Application loads SLA+W into
TWDR, and loads appropriate control
signals into TWCR, makin sure that
TWINT is written to one,
and TWSTA is written to zero.
START
2. TWINT set.
Status code indicates
START condition sent
SLA+W
5. Check TWSR to see if SLA+W was
sent and ACK received.
Application loads data into TWDR, and
loads appropriate control signals into
TWCR, making sure that TWINT is
written to one
A
4. TWINT set.
Status code indicates
SLA+W sent, ACK
received
Data
7. Check TWSR to see if data was sent
and ACK received.
Application loads appropriate control
signals to send STOP into TWCR,
making sure that TWINT is written to one
A
6. TWINT set.
Status code indicates
data sent, ACK received
STOP
Indicates
TWINT set
1. The first step in a TWI transmission is to transmit a START condition. This is done by writing a specific
value into TWCR, instructing the TWI hardware to transmit a START condition. Which value to write is
described later on. However, it is important that the TWINT bit is set in the value written. Writing a one
to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set.
Immediately after the application has cleared TWINT, the TWI will initiate transmission of the START
condition.
2.
When the START condition has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated
with a status code indicating that the START condition has successfully been sent.
3.
The application software should now examine the value of TWSR, to make sure that the START condition
was successfully transmitted. If TWSR indicates otherwise, the application software might take some
special action, like calling an error routine. Assuming that the status code is as expected, the application
must load SLA+W into TWDR. Remember that TWDR is used both for address and data. After TWDR has
been loaded with the desired SLA+W, a specific value must be written to TWCR, instructing the TWI
hardware to transmit the SLA+W present in TWDR. Which value to write is described later on. However, it
is important that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI
will not start any operation as long as the TWINT bit in TWCR is set. Immediately after the application has
cleared TWINT, the TWI will initiate transmission of the address packet.
4.
When the address packet has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated
with a status code indicating that the address packet has successfully been sent. The status code will also
reflect whether a Slave acknowledged the packet or not.
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5.
The application software should now examine the value of TWSR, to make sure that the address packet
was successfully transmitted, and that the value of the ACK bit was as expected. If TWSR indicates
otherwise, the application software might take some special action, like calling an error routine. Assuming
that the status code is as expected, the application must load a data packet into TWDR. Subsequently, a
specific value must be written to TWCR, instructing the TWI hardware to transmit the data packet present
in TWDR. Which value to write is described later on. However, it is important that the TWINT bit is set in
the value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as
the TWINT bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate
transmission of the data packet.
6.
When the data packet has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated with
a status code indicating that the data packet has successfully been sent. The status code will also reflect
whether a Slave acknowledged the packet or not.
7.
The application software should now examine the value of TWSR, to make sure that the data packet was
successfully transmitted, and that the value of the ACK bit was as expected. If TWSR indicates otherwise,
the application software might take some special action, like calling an error routine. Assuming that the
status code is as expected, the application must write a specific value to TWCR, instructing the TWI
hardware to transmit a STOP condition. Which value to write is described later on. However, it is important
that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will not start
any operation as long as the TWINT bit in TWCR is set. Immediately after the application has cleared
TWINT, the TWI will initiate transmission of the STOP condition. Note that TWINT is NOT set after a
STOP condition has been sent.
Even though this example is simple, it shows the principles involved in all TWI transmissions. These can be
summarized as follows:

When the TWI has finished an operation and expects application response, the TWINT Flag is set. The
SCL line is pulled low until TWINT is cleared.

When the TWINT Flag is set, the user must update all TWI Registers with the value relevant for the next
TWI bus cycle. As an example, TWDR must be loaded with the value to be transmitted in the next bus
cycle.

After all TWI Register updates and other pending application software tasks have been completed, TWCR
is written. When writing TWCR, the TWINT bit should be set. Writing a one to TWINT clears the flag. The
TWI will then commence executing whatever operation was specified by the TWCR setting.
In the following an assembly and C implementation of the example is given. Note that the code below assumes
that several definitions have been made, for example by using include-files.
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Assembly Code Example
ldi
r16,
(1<<TWINT)|(1<<TWSTA)|
1
C Example
Comments
TWCR =
(1<<TWINT)|(1<<TWSTA)
|
(1<<TWEN)
Send START condition
(1<<TWEN)
out
TWCR, r16
2
3
4
wait1:
in
r16,TWCR
sbrs
r16,TWINT
rjmp
wait1
while (!(TWCR &
(1<<TWINT)))
in
r16,TWSR
andi
r16, 0xF8
cpi
r16, START
brne
ERROR
if ((TWSR & 0xF8) !=
START)
ldi
r16, SLA_W
out
TWDR, r16
ldi
r16, (1<<TWINT) |
(1<<TWEN)
out
TWCR, r16
TWDR = SLA_W;
TWCR = (1<<TWINT) |
(1<<TWEN);
wait2:
in
r16,TWCR
sbrs
r16,TWINT
rjmp
wait2
while (!(TWCR &
(1<<TWINT)))
;
Wait for TWINT Flag set. This
indicates that the START
condition has been transmitted
Check value of TWI Status
Register. Mask prescaler bits. If
status different from START go to
ERROR
ERROR();
;
Load SLA_W into TWDR
Register. Clear TWINT bit in
TWCR to start transmission of
address
Wait for TWINT Flag set. This
indicates that the SLA+W has
been transmitted, and
ACK/NACK has been received.
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Assembly Code Example
5
6
7
C Example
Comments
in
r16,TWSR
andi
r16, 0xF8
cpi
r16, MT_SLA_ACK
brne
ERROR
if ((TWSR & 0xF8) !=
MT_SLA_ACK)
ldi
r16, DATA
out
TWDR, r16
ldi
r16, (1<<TWINT) |
(1<<TWEN)
out
TWCR, r16
TWDR = DATA;
TWCR = (1<<TWINT) |
(1<<TWEN);
wait3:
in
r16,TWCR
sbrs
r16,TWINT
rjmp
wait3
while (!(TWCR &
(1<<TWINT)))
in
r16,TWSR
andi
r16, 0xF8
cpi
r16, MT_DATA_ACK
brne
ERROR
if ((TWSR & 0xF8) !=
MT_DATA_ACK)
ldi
r16,
(1<<TWINT)|(1<<TWEN)|
TWCR =
(1<<TWINT)|(1<<TWEN)|
(1<<TWSTO);
Check value of TWI Status
Register. Mask prescaler bits. If
status different from
MT_SLA_ACK go to ERROR
ERROR();
Load DATA into TWDR Register.
Clear TWINT bit in TWCR to start
transmission of data
;
Wait for TWINT Flag set. This
indicates that the DATA has been
transmitted, and ACK/NACK has
been received.
Check value of TWI Status
Register. Mask prescaler bits. If
status different from
MT_DATA_ACK go to ERROR
ERROR();
Transmit STOP condition
(1<<TWSTO)
out
TWCR, r16
20.7
Transmission Modes
The TWI can operate in one of four major modes. These are named Master Transmitter (MT), Master Receiver
(MR), Slave Transmitter (ST) and Slave Receiver (SR). Several of these modes can be used in the same
application. As an example, the TWI can use MT mode to write data into a TWI EEPROM, MR mode to read the
data back from the EEPROM. If other masters are present in the system, some of these might transmit data to
the TWI, and then SR mode would be used. It is the application software that decides which modes are legal.
The following sections describe each of these modes. Possible status codes are described along with figures
detailing data transmission in each of the modes. These figures contain the following abbreviations:
S: START condition
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Rs: REPEATED START condition
R: Read bit (high level at SDA)
W: Write bit (low level at SDA)
A: Acknowledge bit (low level at SDA)
A: Not acknowledge bit (high level at SDA)
Data: 8-bit data byte
P: STOP condition
SLA: Slave Address
In Figure 20-12 on page 240 to Figure 20-18 on page 248, circles are used to indicate that the TWINT Flag is
set. The numbers in the circles show the status code held in TWSR, with the prescaler bits masked to zero. At
these points, actions must be taken by the application to continue or complete the TWI transfer. The TWI
transfer is suspended until the TWINT Flag is cleared by software.
When the TWINT Flag is set, the status code in TWSR is used to determine the appropriate software action. For
each status code, the required software action and details of the following serial transfer are given in Table 20-1
on page 238 to Table 20-4 on page 248. Note that the prescaler bits are masked to zero in these tables.
20.7.1 Master Transmitter Mode
In the Master Transmitter mode, a number of data bytes are transmitted to a Slave Receiver (see Figure 20-11).
In order to enter a Master mode, a START condition must be transmitted. The format of the following address
packet determines whether Master Transmitter or Master Receiver mode is to be entered. If SLA+W is
transmitted, MT mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes mentioned
in this section assume that the prescaler bits are zero or are masked to zero.
Figure 20-11. Data Transfer in Master Transmitter Mode
VCC
Device 1
Device 2
MASTER
TRANSMITTER
SLAVE
RECEIVER
Device 3
........
Device n
R1
R2
SDA
SCL
A START condition is sent by writing the following value to TWCR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
1
TWSTO
0
TWWC
X
TWEN
1
–
0
TWIE
X
TWEN must be set to enable the 2-wire Serial Interface, TWSTA must be written to one to transmit a START
condition and TWINT must be written to one to clear the TWINT Flag. The TWI will then test the 2-wire Serial
Bus and generate a START condition as soon as the bus becomes free. After a START condition has been
transmitted, the TWINT Flag is set by hardware, and the status code in TWSR will be 0x08 (see Table 20-1 on
page 238). In order to enter MT mode, SLA+W must be transmitted. This is done by writing SLA+W to TWDR.
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Thereafter the TWINT bit should be cleared (by writing it to one) to continue the transfer. This is accomplished
by writing the following value to TWCR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
0
TWSTO
0
TWWC
X
TWEN
1
–
0
TWIE
X
When SLA+W have been transmitted and an acknowledgement bit has been received, TWINT is set again and
a number of status codes in TWSR are possible. Possible status codes in Master mode are 0x18, 0x20, or 0x38.
The appropriate action to be taken for each of these status codes is detailed in Table 20-1 on page 238.
When SLA+W has been successfully transmitted, a data packet should be transmitted. This is done by writing
the data byte to TWDR. TWDR must only be written when TWINT is high. If not, the access will be discarded,
and the Write Collision bit (TWWC) will be set in the TWCR Register. After updating TWDR, the TWINT bit
should be cleared (by writing it to one) to continue the transfer. This is accomplished by writing the following
value to TWCR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
0
TWSTO
0
TWWC
X
TWEN
1
–
0
TWIE
X
This scheme is repeated until the last byte has been sent and the transfer is ended by generating a STOP
condition or a repeated START condition. A STOP condition is generated by writing the following value to
TWCR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
0
TWSTO
1
TWWC
X
TWEN
1
–
0
TWIE
X
A REPEATED START condition is generated by writing the following value to TWCR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
1
TWSTO
0
TWWC
X
TWEN
1
–
0
TWIE
X
After a repeated START condition (state 0x10) the 2-wire Serial Interface can access the same Slave again, or
a new Slave without transmitting a STOP condition. Repeated START enables the Master to switch between
Slaves, Master Transmitter mode and Master Receiver mode without losing control of the bus
Table 20-1.
Status Code
(TWSR)
Prescaler Bits
are 0
Status codes for Master Transmitter Mode
Application Software Response
Status of the 2-wire Serial Bus
and 2-wire Serial Interface
Hardware
To/from TWDR
To TWCR
STA
STO
TWIN
T
TWE
A
Next Action Taken by TWI Hardware
0x08
A START condition has been
transmitted
Load SLA+W
0
0
1
X
SLA+W will be transmitted;
ACK or NOT ACK will be received
0x10
A repeated START condition
has been transmitted
Load SLA+W or
0
0
1
X
Load SLA+R
0
0
1
X
SLA+W will be transmitted;
ACK or NOT ACK will be received
SLA+R will be transmitted;
Logic will switch to Master Receiver mode
Load data byte or
0
0
1
X
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
1
1
1
X
0x18
SLA+W has been transmitted;
ACK has been received
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
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Table 20-1.
0x20
0x28
0x30
0x38
Status codes for Master Transmitter Mode
SLA+W has been transmitted;
NOT ACK has been received
Data byte has been transmitted;
ACK has been received
Data byte has been transmitted;
NOT ACK has been received
Arbitration lost in SLA+W or
data bytes
Load data byte or
0
0
1
X
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
1
1
1
X
Load data byte or
0
0
1
X
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
1
1
1
X
Load data byte or
0
0
1
X
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
1
1
1
X
No TWDR action or
0
0
1
X
No TWDR action
1
0
1
X
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
2-wire Serial Bus will be released and not addressed
Slave mode entered
A START condition will be transmitted when the bus
becomes free
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Figure 20-12. Formats and States in the Master Transmitter Mode
MT
Successfull
transmission
to a slave
receiver
S
SLA
$08
W
A
DATA
$18
A
P
$28
Next transfer
started with a
repeated start
condition
RS
SLA
W
$10
Not acknowledge
received after the
slave address
A
R
P
$20
MR
Not acknowledge
received after a data
byte
A
P
$30
Arbitration lost in slave
address or data byte
A or A
Other master
continues
$38
Arbitration lost and
addressed as slave
A
$68
From master to slave
From slave to master
A or A
Other master
continues
$38
Other master
continues
$78
DATA
To corresponding
states in slave mode
$B0
A
n
Any number of data bytes
and their associated acknowledge bits
This number (contained in TWSR) corresponds
to a defined state of the Two-Wire Serial Bus. The
prescaler bits are zero or masked to zero
20.7.2 Master Receiver Mode
In the Master Receiver mode, a number of data bytes are received from a Slave Transmitter (see Figure 20-13
on page 241). In order to enter a Master mode, a START condition must be transmitted. The format of the
following address packet determines whether Master Transmitter or Master Receiver mode is to be entered. If
SLA+W is transmitted, MT mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes
mentioned in this section assume that the prescaler bits are zero or are masked to zero.
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Figure 20-13. Data Transfer in Master Receiver Mode
VCC
Device 1
Device 2
MASTER
RECEIVER
SLAVE
TRANSMITTER
Device 3
........
Device n
R1
R2
SDA
SCL
A START condition is sent by writing the following value to TWCR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
1
TWSTO
0
TWWC
X
TWEN
1
–
0
TWIE
X
TWEN must be written to one to enable the 2-wire Serial Interface, TWSTA must be written to one to transmit a
START condition and TWINT must be set to clear the TWINT Flag. The TWI will then test the 2-wire Serial Bus
and generate a START condition as soon as the bus becomes free. After a START condition has been
transmitted, the TWINT Flag is set by hardware, and the status code in TWSR will be 0x08 (See Table 20-1 on
page 238). In order to enter MR mode, SLA+R must be transmitted. This is done by writing SLA+R to TWDR.
Thereafter the TWINT bit should be cleared (by writing it to one) to continue the transfer. This is accomplished
by writing the following value to TWCR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
0
TWSTO
0
TWWC
X
TWEN
1
–
0
TWIE
X
When SLA+R have been transmitted and an acknowledgement bit has been received, TWINT is set again and a
number of status codes in TWSR are possible. Possible status codes in Master mode are 0x38, 0x40, or 0x48.
The appropriate action to be taken for each of these status codes is detailed in Table 20-2 on page 242.
Received data can be read from the TWDR Register when the TWINT Flag is set high by hardware. This
scheme is repeated until the last byte has been received. After the last byte has been received, the MR should
inform the ST by sending a NACK after the last received data byte. The transfer is ended by generating a STOP
condition or a repeated START condition. A STOP condition is generated by writing the following value to
TWCR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
0
TWSTO
1
TWWC
X
TWEN
1
–
0
TWIE
X
A REPEATED START condition is generated by writing the following value to TWCR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
1
TWSTO
0
TWWC
X
TWEN
1
–
0
TWIE
X
After a repeated START condition (state 0x10) the 2-wire Serial Interface can access the same Slave again, or
a new Slave without transmitting a STOP condition. Repeated START enables the Master to switch between
Slaves, Master Transmitter mode and Master Receiver mode without losing control over the bus.
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Table 20-2.
Status Code
(TWSR)
Prescaler Bits
are 0
Status codes for Master Receiver Mode
Application Software Response
Status of the 2-wire Serial Bus
and 2-wire Serial Interface
Hardware
To TWCR
To/from TWDR
STA
STO
TWIN
T
TWE
A
Next Action Taken by TWI Hardware
0x08
A START condition has been
transmitted
Load SLA+R
0
0
1
X
SLA+R will be transmitted
ACK or NOT ACK will be received
0x10
A repeated START condition
has been transmitted
Load SLA+R or
0
0
1
X
Load SLA+W
0
0
1
X
SLA+R will be transmitted
ACK or NOT ACK will be received
SLA+W will be transmitted
Logic will switch to Master Transmitter mode
No TWDR action or
0
0
1
X
No TWDR action
1
0
1
X
No TWDR action or
0
0
1
0
No TWDR action
0
0
1
1
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
1
1
1
X
Read data byte or
0
0
1
0
Read data byte
0
0
1
1
Read data byte or
Read data byte or
1
0
0
1
1
1
X
X
Read data byte
1
1
1
X
0x38
0x40
0x48
0x50
0x58
Arbitration lost in SLA+R or
NOT ACK bit
SLA+R has been transmitted;
ACK has been received
SLA+R has been transmitted;
NOT ACK has been received
Data byte has been received;
ACK has been returned
Data byte has been received;
NOT ACK has been returned
2-wire Serial Bus will be released and not addressed
Slave mode will be entered
A START condition will be transmitted when the bus
becomes free
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Repeated START will be transmitted
STOP condition will be transmitted and TWSTO Flag
will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Repeated START will be transmitted
STOP condition will be transmitted and TWSTO Flag
will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
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Figure 20-14. Formats and States in the Master Receiver Mode
MR
Successfull
reception
from a slave
receiver
S
SLA
R
A
DATA
$40
$08
A
DATA
A
$50
P
$58
Next transfer
started with a
repeated start
condition
RS
SLA
R
$10
Not acknowledge
received after the
slave address
A
W
P
$48
MT
Arbitration lost in slave
address or data byte
Other master
continues
A or A
$38
Arbitration lost and
addressed as slave
From master to slave
Other master
continues
$38
Other master
continues
A
$68
A
$78
DATA
From slave to master
To corresponding
states in slave mode
$B0
A
Any number of data bytes
and their associated acknowledge bits
This number (contained in TWSR) corresponds
to a defined state of the Two-Wire Serial Bus. The
prescaler bits are zero or masked to zero
n
20.7.3 Slave Receiver Mode
In the Slave Receiver mode, a number of data bytes are received from a Master Transmitter (see Figure 20-15).
All the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero.
Figure 20-15. Data transfer in Slave Receiver mode
VCC
Device 1
Device 2
SLAVE
RECEIVER
MASTER
TRANSMITTER
Device 3
........
Device n
R1
R2
SDA
SCL
To initiate the Slave Receiver mode, TWAR and TWCR must be initialized as follows:
TWAR
value
TWA6
TWA5
TWA4
TWA3
TWA2
Device’s Own Slave Address
TWA1
TWA0
TWGCE
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The upper seven bits are the address to which the 2-wire Serial Interface will respond when addressed by a
Master. If the LSB is set, the TWI will respond to the general call address (0x00), otherwise it will ignore the
general call address.
TWCR
value
TWINT
0
TWEA
1
TWSTA
0
TWSTO
0
TWWC
0
TWEN
1
–
0
TWIE
X
TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable the
acknowledgement of the device’s own slave address or the general call address. TWSTA and TWSTO must be
written to zero.
When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own slave address (or
the general call address if enabled) followed by the data direction bit. If the direction bit is “0” (write), the TWI will
operate in SR mode, otherwise ST mode is entered. After its own slave address and the write bit have been
received, the TWINT Flag is set and a valid status code can be read from TWSR. The status code is used to
determine the appropriate software action. The appropriate action to be taken for each status code is detailed in
Table 20-3 on page 245. The Slave Receiver mode may also be entered if arbitration is lost while the TWI is in
the Master mode (see states 0x68 and 0x78).
If the TWEA bit is reset during a transfer, the TWI will return a “Not Acknowledge” (“1”) to SDA after the next
received data byte. This can be used to indicate that the Slave is not able to receive any more bytes. While
TWEA is zero, the TWI does not acknowledge its own slave address. However, the 2-wire Serial Bus is still
monitored and address recognition may resume at any time by setting TWEA. This implies that the TWEA bit
may be used to temporarily isolate the TWI from the 2-wire Serial Bus.
In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA bit is set, the
interface can still acknowledge its own slave address or the general call address by using the 2-wire Serial Bus
clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL clock low during
the wake up and until the TWINT Flag is cleared (by writing it to one). Further data reception will be carried out
as normal, with the AVR clocks running as normal. Observe that if the AVR is set up with a long start-up time,
the SCL line may be held low for a long time, blocking other data transmissions.
Note that the 2-wire Serial Interface Data Register – TWDR does not reflect the last byte present on the bus
when waking up from these Sleep modes.
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Table 20-3.
Status Code
(TWSR)
Prescaler Bits
are 0
Status Codes for Slave Receiver Mode
Application Software Response
Status of the 2-wire Serial Bus
and 2-wire Serial Interface Hardware
To TWCR
To/from TWDR
STA
STO
TWIN
T
TWE
A
No TWDR action or
X
0
1
0
0x60
Own SLA+W has been received;
ACK has been returned
No TWDR action
X
0
1
1
0x68
Arbitration lost in SLA+R/W as
Master; own SLA+W has been
received; ACK has been returned
No TWDR action or
X
0
1
0
No TWDR action
X
0
1
1
0x70
General call address has been
received; ACK has been returned
No TWDR action or
X
0
1
0
No TWDR action
X
0
1
1
0x78
Arbitration lost in SLA+R/W as
Master; General call address has
been received; ACK has been returned
No TWDR action or
X
0
1
0
No TWDR action
X
0
1
1
0x80
Previously addressed with own
SLA+W; data has been received;
ACK has been returned
Read data byte or
X
0
1
0
Read data byte
X
0
1
1
0x88
Previously addressed with own
SLA+W; data has been received;
NOT ACK has been returned
Read data byte or
0
0
1
0
Read data byte or
0
0
1
1
Read data byte or
1
0
1
0
Read data byte
1
0
1
1
0x90
Previously addressed with general call; data has been received;
ACK has been returned
Read data byte or
X
0
1
0
Read data byte
X
0
1
1
0x98
Previously addressed with general call; data has been received;
NOT ACK has been returned
Read data byte or
0
0
1
0
Read data byte or
0
0
1
1
Read data byte or
1
0
1
0
Read data byte
1
0
1
1
No action
0
0
1
0
0
0
1
1
1
0
1
0
1
0
1
1
0xA0
A STOP condition or repeated
START condition has been received while still addressed as
Slave
Next Action Taken by TWI Hardware
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
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Figure 20-16. Formats and States in the Slave Receiver Mode
Reception of the own
slave address and one or
more data bytes. All are
acknowledged
S
SLA
W
A
DATA
$60
A
DATA
$80
Last data byte received
is not acknowledged
A
P or S
$80
$A0
A
P or S
$88
Arbitration lost as master
and addressed as slave
A
$68
Reception of the general call
address and one or more data
bytes
General Call
A
DATA
$70
A
DATA
$90
Last data byte received is
not acknowledged
A
P or S
$90
$A0
A
P or S
$98
Arbitration lost as master and
addressed as slave by general call
A
$78
From master to slave
DATA
From slave to master
A
n
Any number of data bytes
and their associated acknowledge bits
This number (contained in TWSR) corresponds
to a defined state of the Two-Wire Serial Bus. The
prescaler bits are zero or masked to zero
20.7.4 Slave Transmitter Mode
In the Slave Transmitter mode, a number of data bytes are transmitted to a Master Receiver (see Figure 20-17).
All the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero.
Figure 20-17. Data Transfer in Slave Transmitter Mode
VCC
Device 1
Device 2
SLAVE
TRANSMITTER
MASTER
RECEIVER
Device 3
........
Device n
R1
R2
SDA
SCL
To initiate the Slave Transmitter mode, TWAR and TWCR must be initialized as follows:
TWAR
value
TWA6
TWA5
TWA4
TWA3
TWA2
Device’s Own Slave Address
TWA1
TWA0
TWGCE
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The upper seven bits are the address to which the 2-wire Serial Interface will respond when addressed by a
Master. If the LSB is set, the TWI will respond to the general call address (0x00), otherwise it will ignore the
general call address.
TWCR
value
TWINT
0
TWEA
1
TWSTA
0
TWSTO
0
TWWC
0
TWEN
1
–
0
TWIE
X
TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable the
acknowledgement of the device’s own slave address or the general call address. TWSTA and TWSTO must be
written to zero.
When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own slave address (or
the general call address if enabled) followed by the data direction bit. If the direction bit is “1” (read), the TWI will
operate in ST mode, otherwise SR mode is entered. After its own slave address and the write bit have been
received, the TWINT Flag is set and a valid status code can be read from TWSR. The status code is used to
determine the appropriate software action. The appropriate action to be taken for each status code is detailed in
Table 20-4 on page 248. The Slave Transmitter mode may also be entered if arbitration is lost while the TWI is
in the Master mode (see state 0xB0).
If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the transfer. State 0xC0
or state 0xC8 will be entered, depending on whether the Master Receiver transmits a NACK or ACK after the
final byte. The TWI is switched to the not addressed Slave mode, and will ignore the Master if it continues the
transfer. Thus the Master Receiver receives all “1” as serial data. State 0xC8 is entered if the Master demands
additional data bytes (by transmitting ACK), even though the Slave has transmitted the last byte (TWEA zero
and expecting NACK from the Master).
While TWEA is zero, the TWI does not respond to its own slave address. However, the 2-wire Serial Bus is still
monitored and address recognition may resume at any time by setting TWEA. This implies that the TWEA bit
may be used to temporarily isolate the TWI from the 2-wire Serial Bus.
In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA bit is set, the
interface can still acknowledge its own slave address or the general call address by using the 2-wire Serial Bus
clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL clock will low
during the wake up and until the TWINT Flag is cleared (by writing it to one). Further data transmission will be
carried out as normal, with the AVR clocks running as normal. Observe that if the AVR is set up with a long startup time, the SCL line may be held low for a long time, blocking other data transmissions.
Note that the 2-wire Serial Interface Data Register – TWDR does not reflect the last byte present on the bus
when waking up from these sleep modes.
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Table 20-4.
Status Code
(TWSR)
Prescaler
Bits
are 0
0xA8
0xB0
0xB8
0xC0
0xC8
Status Codes for Slave Transmitter Mode
Application Software Response
Status of the 2-wire Serial Bus
and 2-wire Serial Interface Hardware
To TWCR
To/from TWDR
STA
STO
TWIN
T
TWE
A
Load data byte or
X
0
1
0
Load data byte
X
0
1
1
Arbitration lost in SLA+R/W as
Master; own SLA+R has been received; ACK has been returned
Load data byte or
X
0
1
0
Load data byte
X
0
1
1
Data byte in TWDR has been
transmitted; ACK has been received
Load data byte or
X
0
1
0
Load data byte
X
0
1
1
Data byte in TWDR has been
transmitted; NOT ACK has been
received
No TWDR action or
0
0
1
0
No TWDR action or
0
0
1
1
No TWDR action or
1
0
1
0
No TWDR action
1
0
1
1
No TWDR action or
0
0
1
0
No TWDR action or
0
0
1
1
No TWDR action or
1
0
1
0
No TWDR action
1
0
1
1
Own SLA+R has been received;
ACK has been returned
Last data byte in TWDR has been
transmitted (TWEA = “0”); ACK
has been received
Next Action Taken by TWI Hardware
Last data byte will be transmitted and NOT ACK should
be received
Data byte will be transmitted and ACK should be received
Last data byte will be transmitted and NOT ACK should
be received
Data byte will be transmitted and ACK should be received
Last data byte will be transmitted and NOT ACK should
be received
Data byte will be transmitted and ACK should be received
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
Figure 20-18. Formats and States in the Slave Transmitter Mode
Reception of the own
slave address and one or
more data bytes
S
SLA
R
A
DATA
$A8
Arbitration lost as master
and addressed as slave
A
DATA
$B8
A
P or S
$C0
A
$B0
Last data byte transmitted.
Switched to not addressed
slave (TWEA = '0')
A
All 1's
P or S
$C8
From master to slave
From slave to master
DATA
A
n
Any number of data bytes
and their associated acknowledge bits
This number (contained in TWSR) corresponds
to a defined state of the Two-Wire Serial Bus. The
prescaler bits are zero or masked to zero
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20.7.5 Miscellaneous States
There are two status codes that do not correspond to a defined TWI state, see the table below.
Status 0xF8 indicates that no relevant information is available because the TWINT Flag is not set. This occurs
between other states, and when the TWI is not involved in a serial transfer.
Status 0x00 indicates that a bus error has occurred during a 2-wire Serial Bus transfer. A bus error occurs when
a START or STOP condition occurs at an illegal position in the format frame. Examples of such illegal positions
are during the serial transfer of an address byte, a data byte, or an acknowledge bit. When a bus error occurs,
TWINT is set. To recover from a bus error, the TWSTO Flag must set and TWINT must be cleared by writing a
logic one to it. This causes the TWI to enter the not addressed Slave mode and to clear the TWSTO Flag (no
other bits in TWCR are affected). The SDA and SCL lines are released, and no STOP condition is transmitted.
Table 20-5.
Status Code
(TWSR)
Prescaler Bits
are 0
Miscellaneous States
Application Software Response
Status of the 2-wire Serial Bus
and 2-wire Serial Interface
Hardware
To TWCR
To/from TWDR
0xF8
No relevant state information
available; TWINT = “0”
No TWDR action
0x00
Bus error due to an illegal
START or STOP condition
No TWDR action
STA
STO
TWIN
T
TWE
A
No TWCR action
0
1
Next Action Taken by TWI Hardware
Wait or proceed current transfer
1
X
Only the internal hardware is affected, no STOP condition is sent on the bus. In all cases, the bus is released
and TWSTO is cleared.
20.7.6 Combining Several TWI Modes
In some cases, several TWI modes must be combined in order to complete the desired action. Consider for
example reading data from a serial EEPROM. Typically, such a transfer involves the following steps:
1. The transfer must be initiated.
2.
The EEPROM must be instructed what location should be read.
3.
The reading must be performed.
4.
The transfer must be finished.
Note that data is transmitted both from Master to Slave and vice versa. The Master must instruct the Slave what
location it wants to read, requiring the use of the MT mode. Subsequently, data must be read from the Slave,
implying the use of the MR mode. Thus, the transfer direction must be changed. The Master must keep control
of the bus during all these steps, and the steps should be carried out as an atomical operation. If this principle is
violated in a multi master system, another Master can alter the data pointer in the EEPROM between steps 2
and 3, and the Master will read the wrong data location. Such a change in transfer direction is accomplished by
transmitting a REPEATED START between the transmission of the address byte and reception of the data.
After a REPEATED START, the Master keeps ownership of the bus. The following figure shows the flow in this
transfer.
Figure 20-19. Combining Several TWI Modes to Access a Serial EEPROM
Master Transmitter
S
SLA+W
A
ADDRESS
S = START
Transmitted from master to slave
Master Receiver
A
Rs
SLA+R
A
Rs = REPEATED START
DATA
A
P
P = STOP
Transmitted from slave to master
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20.8
Multi-master Systems and Arbitration
If multiple masters are connected to the same bus, transmissions may be initiated simultaneously by one or
more of them. The TWI standard ensures that such situations are handled in such a way that one of the masters
will be allowed to proceed with the transfer, and that no data will be lost in the process. An example of an
arbitration situation is depicted below, where two masters are trying to transmit data to a Slave Receiver.
Figure 20-20. An Arbitration Example
VCC
Device 1
Device 2
Device 3
MASTER
TRANSMITTER
MASTER
TRANSMITTER
SLAVE
RECEIVER
........
Device n
R1
R2
SDA
SCL
Several different scenarios may arise during arbitration, as described below:

Two or more masters are performing identical communication with the same Slave. In this case, neither
the Slave nor any of the masters will know about the bus contention.

Two or more masters are accessing the same Slave with different data or direction bit. In this case,
arbitration will occur, either in the READ/WRITE bit or in the data bits. The masters trying to output a one
on SDA while another Master outputs a zero will lose the arbitration. Losing masters will switch to not
addressed Slave mode or wait until the bus is free and transmit a new START condition, depending on
application software action.

Two or more masters are accessing different slaves. In this case, arbitration will occur in the SLA bits.
Masters trying to output a one on SDA while another Master outputs a zero will lose the arbitration.
Masters losing arbitration in SLA will switch to Slave mode to check if they are being addressed by the
winning Master. If addressed, they will switch to SR or ST mode, depending on the value of the
READ/WRITE bit. If they are not being addressed, they will switch to not addressed Slave mode or wait
until the bus is free and transmit a new START condition, depending on application software action.
This is summarized in Figure 20-21. Possible status values are given in circles.
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Figure 20-21. Possible Status Codes Caused by Arbitration
START
SLA
Data
Arbitration lost in SLA
Own
Address / General Call
received
No
STOP
Arbitration lost in Data
38
TWI bus will be released and not addressed slave mode will be entered
A START condition will be transmitted when the bus becomes free
Yes
Direction
Write
68/78
Read
B0
Data byte will be received and NOT ACK will be returned
Data byte will be received and ACK will be returned
Last data byte will be transmitted and NOT ACK should be received
Data byte will be transmitted and ACK should be received
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20.9
TWI Register Description
20.9.1 TWI Bit Rate Register – TWBR
Bit
Read/Write
Initial Value
7
TWBR7
R/W
0
6
TWBR6
R/W
0
5
TWBR5
R/W
0
4
TWBR4
R/W
0
3
TWBR3
R/W
0
2
TWBR2
R/W
0
1
TWBR1
R/W
0
0
TWBR0
R/W
0
TWBR
• Bits 7..0 – TWI Bit Rate Register
TWBR selects the division factor for the bit rate generator. The bit rate generator is a frequency divider which
generates the SCL clock frequency in the Master modes. See “Bit Rate Generator Unit” on page 231 for
calculating bit rates.
20.9.2 TWI Control Register – TWCR
Bit
Read/Write
Initial Value
7
TWINT
R/W
0
6
TWEA
R/W
0
5
TWSTA
R/W
0
4
TWSTO
R/W
0
3
TWWC
R
0
2
TWEN
R/W
0
1
–
R
0
0
TWIE
R/W
0
TWCR
The TWCR is used to control the operation of the TWI. It is used to enable the TWI, to initiate a Master access
by applying a START condition to the bus, to generate a Receiver acknowledge, to generate a stop condition,
and to control halting of the bus while the data to be written to the bus are written to the TWDR. It also indicates
a write collision if data is attempted written to TWDR while the register is inaccessible.
• Bit 7 – TWINT: TWI Interrupt Flag
This bit is set by hardware when the TWI has finished its current job and expects application software response.
If the I-bit in SREG and TWIE in TWCR are set, the MCU will jump to the TWI Interrupt Vector. While the TWINT
Flag is set, the SCL low period is stretched. The TWINT Flag must be cleared by software by writing a logic one
to it. Note that this flag is not automatically cleared by hardware when executing the interrupt routine. Also note
that clearing this flag starts the operation of the TWI, so all accesses to the TWI Address Register (TWAR), TWI
Status Register (TWSR), and TWI Data Register (TWDR) must be complete before clearing this flag.
• Bit 6 – TWEA: TWI Enable Acknowledge Bit
The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to one, the ACK
pulse is generated on the TWI bus if the following conditions are met:
1. The device’s own slave address has been received.
2.
A general call has been received, while the TWGCE bit in the TWAR is set.
3.
A data byte has been received in Master Receiver or Slave Receiver mode.
By writing the TWEA bit to zero, the device can be virtually disconnected from the 2-wire Serial Bus temporarily.
Address recognition can then be resumed by writing the TWEA bit to one again.
• Bit 5 – TWSTA: TWI START Condition Bit
The application writes the TWSTA bit to one when it desires to become a Master on the 2-wire Serial Bus. The
TWI hardware checks if the bus is available, and generates a START condition on the bus if it is free. However,
if the bus is not free, the TWI waits until a STOP condition is detected, and then generates a new START
condition to claim the bus Master status. TWSTA must be cleared by software when the START condition has
been transmitted.
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• Bit 4 – TWSTO: TWI STOP Condition Bit
Writing the TWSTO bit to one in Master mode will generate a STOP condition on the 2-wire Serial Bus. When
the STOP condition is executed on the bus, the TWSTO bit is cleared automatically. In Slave mode, setting the
TWSTO bit can be used to recover from an error condition. This will not generate a STOP condition, but the TWI
returns to a well-defined unaddressed Slave mode and releases the SCL and SDA lines to a high impedance
state.
• Bit 3 – TWWC: TWI Write Collision Flag
The TWWC bit is set when attempting to write to the TWI Data Register – TWDR when TWINT is low. This flag
is cleared by writing the TWDR Register when TWINT is high.
• Bit 2 – TWEN: TWI Enable Bit
The TWEN bit enables TWI operation and activates the TWI interface. When TWEN is written to one, the TWI
takes control over the I/O pins connected to the SCL and SDA pins, enabling the slew-rate limiters and spike
filters. If this bit is written to zero, the TWI is switched off and all TWI transmissions are terminated, regardless of
any ongoing operation.
• Bit 1 – Res: Reserved Bit
This bit is a reserved bit and will always read as zero.
• Bit 0 – TWIE: TWI Interrupt Enable
When this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will be activated for as long
as the TWINT Flag is high.
20.9.3 TWI Status Register – TWSR
Bit
Read/Write
Initial Value
7
TWS7
R
1
6
TWS6
R
1
5
TWS5
R
1
4
TWS4
R
1
3
TWS3
R
1
2
–
R
0
1
TWPS1
R/W
0
0
TWPS0
R/W
0
TWSR
• Bits 7..3 – TWS: TWI Status
These 5 bits reflect the status of the TWI logic and the 2-wire Serial Bus. The different status codes are
described later in this section. Note that the value read from TWSR contains both the 5-bit status value and the
2-bit prescaler value. The application designer should mask the prescaler bits to zero when checking the Status
bits. This makes status checking independent of prescaler setting. This approach is used in this datasheet,
unless otherwise noted.
• Bit 2 – Res: Reserved Bit
This bit is reserved and will always read as zero.
• Bits 1..0 – TWPS: TWI Prescaler Bits
These bits can be read and written, and control the bit rate prescaler.
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Table 20-6.
TWI Prescaler Bits
TWPS1
TWPS0
Prescaler Value
0
0
1
0
1
4
1
0
16
1
1
64
To calculate bit rates, see “Bit Rate Generator Unit” on page 231. The value of TWPS1..0 is used in the
equation.
20.9.4 TWI Data Register – TWDR
Bit
Read/Write
Initial Value
7
TWD7
R/W
1
6
TWD6
R/W
1
5
TWD5
R/W
1
4
TWD4
R/W
1
3
TWD3
R/W
1
2
TWD2
R/W
1
1
TWD1
R/W
1
0
TWD0
R/W
1
TWDR
In Transmit mode, TWDR contains the next byte to be transmitted. In Receive mode, the TWDR contains the
last byte received. It is writable while the TWI is not in the process of shifting a byte. This occurs when the TWI
Interrupt Flag (TWINT) is set by hardware. Note that the Data Register cannot be initialized by the user before
the first interrupt occurs. The data in TWDR remains stable as long as TWINT is set. While data is shifted out,
data on the bus is simultaneously shifted in. TWDR always contains the last byte present on the bus, except
after a wake up from a sleep mode by the TWI interrupt. In this case, the contents of TWDR is undefined. In the
case of a lost bus arbitration, no data is lost in the transition from Master to Slave. Handling of the ACK bit is
controlled automatically by the TWI logic, the CPU cannot access the ACK bit directly.
• Bits 7..0 – TWD: TWI Data Register
These eight bits constitute the next data byte to be transmitted, or the latest data byte received on the 2-wire
Serial Bus.
20.9.5 TWI (Slave) Address Register – TWAR
Bit
Read/Write
Initial Value
7
6
5
4
3
2
1
0
TWA6
TWA5
TWA4
TWA3
TWA2
TWA1
TWA0
TWGCE
R/W
1
R/W
1
R/W
1
R/W
1
R/W
1
R/W
1
R/W
1
R/W
0
TWAR
The TWAR should be loaded with the 7-bit Slave address (in the seven most significant bits of TWAR) to which
the TWI will respond when programmed as a Slave Transmitter or Receiver, and not needed in the Master
modes. In multi master systems, TWAR must be set in masters which can be addressed as Slaves by other
Masters.
The LSB of TWAR is used to enable recognition of the general call address (0x00). There is an associated
address comparator that looks for the slave address (or general call address if enabled) in the received serial
address. If a match is found, an interrupt request is generated.
• Bits 7..1 – TWA: TWI (Slave) Address Register
These seven bits constitute the slave address of the TWI unit.
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• Bit 0 – TWGCE: TWI General Call Recognition Enable Bit
If set, this bit enables the recognition of a General Call given over the 2-wire Serial Bus.
20.9.6 TWI (Slave) Address Mask Register – TWAMR
Bit
7
6
5
4
3
2
1
0
R/W
0
R/W
0
R/W
0
R
0
TWAM[6:0]
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
R/W
0
–
TWAMR
• Bits 7..1 – TWAM: TWI Address Mask
The TWAMR can be loaded with a 7-bit Slave Address mask. Each of the bits in TWAMR can mask (disable)
the corresponding address bit in the TWI Address Register (TWAR). If the mask bit is set to one then the
address match logic ignores the compare between the incoming address bit and the corresponding bit in TWAR.
Figure 20-22 shows the address match logic in detail.
Figure 20-22. TWI Address Match Logic, Block Diagram
TWAR0
Address
Match
Address
Bit 0
TWAMR0
Address Bit Comparator 0
Address Bit Comparator 6..1
• Bit 0 – Res: Reserved Bit
This bit is reserved and will always read as zero.
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21.
USB Controller
21.1
Features
•
•
•
•
Supports full-speed and low-speed Device role
Complies with USB Specification v2.0
Supports ping-pong mode (dual bank)
832 bytes of DPRAM:
– 1 endpoint 64 bytes max. (default control endpoint)
– 1 endpoints of 256 bytes max., (one or two banks)
– 5 endpoints of 64 bytes max., (one or two banks)
• Crystal-less operation for low-speed mode
21.2
Block Diagram
The USB controller provides the hardware to interface a USB link to a data flow stored in a double port memory
(DPRAM).
The USB controller requires a 48MHz ±0.25% reference clock (for Full-Speed operation), which is the output of
an internal PLL. The on-chip PLL generates the internal high frequency (48MHz) clock for USB interface. The
PLL clock input can be configured to use external low-power crystal oscillator, external source clock or internal
RC (see Section “Crystal-less Operation”, page 259).
The 48MHz clock is used to generate a 12MHz Full-speed (or 1.5MHz Low-Speed) bit clock from the received
USB differential data and to transmit data according to full or low speed USB device tolerance. Clock recovery is
done by a Digital Phase Locked Loop (DPLL) block, which is compliant with the jitter specification of the USB
bus.
To comply with the USB Electrical specification, USB buffers (D+ or D-) should be powered within the 3.0 to
3.6V range. As ATmega16U4/ATmega32U4 can be powered up to 5.5V, an internal regulator provides the USB
buffers power supply.
Figure 21-1.
USB controller Block Diagram Overview
UVCC
AVCC
XT1
IntRC
Clock Mux
UCAP
USB Regulator
PLL
&
Div-by-2
clk
8MHz
PLL clock
Prescaler
clk
48MHz
CPU
D-
DPLL
Clock
Recovery
D+
USB
Interface
VBUS
On-Chip
USB DPRAM
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21.3
Typical Application Implementation
Depending on the target application power supply, the ATmega16U4/ATmega32U4 requires different hardware
typical implementations.
Figure 21-2.
Operating Modes versus Frequency and Power-supply
Max
Operating Frequency (MHz)
VCC (V)
5.5
16 MHz
4.5
USB compliant,
with internal regulator
3.6
8 MHz
3.4
USB compliant,
without internal regulator
3.0
2.7
USB not operational
2 MHz
VCC min
0
21.3.1 Bus Powered Device
Figure 21-3.
Typical Bus Powered Application with 5V I/O
UVCC
AVCC
VCC
UCAP
1µF
VBUS
VBUS
UDP
D+
Rs=22
UDM
DRs=22
UVSS
UID
UGND
UID
XTAL1
XTAL2
GND
GND
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Figure 21-4.
Typical Bus Powered Application with 3V I/O
External
3V Regulator
UVCC
AVCC
VCC
UCAP
1µF
VBUS
VBUS
UDP
D+
Rs=22
UDM
DRs=22
UVSS
UVSS
UGND
UID
UID
XTAL1
XTAL2
GND
GND
21.3.2 Self Powered Device
Figure 21-5.
Typical Self Powered Application with 3.4V to 5.5V I/O
External 3.4V - 5.5V
Power Supply
UVCC
AVCC
VCC
UCAP
1µF
VBUS
VBUS
UDP
D+
Rs=22
UDM
DRs=22
UVSS
UID
UGND
UID
XTAL1
XTAL2
GND
GND
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Figure 21-6.
Typical Self Powered Application with 3.0V to 3.6 I/O
External 3.0V - 3.6V
Power Supply
UVCC
AVCC
VCC
UCAP
1µF
VBUS
VBUS
UDP
D+
Rs=22
UDM
DRs=22
UVSS
UID
UGND
UID
XTAL1
21.4
XTAL2
GND
GND
Crystal-less Operation
To reduce external components count and BOM cost, the USB module can be configured to operate in lowspeed mode with internal RC oscillator as input source clock for the PLL. The internal RC oscillator is factory
calibrated to satisfy the USB low speed frequency accuracy within the 0°C and +40°C temperature range.
For USB full-speed operation only external crystal oscillator or external source clock can be used.
21.5
Design Guidelines

Serial resistors on USB Data lines must have 22 value (±5%)

Traces from the input USB receptable (or from the cable connection in the case of a tethered device) to
the USB microcontroller pads should be as short as possible, and follow differential traces routing rules
(same length, as near as possible, avoid via accumulation)

Voltage transient / ESD suppressors may also be used to prevent USB pads to be damaged by external
disturbances

Ucap capacitor should be 1µF (±10%) for correct operation

A 10µF capacitor is highly recommended on VBUS line
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21.6
General Operating Modes
21.6.1 Introduction
The USB controller is disabled and reset after an hardware reset generated by:
̶
Power on reset
̶
External reset
̶
Watchdog reset
̶
Brown out reset
̶
JTAG reset
But another available and optional CPU reset source is:
̶
USB End Of Reset
In this case, the USB controller is reset, but not disabled (so that the device remains attached).
21.6.2 Power-on and Reset
The next diagram explains the USB controller main states on power-on:
Figure 21-7.
USB Controller States after Reset
Clock stopped
FRZCLK=1
M acro off
<any other
state>
USBE=0
Reset
USBE=1
HW
RESET
USBE=0
USBE=0
Device
USB Controller state after an hardware reset is ‘Reset’. In this state:

USBE is not set

the USB controller clock is stopped in order to minimize the power consumption (FRZCLK=1),

the USB controller is disabled,

the USB pad is in the suspend mode,

the Device USB controller internal state is reset.
After setting USBE, the USB Controller enters the Device state. The controller is ‘Idle’.
The USB Controller can at any time be stopped by clearing USBE. In fact, clearing USBE acts as an hardware
reset.
21.6.3 Interrupts
Two interrupts vectors are assigned to USB interface.
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Figure 21-8.
USB Interrupt System
USB General
Interrupt
USB General
Interrupt Vector
USB Device
Interrupt
USB Endpoint/Pipe
Interrupt Vector
Endpoint
Interrupt
The USB hardware module distinguishes between USB General events and USB Endpoint events that are
relevant with data transfers relative to each endpoint.
Figure 21-9.
USB General Interrupt Vector Sources
VBUSTI
USBINT.0
UPRSMI
UDINT.6
EORSMI
UDINT.5
USB General
Interrupt Vector
VBUSTE
USBCON.0
UPRSME
UDIEN.6
EORSME
UDIEN.5
WAKEUPI
UDINT.4
WAKEUPE
UDIEN.4
EORSTI
UDINT.3
SOFI
UDINT.2
SUSPI
UDINT.0
USB Device
Interrupt
USB General
Interrupt Vector
EORSTE
UDIEN.3
SOFE
UDIEN.2
SUSPE
UDIEN.0
Asynchronous Interrupt source
(allows the CPU to wake up from power down mode)
Almost all these interrupts are time-relative events that will be detected only if the USB clock is enabled
(FRZCLK bit set), except for:

VBUS plug-in detection (insert, remove)

WAKEUP interrupt that will trigger each time a state change is detected on the data lines
This asynchronous interrupts allow to wake-up a device that is in power-down mode, generally after that the
USB has entered the Suspend state.
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Figure 21-10. USB Endpoint Interrupt Vector Sources
Endpoint 6
Endpoint 5
Endpoint 4
Endpoint 3
Endpoint 2
Endpoint 1
Endpoint 0
OVERFI
UESTAX.6
UNDERFI
UESTAX.5
NAKINI
UEINTX.6
NAKOUTI
UEINTX.4
RXSTPI
UEINTX.3
RXOUTI
UEINTX.2
FLERRE
UEIENX.7
NAKINE
UEIENX.6
TXSTPE
UEIENX.4
RXSTPE
UEIENX.3
EPINT
UEINT.X
USB Endpoint
Interrupt Vector
RXOUTE
UEIENX.2
STALLEDI
UEINTX.1
STALLEDE
UEIENX.1
TXINI
UEINTX.0
TXINE
UEIENX.0
Each endpoint has eight interrupts sources associated with flags, and each source can be enabled or not to
trigger the corresponding endpoint interrupt. If, for an endpoint, at least one of the sources is enabled to trigger
interrupt, the corresponding event(s) will make the program branch to the USB Endpoint Interrupt vector. The
user may determine the source (endpoint) of the interrupt by reading the UEINT register, and then handle the
event detected by polling the different flags.
21.7
Power Modes
21.7.1 Idle Mode
In this mode, the CPU core is halted (CPU clock stopped). The Idle mode is taken wether the USB controller is
running or not. The CPU “wakes up” on any USB interrupts.
21.7.2 Power Down
In this mode, the oscillator is stopped and halts all the clocks (CPU and peripherals). The USB controller “wakes
up” when:

the WAKEUPI interrupt is triggered

the VBUSTI interrupt is triggered
21.7.3 Freeze Clock
The firmware has the ability to reduce the power consumption by setting the FRZCLK bit, which freeze the clock
of USB controller. When FRZCLK is set, it is still possible to access to the following registers:
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
USBCON, USBSTA, USBINT

UDCON (detach, ..)

UDINT

UDIEN
Moreover, when FRZCLK is set, only the following interrupts may be triggered:
21.8

WAKEUPI

VBUSTI
Speed Control
The speed selection (Full Speed or Low Speed) depends on the D+/D- pull-up. The LSM bit in UDCON register
allows to select an internal pull up on D- (Low Speed mode) or D+ (Full Speed mode) data lines.
Figure 21-11. Device Mode Speed Selection
UCAP
DETACH
USB
Regulator
UDCON.0
LSM
RPU
RPU
UDCON.2
D+
D-
21.9
Memory Management
The controller only supports the following memory allocation management.
The reservation of a Pipe or an Endpoint can only be made in the increasing order (Pipe/Endpoint 0 to the last
Pipe/Endpoint). The firmware shall thus configure them in the same order.
The reservation of a Pipe or an Endpoint “ki” is done when its ALLOC bit is set. Then, the hardware allocates the
memory and inserts it between the Pipe/Endpoints “ki-1” and “ki+1”. The “ki+1” Pipe/Endpoint memory “slides” up
and its data is lost. Note that the “ki+2” and upper Pipe/Endpoint memory does not slide.
Clearing a Pipe enable (PEN) or an Endpoint enable (EPEN) does not clear either its ALLOC bit, or its
configuration (EPSIZE/PSIZE, EPBK/PBK). To free its memory, the firmware should clear ALLOC. Then, the
“ki+1” Pipe/Endpoint memory automatically “slides” down. Note that the “ki+2” and upper Pipe/Endpoint memory
does not slide.
The following figure illustrates the allocation and reorganization of the USB memory in a typical example:
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Table 21-1.
Allocation and Reorganization USB Memory Flow
Free memory
Free memory
Free memory
Free memory
5
5
5
5
4
4
Lost memory
3
EPEN=0
(ALLOC=1)
4
3 (bigger size)
2
2
2
2
1
1
1
1
0
0
0
0
Endpoints
activation
Endpoint Disable
Free its memory
(ALLOC=0)
Endpoint
Activatation
4
Conflict
EPEN=1
ALLOC=1

First, Endpoint 0 to Endpoint 5 are configured, in the growing order. The memory of each is reserved in
the DPRAM.

Then, the Endpoint 3 is disabled (EPEN=0), but its memory reservation is internally kept by the controller

Its ALLOC bit is cleared: the Endpoint 4 “slides” down, but the Endpoint 5 does not “slide”

Finally, if the firmware chooses to reconfigure the Endpoint 3, with a bigger size. The controller reserved
the memory after the Endpoint 2 memory and automatically “slide” the Endpoint 4. The Endpoint 5 does
not move and a memory conflict appear, in that both Endpoint 4 and 5 use a common area. The data of
those endpoints are potentially lost.
Note that:

the data of Endpoint 0 are never lost whatever the activation or deactivation of the higher Endpoint. Its
data is lost if it is deactivated.

Deactivate and reactivate the same Endpoint with the same parameters does not lead to a “slide” of the
higher endpoints. For those endpoints, the data are preserved.

CFGOK is set by hardware even in the case where there is a “conflict” in the memory allocation
21.10 PAD Suspend
The next figures illustrates the pad behaviour:

In the “idle” mode, the pad is put in low power consumption mode

In the “active” mode, the pad is working
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Figure 21-12. Pad Behaviour
Idle mode
USBE=0
| DETACH=1
| suspend
USBE=1
& DETACH=0
& suspend
Active mode
The SUSPI flag indicated that a suspend state has been detected on the USB bus. This flag automatically put
the USB pad in Idle. The detection of a non-idle event sets the WAKEUPI flag and wakes-up the USB pad.
SUSPI
Suspend detected = USB pad power down
WAKEUPI
Clear Suspend by software
Clear Resume by software
Resume = USB pad wake-up
PAD status
Active
Power Down
Active
Moreover, the pad can also be put in the “idle” mode if the DETACH bit is set. It come back in the active mode
when the DETACH bit is cleared.
21.11 Plug-in Detection
The USB connection is detected by the VBUS pad, thanks to the following architecture:
Figure 21-13. Plug-in Detection Input Block Diagram
RPU
VDD
Session_valid
VBUS
VBUSTI
USBSTA.0
USBINT.0
RPU
VBUS
VSS
Pad logic
The control logic of the VBUS pad outputs a signal regarding the VBUS voltage level:
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
The “Session_valid” signal is active high when the voltage on the UVBUS pad is higher or equal to 1.4V. If
lower than 1.4V, the signal is not active

The VBUS status bit is set when “Session_valid” signal is active (VBUS > 1.4V)

The VBUSTI flag is set each time the VBUS state changes

The USB peripheral cannot attach to the bus while VBUS bit is not set
21.12 USB Software Operating Modes
Depending on the USB operating mode, the software should perform some the following operations:
Power On the USB interface

Power-On USB pads regulator

Configure PLL interface

Enable PLL

Check PLL lock

Enable USB interface

Configure USB interface (USB speed, Endpoints configuration...)

Wait for USB VBUS information connection

Attach USB device
Power Off the USB interface

Detach USB interface

Disable USB interface

Disable PLL

Disable USB pad regulator
Suspending the USB interface

Clear Suspend Bit

Freeze USB clock

Disable PLL

Be sure to have interrupts enable to exit sleep mode

Make the MCU enter sleep mode
Resuming the USB interface

Enable PLL

Wait PLL lock

Unfreeze USB clock

Clear Resume information
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21.13 Registers Description
21.13.1 USB General Registers
Bit
Read/Write
Initial Value
7
R/W
0
6
R/W
0
5
R
0
4
R/W
0
3
R
0
2
R
0
1
R
0
0
UVREGE
R/W
0
UHWCON
• Bits 7:1 – Reserved
These bits are reserved. Do not modify these bits.
• Bit 0 – UVREGE: USB pad regulator Enable
Set to enable the USB pad regulator. Clear to disable the USB pad regulator.
Bit
Read/Write
Initial Value
7
USBE
R/W
0
6
R/W
0
5
FRZCLK
R/W
1
4
OTGPADE
R/W
0
3
R
0
2
R
0
1
R/W
0
0
VBUSTE
R/W
0
USBCON
• Bit 7 – USBE: USB macro Enable Bit
Set to enable the USB controller. Clear to disable and reset the USB controller, to disable the USB transceiver
and to disable the USB controller clock inputs.
• Bit 6 – Reserved
The value read from these bits is always 0. Do not set these bits.
• Bit 5 – FRZCLK: Freeze USB Clock Bit
Set to disable the clock inputs (the ”Resume Detection” is still active). This reduces the power consumption.
Clear to enable the clock inputs.
• Bit 4 – OTGPADE: VBUS Pad Enable
Set to enable the VBUS pad. Clear to disable the VBUS pad.
Note that this bit can be set/cleared even if USBE=0. That allows the VBUS detection even if the USB macro is
disable.
• Bits 3:1 – Reserved
The value read from these bits is always 0. Do not set these bits.
• Bit 0 – VBUSTE: VBUS Transition Interrupt Enable Bit
Set this bit to enable the VBUS Transition interrupt generation.
Clear this bit to disable the VBUS Transition interrupt generation.
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Bit
Read/Write
Initial Value
7
R
0
6
R
0
5
R
0
4
R
0
3
R
0
2
R
0
1
ID
R
1
0
VBUS
R
0
USBSTA
• Bits 7:2 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bit 1 - ID: ID status
This bit is always read as “1”, it has been conserved for compatibility with AT90USB64/128 (in which it indicates
the value of the OTG ID pin).
• Bit 0 – VBUS: VBus Flag
The value read from this bit indicates the state of the VBUS pin. This bit can be used in device mode to monitor
the USB bus connection state of the application. See “Plug-in Detection” on page 265 for more details.
Bit
Read/Write
Initial Value
7
R
0
6
R
0
5
R
0
4
R
0
3
R
0
2
R
0
1
R/W
0
0
VBUSTI
R/W
0
USBINT
• Bits 7:1 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bit 0 – VBUSTI: IVBUS Transition Interrupt Flag
Set by hardware when a transition (high to low, low to high) has been detected on the VBUS pad. This shall be
cleared by software (see “USB Software Operating Modes” on page 266).
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22.
USB Device Operating Modes
22.1
Introduction
The USB device controller supports full speed and low speed data transfers. In addition to the default control
endpoint, it provides six other endpoints, which can be configured in control, bulk, interrupt or isochronous
modes:

Endpoint 0:programmable size FIFO up to 64 bytes, default control endpoint

Endpoints 1 programmable size FIFO up to 256 bytes in ping-pong mode

Endpoints 2 to 6: programmable size FIFO up to 64 bytes in ping-pong mode
The controller starts in the “idle” mode. In this mode, the pad consumption is reduced to the minimum.
22.2
Power-on and Reset
The next diagram explains the USB device controller main states on power-on:
Figure 22-1.
USB Device Controller States after Reset
USBE=0
<any
other
state>
USBE=0
Reset
Idle
USBE=1
HW
RESET
The reset state of the Device controller is:

the macro clock is stopped in order to minimize the power consumption (FRZCLK set)

the USB device controller internal state is reset (all the registers are reset to their default value. Note that
DETACH is set.)

the endpoint banks are reset

the D+ or D- pull up are not activated (mode Detach)
The D+ or D- pull-up will be activated as soon as the DETACH bit is cleared and VBUS is present.
The macro is in the ‘Idle’ state after reset with a minimum power consumption and does not need to have the
PLL activated to enter this state.
The USB device controller can at any time be reset by clearing USBE (disable USB interface).
22.3
Endpoint Reset
An endpoint can be reset at any time by setting in the UERST register the bit corresponding to the endpoint
(EPRSTx). This resets:

the internal state machine on that endpoint

the Rx and Tx banks are cleared and their internal pointers are restored

the UEINTX, UESTA0X and UESTA1X are restored to their reset value
The data toggle field remains unchanged.
The other registers remain unchanged.
The endpoint configuration remains active and the endpoint is still enabled.
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The endpoint reset may be associated with a clear of the data toggle command (RSTDT bit) as an answer to the
CLEAR_FEATURE USB command.
22.4
USB Reset
When an USB reset is detected on the USB line (SE0 state with a minimum duration of 2.5µs), the next
operations are performed by the controller:

all the endpoints are disabled

the default control endpoint remains configured (see “Endpoint Reset” on page 270 for more details)
If the CPU hardware reset function is activated (RSTCPU bit set in UDCON register), a reset is generated to the
CPU core without disabling the USB controller (that follows the same behavior than after a standard USB End of
Reset, and remains attached). That feature may be used to enhance device reliability.
22.5
Endpoint Selection
Prior to any operation performed by the CPU, the endpoint must first be selected. This is done by setting the
EPNUM2:0 bits (UENUM register) with the endpoint number which will be managed by the CPU.
The CPU can then access to the various endpoint registers and data.
22.6
Endpoint Activation
The endpoint is maintained under reset as long as the EPEN bit is not set.
The following flow must be respected in order to activate an endpoint:
Figure 22-2.
Endpoint Activation Flow:
Endpoint
Activation
UENUM
Select the endpoint
EPNUM=x
EPEN=1
Activate the endpoint
UECFG0X
Configure:
- the endpoint direction
- the endpoint type
EPDIR
EPTYPE
...
Configure:
- the endpoint size
- the bank parametrization
Allocation and reorganization of
the memory is made on-the-fly
UECFG1X
ALLOC
EPSIZE
EPBK
CFGOK=1
Yes
Endpoint activated
Test the correct endpoint
configuration
No
ERROR
As long as the endpoint is not correctly configured (CFGOK cleared), the hardware does not acknowledge the
packets sent by the host.
CFGOK is will not be sent if the Endpoint size parameter is bigger than the DPRAM size.
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A clear of EPEN acts as an endpoint reset (see “Endpoint Reset” on page 270 for more details). It also performs
the next operation:

The configuration of the endpoint is kept (EPSIZE, EPBK, ALLOC kept)

It resets the data toggle field

The DPRAM memory associated to the endpoint is still reserved
See “Memory Management” on page 263 for more details about the memory allocation/reorganization.
22.7
Address Setup
The USB device address is set up according to the USB protocol:

the USB device, after power-up, responds at address 0

the host sends a SETUP command (SET_ADDRESS(addr))

the firmware handles this request, and records that address in UADD, but keep ADDEN cleared

the USB device firmware sends an IN command of 0 bytes (IN 0 Zero Length Packet)

then, the firmware can enable the USB device address by setting ADDEN. The only accepted address by
the controller is the one stored in UADD.
ADDEN and UADD shall not be written at the same time.
UADD contains the default address 00h after a power-up or USB reset.
ADDEN is cleared by hardware:

after a power-up reset

when an USB reset is received

or when the macro is disabled (USBE cleared)
When this bit is cleared, the default device address 00h is used.
22.8
Suspend, Wake-up and Resume
After a period of 3ms during which the USB line was inactive, the controller switches to the full-speed mode and
triggers (if enabled) the SUSPI (suspend) interrupt. The firmware may then set the FRZCLK bit.
The CPU can also, depending on software architecture, enter in the idle mode to lower again the power
consumption.
There are two ways to recover from the “Suspend” mode:

First one is to clear the FRZCLK bit. This is possible if the CPU is not in the Idle mode.

Second way, if the CPU is “idle”, is to enable the WAKEUPI interrupt (WAKEUPE set). Then, as soon as
an non-idle signal is seen by the controller, the WAKEUPI interrupt is triggered. The firmware shall then
clear the FRZCLK bit to restart the transfer.
There are no relationship between the SUSPI interrupt and the WAKEUPI interrupt: the WAKEUPI interrupt is
triggered as soon as there are non-idle patterns on the data lines. Thus, the WAKEUPI interrupt can occurs
even if the controller is not in the “suspend” mode.
When the WAKEUPI interrupt is triggered, if the SUSPI interrupt bit was already set, it is cleared by hardware.
When the SUSPI interrupt is triggered, if the WAKEUPI interrupt bit was already set, it is cleared by hardware.
22.9
Detach
The reset value of the DETACH bit is 1.
It is possible to re-enumerate a device, simply by setting and clearing the DETACH bit (but firmware must take
in account a debouncing delay of some milliseconds).
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
Setting DETACH will disconnect the pull-up on the D+ or D- pad (depending on full or low speed mode
selected). Then, clearing DETACH will connect the pull-up on the D+ or D- pad.
Figure 22-3.
Detach a Device in Full-speed
UVREF
UVREF
D+
D+
D-
D-
EN=1
Detach, then
Attach
EN=1
22.10 Remote Wake-up
The “Remote Wake-up” (or “upstream resume”) feature is the only operation allowed to be sent by the device on
its own initiative. Anyway, to do that, the device should first have received a DEVICE_REMOTE_WAKEUP
request from the host.

First, the USB controller must have detected the “suspend” state of the line: the remote wake-up can only
be sent when a SUSPI flag is set

The firmware has then the ability to set RMWKUP to send the “upstream resume” stream. This will
automatically be done by the controller after 5ms of inactivity on the USB line.

When the controller starts to send the “upstream resume”, the UPRSMI interrupt is triggered (if enabled).
SUSPI is cleared by hardware

RMWKUP is cleared by hardware at the end of the “upstream resume”

If the controller detects a good “End Of Resume” signal from the host, an EORSMI interrupt is triggered (if
enabled)
22.11 STALL Request
For each endpoint, the STALL management is performed using two bits:
̶
STALLRQ (enable stall request)
̶
STALLRQC (disable stall request)
̶
STALLEDI (stall sent interrupt)
To send a STALL handshake at the next request, the STALLRQ request bit has to be set. All following requests
will be handshak’ed with a STALL until the STALLRQC bit is set.
Setting STALLRQC automatically clears the STALLRQ bit. The STALLRQC bit is also immediately cleared by
hardware after being set by software. Thus, the firmware will never read this bit as set.
Each time the STALL handshake is sent, the STALLEDI flag is set by the USB controller and the EPINTx
interrupt will be triggered (if enabled).
The incoming packets will be discarded (RXOUTI and RWAL will not be set).
The host will then send a command to reset the STALL: the firmware just has to set the STALLRQC bit and to
reset the endpoint.
22.11.1 Special Consideration for Control Endpoints
A SETUP request is always ACK’ed.
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If a STALL request is set for a Control Endpoint and if a SETUP request occurs, the SETUP request has to be
ACK’ed and the STALLRQ request and STALLEDI sent flags are automatically reset (RXSETUPI set, TXIN
cleared, STALLED cleared, TXINI cleared...).
This management simplifies the enumeration process management. If a command is not supported or contains
an error, the firmware set the STALL request flag and can return to the main task, waiting for the next SETUP
request.
This function is compliant with the Chapter 8 test that may send extra status for a GET_DESCRIPTOR. The
firmware sets the STALL request just after receiving the status. All extra status will be automatically STALL’ed
until the next SETUP request.
22.11.2 STALL Handshake and Retry Mechanism
The Retry mechanism has priority over the STALL handshake. A STALL handshake is sent if the STALLRQ
request bit is set and if there is no retry required.
22.12 CONTROL Endpoint Management
A SETUP request is always ACK’ed. When a new setup packet is received, the RXSTPI interrupt is triggered (if
enabled). The RXOUTI interrupt is not triggered.
The FIFOCON and RWAL fields are irrelevant with CONTROL endpoints. The firmware shall thus never use
them on that endpoints. When read, their value is always 0.
CONTROL endpoints are managed by the following bits:

RXSTPI is set when a new SETUP is received. It shall be cleared by firmware to acknowledge the packet
and to clear the endpoint bank.

RXOUTI is set when a new OUT data is received. It shall be cleared by firmware to acknowledge the
packet and to clear the endpoint bank.

TXINI is set when the bank is ready to accept a new IN packet. It shall be cleared by firmware to send the
packet and to clear the endpoint bank.
22.12.1 Control Write
The next figure shows a control write transaction. During the status stage, the controller will not necessary send
a NAK at the first IN token:

If the firmware knows the exact number of descriptor bytes that must be read, it can then anticipate on the
status stage and send a ZLP for the next IN token

or it can read the bytes and poll NAKINI, which tells that all the bytes have been sent by the host, and the
transaction is now in the status stage
SETUP
USB line
RXSTPI
RXOUTI
TXINI
DATA
SETUP
HW
OUT
STATUS
OUT
IN
IN
NAK
SW
HW
SW
HW
SW
SW
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22.12.2 Control Read
The next figure shows a control read transaction. The USB controller has to manage the simultaneous write
requests from the CPU and the USB host:
SETUP
USB line
DATA
SETUP
RXSTPI
HW
IN
STATUS
IN
OUT
NAK
SW
RXOUTI
TXINI
OUT
HW
SW
HW
SW
SW
Wr Enable
HOST
Wr Enable
CPU
A NAK handshake is always generated at the first status stage command.
When the controller detect the status stage, all the data written by the CPU are erased, and clearing TXINI has
no effects.
The firmware checks if the transmission is complete or if the reception is complete.
The OUT retry is always ack’ed. This reception:
- set the RXOUTI flag (received OUT data)
- set the TXINI flag (data sent, ready to accept new data)
software algorithm:
set transmit ready
wait (transmit complete OR Receive complete)
if receive complete, clear flag and return
if transmit complete, continue
Once the OUT status stage has been received, the USB controller waits for a SETUP request. The SETUP
request have priority over any other request and has to be ACK’ed. This means that any other flag should be
cleared and the fifo reset when a SETUP is received.
WARNING: the byte counter is reset when the OUT Zero Length Packet is received. The firmware has to take
care of this.
22.13 OUT Endpoint Management
OUT packets are sent by the host. All the data can be read by the CPU, which acknowledges or not the bank
when it is empty.
22.13.1 Overview
The Endpoint must be configured first.
Each time the current bank is full, the RXOUTI and the FIFOCON bits are set. This triggers an interrupt if the
RXOUTE bit is set. The firmware can acknowledge the USB interrupt by clearing the RXOUTI bit. The Firmware
read the data and clear the FIFOCON bit in order to free the current bank. If the OUT Endpoint is composed of
multiple banks, clearing the FIFOCON bit will switch to the next bank. The RXOUTI and FIFOCON bits are then
updated by hardware in accordance with the status of the new bank.
RXOUTI shall always be cleared before clearing FIFOCON.
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The RWAL bit always reflects the state of the current bank. This bit is set if the firmware can read data from the
bank, and cleared by hardware when the bank is empty.
Example with 1 OUT data bank
OUT
DATA
(to bank 0)
NAK
ACK
OUT
DATA
(to bank 0)
ACK
HW
RXOUTI
HW
SW
SW
read data from CPU
BANK 0
FIFOCON
SW
read data from CPU
BANK 0
Example with 2 OUT data banks
OUT
DATA
(to bank 0)
ACK
OUT
DATA
(to bank 1)
ACK
HW
RXOUTI
FIFOCON
HW
SW
SW
read data from CPU
BANK 0
SW
read data from CPU
BANK 1
22.13.2 Detailed description
22.13.2.1
The data are read by the CPU, following the next flow:

When the bank is filled by the host, an endpoint interrupt (EPINTx) is triggered, if enabled (RXOUTE set)
and RXOUTI is set. The CPU can also poll RXOUTI or FIFOCON, depending on the software architecture

The CPU acknowledges the interrupt by clearing RXOUTI

The CPU can read the number of byte (N) in the current bank (N=BYCT)

The CPU can read the data from the current bank (“N” read of UEDATX)

The CPU can free the bank by clearing FIFOCON when all the data is read, that is:
̶
after “N” read of UEDATX
̶
as soon as RWAL is cleared by hardware
If the endpoint uses 2 banks, the second one can be filled by the HOST while the current one is being read by
the CPU. Then, when the CPU clear FIFOCON, the next bank may be already ready and RXOUTI is set
immediately.
22.14 IN endpoint management
IN packets are sent by the USB device controller, upon an IN request from the host. All the data can be written
by the CPU, which acknowledge or not the bank when it is full.Overview
The Endpoint must be configured first.
The TXINI bit is set by hardware when the current bank becomes free. This triggers an interrupt if the TXINE bit
is set. The FIFOCON bit is set at the same time. The CPU writes into the FIFO and clears the FIFOCON bit to
allow the USB controller to send the data. If the IN Endpoint is composed of multiple banks, this also switches to
the next data bank. The TXINI and FIFOCON bits are automatically updated by hardware regarding the status
of the next bank.
TXINI shall always be cleared before clearing FIFOCON.
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The RWAL bit always reflects the state of the current bank. This bit is set if the firmware can write data to the
bank, and cleared by hardware when the bank is full
.
Example with 1 IN data bank
NAK
DATA
(bank 0)
IN
ACK
IN
HW
TXINI
FIFOCON
SW
write data from CPU
BANK 0
SW
SW
SW
write data from CPU
BANK 0
Example with 2 IN data banks
DATA
(bank 0)
IN
ACK
IN
DATA
(bank 1)
ACK
HW
TXINI
FIFOCON
SW
write data from CPU
BANK 0
SW
SW
write data from CPU
BANK 1
SW
write data from CPU
BANK0
SW
22.14.1 Detailed Description
The data are written by the CPU, following the next flow:

When the bank is empty, an endpoint interrupt (EPINTx) is triggered, if enabled (TXINE set) and TXINI is
set. The CPU can also poll TXINI or FIFOCON, depending the software architecture choice

The CPU acknowledges the interrupt by clearing TXINI

The CPU can write the data into the current bank (write in UEDATX)

The CPU can free the bank by clearing FIFOCON when all the data are written, that is:

after “N” write into UEDATX

as soon as RWAL is cleared by hardware
If the endpoint uses two banks, the second one can be read by the HOST while the current is being written by
the CPU. Then, when the CPU clears FIFOCON, the next bank may be already ready (free) and TXINI is set
immediately.
22.14.1.1Abort
An “abort” stage can be produced by the host in some situations:

In a control transaction: ZLP data OUT received during a IN stage

In an isochronous IN transaction: ZLP data OUT received on the OUT endpoint during a IN stage on the
IN endpoint

...
The KILLBK bit is used to kill the last “written” bank. The best way to manage this abort is to perform the
following operations:
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Table 22-1.
Abort Flow
Endpoint
Abort
Clear
UEIENX.
TXINE
NBUSYBK
=0
Yes
Disable the TXINI interrupt.
Abort is based on the fact
that no banks are busy,
meaning that nothing has to
be sent.
No
Endpoint
reset
Yes
KILLBK=1
Kill the last written
bank.
KILLBK=1
Wait for the end of the
procedure.
No
Abort done
22.15 Isochronous Mode
22.15.1 Underflow
An underflow can occur during IN stage if the host attempts to read a bank which is empty. In this situation, the
UNDERFI interrupt is triggered.
An underflow can also occur during OUT stage if the host send a packet while the banks are already full.
Typically, the CPU is not fast enough. The packet is lost.
It is not possible to have underflow error during OUT stage, in the CPU side, since the CPU should read only if
the bank is ready to give data (RXOUTI=1 or RWAL=1)
22.15.2 CRC Error
A CRC error can occur during OUT stage if the USB controller detects a bad received packet. In this situation,
the STALLEDI interrupt is triggered. This does not prevent the RXOUTI interrupt from being triggered.
22.16 Overflow
In Control, Isochronous, Bulk or Interrupt Endpoint, an overflow can occur during OUT stage, if the host
attempts to write in a bank that is too small for the packet. In this situation, the OVERFI interrupt is triggered (if
enabled). The packet is acknowledged and the RXOUTI interrupt is also triggered (if enabled). The bank is filled
with the first bytes of the packet.
It is not possible to have overflow error during IN stage, in the CPU side, since the CPU should write only if the
bank is ready to access data (TXINI=1 or RWAL=1).
22.17 Interrupts
Figure 22-4 shows all the interrupts sources.
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Figure 22-4.
USB Device Controller Interrupt System
UPRSMI
UDINT.6
EORSMI
UDINT.5
UPRSME
UDIEN.6
EORSME
UDIEN.5
WAKEUPI
UDINT.4
WAKEUPE
UDIEN.4
EORSTI
UDINT.3
SOFI
UDINT.2
SUSPI
UDINT.0
USB Device
Interrupt
EORSTE
UDIEN.3
SOFE
UDIEN.2
SUSPE
UDIEN.0
There are two kind of interrupts: processing (i.e. their generation are part of the normal processing) and
exception (errors).
Processing interrupts are generated when:

VBUS plug-in detection (insert, remove)(VBUSTI)

Upstream resume(UPRSMI)

End of resume(EORSMI)

Wake up(WAKEUPI)

End of reset (Speed Initialization)(EORSTI)

Start of frame(SOFI, if FNCERR=0)

Suspend detected after 3ms of inactivity(SUSPI)
Exception Interrupts are generated when:

CRC error in frame number of SOF(SOFI, FNCERR=1)
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Figure 22-5.
USB Device Controller Endpoint Interrupt System
Endpoint 6
Endpoint 5
Endpoint 4
Endpoint 3
Endpoint 2
Endpoint 1
Endpoint 0
OVERFI
UESTAX.6
UNDERFI
UESTAX.5
NAKINI
UEINTX.6
NAKOUTI
UEINTX.4
RXSTPI
UEINTX.3
RXOUTI
UEINTX.2
FLERRE
UEIENX.7
NAKINE
UEIENX.6
TXSTPE
UEIENX.4
TXOUTE
UEIENX.3
Endpoint Interrupt
EPINT
UEINT.X
RXOUTE
UEIENX.2
STALLEDI
UEINTX.1
STALLEDE
UEIENX.1
TXINI
UEINTX.0
TXINE
UEIENX.0
Processing interrupts are generated when:

Ready to accept IN data(EPINTx, TXINI=1)

Received OUT data(EPINTx, RXOUTI=1)

Received SETUP(EPINTx, RXSTPI=1)
Exception Interrupts are generated when:

Stalled packet(EPINTx, STALLEDI=1)

CRC error on OUT in isochronous mode(EPINTx, STALLEDI=1)

Overflow in isochronous mode(EPINTx, OVERFI=1)

Underflow in isochronous mode(EPINTx, UNDERFI=1)

NAK IN sent(EPINTx, NAKINI=1)

NAK OUT sent(EPINTx, NAKOUTI=1)
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22.18 Registers
22.18.1 USB Device General Registers
Bit
7
6
5
4
3
2
1
0
-
-
-
-
RSTCPU
LSM
RMWKUP
DETACH
Read/Write
R
R
R
R
R
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
1
UDCON
• Bits 7:4 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bit 3 - RSTCPU - USB Reset CPU bit
Set this bit to 1 by firmware in order to reset the CPU on the detection of a USB End of Reset signal (without
disabling the USB controller and Attached state). This bit is reset when the USB controller is disabled, but is not
affected by the CPU reset generated after a USB End of Reset (remains enabled).
• Bit 2 - LSM - USB Device Low Speed Mode Selection
When configured USB is configured in device mode, this bit allows to select the USB the USB Low Speed or Full
Speed Mod.
Clear to select full speed mode (D+ internal pull-up will be activate with the ATTACH bit will be set).
Set to select low speed mode (D- internal pull-up will be activate with the ATTACH bit will be set). This bit has no
effect when the USB interface is configured in HOST mode.
• Bit 1- RMWKUP - Remote Wake-up Bit
Set to send an “upstream-resume” to the host for a remote wake-up (the SUSPI bit must be set).
Cleared by hardware when signalling finished. Clearing by software has no effect.
See Section 22.10, page 273 for more details.
• Bit 0 - DETACH - Detach Bit
Set to physically detach de device (disconnect internal pull-up on D+ or D-).
Clear to reconnect the device. See Section 22.9, page 272 for more details
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Bit
7
6
5
4
3
2
1
0
-
UPRSMI
EORSMI
WAKEUPI
EORSTI
SOFI
-
SUSPI
0
0
0
0
0
0
0
0
UDINT
Read/Write
Initial Value
• Bit 7 - Reserved
The value read from this bits is always 0. Do not set this bit.
• Bit 6 - UPRSMI - Upstream Resume Interrupt Flag
Set by hardware when the USB controller is sending a resume signal called “Upstream Resume”. This triggers
an USB interrupt if UPRSME is set.
Shall be cleared by software (USB clocks must be enabled before). Setting by software has no effect.
• Bit 5 - EORSMI - End Of Resume Interrupt Flag
Set by hardware when the USB controller detects a good “End Of Resume” signal initiated by the host. This
triggers an USB interrupt if EORSME is set.
Shall be cleared by software. Setting by software has no effect.
• Bit 4 - WAKEUPI - Wake-up CPU Interrupt Flag
Set by hardware when the USB controller is re-activated by a filtered non-idle signal from the lines (not by an
upstream resume). This triggers an interrupt if WAKEUPE is set.
Shall be cleared by software (USB clock inputs must be enabled before). Setting by software has no effect.
See “Suspend, Wake-up and Resume” on page 272 for more details.
• Bit 3 - EORSTI - End Of Reset Interrupt Flag
Set by hardware when an “End Of Reset” has been detected by the USB controller. This triggers an USB
interrupt if EORSTE is set.
Shall be cleared by software. Setting by software has no effect.
• Bit 2 - SOFI - Start Of Frame Interrupt Flag
Set by hardware when an USB “Start Of Frame” PID (SOF) has been detected (every 1ms). This triggers an
USB interrupt if SOFE is set.
• Bit 1 - Reserved
The value read from this bits is always 0. Do not set this bit
• Bit 0 - SUSPI - Suspend Interrupt Flag
Shall be cleared by software. Setting by software has no effect. The interrupt bits are set even if their
corresponding ‘Enable’ bits is not set.
See “Suspend, Wake-up and Resume” on page 272 for more details.
See for more details.
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Bit
7
6
5
4
3
2
1
0
-
UPRSME
EORSME
WAKEUPE
EORSTE
SOFE
-
SUSPE
0
0
0
0
0
0
0
0
UDIEN
Read/Write
Initial Value
• Bit 7 - Reserved
The value read from this bits is always 0. Do not set this bit.
• Bit 6 - UPRSME - Upstream Resume Interrupt Enable Bit
Set to enable the UPRSMI interrupt.
Clear to disable the UPRSMI interrupt.
• Bit 5 - EORSME - End Of Resume Interrupt Enable Bit
Set to enable the EORSMI interrupt.
Clear to disable the EORSMI interrupt.
• Bit 4 - WAKEUPE - Wake-up CPU Interrupt Enable Bit
Set to enable the WAKEUPI interrupt.
Clear to disable the WAKEUPI interrupt.
• Bit 3 - EORSTE - End Of Reset Interrupt Enable Bit
Set to enable the EORSTI interrupt. This bit is set after a reset.
Clear to disable the EORSTI interrupt.
• Bit 2 - SOFE - Start Of Frame Interrupt Enable Bit
Set to enable the SOFI interrupt.
Clear to disable the SOFI interrupt.
• Bit 1 - Reserved
The value read from this bits is always 0. Do not set this bit.
• Bit 0 - SUSPE - Suspend Interrupt Enable Bit
Set to enable the SUSPI interrupt.
Clear to disable the SUSPI interrupt.
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Bit
7
6
5
4
3
ADDEN
2
1
0
UADD6:0
UDADDR
Read/Write
W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
• Bit 7 - ADDEN - Address Enable Bit
Set to activate the UADD (USB address).
Cleared by hardware. Clearing by software has no effect.
See “Address Setup” on page 272 for more details.
• Bits 6-0 - UADD6:0 - USB Address Bits
Load by software to configure the device address
Bit
7
6
5
4
3
-
-
-
-
-
2
1
0
Read/Write
R
R
R
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
FNUM10:8
UDFNUMH
• 7-3 - Reserved
The value read from these bits is always 0. Do not set these bits.
• 2-0 - FNUM10:8 - Frame Number Upper Value
Set by hardware. These bits are the three MSB of the 11-bits Frame Number information. They are provided in
the last received SOF packet. FNUM is updated if a corrupted SOF is received.
Bit
7
6
5
4
3
2
1
0
FNUM7:0
UDFNUML
Read/Write
R
R
R
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
• Bits 7:0 - FNUM7:0 - Frame Number Lower Value
Set by hardware. These bits are the 8 LSB of the 11-bits Frame Number information
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Bit
7
6
5
4
3
2
1
0
-
-
-
FNCERR
-
-
-
-
0
0
0
0
Read/Wri
te
Initial
Value
UDMFN
R
0
0
0
0
• Bits 7:5 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bit 4 - FNCERR -Frame Number CRC Error Flag
Set by hardware when a corrupted Frame Number in start of frame packet is received.
This bit and the SOFI interrupt are updated at the same time.
• Bits 3:0 - Reserved
The value read from these bits is always 0. Do not set these bits.
22.18.2 USB Device Endpoint Registers
Bit
7
6
5
4
3
-
-
-
-
-
2
1
0
Read/Write
R
R
R
R
R
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
EPNUM2:0
UENUM
• Bits 7:3 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bits 2-0 - EPNUM2:0 Endpoint Number Bits
Load by software to select the number of the endpoint which shall be accessed by the CPU. See “Endpoint
Reset” on page 270 for more details.
EPNUM = 111b is forbidden
Bit
7
6
5
4
3
2
1
0
-
EPRST6
EPRST5
EPRST4
EPRST3
EPRST2
EPRST1
EPRST0
Read/Write
R
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
UERST
• Bit 7 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bits 6-0 - EPRST6:0 - Endpoint FIFO Reset Bits
Set to reset the selected endpoint FIFO prior to any other operation, upon hardware reset or when an USB bus
reset has been received. See “Endpoint Reset” on page 270 for more information
Then, clear by software to complete the reset operation and start using the endpoint.
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Bit
7
6
5
4
3
2
1
0
-
-
STALLRQ
STALLRQC
RSTDT
-
-
EPEN
Read/Write
R
R
W
W
W
R
R
R/W
Initial Value
0
0
0
0
0
0
0
0
UECONX
• Bits 7:6 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bit 5 - STALLRQ - STALL Request Handshake Bit
Set to request a STALL answer to the host for the next handshake.
Cleared by hardware when a new SETUP is received. Clearing by software has no effect.
See “STALL Request” on page 273 for more details.
• Bit 4 - STALLRQC - STALL Request Clear Handshake Bit
Set to disable the STALL handshake mechanism.
Cleared by hardware immediately after the set. Clearing by software has no effect.
See “STALL Request” on page 273 for more details.
• Bit 3 - RSTDT - Reset Data Toggle Bit
Set to automatically clear the data toggle sequence:
For OUT endpoint: the next received packet will have the data toggle 0.
For IN endpoint: the next packet to be sent will have the data toggle 0.
Cleared by hardware instantaneously. The firmware does not have to wait that the bit is cleared. Clearing by
software has no effect.
• 2 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bit 1 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bit 0 - EPEN - Endpoint Enable Bit
Set to enable the endpoint according to the device configuration. Endpoint 0 shall always be enabled after a
hardware or USB reset and participate in the device configuration.
Clear this bit to disable the endpoint. See “Endpoint Activation” on page 271 for more details.
Bit
7
6
EPTYPE1:0
5
4
3
2
1
0
-
-
-
-
-
EPDIR
Read/Write
R/W
R/W
R
R
R
R
R
R/W
Initial Value
0
0
0
0
0
0
0
0
UECFG0X
• Bits 7:6 - EPTYPE1:0 - Endpoint Type Bits
Set this bit according to the endpoint configuration:
00b: Control10b: Bulk
01b: Isochronous11b: Interrupt
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• Bits 5:1 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bit 0 - EPDIR - Endpoint Direction Bit
Set to configure an IN direction for bulk, interrupt or isochronous endpoints.
Clear to configure an OUT direction for bulk, interrupt, isochronous or control endpoints.
Bit
7
6
-
5
4
3
EPSIZE2:0
2
EPBK1:0
1
0
ALLOC
-
Read/Write
R
R/W
R/W
R/W
R/W
R/W
R/W
R
Initial Value
0
0
0
0
0
0
0
0
1
0
UECFG1X
• Bit 7 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bits 6-4 - EPSIZE2:0 - Endpoint Size Bits
Set this bit according to the endpoint size:
000b: 8 bytes100b: 128 bytes
001b: 16 bytes101b: 256 bytes
010b: 32 bytes110b: 512 bytes
011b: 64 bytes111b: Reserved. Do not use this configuration.
• Bits 3:2 - EPBK1:0 - Endpoint Bank Bits
Set this field according to the endpoint size:
00b: One bank
01b: Double bank
1xb: Reserved. Do not use this configuration.
• Bit 1 - ALLOC - Endpoint Allocation Bit
Set this bit to allocate the endpoint memory.
Clear to free the endpoint memory.
See “Endpoint Activation” on page 271 for more details.
• Bit 0 - Reserved
The value read from these bits is always 0. Do not set these bits.
Bit
7
6
5
4
CFGOK
OVERFI
UNDERFI
-
3
2
Read/Write
R
R/W
R/W
R/W
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
DTSEQ1:0
NBUSYBK1:0
UESTA0X
• Bit 7 - CFGOK - Configuration Status Flag
Set by hardware when the endpoint X size parameter (EPSIZE) and the bank parametrization (EPBK) are
correct compared to the maximum FIFO capacity and the maximum number of allowed bank. This bit is updated
when the bit ALLOC is set.
If this bit is cleared, the user should reprogram the UECFG1X register with correct EPSIZE and EPBK values.
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• Bit 6 - OVERFI - Overflow Error Interrupt Flag
Set by hardware when an overflow error occurs in an isochronous endpoint. An interrupt (EPINTx) is triggered
(if enabled).
See Section 22.15, page 278 for more details.
Shall be cleared by software. Setting by software has no effect.
• Bit 5 - UNDERFI - Flow Error Interrupt Flag
Set by hardware when an underflow error occurs in an isochronous endpoint. An interrupt (EPINTx) is triggered
(if enabled).
See Section 22.15, page 278 for more details.
Shall be cleared by software. Setting by software has no effect.
• Bit 4 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bits 3-2 - DTSEQ1:0 - Data Toggle Sequencing Flag
Set by hardware to indicate the PID data of the current bank:
00bData0
01bData1
1xbReserved
For OUT transfer, this value indicates the last data toggle received on the current bank.
For IN transfer, it indicates the Toggle that will be used for the next packet to be sent. This is not relative to the
current bank.
• Bits 1:0 - NBUSYBK1:0 - Busy Bank Flag
Set by hardware to indicate the number of busy bank.
For IN endpoint, it indicates the number of busy bank(s), filled by the user, ready for IN transfer.
For OUT endpoint, it indicates the number of busy bank(s) filled by OUT transaction from the host.
00bAll banks are free
01b1 busy bank
10b2 busy banks
11bReserved
Bit
7
6
5
4
3
2
-
-
-
-
-
CTRLDIR
1
0
Read/Write
R
R
R
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
CURRBK1:0
UESTA1X
• Bits 7:3 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bit 2 - CTRLDIR - Control Direction (Flag, and bit for debug purpose)
Set by hardware after a SETUP packet, and gives the direction of the following packet:
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- 1 for IN endpoint
- 0 for OUT endpoint
Can not be set or cleared by software.
• Bits 1:0 - CURRBK1:0 - Current Bank (all endpoints except Control endpoint) Flag
Set by hardware to indicate the number of the current bank:
00bBank0
01bBank1
1xbReserved
Can not be set or cleared by software.
Bit
7
6
5
4
3
2
1
0
FIFOCON
NAKINI
RWAL
NAKOUTI
RXSTPI
RXOUTI
STALLEDI
TXINI
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
UEINTX
• Bit 7 - FIFOCON - FIFO Control Bit
For OUT and SETUP Endpoint:
Set by hardware when a new OUT message is stored in the current bank, at the same time than RXOUT or
RXSTP.
Clear to free the current bank and to switch to the following bank. Setting by software has no effect.
For IN Endpoint:
Set by hardware when the current bank is free, at the same time than TXIN.
Clear to send the FIFO data and to switch the bank. Setting by software has no effect.
• Bit 6 - NAKINI - NAK IN Received Interrupt Flag
Set by hardware when a NAK handshake has been sent in response of a IN request from the host. This triggers
an USB interrupt if NAKINE is sent.
Shall be cleared by software. Setting by software has no effect.
• Bit 5 - RWAL - Read/Write Allowed Flag
Set by hardware to signal:
- for an IN endpoint: the current bank is not full i.e. the firmware can push data into the FIFO
- for an OUT endpoint: the current bank is not empty, i.e. the firmware can read data from the FIFO
The bit is never set if STALLRQ is set, or in case of error.
Cleared by hardware otherwise.
This bit shall not be used for the control endpoint.
• Bit 4 - NAKOUTI - NAK OUT Received Interrupt Flag
Set by hardware when a NAK handshake has been sent in response of a OUT/PING request from the host. This
triggers an USB interrupt if NAKOUTE is sent.
Shall be cleared by software. Setting by software has no effect.
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• Bit 3 - RXSTPI - Received SETUP Interrupt Flag
Set by hardware to signal that the current bank contains a new valid SETUP packet. An interrupt (EPINTx) is
triggered (if enabled).
Shall be cleared by software to handshake the interrupt. Setting by software has no effect.
This bit is inactive (cleared) if the endpoint is an IN endpoint.
• Bit 2 - RXOUTI / KILLBK - Received OUT Data Interrupt Flag
Set by hardware to signal that the current bank contains a new packet. An interrupt (EPINTx) is triggered (if
enabled).
Shall be cleared by software to handshake the interrupt. Setting by software has no effect.
Kill Bank IN Bit
Set this bit to kill the last written bank.
Cleared by hardware when the bank is killed. Clearing by software has no effect.
See “Abort” on page 277 for more details on the Abort.
• Bit 1 - STALLEDI - STALLEDI Interrupt Flag
Set by hardware to signal that a STALL handshake has been sent, or that a CRC error has been detected in a
OUT isochronous endpoint.
Shall be cleared by software. Setting by software has no effect.
• Bit 0 - TXINI - Transmitter Ready Interrupt Flag
Set by hardware to signal that the current bank is free and can be filled. An interrupt (EPINTx) is triggered (if
enabled).
Shall be cleared by software to handshake the interrupt. Setting by software has no effect.
This bit is inactive (cleared) if the endpoint is an OUT endpoint.
Bit
7
6
5
4
3
2
1
0
FLERRE
NAKINE
-
NAKOUTE
RXSTPE
RXOUTE
STALLEDE
TXINE
Read/Write
R/W
R/W
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
UEIENX
• Bit 7 - FLERRE - Flow Error Interrupt Enable Flag
Set to enable an endpoint interrupt (EPINTx) when OVERFI or UNDERFI are sent.
Clear to disable an endpoint interrupt (EPINTx) when OVERFI or UNDERFI are sent.
• Bit 6 - NAKINE - NAK IN Interrupt Enable Bit
Set to enable an endpoint interrupt (EPINTx) when NAKINI is set.
Clear to disable an endpoint interrupt (EPINTx) when NAKINI is set.
• Bit 5 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bit 4 - NAKOUTE - NAK OUT Interrupt Enable Bit
Set to enable an endpoint interrupt (EPINTx) when NAKOUTI is set.
Clear to disable an endpoint interrupt (EPINTx) when NAKOUTI is set.
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• Bit 3 - RXSTPE - Received SETUP Interrupt Enable Flag
Set to enable an endpoint interrupt (EPINTx) when RXSTPI is sent.
Clear to disable an endpoint interrupt (EPINTx) when RXSTPI is sent.
• Bit 2 - RXOUTE - Received OUT Data Interrupt Enable Flag
Set to enable an endpoint interrupt (EPINTx) when RXOUTI is sent.
Clear to disable an endpoint interrupt (EPINTx) when RXOUTI is sent.
• Bit 1 - STALLEDE - Stalled Interrupt Enable Flag
Set to enable an endpoint interrupt (EPINTx) when STALLEDI is sent.
Clear to disable an endpoint interrupt (EPINTx) when STALLEDI is sent.
• Bit 0 - TXINE - Transmitter Ready Interrupt Enable Flag
Set to enable an endpoint interrupt (EPINTx) when TXINI is sent.
Clear to disable an endpoint interrupt (EPINTx) when TXINI is sent.
Bit
7
6
5
4
3
2
1
0
DAT D7
DAT D6
DAT D5
DAT D4
DAT D3
DAT D2
DAT D1
DAT D0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
UEDATX
• Bits 7:0 - DAT7:0 -Data Bits
Set by the software to read/write a byte from/to the endpoint FIFO selected by EPNUM
Bit
7
-
6
-
5
-
4
3
-
2
-
1
BYCT D10
0
BYCT D9
BYCT D8
Read/Write
R
R
R
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
UEBCHX
• Bits 7:3 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bits 2:0 - BYCT10:8 - Byte count (high) Bits
Set by hardware. This field is the MSB of the byte count of the FIFO endpoint. The LSB part is provided by the
UEBCLX register
Bit
7
6
5
4
3
2
1
0
BYCT D7
BYCT D6
BYCT D5
BYCT D4
BYCT D3
BYCT D2
BYCT D1
BYCT D0
Read/Write
R
R
R
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
UEBCLX
• Bits 7-0 - BYCT7:0 - Byte Count (low) Bits
Set by the hardware. BYCT10:0 is:

(for IN endpoint) increased after each writing into the endpoint and decremented after each byte sent,

(for OUT endpoint) increased after each byte sent by the host, and decremented after each byte read by
the software.
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Bit
7
6
5
4
3
2
1
0
-
EPINT D6
EPINT D5
EPINT D4
EPINT D3
EPINT D2
EPINT D1
EPINT D0
Read/Write
R
R
R
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
UEINT
• Bit 7 - Reserved
The value read from these bits is always 0. Do not set these bits.
• Bits 6:0 - EPINT6:0 - Endpoint Interrupts Bits
Set by hardware when an interrupt is triggered by the UEINTX register and if the corresponding endpoint
interrupt enable bit is set.
Cleared by hardware when the interrupt source is served.
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23.
Analog Comparator
The Analog Comparator compares the input values on the positive pin AIN+ and negative pin AIN-. When the
voltage on the positive pin AIN+ is higher than the voltage on the negative pin AIN-, the Analog Comparator
output, ACO, is set. The comparator’s output can be set to trigger the Timer/Counter1 Input Capture function. In
addition, the comparator can trigger a separate interrupt, exclusive to the Analog Comparator. The user can
select Interrupt triggering on comparator output rise, fall or toggle. A block diagram of the comparator and its
surrounding logic is shown in Figure 23-1. AIN+ can be connected either to the AIN0 (PE6) pin, or to the internal
Bandgap reference. AIN- can only be connected to the ADC multiplexer.
The Power Reduction ADC bit, PRADC, in “Power Reduction Register 0 - PRR0” on page 47 must be disabled
by writing a logical zero to be able to use the ADC input MUX.
Figure 23-1.
Analog Comparator Block Diagram(2)
BANDGAP
REFERENCE
ACBG
AIN+
BANDGAP
REFERENCE
AIN-
ACME
ADEN
ADC MULTIPLEXER
OUTPUT (1)
Notes:
1.
2.
See Table 23-2 on page 295.
Refer to “Pinout” on page 3 and Table 10-3 on page 74 for Analog Comparator pin placement.
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23.1
Register Description
23.1.1 ADC Control and Status Register B – ADCSRB
Bit
Read/Write
Initial Value
7
ADHSM
R
0
6
ACME
R/W
0
5
MUX5
R/W
0
4
–
R
0
3
ADTS3
R
0
2
ADTS2
R/W
0
1
ADTS1
R/W
0
0
ADTS0
R/W
0
ADCSRB
• Bit 6 – ACME: Analog Comparator Multiplexer Enable
When this bit is written logic one and the ADC is switched off (ADEN in ADCSRA is zero), the ADC multiplexer
is connected to the negative input to the Analog Comparator. When this bit is written logic zero, the Bandgap
reference is connected to the negative input of the Analog Comparator (See “Internal Voltage Reference” on
page 54.) For a detailed description of this bit, see “Analog Comparator Multiplexed Input” on page 295.
23.1.2 Analog Comparator Control and Status Register – ACSR
Bit
Read/Write
Initial Value
7
ACD
R/W
0
6
ACBG
R/W
0
5
ACO
R
N/A
4
ACI
R/W
0
3
ACIE
R/W
0
2
ACIC
R/W
0
1
ACIS1
R/W
0
0
ACIS0
R/W
0
ACSR
• Bit 7 – ACD: Analog Comparator Disable
When this bit is written logic one, the power to the Analog Comparator is switched off. This bit can be set at any
time to turn off the Analog Comparator. This will reduce power consumption in Active and Idle mode. When
changing the ACD bit, the Analog Comparator Interrupt must be disabled by clearing the ACIE bit in ACSR.
Otherwise an interrupt can occur when the bit is changed.
• Bit 6 – ACBG: Analog Comparator Bandgap Select
When this bit is set, a fixed bandgap reference voltage replaces the positive input to the Analog Comparator.
When this bit is cleared, AIN0 is applied to the positive input of the Analog Comparator. See “Internal Voltage
Reference” on page 54.
• Bit 5 – ACO: Analog Comparator Output
The output of the Analog Comparator is synchronized and then directly connected to ACO. The synchronization
introduces a delay of 1 - 2 clock cycles.
• Bit 4 – ACI: Analog Comparator Interrupt Flag
This bit is set by hardware when a comparator output event triggers the interrupt mode defined by ACIS1 and
ACIS0. The Analog Comparator interrupt routine is executed if the ACIE bit is set and the I-bit in SREG is set.
ACI is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, ACI is
cleared by writing a logic one to the flag.
• Bit 3 – ACIE: Analog Comparator Interrupt Enable
When the ACIE bit is written logic one and the I-bit in the Status Register is set, the Analog Comparator interrupt
is activated. When written logic zero, the interrupt is disabled.
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• Bit 2 – ACIC: Analog Comparator Input Capture Enable
When written logic one, this bit enables the input capture function in Timer/Counter1 to be triggered by the
Analog Comparator. The comparator output is in this case directly connected to the input capture front-end
logic, making the comparator utilize the noise canceler and edge select features of the Timer/Counter1 Input
Capture interrupt. When written logic zero, no connection between the Analog Comparator and the input
capture function exists. To make the comparator trigger the Timer/Counter1 Input Capture interrupt, the ICIE1
bit in the Timer Interrupt Mask Register (TIMSK1) must be set.
• Bits 1, 0 – ACIS1, ACIS0: Analog Comparator Interrupt Mode Select
These bits determine which comparator events that trigger the Analog Comparator interrupt. The different
settings are shown in the table below.
Table 23-1.
ACIS1/ACIS0 Settings
ACIS1
ACIS0
Interrupt Mode
0
0
Comparator Interrupt on Output Toggle
0
1
Reserved
1
0
Comparator Interrupt on Falling Output Edge
1
1
Comparator Interrupt on Rising Output Edge
When changing the ACIS1/ACIS0 bits, the Analog Comparator Interrupt must be disabled by clearing its
Interrupt Enable bit in the ACSR Register. Otherwise an interrupt can occur when the bits are changed.
23.2
Analog Comparator Multiplexed Input
It is possible to select any of the ADC13..0 pins to replace the negative input to the Analog Comparator. The
ADC multiplexer is used to select this input, and consequently, the ADC must be switched off to utilize this
feature. If the Analog Comparator Multiplexer Enable bit (ACME in ADCSRB) is set and the ADC is switched off
(ADEN in ADCSRA is zero), and MUX2..0 in ADMUX select the input pin to replace the negative input to the
Analog Comparator, as shown in the table. If ACME is cleared or ADEN is set, the Bandgap reference is applied
to the negative input to the Analog Comparator.
Table 23-2.
Analog Comparator Multiplexed Input
ACME
ADEN
MUX2..0
Analog Comparator Negative Input
0
x
xxx
Bandgap Ref.
1
1
xxx
Bandgap Ref.
1
0
000
ADC0
1
0
001
ADC1
1
0
010
1
0
011
1
0
100
ADC4
1
0
101
ADC5
1
0
110
ADC6
1
0
111
ADC7
N/A
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23.2.1 Digital Input Disable Register 1 – DIDR1
Bit
Read/Write
Initial Value
7
–
R
0
6
–
R
0
5
–
R
0
4
–
R
0
3
–
R
0
2
–
R
0
1
–
R
0
0
AIN0D
R/W
0
DIDR1
• Bit 0 – AIN0D: AIN0 Digital Input Disable
When this bit is written logic one, the digital input buffer on the AIN0 pin is disabled. The corresponding PIN
Register bit will always read as zero when this bit is set. When an analog signal is applied to the AIN0 pin and
the digital input from this pin is not needed, this bit should be written logic one to reduce power consumption in
the digital input buffer.
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24.
Analog to Digital Converter - ADC
24.1
Features
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
10/8-bit Resolution
0.5LSB Integral Non-linearity
±2LSB Absolute Accuracy
65 - 260µs Conversion Time
Up to 15kSPS at Maximum Resolution
Twelve Multiplexed Single-Ended Input Channels
One Differential amplifier providing gain of 1x - 10x - 40x - 200x
Temperature sensor
Optional Left Adjustment for ADC Result Readout
0 - VCC ADC Input Voltage Range
Selectable 2.56V ADC Reference Voltage
Free Running or Single Conversion Mode
ADC Start Conversion by Auto Triggering on Interrupt Sources
Interrupt on ADC Conversion Complete
Sleep Mode Noise Canceler
The ATmega16U4/ATmega32U4 features a 10-bit successive approximation ADC. The ADC is connected to an
12-channel Analog Multiplexer which allows six single-ended voltage inputs constructed from several pins of
Port B, D, and F. The single-ended voltage inputs refer to 0V (GND).
The device also supports 32 differential voltage input combinations, thanks to a differential amplifier equipped
with a programmable gain stage, providing amplification steps of 0 dB (1x), 10 dB (10x), 16dB (40x), or 23dB
(200x) on the differential input voltage before the A/D conversion. Two differential analog input channels share a
common negative terminal (ADC0/ADC1), while any other ADC input can be selected as the positive input
terminal. If 1x, 10x, or 40x gain is used, 8-bit resolution can be expected. If 200x gain is used, 7-bit resolution
can be expected.
The ADC contains a Sample and Hold circuit which ensures that the input voltage to the ADC is held at a
constant level during conversion. A block diagram of the ADC is shown in Figure 24-1.
The ADC has a separate analog supply voltage pin, AVCC. AVCC must not differ more than ± 0.3V from VCC. See
the paragraph “ADC Noise Canceler” on page 305 on how to connect this pin.
Internal reference voltages of nominally 2.56V or AVCC are provided On-chip. The voltage reference may be
externally decoupled at the AREF pin by a capacitor for better noise performance.
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Figure 24-1.
Analog to Digital Converter Block Schematic
ADC CONVERSION
COMPLETE IRQ
INTERRUPT
FLAGS
ADTS[3:0]
TRIGGER
SELECT
ADC[9:0]
ADPS1
0
ADC DATA REGISTER
(ADCH/ADCL)
ADPS0
ADPS2
ADIF
ADATE
ADEN
ADSC
MUX0
MUX2
15
ADC CTRL. & STATUS
REGISTER (ADCSRA)
MUX1
MUX3
MUX5
MUX4
REFS0
ADLAR
REFS1
ADC MULTIPLEXER
SELECT (ADMUX)
ADIE
ADIF
8-BIT DATA BUS
AVCC
PRESCALER
START
GAIN SELECTION
CHANNEL SELECTION
MUX DECODER
CONVERSION LOGIC
INTERNAL
REFERENCE
SAMPLE & HOLD
COMPARATOR
AREF
10-BIT DAC
+
ADHSM
GND
BANDGAP
REFERENCE
TEMPERATURE
SENSOR
ADC13
ADC12
SINGLE ENDED / DIFFERENTIAL SELECTION
POS.
INPUT
MUX
ADC MULTIPLEXER
OUTPUT
ADC11
ADC10
+
ADC9
-
DIFFERENTIAL
AMPLIFIER
ADC8
ADC7
ADC6
ADC5
ADC4
ADC1
ADC0
NEG.
INPUT
MUX
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24.2
Operation
The ADC converts an analog input voltage to a 10-bit digital value through successive approximation. The
minimum value represents GND and the maximum value represents the voltage on the AREF pin minus 1 LSB.
Optionally, AVCC or an internal 2.56V reference voltage may be connected to the AREF pin by writing to the
REFSn bits in the ADMUX Register. The internal voltage reference may thus be decoupled by an external
capacitor at the AREF pin to improve noise immunity.
The analog input channel and differential gain are selected by writing to the MUX bits in ADMUX. Any of the
ADC input pins, as well as GND and a fixed bandgap voltage reference, can be selected as single ended inputs
to the ADC. A selection of ADC input pins can be selected as positive and negative inputs to the differential
amplifier.
The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Voltage reference and input channel
selections will not go into effect until ADEN is set. The ADC does not consume power when ADEN is cleared, so
it is recommended to switch off the ADC before entering power saving sleep modes.
The ADC generates a 10-bit result which is presented in the ADC Data Registers, ADCH and ADCL. By default,
the result is presented right adjusted, but can optionally be presented left adjusted by setting the ADLAR bit in
ADMUX.
If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read ADCH. Otherwise,
ADCL must be read first, then ADCH, to ensure that the content of the Data Registers belongs to the same
conversion. Once ADCL is read, ADC access to Data Registers is blocked. This means that if ADCL has been
read, and a conversion completes before ADCH is read, neither register is updated and the result from the
conversion is lost. When ADCH is read, ADC access to the ADCH and ADCL Registers is re-enabled.
The ADC has its own interrupt which can be triggered when a conversion completes. The ADC access to the
Data Registers is prohibited between reading of ADCH and ADCL, the interrupt will trigger even if the result is
lost.
24.3
Starting a Conversion
A single conversion is started by writing a logical one to the ADC Start Conversion bit, ADSC. This bit stays high
as long as the conversion is in progress and will be cleared by hardware when the conversion is completed. If a
different data channel is selected while a conversion is in progress, the ADC will finish the current conversion
before performing the channel change.
Alternatively, a conversion can be triggered automatically by various sources. Auto Triggering is enabled by
setting the ADC Auto Trigger Enable bit, ADATE in ADCSRA. The trigger source is selected by setting the ADC
Trigger Select bits, ADTS in ADCSRB (See description of the ADTS bits for a list of the trigger sources). When
a positive edge occurs on the selected trigger signal, the ADC prescaler is reset and a conversion is started.
This provides a method of starting conversions at fixed intervals. If the trigger signal is still set when the
conversion completes, a new conversion will not be started. If another positive edge occurs on the trigger signal
during conversion, the edge will be ignored. Note that an interrupt flag will be set even if the specific interrupt is
disabled or the Global Interrupt Enable bit in SREG is cleared. A conversion can thus be triggered without
causing an interrupt. However, the interrupt flag must be cleared in order to trigger a new conversion at the next
interrupt event.
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Figure 24-2.
ADC Auto Trigger Logic
ADTS[2:0]
PRESCALER
START
CLKADC
ADATE
ADIF
SOURCE 1
.
.
.
.
CONVERSION
LOGIC
EDGE
DETECTOR
SOURCE n
ADSC
Using the ADC Interrupt Flag as a trigger source makes the ADC start a new conversion as soon as the ongoing
conversion has finished. The ADC then operates in Free Running mode, constantly sampling and updating the
ADC Data Register. The first conversion must be started by writing a logical one to the ADSC bit in ADCSRA. In
this mode the ADC will perform successive conversions independently of whether the ADC Interrupt Flag, ADIF
is cleared or not.
If Auto Triggering is enabled, single conversions can be started by writing ADSC in ADCSRA to one. ADSC can
also be used to determine if a conversion is in progress. The ADSC bit will be read as one during a conversion,
independently of how the conversion was started.
Prescaling and Conversion Timing
Figure 24-3.
ADC Prescaler
ADEN
START
Reset
7-BIT ADC PRESCALER
CK/64
CK/128
CK/32
CK/8
CK/16
CK/4
CK
CK/2
24.4
ADPS0
ADPS1
ADPS2
ADC CLOCK SOURCE
By default, the successive approximation circuitry requires an input clock frequency between 50kHz and
200kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the input clock frequency to the
ADC can be higher than 200kHz to get a higher sample rate. Alternatively, setting the ADHSM bit in ADCSRB
allows an increased ADC clock frequency at the expense of higher power consumption.
The ADC module contains a prescaler, which generates an acceptable ADC clock frequency from any CPU
frequency above 100kHz. The prescaling is set by the ADPS bits in ADCSRA. The prescaler starts counting
from the moment the ADC is switched on by setting the ADEN bit in ADCSRA. The prescaler keeps running for
as long as the ADEN bit is set, and is continuously reset when ADEN is low.
When initiating a single ended conversion by setting the ADSC bit in ADCSRA, the conversion starts at the
following rising edge of the ADC clock cycle. See “Differential Channels” on page 302 for details on differential
conversion timing.
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A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is switched on (ADEN in
ADCSRA is set) takes 25 ADC clock cycles in order to initialize the analog circuitry.
The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal conversion and 13.5
ADC clock cycles after the start of an first conversion. When a conversion is complete, the result is written to the
ADC Data Registers, and ADIF is set. In Single Conversion mode, ADSC is cleared simultaneously. The
software may then set ADSC again, and a new conversion will be initiated on the first rising ADC clock edge.
When Auto Triggering is used, the prescaler is reset when the trigger event occurs. This assures a fixed delay
from the trigger event to the start of conversion. In this mode, the sample-and-hold takes place two ADC clock
cycles after the rising edge on the trigger source signal. Three additional CPU clock cycles are used for
synchronization logic.
In Free Running mode, a new conversion will be started immediately after the conversion completes, while
ADSC remains high. For a summary of conversion times, see the table below.
Figure 24-4.
ADC Timing Diagram, First Conversion (Single Conversion Mode)
Next
Conversion
First Conversion
Cycle Number
1
2
12
13
14
15
16
17
18
19
20
21
22
23
24
25
1
2
3
ADC Clock
ADEN
ADSC
ADIF
Sign and MSB of Result
ADCH
LSB of Result
ADCL
MUX and REFS
Update
Figure 24-5.
MUX
and REFS
Update
Conversion
Complete
Sample & Hold
ADC Timing Diagram, Single Conversion
One Conversion
Cycle Number
1
2
3
4
5
6
7
8
9
Next Conversion
10
11
12
13
1
2
3
ADC Clock
ADSC
ADIF
ADCH
Sign and MSB of Result
ADCL
LSB of Result
Sample & Hold
MUX and REFS
Update
Conversion
Complete
MUX and REFS
Update
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Figure 24-6.
ADC Timing Diagram, Auto Triggered Conversion
One Conversion
Cycle Number
1
2
3
4
5
6
7
8
Next Conversion
10
9
11
12
13
1
2
ADC Clock
Trigger
Source
ADATE
ADIF
ADCH
Sign and MSB of Result
ADCL
LSB of Result
Prescaler
Reset
Figure 24-7.
Sample &
Hold
Prescaler
Reset
Conversion
Complete
MUX and REFS
Update
ADC Timing Diagram, Free Running Conversion
One Conversion
Cycle Number
11
12
Next Conversion
13
1
2
3
4
ADC Clock
ADSC
ADIF
ADCH
Sign and MSB of Result
ADCL
LSB of Result
Conversion
Complete
Table 24-1.
Sample & Hold
MUX and REFS
Update
ADC Conversion Time
Condition
Sample and Hold
(Cycles from Start of Convention)
Conversion Time
(Cycles)
First Conversion
Normal Conversion,
Single Ended
Auto Triggered
Conversion
14.5
1.5
2
25
13
13.5
24.4.1 Differential Channels
When using differential channels, certain aspects of the conversion need to be taken into consideration.
Differential conversions are synchronized to the internal clock CKADC2 equal to half the ADC clock frequency.
This synchronization is done automatically by the ADC interface in such a way that the sample-and-hold occurs
at a specific phase of CKADC2. A conversion initiated by the user (i.e., all single conversions, and the first free
running conversion) when CKADC2 is low will take the same amount of time as a single ended conversion (13
ADC clock cycles from the next prescaled clock cycle). A conversion initiated by the user when CKADC2 is high
will take 14 ADC clock cycles due to the synchronization mechanism. In Free Running mode, a new conversion
is initiated immediately after the previous conversion completes, and since CKADC2 is high at this time, all
automatically started (i.e., all but the first) Free Running conversions will take 14 ADC clock cycles.
If differential channels are used and conversions are started by Auto Triggering, the ADC must be switched off
between conversions. When Auto Triggering is used, the ADC prescaler is reset before the conversion is
started. Since the stage is dependent of a stable ADC clock prior to the conversion, this conversion will not be
valid. By disabling and then re-enabling the ADC between each conversion (writing ADEN in ADCSRA to “0”
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then to “1”), only extended conversions are performed. The result from the extended conversions will be valid.
See “Prescaling and Conversion Timing” on page 300 for timing details.
The gain stage is optimized for a bandwidth of 4kHz at all gain settings. Higher frequencies may be subjected to
non-linear amplification. An external low-pass filter should be used if the input signal contains higher frequency
components than the gain stage bandwidth. Note that the ADC clock frequency is independent of the gain stage
bandwidth limitation. E.g. the ADC clock period may be 6µs, allowing a channel to be sampled at 12kSPS,
regardless of the bandwidth of this channel.
24.5
Changing Channel or Reference Selection
The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a temporary register to which
the CPU has random access. This ensures that the channels and reference selection only takes place at a safe
point during the conversion. The channel and reference selection is continuously updated until a conversion is
started. Once the conversion starts, the channel and reference selection is locked to ensure a sufficient
sampling time for the ADC. Continuous updating resumes in the last ADC clock cycle before the conversion
completes (ADIF in ADCSRA is set). Note that the conversion starts on the following rising ADC clock edge after
ADSC is written. The user is thus advised not to write new channel or reference selection values to ADMUX until
one ADC clock cycle after ADSC is written.
If Auto Triggering is used, the exact time of the triggering event can be indeterministic. Special care must be
taken when updating the ADMUX Register, in order to control which conversion will be affected by the new
settings.
If both ADATE and ADEN is written to one, an interrupt event can occur at any time. If the ADMUX Register is
changed in this period, the user cannot tell if the next conversion is based on the old or the new settings.
ADMUX can be safely updated in the following ways:
a. When ADATE or ADEN is cleared.
3. During conversion, minimum one ADC clock cycle after the trigger event.
4. After a conversion, before the interrupt flag used as trigger source is cleared.
When updating ADMUX in one of these conditions, the new settings will affect the next ADC conversion.
Special care should be taken when changing differential channels. Once a differential channel has been
selected, the stage may take as much as 125µs to stabilize to the new value. Thus conversions should not be
started within the first 125µs after selecting a new differential channel. Alternatively, conversion results obtained
within this period should be discarded.
The same settling time should be observed for the first differential conversion after changing ADC reference (by
changing the REFS1:0 bits in ADMUX).
The settling time and gain stage bandwidth is independent of the ADHSM bit setting.
24.5.1 ADC Input Channels
When changing channel selections, the user should observe the following guidelines to ensure that the correct
channel is selected:

In Single Conversion mode, always select the channel before starting the conversion. The channel
selection may be changed one ADC clock cycle after writing one to ADSC. However, the simplest method
is to wait for the conversion to complete before changing the channel selection.

In Free Running mode, always select the channel before starting the first conversion. The channel
selection may be changed one ADC clock cycle after writing one to ADSC. However, the simplest method
is to wait for the first conversion to complete, and then change the channel selection. Since the next
conversion has already started automatically, the next result will reflect the previous channel selection.
Subsequent conversions will reflect the new channel selection.
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When switching to a differential gain channel, the first conversion result may have a poor accuracy due to the
required settling time for the automatic offset cancellation circuitry. The user should preferably disregard the first
conversion result.
24.5.2 ADC Voltage Reference
The reference voltage for the ADC (VREF) indicates the conversion range for the ADC. Single ended channels
that exceed VREF will result in codes close to 0x3FF. VREF can be selected as either AVCC, internal 2.56V
reference, or external AREF pin.
AVCC is connected to the ADC through a passive switch. The internal 2.56V reference is generated from the
internal bandgap reference (VBG) through an internal amplifier. In either case, the external AREF pin is directly
connected to the ADC, and the reference voltage can be made more immune to noise by connecting a capacitor
between the AREF pin and ground. VREF can also be measured at the AREF pin with a high impedance
voltmeter. Note that VREF is a high impudent source, and only a capacitive load should be connected in a
system.
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other reference
voltage options in the application, as they will be shorted to the external voltage. If no external voltage is applied
to the AREF pin, the user may switch between AVCC and 2.56V as reference selection. The first ADC
conversion result after switching reference voltage source may be inaccurate, and the user is advised to discard
this result.
If differential channels are used, the selected reference should not be closer to AVCC than indicated in Table 297 on page 390.
24.6
Temperature Sensor
The ATmega16U4/ATmega32U4 includes an on-chip temperature sensor, whose the value can be read through
the A/D Converter.
The temperature measurement is based on an on-chip temperature sensor that is coupled to a single ended
ADC input. MUX[5..0] bits in ADMUX register enables the temperature sensor. The internal 2.56V voltage
reference must also be selected for the ADC voltage reference source in he temperature sensor measurement.
When the temperature sensor is enabled, the ADC converter can be used in single conversion mode to
measure the voltage over the temperature sensor.
The temperature sensor and its internal driver are enabled when ADMUX value selects the temperature sensor
as ADC input. The propagation delay of this driver is approximately 2µS. Therefore two successive conversions
are required. The correct temperature measurement will be the second one.
One can also reduce this timing to one conversion by setting the ADMUX during the previous conversion.
Indeed the ADMUX can be programmed to select the temperature sensor just after the beginning of the
previous conversion start event and then the driver will be enabled 2µS before sampling and hold phase of
temperature sensor measurement.
24.6.1 Sensor Calibration
The sensor initial tolerance is large (±10°C), but its characteristic is linear. Thus, if the application requires
accuracy, the firmware must include a calibration stage to use the sensor for direct temperature measurement.
Another application of this sensor may concern the Internal Calibrated RC Oscillator, whose the frequency can
be adjusted by the user through the OSCCAL register (see “Oscillator Calibration Register – OSCCAL” on
page 32). During the production, a calibration is done at two temperatures (+25°C and +85°C, with a tolerance
of ±10°C(1)). At each temperature, the temperature sensor value Ti is measured and stored in EEPROM
memory(2), and the OSCCAL calibration value Oi (i.e. the value that should be set in OSCCAL register at this
temperature to have an accurate 8MHz output) is stored in another memory zone.
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Thanks to these four values and the linear characteristics of the temperature sensor and Internal RC Oscillator,
firmware can easily recalibrate the RC Oscillator on-the-go in function of the temperature sensor measure(3) (an
application note describes the operation):
Figure 24-8.
Linear Characterization of OSCCAL in Function of T° Measurement from ADC
OSCCAL
O2
O1
T1
Notes:
1.
2.
3.
24.7
T2
T (ADC
The temperature sensor calibration values cannot be used to do accurate temperature measurements since
the calibration temperature during production is not accurate (±10°C)
Be aware that if EESAVE fuse is left unprogrammed, any chip erase operation will clear the temperature
sensor calibration values contained in EEPROM memory.
Accuracy results after a software recalibration of OSCCAL in function of Temperature is given by
characterization.
ADC Noise Canceler
The ADC features a noise canceler that enables conversion during sleep mode to reduce noise induced from
the CPU core and other I/O peripherals. The noise canceler can be used with ADC Noise Reduction and Idle
mode. To make use of this feature, the following procedure should be used:
a. Make sure that the ADC is enabled and is not busy converting. Single Conversion mode must be
selected and the ADC conversion complete interrupt must be enabled.
5. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion once the CPU
has been halted.
6. If no other interrupts occur before the ADC conversion completes, the ADC interrupt will wake up
the CPU and execute the ADC Conversion Complete interrupt routine. If another interrupt wakes up
the CPU before the ADC conversion is complete, that interrupt will be executed, and an ADC
Conversion Complete interrupt request will be generated when the ADC conversion completes. The
CPU will remain in active mode until a new sleep command is executed.
Note that the ADC will not be automatically turned off when entering other sleep modes than Idle mode and
ADC Noise Reduction mode. The user is advised to write zero to ADEN before entering such sleep modes to
avoid excessive power consumption.
If the ADC is enabled in such sleep modes and the user wants to perform differential conversions, the user is
advised to switch the ADC off and on after waking up from sleep to prompt an extended conversion to get a
valid result.
24.7.1 Analog Input Circuitry
The analog input circuitry for single ended channels is illustrated in Figure 24-9. An analog source applied to
ADCn is subjected to the pin capacitance and input leakage of that pin, regardless of whether that channel is
selected as input for the ADC. When the channel is selected, the source must drive the S/H capacitor through
the series resistance (combined resistance in the input path).
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The ADC is optimized for analog signals with an output impedance of approximately 10k or less. If such a
source is used, the sampling time will be negligible. If a source with higher impedance is used, the sampling
time will depend on how long time the source needs to charge the S/H capacitor, with can vary widely. The user
is recommended to only use low impedance sources with slowly varying signals, since this minimizes the
required charge transfer to the S/H capacitor.
If differential gain channels are used, the input circuitry looks somewhat different, although source impedances
of a few hundred k or less is recommended.
Signal components higher than the Nyquist frequency (fADC/2) should not be present for either kind of channels,
to avoid distortion from unpredictable signal convolution. The user is advised to remove high frequency
components with a low-pass filter before applying the signals as inputs to the ADC.
Figure 24-9.
Analog Input Circuitry
IIH
ADCn
1..100 kΩ
CS/H= 14 pF
IIL
VCC/2
24.7.2 Analog Noise Canceling Techniques
Digital circuitry inside and outside the device generates EMI which might affect the accuracy of analog
measurements. If conversion accuracy is critical, the noise level can be reduced by applying the following
techniques:
a. Keep analog signal paths as short as possible. Make sure analog tracks run over the analog
ground plane, and keep them well away from high-speed switching digital tracks.
7. The AVCC pin on the device should be connected to the digital VCC supply voltage via an LC network
as shown in Figure 24-10.
8. Use the ADC noise canceler function to reduce induced noise from the CPU.
9. If any ADC port pins are used as digital outputs, it is essential that these do not switch while a
conversion is in progress.
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Figure 24-10. ADC Power Connections
VCC
34
GND 35
(ADC7) PF7 36
(ADC6) PF6 37
(ADC5) PF5 38
(ADC4) PF4 39
(ADC1) PF1 40
(ADC0) PF0 41
10µH
AREF 42
GND
AVCC
100nF
43
44
1
Analog Ground Plane
Note:
The same circuitry should be used for AVCC filtering on the ADC8-ADC13 side.
24.7.3 Offset Compensation Schemes
The gain stage has a built-in offset cancellation circuitry that nulls the offset of differential measurements as
much as possible. The remaining offset in the analog path can be measured directly by selecting the same
channel for both differential inputs. This offset residue can be then subtracted in software from the
measurement results. Using this kind of software based offset correction, offset on any channel can be reduced
below one LSB.
24.7.4 ADC Accuracy Definitions
An n-bit single-ended ADC converts a voltage linearly between GND and VREF in 2n steps (LSBs). The lowest
code is read as 0, and the highest code is read as 2n-1.
Several parameters describe the deviation from the ideal behavior:

Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition (at 0.5 LSB).
Ideal value: 0 LSB.
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Figure 24-11. Offset Error
Output Code
Ideal ADC
Actual ADC
Offset
Error

VREF Input Voltage
Gain Error: After adjusting for offset, the Gain Error is found as the deviation of the last transition (0x3FE
to 0x3FF) compared to the ideal transition (at 1.5 LSB below maximum). Ideal value: 0 LSB.
Figure 24-12. Gain Error
Output Code
Gain
Error
Ideal ADC
Actual ADC
VREF Input Voltage

Integral Non-linearity (INL): After adjusting for offset and gain error, the INL is the maximum deviation of
an actual transition compared to an ideal transition for any code. Ideal value: 0 LSB.
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Figure 24-13. Integral Non-linearity (INL)
Output Code
INL
Ideal ADC
Actual ADC
VREF

Input Voltage
Differential Non-linearity (DNL): The maximum deviation of the actual code width (the interval between
two adjacent transitions) from the ideal code width (1 LSB). Ideal value: 0 LSB.
Figure 24-14. Differential Non-linearity (DNL)
Output Code
0x3FF
1 LSB
DNL
0x000
0
24.8
VREF Input Voltage

Quantization Error: Due to the quantization of the input voltage into a finite number of codes, a range of
input voltages (1 LSB wide) will code to the same value. Always ±0.5 LSB.

Absolute Accuracy: The maximum deviation of an actual (unadjusted) transition compared to an ideal
transition for any code. This is the compound effect of offset, gain error, differential error, non-linearity, and
quantization error. Ideal value: ±0.5 LSB.
ADC Conversion Result
After the conversion is complete (ADIF is high), the conversion result can be found in the ADC Result Registers
(ADCL, ADCH).
For single ended conversion, the result is:
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V IN  1023
ADC = -------------------------V REF
where VIN is the voltage on the selected input pin and VREF the selected voltage reference (see Table 24-3 on
page 313 and Table 24-4 on page 313). 0x000 represents analog ground, and 0x3FF represents the selected
reference voltage minus one LSB.
If differential channels are used, the result is:
 V POS – V NEG   GAIN  512
ADC = -----------------------------------------------------------------------V REF
where VPOS is the voltage on the positive input pin, VNEG the voltage on the negative input pin, GAIN the
selected gain factor and VREF the selected voltage reference. The result is presented in two’s complement form,
from 0x200 (-512d) through 0x1FF (+511d). Note that if the user wants to perform a quick polarity check of the
result, it is sufficient to read the MSB of the result (ADC9 in ADCH). If the bit is one, the result is negative, and if
this bit is zero, the result is positive. Figure 24-15 on page 311 shows the decoding of the differential input
range.
Table 82 shows the resulting output codes if the differential input channel pair (ADCn - ADCm) is selected with
a reference voltage of VREF.
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Figure 24-15. Differential Measurement Range
Output Code
0x1FF
0x000
- VREF
0x3FF
0
VREF
Differential Input
Voltage (Volts)
0x200
Table 24-2.
Correlation Between Input Voltage and Output Codes
VADCn
Read code
Corresponding decimal value
VADCm + VREF /GAIN
0x1FF
511
VADCm + 0.999 VREF /GAIN
0x1FF
511
VADCm + 0.998 VREF /GAIN
0x1FE
510
...
...
...
VADCm + 0.001 VREF /GAIN
0x001
1
VADCm
0x000
0
VADCm - 0.001 VREF /GAIN
0x3FF
-1
...
...
...
VADCm - 0.999 VREF /GAIN
0x201
-511
VADCm - VREF /GAIN
0x200
-512
Example 1:
̶
ADMUX = 0xE9, MUX5 = 0 (ADC1 - ADC0, 10x gain, 2.56V reference, left adjusted result)
̶
Voltage on ADC1 is 300mV, voltage on ADC0 is 500mV
̶
ADCR = 512 * 10 * (300 - 500) / 2560 = -400 = 0x270
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̶
ADCL will thus read 0x00, and ADCH will read 0x9C.
Writing zero to ADLAR right adjusts the result: ADCL = 0x70, ADCH = 0x02.
Example 2:
̶
ADMUX = 0xF0, MUX5 = 0 (ADC0 - ADC1, 1x gain, 2.56V reference, left adjusted result)
̶
Voltage on ADC0 is 300mV, voltage on ADC1 is 500mV
̶
ADCR = 512 * 1 * (300 - 500) / 2560 = -41 = 0x029
̶
ADCL will thus read 0x40, and ADCH will read 0x0A.
Writing zero to ADLAR right adjusts the result: ADCL = 0x00, ADCH = 0x29.
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24.9
ADC Register Description
24.9.1 ADC Multiplexer Selection Register – ADMUX
Bit
Read/Write
Initial Value
7
REFS1
R/W
0
6
REFS0
R/W
0
5
ADLAR
R/W
0
4
MUX4
R/W
0
3
MUX3
R/W
0
2
MUX2
R/W
0
1
MUX1
R/W
0
0
MUX0
R/W
0
ADMUX
• Bit 7:6 – REFS1:0: Reference Selection Bits
These bits select the voltage reference for the ADC, as shown in the table. If these bits are changed during a
conversion, the change will not go in effect until this conversion is complete (ADIF in ADCSRA is set). The
internal voltage reference options may not be used if an external reference voltage is being applied to the AREF
pin.
Table 24-3.
•
Voltage Reference Selections for ADC
REFS1
REFS0
Voltage Reference Selection
0
0
AREF, Internal VREF turned off
0
1
AVCC with external capacitor on AREF pin
1
0
Reserved
1
1
Internal 2.56V Voltage Reference with external capacitor on AREF pin
Bit 5 – ADLAR: ADC Left Adjust Result
The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register. Write one to
ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the ADLAR bit will affect the
ADC Data Register immediately, regardless of any ongoing conversions. For a complete description of this bit,
see “The ADC Data Register – ADCL and ADCH” on page 316.
• Bits 4:0 – MUX4:0: Analog Channel Selection Bits
The value of these bits selects which combination of analog inputs are connected to the ADC. These bits also
select the gain for the differential channels as shown in the table. If these bits are changed during a conversion,
the change will not go in effect until this conversion is complete (ADIF in ADCSRA is set).
Table 24-4.
Input Channel and Gain Selections
MUX5..0(1)
Single Ended Input
000000
ADC0
000001
ADC1
000010
000011
Positive Differential Input
Negative Differential Input
Gain
N/A
000100
ADC4
000101
ADC5
000110
ADC6
000111
ADC7
N/A
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Table 24-4.
MUX5..0(1)
Input Channel and Gain Selections
Positive Differential Input
Negative Differential Input
Gain
001000
N/A
N/A
N/A
001001
ADC1
ADC0
10x
001010
N/A
N/A
N/A
001011
ADC1
ADC0
200x
ADC1
1x
001100
Single Ended Input
N/A
001101
N/A
001110
001111
010000
ADC0
010001
010010
N/A
010011
010100
ADC4
ADC1
1x
010101
ADC5
ADC1
1x
010110
ADC6
ADC1
1x
ADC7
ADC1
1x
ADC0
40x
010111
N/A
011000
011001
011010
011011
011100
011101
011110
1.1V (VBand Gap)
011111
0V (GND)
100000
ADC8
100001
ADC9
100010
ADC10
100011
ADC11
100100
ADC12
100101
ADC13
100110
N/A
100111
Temperature Sensor
N/A
ADC1
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Table 24-4.
MUX5..0(1)
Input Channel and Gain Selections
Positive Differential Input
Negative Differential Input
Gain
101000
ADC4
ADC0
10x
101001
ADC5
ADC0
10x
101010
ADC6
ADC0
10x
101011
ADC7
ADC0
10x
101100
ADC4
ADC1
10x
ADC5
ADC1
10x
ADC6
ADC1
10x
101111
ADC7
ADC1
10x
110000
ADC4
ADC0
40x
110001
ADC5
ADC0
40x
110010
ADC6
ADC0
40x
110011
ADC7
ADC0
40x
110100
ADC4
ADC1
40x
110101
ADC5
ADC1
40x
110110
ADC6
ADC1
40x
110111
ADC7
ADC1
40x
111000
ADC4
ADC0
200x
ADC5
ADC0
200x
ADC6
ADC0
200x
111011
ADC7
ADC0
200x
111100
ADC4
ADC1
200x
111101
ADC5
ADC1
200x
111110
ADC6
ADC1
200x
111111
ADC7
ADC1
200x
101101
N/A
101110
111001
N/A
111010
Note:
Single Ended Input
1.
MUX5 bit make part of ADCSRB register.
24.9.2 ADC Control and Status Register A – ADCSRA
Bit
Read/Write
Initial Value
7
ADEN
R/W
0
6
ADSC
R/W
0
5
ADATE
R/W
0
4
ADIF
R/W
0
3
ADIE
R/W
0
2
ADPS2
R/W
0
1
ADPS1
R/W
0
0
ADPS0
R/W
0
ADCSRA
• Bit 7 – ADEN: ADC Enable
Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off. Turning the ADC off while a
conversion is in progress, will terminate this conversion.
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• Bit 6 – ADSC: ADC Start Conversion
In Single Conversion mode, write this bit to one to start each conversion. In Free Running mode, write this bit to
one to start the first conversion. The first conversion after ADSC has been written after the ADC has been
enabled, or if ADSC is written at the same time as the ADC is enabled, will take 25 ADC clock cycles instead of
the normal 13. This first conversion performs initialization of the ADC.
ADSC will read as one as long as a conversion is in progress. When the conversion is complete, it returns to
zero. Writing zero to this bit has no effect.
• Bit 5 – ADATE: ADC Auto Trigger Enable
When this bit is written to one, Auto Triggering of the ADC is enabled. The ADC will start a conversion on a
positive edge of the selected trigger signal. The trigger source is selected by setting the ADC Trigger Select bits,
ADTS in ADCSRB.
• Bit 4 – ADIF: ADC Interrupt Flag
This bit is set when an ADC conversion completes and the Data Registers are updated. The ADC Conversion
Complete Interrupt is executed if the ADIE bit and the I-bit in SREG are set. ADIF is cleared by hardware when
executing the corresponding interrupt handling vector. Alternatively, ADIF is cleared by writing a logical one to
the flag. Beware that if doing a Read-Modify-Write on ADCSRA, a pending interrupt can be disabled. This also
applies if the SBI and CBI instructions are used.
• Bit 3 – ADIE: ADC Interrupt Enable
When this bit is written to one and the I-bit in SREG is set, the ADC Conversion Complete Interrupt is activated.
• Bits 2:0 – ADPS2:0: ADC Prescaler Select Bits
These bits determine the division factor between the XTAL frequency and the input clock to the ADC.
Table 24-5.
ADC Prescaler Selections
ADPS2
ADPS1
ADPS0
Division Factor
0
0
0
2
0
0
1
2
0
1
0
4
0
1
1
8
1
0
0
16
1
0
1
32
1
1
0
64
1
1
1
128
24.9.3 The ADC Data Register – ADCL and ADCH
24.9.3.1 ADLAR = 0
Bit
Bit
Read/Write
Initial Value
15
–
ADC7
7
R
R
0
0
14
–
ADC6
6
R
R
0
0
13
–
ADC5
5
R
R
0
0
12
–
ADC4
4
R
R
0
0
11
–
ADC3
3
R
R
0
0
10
–
ADC2
2
R
R
0
0
9
ADC9
ADC1
1
R
R
0
0
8
ADC8
ADC0
0
R
R
0
0
ADCH
ADCL
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24.9.3.2 ADLAR = 1
Bit
Bit
Read/Write
Initial Value
15
ADC9
ADC1
7
R
R
0
0
14
ADC8
ADC0
6
R
R
0
0
13
ADC7
–
5
R
R
0
0
12
ADC6
–
4
R
R
0
0
11
ADC5
–
3
R
R
0
0
10
ADC4
–
2
R
R
0
0
9
ADC3
–
1
R
R
0
0
8
ADC2
–
0
R
R
0
0
ADCH
ADCL
When an ADC conversion is complete, the result is found in these two registers. If differential channels are
used, the result is presented in two’s complement form.
When ADCL is read, the ADC Data Register is not updated until ADCH is read. Consequently, if the result is left
adjusted and no more than 8-bit precision (7 bit + sign bit for differential input channels) is required, it is
sufficient to read ADCH. Otherwise, ADCL must be read first, then ADCH.
The ADLAR bit in ADMUX, and the MUXn bits in ADMUX affect the way the result is read from the registers. If
ADLAR is set, the result is left adjusted. If ADLAR is cleared (default), the result is right adjusted.
• ADC9:0: ADC Conversion Result
These bits represent the result from the conversion, as detailed in “ADC Conversion Result” on page 309.
24.9.4 ADC Control and Status Register B – ADCSRB
Bit
Read/Write
Initial Value
7
ADHSM
R/W
0
6
ACME
R/W
0
5
MUX5
R
0
4
–
R
0
3
ADTS3
R
0
2
ADTS2
R/W
0
1
ADTS1
R/W
0
0
ADTS0
R/W
0
ADCSRB
• Bit 7 – ADHSM: ADC High Speed Mode
Writing this bit to one enables the ADC High Speed mode. This mode enables higher conversion rate at the
expense of higher power consumption.
• Bit 5 – MUX5: Analog Channel Additional Selection Bits
This bit make part of MUX5:0 bits of ADRCSRB and ADMUX register, that select the combination of analog
inputs connected to the ADC (including differential amplifier configuration).
• Bit 3:0 – ADTS3:0: ADC Auto Trigger Source
If ADATE in ADCSRA is written to one, the value of these bits selects which source will trigger an ADC
conversion. If ADATE is cleared, the ADTS3:0 settings will have no effect. A conversion will be triggered by the
rising edge of the selected interrupt flag. Note that switching from a trigger source that is cleared to a trigger
source that is set, will generate a positive edge on the trigger signal. If ADEN in ADCSRA is set, this will start a
conversion. Switching to Free Running mode (ADTS[3:0]=0) will not cause a trigger event, even if the ADC
Interrupt Flag is set.
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Table 24-6.
ADC Auto Trigger Source Selections
ADTS3
ADTS2
ADTS1
ADTS0
Trigger Source
0
0
0
0
Free Running mode
0
0
0
1
Analog Comparator
0
0
1
0
External Interrupt Request 0
0
0
1
1
Timer/Counter0 Compare Match A
0
1
0
0
Timer/Counter0 Overflow
0
1
0
1
Timer/Counter1 Compare Match B
0
1
1
0
Timer/Counter1 Overflow
0
1
1
1
Timer/Counter1 Capture Event
1
0
0
0
Timer/Counter4 Overflow
1
0
0
1
Timer/Counter4 Compare Match A
1
0
1
0
Timer/Counter4 Compare Match B
1
0
1
1
Timer/Counter4 Compare Match D
3
R/W
0
2
R/W
0
24.9.5 Digital Input Disable Register 0 – DIDR0
Bit
Read/Write
Initial Value
7
ADC7D
R/W
0
6
ADC6D
R/W
0
5
ADC5D
R/W
0
4
ADC4D
R/W
0
1
ADC1D
R/W
0
0
ADC0D
R/W
0
DIDR0
• Bit 7:4, 1:0 – ADC7D..4D - ADC1D..0D: ADC7:4 - ADC1:0 Digital Input Disable
When this bit is written logic one, the digital input buffer on the corresponding ADC pin is disabled. The
corresponding PIN Register bit will always read as zero when this bit is set. When an analog signal is applied to
the ADC7..4 / ADC1..0 pin and the digital input from this pin is not needed, this bit should be written logic one to
reduce power consumption in the digital input buffer.
24.9.6 Digital Input Disable Register 2 – DIDR2
Bit
Read/Write
Initial Value
7
R/W
0
6
R/W
0
5
ADC13D
R/W
0
4
ADC12D
R/W
0
3
ADC11D
R/W
0
2
ADC10D
R/W
0
1
ADC9D
R/W
0
0
ADC8D
R/W
0
DIDR2
• Bit 5:0 – ADC13D..ADC8D: ADC13:8 Digital Input Disable
When this bit is written logic one, the digital input buffer on the corresponding ADC pin is disabled. The
corresponding PIN Register bit will always read as zero when this bit is set. When an analog signal is applied to
the ADC13..8 pin and the digital input from this pin is not needed, this bit should be written logic one to reduce
power consumption in the digital input buffer.
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25.
JTAG Interface and On-chip Debug System
25.0.1 Features
• JTAG (IEEE std. 1149.1 Compliant) Interface
• Boundary-scan Capabilities According to the IEEE std. 1149.1 (JTAG) Standard
• Debugger Access to:
– All Internal Peripheral Units
– Internal and External RAM
– The Internal Register File
– Program Counter
– EEPROM and Flash Memories
• Extensive On-chip Debug Support for Break Conditions, Including
– AVR Break Instruction
– Break on Change of Program Memory Flow
– Single Step Break
– Program Memory Break Points on Single Address or Address Range
– Data Memory Break Points on Single Address or Address Range
• Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface
• On-chip Debugging Supported by AVR Studio®
25.1
Overview
The AVR IEEE std. 1149.1 compliant JTAG interface can be used for:

Testing PCBs by using the JTAG Boundary-scan capability

Programming the non-volatile memories, Fuses and Lock bits

On-chip debugging
A brief description is given in the following sections. Detailed descriptions for Programming via the JTAG
interface, and using the Boundary-scan Chain can be found in the sections “Programming via the JTAG
Interface” on page 371 and “IEEE 1149.1 (JTAG) Boundary-scan” on page 325, respectively. The On-chip
Debug support is considered being private JTAG instructions, and distributed within Atmel and to selected third
party vendors only.
Figure 25-1 shows a block diagram of the JTAG interface and the On-chip Debug system. The TAP Controller is
a state machine controlled by the TCK and TMS signals. The TAP Controller selects either the JTAG Instruction
Register or one of several Data Registers as the scan chain (Shift Register) between the TDI – input and TDO –
output. The Instruction Register holds JTAG instructions controlling the behavior of a Data Register.
The ID-Register, Bypass Register, and the Boundary-scan Chain are the Data Registers used for board-level
testing. The JTAG Programming Interface (actually consisting of several physical and virtual Data Registers) is
used for serial programming via the JTAG interface. The Internal Scan Chain and Break Point Scan Chain are
used for On-chip debugging only.
25.2
Test Access Port – TAP
The JTAG interface is accessed through four of the AVR’s pins. In JTAG terminology, these pins constitute the
Test Access Port – TAP. These pins are:

TMS: Test mode select. This pin is used for navigating through the TAP-controller state machine.

TCK: Test Clock. JTAG operation is synchronous to TCK.

TDI: Test Data In. Serial input data to be shifted in to the Instruction Register or Data Register (Scan
Chains).

TDO: Test Data Out. Serial output data from Instruction Register or Data Register.
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The IEEE std. 1149.1 also specifies an optional TAP signal; TRST – Test ReSeT – which is not provided.
When the JTAGEN Fuse is unprogrammed, these four TAP pins are normal port pins, and the TAP controller is
in reset. When programmed, the input TAP signals are internally pulled high and the JTAG is enabled for
Boundary-scan and programming. The device is shipped with this fuse programmed.
For the On-chip Debug system, in addition to the JTAG interface pins, the RESET pin is monitored by the
debugger to be able to detect external reset sources. The debugger can also pull the RESET pin low to reset the
whole system, assuming only open collectors on the reset line are used in the application.
Figure 25-1.
Block Diagram
I/O PORT 0
DEVICE BOUNDARY
BOUNDARY SCAN CHAIN
TDI
TDO
TCK
TMS
JTAG PROGRAMMING
INTERFACE
TAP
CONTROLLER
AVR CPU
INSTRUCTION
REGISTER
ID
REGISTER
M
U
X
FLASH
MEMORY
Address
Data
BREAKPOINT
UNIT
BYPASS
REGISTER
INTERNAL
SCAN
CHAIN
PC
Instruction
FLOW CONTROL
UNIT
DIGITAL
PERIPHERAL
UNITS
ANALOG
PERIPHERIAL
UNITS
Analog inputs
BREAKPOINT
SCAN CHAIN
ADDRESS
DECODER
JTAG / AVR CORE
COMMUNICATION
INTERFACE
OCD STATUS
AND CONTROL
Control & Clock lines
I/O PORT n
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Figure 25-2.
TAP Controller State Diagram
1
Test-Logic-Reset
0
0
Run-Test/Idle
1
Select-DR Scan
1
Select-IR Scan
0
0
1
1
Capture-DR
Capture-IR
0
0
0
Shift-DR
1
1
Exit1-DR
0
0
Pause-DR
0
Pause-IR
1
1
0
Exit2-DR
Exit2-IR
1
1
Update-DR
25.3
1
Exit1-IR
0
1
0
Shift-IR
1
0
1
Update-IR
0
1
0
TAP Controller
The TAP controller is a 16-state finite state machine that controls the operation of the Boundary-scan circuitry,
JTAG programming circuitry, or On-chip Debug system. The state transitions depicted in Figure 25-2 depend on
the signal present on TMS (shown adjacent to each state transition) at the time of the rising edge at TCK. The
initial state after a Power-on Reset is Test-Logic-Reset.
As a definition in this document, the LSB is shifted in and out first for all Shift Registers.
Assuming Run-Test/Idle is the present state, a typical scenario for using the JTAG interface is:

At the TMS input, apply the sequence 1, 1, 0, 0 at the rising edges of TCK to enter the Shift Instruction
Register – Shift-IR state. While in this state, shift the four bits of the JTAG instructions into the JTAG
Instruction Register from the TDI input at the rising edge of TCK. The TMS input must be held low during
input of the three LSBs in order to remain in the Shift-IR state. The MSB of the instruction is shifted in
when this state is left by setting TMS high. While the instruction is shifted in from the TDI pin, the captured
IR-state 0x01 is shifted out on the TDO pin. The JTAG Instruction selects a particular Data Register as
path between TDI and TDO and controls the circuitry surrounding the selected Data Register.

Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. The instruction is latched onto the
parallel output from the Shift Register path in the Update-IR state. The Exit-IR, Pause-IR, and Exit2-IR
states are only used for navigating the state machine.
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
At the TMS input, apply the sequence 1, 0, 0 at the rising edges of TCK to enter the Shift Data Register –
Shift-DR state. While in this state, upload the selected Data Register (selected by the present JTAG
instruction in the JTAG Instruction Register) from the TDI input at the rising edge of TCK. In order to
remain in the Shift-DR state, the TMS input must be held low during input of all bits except the MSB. The
MSB of the data is shifted in when this state is left by setting TMS high. While the Data Register is shifted
in from the TDI pin, the parallel inputs to the Data Register captured in the Capture-DR state is shifted out
on the TDO pin.

Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. If the selected Data Register has a
latched parallel-output, the latching takes place in the Update-DR state. The Exit-DR, Pause-DR, and
Exit2-DR states are only used for navigating the state machine.
As shown in the state diagram, the Run-Test/Idle state need not be entered between selecting JTAG instruction
and using Data Registers, and some JTAG instructions may select certain functions to be performed in the RunTest/Idle, making it unsuitable as an Idle state.
Note:
Independent of the initial state of the TAP Controller, the Test-Logic-Reset state can always be entered by holding
TMS high for five TCK clock periods.
For detailed information on the JTAG specification, refer to the literature listed in “Bibliography” on page 324.
25.4
Using the Boundary-scan Chain
A complete description of the Boundary-scan capabilities are given in the section “IEEE 1149.1 (JTAG)
Boundary-scan” on page 325.
25.5
Using the On-chip Debug System
As shown in Figure 25-1 on page 320, the hardware support for On-chip Debugging consists mainly of

A scan chain on the interface between the internal AVR CPU and the internal peripheral units

Break Point unit

Communication interface between the CPU and JTAG system
All read or modify/write operations needed for implementing the Debugger are done by applying AVR
instructions via the internal AVR CPU Scan Chain. The CPU sends the result to an I/O memory mapped location
which is part of the communication interface between the CPU and the JTAG system.
The Break Point Unit implements Break on Change of Program Flow, Single Step Break, two Program Memory
Break Points, and two combined Break Points. Together, the four Break Points can be configured as either:

Four single Program Memory Break Points

Three Single Program Memory Break Point + one single Data Memory Break Point

Two Single Program Memory Break Points + two single Data Memory Break Points

Two Single Program Memory Break Points + one Program Memory Break Point with mask (“range Break
Point”)

Two Single Program Memory Break Points + 1 Data Memory Break Point with mask (“range Break Point”).
A debugger, like the AVR Studio, may however use one or more of these resources for its internal purpose,
leaving less flexibility to the end-user.
A list of the On-chip Debug specific JTAG instructions is given in “On-chip Debug Specific JTAG Instructions” on
page 323.
The JTAGEN Fuse must be programmed to enable the JTAG Test Access Port. In addition, the OCDEN Fuse
must be programmed and no Lock bits must be set for the On-chip debug system to work. As a security feature,
the On-chip debug system is disabled when either of the LB1 or LB2 Lock bits are set. Otherwise, the On-chip
debug system would have provided a back-door into a secured device.
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The AVR Studio enables the user to fully control execution of programs on an AVR device with On-chip Debug
capability, AVR In-Circuit Emulator, or the built-in AVR Instruction Set Simulator. AVR Studio® supports source
level execution of Assembly programs assembled with Atmel Corporation’s AVR Assembler and C programs
compiled with third party vendors’ compilers.
AVR Studio runs under Microsoft® Windows® 95/98/2000 and Microsoft Windows NT.
For a full description of the AVR Studio, refer to the AVR Studio User Guide. Only highlights are presented in
this document.
All necessary execution commands are available in AVR Studio, both on source level and on disassembly level.
The user can execute the program, single step through the code either by tracing into or stepping over
functions, step out of functions, place the cursor on a statement and execute until the statement is reached, stop
the execution, and reset the execution target. In addition, the user can have an unlimited number of code Break
Points (using the BREAK instruction) and up to two data memory Break Points, alternatively combined as a
mask (range) Break Point.
25.6
On-chip Debug Specific JTAG Instructions
The On-chip debug support is considered being private JTAG instructions, and distributed within Atmel and to
selected third party vendors only. Instruction opcodes are listed for reference.
25.6.1 PRIVATE0; 0x8
Private JTAG instruction for accessing On-chip debug system.
25.6.2 PRIVATE1; 0x9
Private JTAG instruction for accessing On-chip debug system.
25.6.3 PRIVATE2; 0xA
Private JTAG instruction for accessing On-chip debug system.
25.6.4 PRIVATE3; 0xB
Private JTAG instruction for accessing On-chip debug system.
25.7
On-chip Debug Related Register in I/O Memory
25.7.1 On-chip Debug Register – OCDR
Bit
Read/Write
Initial Value
7
MSB/IDRD
R/W
0
6
5
4
3
2
1
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
0
LSB
R/W
0
OCDR
The OCDR Register provides a communication channel from the running program in the microcontroller to the
debugger. The CPU can transfer a byte to the debugger by writing to this location. At the same time, an internal
flag; I/O Debug Register Dirty – IDRD – is set to indicate to the debugger that the register has been written.
When the CPU reads the OCDR Register the seven LSB will be from the OCDR Register, while the MSB is the
IDRD bit. The debugger clears the IDRD bit when it has read the information.
In some AVR devices, this register is shared with a standard I/O location. In this case, the OCDR Register can
only be accessed if the OCDEN Fuse is programmed, and the debugger enables access to the OCDR Register.
In all other cases, the standard I/O location is accessed.
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Refer to the debugger documentation for further information on how to use this register.
25.8
Using the JTAG Programming Capabilities
Programming of AVR parts via JTAG is performed via the 4-pin JTAG port, TCK, TMS, TDI, and TDO. These
are the only pins that need to be controlled/observed to perform JTAG programming (in addition to power pins).
It is not required to apply 12V externally. The JTAGEN Fuse must be programmed and the JTD bit in the
MCUCR Register must be cleared to enable the JTAG Test Access Port.
The JTAG programming capability supports:

Flash programming and verifying

EEPROM programming and verifying

Fuse programming and verifying

Lock bit programming and verifying
The Lock bit security is exactly as in parallel programming mode. If the Lock bits LB1 or LB2 are programmed,
the OCDEN Fuse cannot be programmed unless first doing a chip erase. This is a security feature that ensures
no back-door exists for reading out the content of a secured device.
The details on programming through the JTAG interface and programming specific JTAG instructions are given
in the section “Programming via the JTAG Interface” on page 371.
25.9
Bibliography
For more information about general Boundary-scan, the following literature can be consulted:

IEEE: IEEE Std. 1149.1-1990. IEEE Standard Test Access Port and Boundary-scan Architecture, IEEE,
1993.

Colin Maunder: The Board Designers Guide to Testable Logic Circuits, Addison-Wesley, 1992.
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26.
IEEE 1149.1 (JTAG) Boundary-scan
26.1
Features
•
•
•
•
•
26.2
JTAG (IEEE std. 1149.1 compliant) Interface
Boundary-scan Capabilities According to the JTAG Standard
Full Scan of all Port Functions as well as Analog Circuitry having Off-chip Connections
Supports the Optional IDCODE Instruction
Additional Public AVR_RESET Instruction to Reset the AVR
System Overview
The Boundary-scan chain has the capability of driving and observing the logic levels on the digital I/O pins, as
well as the boundary between digital and analog logic for analog circuitry having off-chip connections. At system
level, all ICs having JTAG capabilities are connected serially by the TDI/TDO signals to form a long Shift
Register. An external controller sets up the devices to drive values at their output pins, and observe the input
values received from other devices. The controller compares the received data with the expected result. In this
way, Boundary-scan provides a mechanism for testing interconnections and integrity of components on Printed
Circuits Boards by using the four TAP signals only.
The four IEEE 1149.1 defined mandatory JTAG instructions IDCODE, BYPASS, SAMPLE/PRELOAD, and
EXTEST, as well as the AVR specific public JTAG instruction AVR_RESET can be used for testing the Printed
Circuit Board. Initial scanning of the Data Register path will show the ID-Code of the device, since IDCODE is
the default JTAG instruction. It may be desirable to have the AVR device in reset during test mode. If not reset,
inputs to the device may be determined by the scan operations, and the internal software may be in an
undetermined state when exiting the test mode. Entering reset, the outputs of any port pin will instantly enter the
high impedance state, making the HIGHZ instruction redundant. If needed, the BYPASS instruction can be
issued to make the shortest possible scan chain through the device. The device can be set in the reset state
either by pulling the external RESET pin low, or issuing the AVR_RESET instruction with appropriate setting of
the Reset Data Register.
The EXTEST instruction is used for sampling external pins and loading output pins with data. The data from the
output latch will be driven out on the pins as soon as the EXTEST instruction is loaded into the JTAG IRRegister. Therefore, the SAMPLE/PRELOAD should also be used for setting initial values to the scan ring, to
avoid damaging the board when issuing the EXTEST instruction for the first time. SAMPLE/PRELOAD can also
be used for taking a snapshot of the external pins during normal operation of the part.
The JTAGEN Fuse must be programmed and the JTD bit in the I/O Register MCUCR must be cleared to enable
the JTAG Test Access Port.
When using the JTAG interface for Boundary-scan, using a JTAG TCK clock frequency higher than the internal
chip frequency is possible. The chip clock is not required to run.
26.3
Data Registers
The Data Registers relevant for Boundary-scan operations are:

Bypass Register

Device Identification Register

Reset Register

Boundary-scan Chain
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26.3.1 Bypass Register
The Bypass Register consists of a single Shift Register stage. When the Bypass Register is selected as path
between TDI and TDO, the register is reset to 0 when leaving the Capture-DR controller state. The Bypass
Register can be used to shorten the scan chain on a system when the other devices are to be tested.
26.3.2 Device Identification Register
Figure 26-1 shows the structure of the Device Identification Register.
Figure 26-1.
The Format of the Device Identification Register
LSB
MSB
Bit
31
Device ID
Version
4 bits
28
27
Part Number
16 bits
12
11
1
Manufacturer ID
11 bits
0
1
1-bit
26.3.2.1 Version
Version is a 4-bit number identifying the revision of the component. The JTAG version number follows the
revision of the device. Revision A is 0x0, revision B is 0x1 and so on.
26.3.2.2 Part Number
The part number is a 16-bit code identifying the component. The JTAG Part Number for
ATmega16U4/ATmega32U4 is listed in this table.
Part Number
AVR USB
JTAG Part Number (Hex)
0x9782
26.3.2.3 Manufacturer ID
The Manufacturer ID is a 11-bit code identifying the manufacturer. The JTAG manufacturer ID for Atmel is listed
below.
Manufacturer
Atmel
JTAG Manufacturer ID (Hex)
0x01F
26.3.3 Reset Register
The Reset Register is a test Data Register used to reset the part. Since the AVR tri-states Port Pins when reset,
the Reset Register can also replace the function of the un-implemented optional JTAG instruction HIGHZ.
A high value in the Reset Register corresponds to pulling the external Reset low. The part is reset as long as
there is a high value present in the Reset Register. Depending on the fuse settings for the clock options, the part
will remain reset for a reset time-out period (refer to “Clock Sources” on page 28) after releasing the Reset
Register. The output from this Data Register is not latched, so the reset will take place immediately, as shown in
Figure 26-2.
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Figure 26-2.
Reset Register
To
TDO
From Other Internal and
External Reset Sources
From
TDI
D
Internal reset
Q
ClockDR · AVR_RESET
26.3.4 Boundary-scan Chain
The Boundary-scan Chain has the capability of driving and observing the logic levels on the digital I/O pins, as
well as the boundary between digital and analog logic for analog circuitry having off-chip connections.
See “Boundary-scan Chain” on page 329 for a complete description.
26.4
Boundary-scan Specific JTAG Instructions
The Instruction Register is 4-bit wide, supporting up to 16 instructions. Listed below are the JTAG instructions
useful for Boundary-scan operation. Note that the optional HIGHZ instruction is not implemented, but all outputs
with tri-state capability can be set in high-impedance state by using the AVR_RESET instruction, since the initial
state for all port pins is tri-state.
As a definition in this datasheet, the LSB is shifted in and out first for all Shift Registers.
The OPCODE for each instruction is shown behind the instruction name in hex format. The text describes which
Data Register is selected as path between TDI and TDO for each instruction.
26.4.1 EXTEST; 0x0
Mandatory JTAG instruction for selecting the Boundary-scan Chain as Data Register for testing circuitry
external to the AVR package. For port-pins, Pull-up Disable, Output Control, Output Data, and Input Data are all
accessible in the scan chain. For Analog circuits having off-chip connections, the interface between the analog
and the digital logic is in the scan chain. The contents of the latched outputs of the Boundary-scan chain is
driven out as soon as the JTAG IR-Register is loaded with the EXTEST instruction.
The active states are:

Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain

Shift-DR: The Internal Scan Chain is shifted by the TCK input

Update-DR: Data from the scan chain is applied to output pins
26.4.2 IDCODE; 0x1
Optional JTAG instruction selecting the 32-bit ID-Register as Data Register. The ID-Register consists of a
version number, a device number and the manufacturer code chosen by JEDEC. This is the default instruction
after power-up.
The active states are:

Capture-DR: Data in the IDCODE Register is sampled into the Boundary-scan Chain

Shift-DR: The IDCODE scan chain is shifted by the TCK input
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26.4.3 SAMPLE_PRELOAD; 0x2
Mandatory JTAG instruction for pre-loading the output latches and taking a snap-shot of the input/output pins
without affecting the system operation. However, the output latches are not connected to the pins. The
Boundary-scan Chain is selected as Data Register.
The active states are:

Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain

Shift-DR: The Boundary-scan Chain is shifted by the TCK input

Update-DR: Data from the Boundary-scan chain is applied to the output latches. However, the output
latches are not connected to the pins.
26.4.4 AVR_RESET; 0xC
The AVR specific public JTAG instruction for forcing the AVR device into the Reset mode or releasing the JTAG
reset source. The TAP controller is not reset by this instruction. The one bit Reset Register is selected as Data
Register. Note that the reset will be active as long as there is a logic “one” in the Reset Chain. The output from
this chain is not latched.
The active states are:

Shift-DR: The Reset Register is shifted by the TCK input
26.4.5 BYPASS; 0xF
Mandatory JTAG instruction selecting the Bypass Register for Data Register.
The active states are:
26.5

Capture-DR: Loads a logic “0” into the Bypass Register

Shift-DR: The Bypass Register cell between TDI and TDO is shifted
Boundary-scan Related Register in I/O Memory
26.5.1 MCU Control Register – MCUCR
The MCU Control Register contains control bits for general MCU functions.
Bit
Read/Write
Initial Value
7
JTD
R/W
0
6
–
R
0
5
–
R
0
4
PUD
R/W
0
3
–
R
0
2
–
R
0
1
IVSEL
R/W
0
0
IVCE
R/W
0
MCUCR
• Bits 7 – JTD: JTAG Interface Disable
When this bit is zero, the JTAG interface is enabled if the JTAGEN Fuse is programmed. If this bit is one, the
JTAG interface is disabled. In order to avoid unintentional disabling or enabling of the JTAG interface, a timed
sequence must be followed when changing this bit: The application software must write this bit to the desired
value twice within four cycles to change its value. Note that this bit must not be altered when using the On-chip
Debug system.
26.5.2 MCU Status Register – MCUSR
The MCU Status Register provides information on which reset source caused an MCU reset.
Bit
Read/Write
Initial Value
7
–
R
0
6
–
R
0
5
–
R
0
4
3
JTRF
WDRF
R/W
R/W
See Bit Description
2
BORF
R/W
1
EXTRF
R/W
0
PORF
R/W
MCUSR
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• Bit 4 – JTRF: JTAG Reset Flag
This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by the JTAG
instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic zero to the flag.
26.6
Boundary-scan Chain
The Boundary-scan chain has the capability of driving and observing the logic levels on the digital I/O pins, as
well as the boundary between digital and analog logic for analog circuitry having off-chip connection.
26.6.1 Scanning the Digital Port Pins
Figure 26-3 on page 330 shows the Boundary-scan Cell for a bi-directional port pin. The pull-up function is
disabled during Boundary-scan when the JTAG IC contains EXTEST or SAMPLE_PRELOAD. The cell consists
of a bi-directional pin cell that combines the three signals Output Control - OCxn, Output Data - ODxn, and Input
Data - IDxn, into only a two-stage Shift Register. The port and pin indexes are not used in the following
description.
The Boundary-scan logic is not included in the figures in the datasheet. Figure 26-4 on page 331 shows a
simple digital port pin as described in the section “I/O-Ports” on page 67. The Boundary-scan details from Figure
26-3 on page 330 replaces the dashed box in Figure 26-4 on page 331.
When no alternate port function is present, the Input Data - ID - corresponds to the PINxn Register value (but ID
has no synchronizer), Output Data corresponds to the PORT Register, Output Control corresponds to the Data
Direction - DD Register, and the Pull-up Enable - PUExn - corresponds to logic expression PUD · DDxn ·
PORTxn.
Digital alternate port functions are connected outside the dotted box in Figure 26-4 on page 331 to make the
scan chain read the actual pin value. For analog function, there is a direct connection from the external pin to
the analog circuit. There is no scan chain on the interface between the digital and the analog circuitry, but some
digital control signal to analog circuitry are turned off to avoid driving contention on the pads.
When JTAG IR contains EXTEST or SAMPLE_PRELOAD the clock is not sent out on the port pins even if the
CKOUT fuse is programmed. Even though the clock is output when the JTAG IR contains SAMPLE_PRELOAD,
the clock is not sampled by the boundary scan.
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Boundary-scan Cell for bi-directional Port Pin with Pull-up Function
To Next Cell
ShiftDR
EXTEST
Pull-up Enable (PUE)
Vcc
0
1
Output Control (OC)
FF1
LD1
0
D
Q
D
Q
0
1
1
G
Output Data (OD)
0
1
FF0
LD0
0
D
Q
D
1
Q
0
1
Port Pin (PXn)
Figure 26-3.
G
Input Data (ID)
From Last Cell
ClockDR
UpdateDR
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Figure 26-4.
General Port Pin Schematic Diagram
See Boundary-scan
Description for Details!
PUExn
PUD
Q
D
DDxn
Q CLR
RESET
OCxn
WDx
Q
Pxn
ODxn
D
PORTxn
Q CLR
WRx
IDxn
DATA BUS
RDx
RESET
RRx
SLEEP
SYNCHRONIZER
D
Q
L
Q
D
RPx
Q
PINxn
Q
CLK I/O
PUD:
PUExn:
OCxn:
ODxn:
IDxn:
SLEEP:
PULLUP DISABLE
PULLUP ENABLE for pin Pxn
OUTPUT CONTROL for pin Pxn
OUTPUT DATA to pin Pxn
INPUT DATA from pin Pxn
SLEEP CONTROL
WDx:
RDx:
WRx:
RRx:
RPx:
CLK I/O :
WRITE DDRx
READ DDRx
WRITE PORTx
READ PORTx REGISTER
READ PORTx PIN
I/O CLOCK
26.6.2 Scanning the RESET Pin
The RESET pin accepts 5V active low logic for standard reset operation, and 12V active high logic for High
Voltage Parallel programming. An observe-only cell as shown in Figure 26-5 is inserted for the 5V reset signal.
Figure 26-5.
Observe-only Cell
To
Next
Cell
ShiftDR
From System Pin
To System Logic
FF1
0
D
Q
1
From
Previous
Cell
ClockDR
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26.7
Boundary-scan Order
The table below shows the Scan order between TDI and TDO when the Boundary-scan chain is selected as
data path. Bit 0 is the LSB; the first bit scanned in, and the first bit scanned out. The scan order follows the pinout order as far as possible. Exceptions from the rules are the Scan chains for the analog circuits, which
constitute the most significant bits of the scan chain regardless of which physical pin they are connected to. In
Figure 26-3 on page 330, PXn. Data corresponds to FF0, PXn. Control corresponds to FF1, PXn. Bit 4, 5, 6, and
7 of Port F is not in the scan chain, since these pins constitute the TAP pins when the JTAG is enabled. The
USB pads are not included in the boundary-scan.
Table 26-1.
ATmega16U4/ATmega32U4 Boundary-scan Order
Bit Number
Signal Name
88
PE6.Data
87
PE6.Control
86
Reserved
85
Reserved
84
Reserved
83
Reserved
82
PB0.Data
81
PB0.Control
80
PB1.Data
79
PB1.Control
78
PB2.Data
77
PB2.Control
76
PB3.Data
75
PB3.Control
74
PB4.Data
73
PB4.Control
72
PB5.Data
71
PB5.Control
70
PB6.Data
69
PB6.Control
68
PB7.Data
67
PB7.Control
66
Reserved
65
Reserved
64
Reserved
63
Reserved
62
RSTT
Module
Port E
Port B
PORTE
Reset Logic (Observe Only)
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Table 26-1.
ATmega16U4/ATmega32U4 Boundary-scan Order
Bit Number
Signal Name
61
PD0.Data
60
PD0.Control
59
PD1.Data
58
PD1.Control
57
PD2.Data
56
PD2.Control
55
PD3.Data
54
PD3.Control
53
PD4.Data
52
PD4.Control
51
PD5.Data
50
PD5.Control
49
PD6.Data
48
PD6.Control
47
PD7.Data
46
PD7.Control
45
Reserved
44
Reserved
43
Reserved
42
Reserved
Module
Port D
Port E
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Table 26-1.
ATmega16U4/ATmega32U4 Boundary-scan Order
Bit Number
Signal Name
41
Reserved
40
Reserved
39
Reserved
38
Reserved
37
Reserved
36
Reserved
35
Reserved
34
Reserved
33
Reserved
32
Reserved
31
Reserved
30
Reserved
29
Reserved
28
Reserved
27
Reserved
26
Reserved
25
PE2.Data
24
PE2.Control
Module
Reserved
Port E
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Table 26-1.
26.8
ATmega16U4/ATmega32U4 Boundary-scan Order
Bit Number
Signal Name
23
Reserved
22
Reserved
21
Reserved
20
Reserved
19
Reserved
18
Reserved
17
Reserved
16
Reserved
15
Reserved
14
Reserved
13
Reserved
12
Reserved
11
Reserved
10
Reserved
9
Reserved
8
Reserved
7
Reserved
6
Reserved
5
Reserved
4
Reserved
3
PF1.Data
2
PF1.Control
1
PF0.Data
0
PF0.Control
Module
Reserved
Port F
Boundary-scan Description Language Files
Boundary-scan Description Language (BSDL) files describe Boundary-scan capable devices in a standard
format used by automated test-generation software. The order and function of bits in the Boundary-scan Data
Register are included in this description. BSDL files are available for the device.
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27.
Boot Loader Support – Read-While-Write Self-Programming
The Boot Loader Support provides a real Read-While-Write Self-Programming mechanism for downloading and
uploading program code by the MCU itself. This feature allows flexible application software updates controlled
by the MCU using a Flash-resident Boot Loader program. The Boot Loader program can use any available data
interface and associated protocol to read code and write (program) that code into the Flash memory, or read the
code from the program memory. The program code within the Boot Loader section has the capability to write
into the entire Flash, including the Boot Loader memory. The Boot Loader can thus even modify itself, and it can
also erase itself from the code if the feature is not needed anymore. The size of the Boot Loader memory is
configurable with fuses and the Boot Loader has two separate sets of Boot Lock bits which can be set
independently. This gives the user a unique flexibility to select different levels of protection.
General information on SPM and ELPM is provided in “AVR CPU Core” on page 9.
27.1
Boot Loader Features
•
•
•
•
•
•
•
Read-While-Write Self-Programming
Flexible Boot Memory Size
High Security (Separate Boot Lock Bits for a Flexible Protection)
Separate Fuse to Select Reset Vector
Optimized Page(1) Size
Code Efficient Algorithm
Efficient Read-Modify-Write Support
Note:
27.2
1.
A page is a section in the Flash consisting of several bytes (see Table 28-11 on page 359) used during
programming. The page organization does not affect normal operation.
Application and Boot Loader Flash Sections
The Flash memory is organized in two main sections, the Application section and the Boot Loader section (see
Figure 27-2 on page 338). The size of the different sections is configured by the BOOTSZ Fuses as shown in
Table 27-8 on page 349 and Figure 27-2 on page 338. These two sections can have different level of protection
since they have different sets of Lock bits.
27.2.1 Application Section
The Application section is the section of the Flash that is used for storing the application code. The protection
level for the Application section can be selected by the application Boot Lock bits (Boot Lock bits 0). The
Application section can never store any Boot Loader code since the SPM instruction is disabled when executed
from the Application section.
Refer to Table 27-2 on page 339.
27.2.2 BLS – Boot Loader Section
While the Application section is used for storing the application code, the The Boot Loader software must be
located in the BLS since the SPM instruction can initiate a programming when executing from the BLS only. The
SPM instruction can access the entire Flash, including the BLS itself. The protection level for the Boot Loader
section can be selected by the Boot Loader Lock bits (Boot Lock bits 1). Refer to Table 27-3 on page 339.
27.3
Read-While-Write and No Read-While-Write Flash Sections
Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot Loader software update is
dependent on which address that is being programmed. In addition to the two sections that are configurable by
the BOOTSZ Fuses as described above, the Flash is also divided into two fixed sections, the Read-While-Write
(RWW) section and the No Read-While-Write (NRWW) section.
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The limit between the RWW- and NRWW sections is given in Table 27-1 on page 337 and Figure 27-1 on page
338. The main difference between the two sections is:

When erasing or writing a page located inside the RWW section, the NRWW section can be read during
the operation

When erasing or writing a page located inside the NRWW section, the CPU is halted during the entire
operation
Note that the user software can never read any code that is located inside the RWW section during a Boot
Loader software operation. The syntax “Read-While-Write section” refers to which section that is being
programmed (erased or written), not which section that actually is being read during a Boot Loader software
update.
27.3.1 RWW – Read-While-Write Section
If a Boot Loader software update is programming a page inside the RWW section, it is possible to read code
from the Flash, but only code that is located in the NRWW section. During an on-going programming, the
software must ensure that the RWW section never is being read. If the user software is trying to read code that
is located inside the RWW section (i.e., by load program memory, call, or jump instructions or an interrupt)
during programming, the software might end up in an unknown state. To avoid this, the interrupts should either
be disabled or moved to the Boot Loader section. The Boot Loader section is always located in the NRWW
section. The RWW Section Busy bit (RWWSB) in the Store Program Memory Control and Status Register
(SPMCSR) will be read as logical one as long as the RWW section is blocked for reading. After a programming
is completed, the RWWSB must be cleared by software before reading code located in the RWW section.
Refer to “Store Program Memory Control and Status Register – SPMCSR” on page 341 for details on how to
clear RWWSB.
27.3.2 NRWW – No Read-While-Write Section
The code located in the NRWW section can be read when the Boot Loader software is updating a page in the
RWW section. When the Boot Loader code updates the NRWW section, the CPU is halted during the entire
Page Erase or Page Write operation.
Table 27-1.
Read-While-Write Features
Which Section does the Z-pointer
Address During the Programming?
Which Section Can
be Read During
Programming?
Is the CPU
Halted?
Read-While-Write
Supported?
RWW Section
NRWW Section
No
Yes
NRWW Section
None
Yes
No
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Figure 27-1.
Read-While-Write vs. No Read-While-Write
Read-While-Write
(RWW) Section
Z-pointer
Addresses NRWW
Section
Z-pointer
Addresses RWW
Section
No Read-While-Write
(NRWW) Section
CPU is Halted
During the Operation
Code Located in
NRWW Section
Can be Read During
the Operation
Figure 27-2.
Memory Sections
Program Memory
BOOTSZ = '10'
Program Memory
BOOTSZ = '11'
0x0000
Read-While-Write Section
Application Flash Section
End RWW
Start NRWW
Application Flash Section
Boot Loader Flash Section
End Application
Start Boot Loader
Flashend
No Read-While-Write Section
No Read-While-Write Section
Read-While-Write Section
0x0000
Program Memory
BOOTSZ = '01'
Application Flash Section
End RWW
Start NRWW
Application Flash Section
End Application
Start Boot Loader
Boot Loader Flash Section
Flashend
Program Memory
BOOTSZ = '00'
Note:
1.
Read-While-Write Section
0x0000
Application Flash Section
End RWW
Start NRWW
Application Flash Section
End Application
Start Boot Loader
Boot Loader Flash Section
Flashend
No Read-While-Write Section
No Read-While-Write Section
Read-While-Write Section
0x0000
Application Flash Section
End RWW, End Application
Start NRWW, Start Boot Loader
Boot Loader Flash Section
Flashend
The parameters in Figure 27-2 are given in Table 27-8 on page 349.
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27.4
Boot Loader Lock Bits
If no Boot Loader capability is needed, the entire Flash is available for application code. The Boot Loader has
two separate sets of Boot Lock bits which can be set independently. This gives the user a unique flexibility to
select different levels of protection.
The user can select:

To protect the entire Flash from a software update by the MCU

To protect only the Boot Loader Flash section from a software update by the MCU

To protect only the Application Flash section from a software update by the MCU

Allow software update in the entire Flash
See the following two tables for further details. The Boot Lock bits can be set by software and in Serial or in
Parallel Programming mode. They can only be cleared by a Chip Erase command only. The general Write Lock
(Lock Bit mode 2) does not control the programming of the Flash memory by SPM instruction. Similarly, the
general Read/Write Lock (Lock Bit mode 1) does not control reading nor writing by (E)LPM/SPM, if it is
attempted.
Boot Lock Bit0 Protection Modes (Application Section)(1)
Table 27-2.
BLB0 Mode
BLB02
BLB01
Protection
1
1
1
No restrictions for SPM or (E)LPM accessing the Application section
2
1
0
SPM is not allowed to write to the Application section
3
0
0
SPM is not allowed to write to the Application section, and (E)LPM executing from
the Boot Loader section is not allowed to read from the Application section. If
Interrupt Vectors are placed in the Boot Loader section, interrupts are disabled while
executing from the Application section.
4
0
1
(E)LPM executing from the Boot Loader section is not allowed to read from the
Application section. If Interrupt Vectors are placed in the Boot Loader section,
interrupts are disabled while executing from the Application section.
Note:
1.
“1” means unprogrammed, “0” means programmed
Boot Lock Bit1 Protection Modes (Boot Loader Section)(1)
Table 27-3.
BLB1 Mode
BLB12
BLB11
1
1
1
No restrictions for SPM or (E)LPM accessing the Boot Loader section
2
1
0
SPM is not allowed to write to the Boot Loader section
Note:
Protection
3
0
0
SPM is not allowed to write to the Boot Loader section, and (E)LPM executing from
the Application section is not allowed to read from the Boot Loader section. If
Interrupt Vectors are placed in the Application section, interrupts are disabled while
executing from the Boot Loader section.
4
0
1
(E)LPM executing from the Application section is not allowed to read from the Boot
Loader section. If Interrupt Vectors are placed in the Application section, interrupts
are disabled while executing from the Boot Loader section.
1.
“1” means unprogrammed, “0” means programmed.
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27.5
Entering the Boot Loader Program
The bootloader can be executed with three different conditions:
27.5.1 Regular Application Conditions
A jump or call from the application program. This may be initiated by a trigger such as a command received via
USART, SPI, or USB.
27.5.2 Boot Reset Fuse
The Boot Reset Fuse (BOOTRST) can be programmed so that the Reset Vector is pointing to the Boot Flash
start address after a reset. In this case, the Boot Loader is started after a reset. After the application code is
loaded, the program can start executing the application code. Note that the fuses cannot be changed by the
MCU itself. This means that once the Boot Reset Fuse is programmed, the Reset Vector will always point to the
Boot Loader Reset and the fuse can only be changed through the serial or parallel programming interface.
Boot Reset Fuse(1)
Table 27-4.
BOOTRST
Note:
Reset Address
1
Reset Vector = Application Reset (address 0x0000)
0
Reset Vector = Boot Loader Reset (see Table 27-8 on page 349)
1.
“1” means unprogrammed, “0” means programmed.
27.5.3 External Hardware conditions
The Hardware Boot Enable Fuse (HWBE) can be programmed (see the table below) so that upon special
hardware conditions under reset, the bootloader execution is forced after reset.
Table 27-5.
HWBE
Note:
Hardware Boot Enable Fuse(1)
Reset Address
1
ALE/HWB pin can not be used to force Boot Loader execution after reset
0
ALE/HWB pin is used during reset to force bootloader execution after reset
1.
“1” means unprogrammed, “0” means programmed.
When the HWBE fuse is enable the ALE/HWB pin is configured as input during reset and sampled during reset
rising edge. When ALE/HWB pin is ‘0’ during reset rising edge, the reset vector will be set as the Boot Loader
Reset address and the Boot Loader will be executed (See Figure 27-3).
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Figure 27-3.
Boot Process Description
RESET
tHHRH
tSHRH
ALE/HWB
HWBE ?
Ext. Hardware
Conditions ?
BOOTRST ?
Reset Vector = Application Reset
Reset Vector =Boot Lhoader Reset
27.5.4 Store Program Memory Control and Status Register – SPMCSR
The Store Program Memory Control and Status Register contains the control bits needed to control the Boot
Loader operations.
Bit
Read/Write
Initial Value
7
SPMIE
R/W
0
6
RWWSB
R
0
5
SIGRD
R/W
0
4
RWWSRE
R/W
0
3
BLBSET
R/W
0
2
PGWRT
R/W
0
1
PGERS
R/W
0
0
SPMEN
R/W
0
SPMCSR
• Bit 7 – SPMIE: SPM Interrupt Enable
When the SPMIE bit is written to one, and the I-bit in the Status Register is set (one), the SPM ready interrupt
will be enabled. The SPM ready Interrupt will be executed as long as the SPMEN bit in the SPMCSR Register is
cleared.
• Bit 6 – RWWSB: Read-While-Write Section Busy
When a Self-Programming (Page Erase or Page Write) operation to the RWW section is initiated, the RWWSB
will be set (one) by hardware. When the RWWSB bit is set, the RWW section cannot be accessed. The
RWWSB bit will be cleared if the RWWSRE bit is written to one after a Self-Programming operation is
completed. Alternatively the RWWSB bit will automatically be cleared if a page load operation is initiated.
• Bit 5 – SIGRD: Signature Row Read
If this bit is written to one at the same time as SPMEN, the next LPM instruction within three clock cycles will
read a byte from the signature row into the destination register. see “Reading the Signature Row from Software”
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on page 346 for details. An SPM instruction within four cycles after SIGRD and SPMEN are set will have no
effect. This operation is reserved for future use and should not be used.
• Bit 4 – RWWSRE: Read-While-Write Section Read Enable
When programming (Page Erase or Page Write) to the RWW section, the RWW section is blocked for reading
(the RWWSB will be set by hardware). To re-enable the RWW section, the user software must wait until the
programming is completed (SPMEN will be cleared). Then, if the RWWSRE bit is written to one at the same
time as SPMEN, the next SPM instruction within four clock cycles re-enables the RWW section. The RWW
section cannot be re-enabled while the Flash is busy with a Page Erase or a Page Write (SPMEN is set). If the
RWWSRE bit is written while the Flash is being loaded, the Flash load operation will abort and the data loaded
will be lost.
• Bit 3 – BLBSET: Boot Lock Bit Set
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles sets
Boot Lock bits, according to the data in R0. The data in R1 and the address in the Z-pointer are ignored. The
BLBSET bit will automatically be cleared upon completion of the Lock bit set, or if no SPM instruction is
executed within four clock cycles.
An (E)LPM instruction within three cycles after BLBSET and SPMEN are set in the SPMCSR Register, will read
either the Lock bits or the Fuse bits (depending on Z0 in the Z-pointer) into the destination register. See
“Reading the Fuse and Lock Bits from Software” on page 345 for details.
• Bit 2 – PGWRT: Page Write
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles
executes Page Write, with the data stored in the temporary buffer. The page address is taken from the high part
of the Z-pointer. The data in R1 and R0 are ignored. The PGWRT bit will auto-clear upon completion of a Page
Write, or if no SPM instruction is executed within four clock cycles. The CPU is halted during the entire Page
Write operation if the NRWW section is addressed.
• Bit 1 – PGERS: Page Erase
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles
executes Page Erase. The page address is taken from the high part of the Z-pointer. The data in R1 and R0 are
ignored. The PGERS bit will auto-clear upon completion of a Page Erase, or if no SPM instruction is executed
within four clock cycles. The CPU is halted during the entire Page Write operation if the NRWW section is
addressed.
• Bit 0 – SPMEN: Store Program Memory Enable
This bit enables the SPM instruction for the next four clock cycles. If written to one together with either
RWWSRE, BLBSET, PGWRT’ or PGERS, the following SPM instruction will have a special meaning, see
description above. If only SPMEN is written, the following SPM instruction will store the value in R1:R0 in the
temporary page buffer addressed by the Z-pointer. The LSB of the Z-pointer is ignored. The SPMEN bit will
auto-clear upon completion of an SPM instruction, or if no SPM instruction is executed within four clock cycles.
During Page Erase and Page Write, the SPMEN bit remains high until the operation is completed.
Writing any other combination than “10001”, “01001”, “00101”, “00011”, or “00001” in the lower five bits will have
no effect.
Note:
Only one SPM instruction should be active at any time.
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27.6
Addressing the Flash During Self-Programming
The Z-pointer is used to address the SPM commands. The Z pointer consists of the Z-registers ZL and ZH in the
register file, and RAMPZ in the I/O space. The number of bits actually used is implementation dependent. Note
that the RAMPZ register is only implemented when the program space is larger than 64KB.
Bit
23
15
RAMPZ7
Z15
Z7
7
RAMPZ
ZH (R31)
ZL (R30)
22
14
RAMPZ6
Z14
Z6
6
21
13
RAMPZ5
Z13
Z5
5
20
12
RAMPZ4
Z12
Z4
4
19
11
RAMPZ3
Z11
Z3
3
18
10
RAMPZ2
Z10
Z2
2
17
9
RAMPZ1
Z9
Z1
1
16
8
RAMPZ0
Z8
Z0
0
Since the Flash is organized in pages, the Program Counter can be treated as having two different sections.
One section, consisting of the least significant bits, is addressing the words within a page, while the most
significant bits are addressing the pages. This is shown in Figure 27-4 on page 343. Note that the Page Erase
and Page Write operations are addressed independently. Therefore it is of major importance that the Boot
Loader software addresses the same page in both the Page Erase and Page Write operation. Once a
programming operation is initiated, the address is latched and the Z-pointer can be used for other operations.
The (E)LPM instruction use the Z-pointer to store the address. Since this instruction addresses the Flash byteby-byte, also bit Z0 of the Z-pointer is used.
Figure 27-4.
Addressing the Flash During SPM(1)
BIT
23
ZPCMSB
ZPAGEMSB
1 0
0
PCMSB
PROGRAM COUNTER
Z - POINTER
PAGEMSB
PCPAGE
PAGE ADDRESS
WITHIN THE FLASH
PCWORD
WORD ADDRESS
WITHIN A PAGE
PROGRAM MEMORY
PAGE
PAGE
INSTRUCTION WORD
PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
Note:
1.
The different variables used in Figure 27-4 are listed in Table 27-10 on page 350.
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27.7
Self-Programming the Flash
The program memory is updated in a page by page fashion. Before programming a page with the data stored in
the temporary page buffer, the page must be erased. The temporary page buffer is filled one word at a time
using SPM and the buffer can be filled either before the Page Erase command or between a Page Erase and a
Page Write operation:
Alternative 1, fill the buffer before a Page Erase

Fill temporary page buffer

Perform a Page Erase

Perform a Page Write
Alternative 2, fill the buffer after Page Erase

Perform a Page Erase

Fill temporary page buffer

Perform a Page Write
If only a part of the page needs to be changed, the rest of the page must be stored (for example in the
temporary page buffer) before the erase, and then be rewritten. When using alternative 1, the Boot Loader
provides an effective Read-Modify-Write feature which allows the user software to first read the page, do the
necessary changes, and then write back the modified data. If alternative 2 is used, it is not possible to read the
old data while loading since the page is already erased. The temporary page buffer can be accessed in a
random sequence. It is essential that the page address used in both the Page Erase and Page Write operation
is addressing the same page. See “Simple Assembly Code Example for a Boot Loader” on page 347 for an
assembly code example.
27.7.1 Performing Page Erase by SPM
To execute Page Erase, set up the address in the Z-pointer, write “X0000011” to SPMCSR and execute SPM
within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored. The page address must be
written to PCPAGE in the Z-register. Other bits in the Z-pointer will be ignored during this operation.

Page Erase to the RWW section: The NRWW section can be read during the Page Erase

Page Erase to the NRWW section: The CPU is halted during the operation
27.7.2 Filling the Temporary Buffer (Page Loading)
To write an instruction word, set up the address in the Z-pointer and data in R1:R0, write “00000001” to
SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The content of PCWORD in the Zregister is used to address the data in the temporary buffer. The temporary buffer will auto-erase after a Page
Write operation or by writing the RWWSRE bit in SPMCSR. It is also erased after a system reset. Note that it is
not possible to write more than one time to each address without erasing the temporary buffer.
If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be lost.
27.7.3 Performing a Page Write
To execute Page Write, set up the address in the Z-pointer, write “X0000101” to SPMCSR and execute SPM
within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored. The page address must be
written to PCPAGE. Other bits in the Z-pointer must be written to zero during this operation.

Page Write to the RWW section: The NRWW section can be read during the Page Write

Page Write to the NRWW section: The CPU is halted during the operation
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27.7.4 Using the SPM Interrupt
If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt when the SPMEN bit in
SPMCSR is cleared. This means that the interrupt can be used instead of polling the SPMCSR Register in
software. When using the SPM interrupt, the Interrupt Vectors should be moved to the BLS section to avoid that
an interrupt is accessing the RWW section when it is blocked for reading. How to move the interrupts is
described in “Interrupts” on page 63.
27.7.5 Consideration While Updating BLS
Special care must be taken if the user allows the Boot Loader section to be updated by leaving Boot Lock bit11
unprogrammed. An accidental write to the Boot Loader itself can corrupt the entire Boot Loader, and further
software updates might be impossible. If it is not necessary to change the Boot Loader software itself, it is
recommended to program the Boot Lock bit11 to protect the Boot Loader software from any internal software
changes.
27.7.6 Prevent Reading the RWW Section During Self-Programming
During Self-Programming (either Page Erase or Page Write), the RWW section is always blocked for reading.
The user software itself must prevent that this section is addressed during the self programming operation. The
RWWSB in the SPMCSR will be set as long as the RWW section is busy. During Self-Programming the Interrupt
Vector table should be moved to the BLS as described in “Interrupts” on page 63, or the interrupts must be
disabled. Before addressing the RWW section after the programming is completed, the user software must clear
the RWWSB by writing the RWWSRE. See “Simple Assembly Code Example for a Boot Loader” on page 347
for an example.
27.7.7 Setting the Boot Loader Lock Bits by SPM
To set the Boot Loader Lock bits, write the desired data to R0, write “X0001001” to SPMCSR and execute SPM
within four clock cycles after writing SPMCSR. The only accessible Lock bits are the Boot Lock bits that may
prevent the Application and Boot Loader section from any software update by the MCU.
Bit
R0
7
1
6
1
5
BLB12
4
BLB11
3
BLB02
2
BLB01
1
1
0
1
See Table 27-2 on page 339 and Table 27-3 on page 339 for how the different settings of the Boot Loader bits
affect the Flash access.
If bits 5..2 in R0 are cleared (zero), the corresponding Boot Lock bit will be programmed if an SPM instruction is
executed within four cycles after BLBSET and SPMEN are set in SPMCSR. The Z-pointer is don’t care during
this operation, but for future compatibility it is recommended to load the Z-pointer with 0x0001 (same as used for
reading the lOck bits). For future compatibility it is also recommended to set bits 7, 6, 1, and 0 in R0 to “1” when
writing the Lock bits. When programming the Lock bits the entire Flash can be read during the operation.
27.7.8 EEPROM Write Prevents Writing to SPMCSR
Note that an EEPROM write operation will block all software programming to Flash. Reading the Fuses and
Lock bits from software will also be prevented during the EEPROM write operation. It is recommended that the
user checks the status bit (EEPE) in the EECR Register and verifies that the bit is cleared before writing to the
SPMCSR Register.
27.7.9 Reading the Fuse and Lock Bits from Software
It is possible to read both the Fuse and Lock bits from software. To read the Lock bits, load the Z-pointer with
0x0001 and set the BLBSET and SPMEN bits in SPMCSR. When an (E)LPM instruction is executed within
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three CPU cycles after the BLBSET and SPMEN bits are set in SPMCSR, the value of the Lock bits will be
loaded in the destination register. The BLBSET and SPMEN bits will auto-clear upon completion of reading the
Lock bits or if no (E)LPM instruction is executed within three CPU cycles or no SPM instruction is executed
within four CPU cycles. When BLBSET and SPMEN are cleared, (E)LPM will work as described in the
Instruction set Manual.
Bit
Rd
7
–
6
–
5
BLB12
4
BLB11
3
BLB02
2
BLB01
1
LB2
0
LB1
The algorithm for reading the Fuse Low byte is similar to the one described above for reading the Lock bits. To
read the Fuse Low byte, load the Z-pointer with 0x0000 and set the BLBSET and SPMEN bits in SPMCSR.
When an (E)LPM instruction is executed within three cycles after the BLBSET and SPMEN bits are set in the
SPMCSR, the value of the Fuse Low byte (FLB) will be loaded in the destination register as shown below. Refer
to Table 27-5 on page 340 for a detailed description and mapping of the Fuse Low byte.
Bit
Rd
7
FLB7
6
FLB6
5
FLB5
4
FLB4
3
FLB3
2
FLB2
1
FLB1
0
FLB0
Similarly, when reading the Fuse High byte, load 0x0003 in the Z-pointer. When an (E)LPM instruction is
executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the value of the Fuse
High byte (FHB) will be loaded in the destination register as shown below. Refer to Table 27-4 on page 340 for
detailed description and mapping of the Fuse High byte.
Bit
Rd
7
FHB7
6
FHB6
5
FHB5
4
FHB4
3
FHB3
2
FHB2
1
FHB1
0
FHB0
When reading the Extended Fuse byte, load 0x0002 in the Z-pointer. When an (E)LPM instruction is executed
within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the value of the Extended Fuse
byte (EFB) will be loaded in the destination register as shown below. Refer to Table 27-3 on page 339 for
detailed description and mapping of the Extended Fuse byte.
Bit
Rd
7
–
6
–
5
–
4
–
3
–
2
EFB2
1
EFB1
0
EFB0
Fuse and Lock bits that are programmed, will be read as zero. Fuse and Lock bits that are unprogrammed, will
be read as one.
27.7.10 Reading the Signature Row from Software
To read the Signature Row from software, load the Z-pointer with the signature byte address given in the table
below and set the SIGRD and SPMEN bits in SPMCSR. When an LPM instruction is executed within three CPU
cycles after the SIGRD and SPMEN bits are set in SPMCSR, the signature byte value will be loaded in the
destination register. The SIGRD and SPMEN bits will auto-clear upon completion of reading the Signature Row
Lock bits or if no LPM instruction is executed within three CPU cycles. When SIGRD and SPMEN are cleared,
LPM will work as described in the Instruction set Manual.
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Table 27-6.
Signature Row Addressing
Signature Byte
Z-Pointer Address
Device Signature Byte 1
0x0000
Device Signature Byte 2
0x0002
Device Signature Byte 3
0x0004
RC Oscillator Calibration Byte
0x0001
Note:
All other addresses are reserved for future use.
27.7.11 Preventing Flash Corruption
During periods of low VCC, the Flash program can be corrupted because the supply voltage is too low for the
CPU and the Flash to operate properly. These issues are the same as for board level systems using the Flash,
and the same design solutions should be applied.
A Flash program corruption can be caused by two situations when the voltage is too low. First, a regular write
sequence to the Flash requires a minimum voltage to operate correctly. Secondly, the CPU itself can execute
instructions incorrectly, if the supply voltage for executing instructions is too low.
Flash corruption can easily be avoided by following these design recommendations (one is sufficient):
1. If there is no need for a Boot Loader update in the system, program the Boot Loader Lock bits to prevent any Boot Loader software updates.
2.
Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done
by enabling the internal Brown-out Detector (BOD) if the operating voltage matches the detection level. If
not, an external low VCC reset protection circuit can be used. If a reset occurs while a write operation is in
progress, the write operation will be completed provided that the power supply voltage is sufficient.
3.
Keep the AVR core in Power-down sleep mode during periods of low VCC. This will prevent the CPU from
attempting to decode and execute instructions, effectively protecting the SPMCSR Register and thus the
Flash from unintentional writes.
27.7.12 Programming Time for Flash when Using SPM
The calibrated RC Oscillator is used to time Flash accesses. This table shows the typical programming time for
Flash accesses from the CPU.
Table 27-7.
SPM Programming Time
Symbol
Min. Programming Time
Max. Programming Time
Flash write (Page Erase, Page Write, and write Lock bits by
SPM)
3.7ms
4.5ms
27.7.13 Simple Assembly Code Example for a Boot Loader
;-the routine writes one page of data from RAM to Flash
; the first data location in RAM is pointed to by the Y pointer
; the first data location in Flash is pointed to by the Z-pointer
;-error handling is not included
;-the routine must be placed inside the Boot space
; (at least the Do_spm sub routine). Only code inside NRWW section can
; be read during Self-Programming (Page Erase and Page Write).
;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24),
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; loophi (r25), spmcrval (r20)
; storing and restoring of registers is not included in the routine
; register usage can be optimized at the expense of code size
;-It is assumed that either the interrupt table is moved to the Boot
; loader section or that the interrupts are disabled.
.equ PAGESIZEB = PAGESIZE*2
;PAGESIZEB is page size in BYTES, not words
.org SMALLBOOTSTART
Write_page:
; Page Erase
ldi
spmcrval, (1<<PGERS) | (1<<SPMEN)
call Do_spm
; re-enable the RWW section
ldi
spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call
Do_spm
;transfer data from RAM to Flash page buffer
ldi
looplo, low(PAGESIZEB)
;init loop variable
ldi
loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256
Wrloop:
ld
r0, Y+
ld
r1, Y+
ldi
spmcrval, (1<<SPMEN)
call Do_spm
adiw ZH:ZL, 2
sbiw loophi:looplo, 2
;use subi for PAGESIZEB<=256
brne Wrloop
; execute Page Write
subi ZL, low(PAGESIZEB)
;restore pointer
sbci ZH, high(PAGESIZEB)
;not required for PAGESIZEB<=256
ldi
spmcrval, (1<<PGWRT) | (1<<SPMEN)
call Do_spm
; re-enable the RWW section
ldi
spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
; read back and check, optional
ldi
looplo, low(PAGESIZEB)
ldi
loophi, high(PAGESIZEB)
subi YL, low(PAGESIZEB)
sbci YH, high(PAGESIZEB)
Rdloop:
elpm r0, Z+
ld
r1, Y+
cpse r0, r1
jmp
Error
sbiw loophi:looplo, 1
brne Rdloop
;init loop variable
;not required for PAGESIZEB<=256
;restore pointer
;use subi for PAGESIZEB<=256
; return to RWW section
; verify that RWW section is safe to read
Return:
in
temp1, SPMCSR
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sbrs temp1, RWWSB
; If RWWSB is set, the RWW
section is not ready yet
ret
; re-enable the RWW section
ldi
spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
rjmp Return
Do_spm:
; check for previous SPM complete
Wait_spm:
in
temp1, SPMCSR
sbrc
temp1, SPMEN
rjmp
Wait_spm
; input: spmcrval determines SPM action
; disable interrupts if enabled, store status
in
temp2, SREG
cli
; check that no EEPROM write access is present
Wait_ee:
sbic EECR, EEPE
rjmp Wait_ee
; SPM timed sequence
out
SPMCSR, spmcrval
spm
; restore SREG (to enable interrupts if originally enabled)
out
SREG, temp2
ret
27.7.14 Boot Loader Parameters
The parameters used in the description of the Self-Programming are given throughout the following tables.
Note:
(Start Boot
Loader Section)
Boot
Reset Address
Boot Loader
Flash Section
256 words
4
0x0000 - 0x3EFF
0x3F00 - 0x3FFF
0x3EFF
0x3F00
1
0
512 words
8
0x0000 - 0x3DFF
0x3E00 - 0x3FFF
0x3DFF
0x3E00
0
1
1024 words
16
0x0000 - 0x3BFF
0x3C00 - 0x3FFF
0x3BFF
0x3C00
0
0
2048 words
32
0x0000 - 0x37FF
0x3800 - 0x3FFF
0x37FF
0x3800
1
1
256 words
4
0x0000 - 0x1EFF
0x1F00 - 0x1FFF
0x1EFF
0x1F00
1
0
512 words
8
0x0000 - 0x1DFF
0x1E00 - 0x1FFF
0x1DFF
0x1E00
0
1
1024 words
16
0x0000 - 0x1BFF
0x1C00 - 0x1FFF
0x1BFF
0x1C00
0
0
2048 words
32
0x0000 - 0x17FF
0x1800 - 0x1FFF
0x17FF
0x1800
1.
End
Application
Section
1
Pages
1
Boot Size
BOOTSZ0
Application
Flash Section
Boot Size Configuration (Word Addresses)(1)
BOOTSZ1
ATmega16U4
ATmega32U4
Device
Table 27-8.
The different BOOTSZ Fuse configurations are shown in Figure 27-2 on page 338
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Read-While-Write Limit (Word Addresses)(1)
Table 27-9.
Device
Section
ATmega32U4
ATmega16U4
Note:
1.
Table 27-10.
Pages
Address
Read-While-Write section (RWW)
224
0x0000 - 0x37FF
No Read-While-Write section (NRWW)
32
0x3800 - 0x3FFF
Read-While-Write section (RWW)
97
0x0000 - 0x17FF
No Read-While-Write section (NRWW)
32
0x1800 - 0x1FFF
For details about these two section, see “NRWW – No Read-While-Write Section” on page 337 and “RWW –
Read-While-Write Section” on page 337.
Explanation of different variables used in Figure 27-4 and the mapping to the Z-pointer
Corresponding
Z-value(1)
Variable
Description
PCMSB
13
Most significant bit in the Program Counter. (The Program Counter is 14
bits PC[13:0])
PAGEMSB
6
Most significant bit which is used to address the words within one page
(64 words in a page requires six bits PC [5:0])
ZPCMSB
Z14
Bit in Z-pointer that is mapped to PCMSB. Because Z0 is not used, the
ZPCMSB equals PCMSB + 1
ZPAGEMS
B
Z7
Bit in Z-pointer that is mapped to PCMSB. Because Z0 is not used, the
ZPAGEMSB equals PAGEMSB + 1
PCPAGE
PC[13:6]
Z14:Z7
Program Counter page address: Page select, for Page Erase and Page
Write
PCWORD
PC[5:0]
Z6:Z1
Program Counter word address: Word select, for filling temporary buffer
(must be zero during Page Write operation)
Note:
Note:
1. Z0: should be zero for all SPM commands, byte select for the (E)LPM instruction.
See “Addressing the Flash During Self-Programming” on page 343 for details about the use of Z-pointer during
Self-Programming.
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28.
Memory Programming
28.1
Program And Data Memory Lock Bits
The device provides six Lock bits which can be left unprogrammed (“1”) or can be programmed (“0”) to obtain
the additional features listed in Table 28-2. The Lock bits can only be erased to “1” with the Chip Erase
command.
Lock Bit Byte()
Table 28-1.
Lock Bit Byte
Bit No
Description
Default Value
ATmega16U4/32U4
ATmega16U4RC/32U4RC
7
–
1
6
–
1
BLB12
5
Boot Lock bit
1
BLB11
4
Boot Lock bit
BLB02
3
Boot Lock bit
1
BLB01
2
Boot Lock bit
1
LB2
1
Lock bit
0
1
LB1
0
Lock bit
0
1
Note:
1.
0
1
“1”: unprogrammed, “0”: programmed
Table 28-2.
Lock Bit Protection Modes(1)(2)
Memory Lock Bits
Protection Type
LB Mode
LB2
LB1
1
1
1
No memory lock features enabled.
2
1
0
Further programming of the Flash and EEPROM is disabled in
Parallel and Serial Programming mode. The Fuse bits are locked in
both Serial and Parallel Programming mode.(1)
Further programming and verification of the Flash and EEPROM is
disabled in Parallel and Serial Programming mode. The Boot Lock
bits and Fuse bits are locked in both Serial and Parallel
Programming mode.(1)
3
0
0
BLB0 Mode
BLB02
BLB01
1
1
1
No restrictions for SPM or (E)LPM accessing the Application
section.
2
1
0
SPM is not allowed to write to the Application section.
0
SPM is not allowed to write to the Application section, and (E)LPM
executing from the Boot Loader section is not allowed to read from
the Application section. If Interrupt Vectors are placed in the Boot
Loader section, interrupts are disabled while executing from the
Application section.
3
0
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Lock Bit Protection Modes(1)(2)
Table 28-2.
Memory Lock Bits
(E)LPM executing from the Boot Loader section is not allowed to
read from the Application section. If Interrupt Vectors are placed in
the Boot Loader section, interrupts are disabled while executing
from the Application section.
4
0
1
BLB1 Mode
BLB12
BLB11
1
1
1
No restrictions for SPM or (E)LPM accessing the Boot Loader
section.
2
1
0
SPM is not allowed to write to the Boot Loader section.
0
SPM is not allowed to write to the Boot Loader section, and (E)LPM
executing from the Application section is not allowed to read from
the Boot Loader section. If Interrupt Vectors are placed in the
Application section, interrupts are disabled while executing from the
Boot Loader section.
1
(E)LPM executing from the Application section is not allowed to
read from the Boot Loader section. If Interrupt Vectors are placed in
the Application section, interrupts are disabled while executing from
the Boot Loader section.
3
0
4
Notes:
28.2
Protection Type
0
1.
2.
Program the Fuse bits and Boot Lock bits before programming the LB1 and LB2.
“1” means unprogrammed, “0” means programmed.
Fuse Bits
The device has three bytes. Table 28-3 to Table 28-5 on page 355 describe briefly the functionality of all the
fuses and how they are mapped into the Fuse bytes. Note that the fuses are read as logical zero, “0”, if they are
programmed.
Table 28-3.
Extended Fuse Byte(1)(2)
Fuse Low Byte
Bit No
Description
Default Value
ATmega16/32U4
ATmega16/32U4RC
–
7
–
1
–
6
–
1
–
5
–
1
–
4
–
1
HWBE
3
Hardware Boot Enable
BODLEVEL2(1)
2
Brown-out Detector trigger
level
0 (programmed)
BODLEVEL1(1)
1
Brown-out Detector trigger
level
1 (unprogrammed)
BODLEVEL0(1)
0
Brown-out Detector trigger
level
1 (unprogrammed)
Notes:
1.
2.
0 (programmed)
1 (unprogrammed)
See Table 8-1 on page 53 for BODLEVEL Fuse decoding.
“1” means unprogrammed, “0” means programmed.
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Table 28-4.
Fuse High Byte
Fuse High
Byte
Bit No
Description
Default Value
OCDEN(4)
7
Enable OCD
1 (unprogrammed, OCD disabled)
JTAGEN
6
Enable JTAG
0 (programmed, JTAG enabled)
SPIEN(1)
5
Enable Serial Program and Data
Downloading
0 (programmed, SPI prog. enabled)
WDTON(3)
4
Watchdog Timer always on
1 (unprogrammed)
EESAVE
3
EEPROM memory is preserved
through the Chip Erase
1 (unprogrammed, EEPROM
preserved)
BOOTSZ1
2
Select Boot Size (see Table 28-7
for details)
0 (programmed)(2)
BOOTSZ0
1
Select Boot Size (see Table 28-7
for details)
0 (programmed)(2)
BOOTRST
0
Select Bootloader Address as
Reset Vector
1 (unprogrammed, Reset vector
@0x0000)
Note:
1.
2.
3.
4.
Table 28-5.
The SPIEN Fuse is not accessible in serial programming mode.
The default value of BOOTSZ1..0 results in maximum Boot Size. See Table 28-7 for details.
See “Watchdog Timer” on page 55 for details.
Never ship a product with the OCDEN Fuse programmed regardless of the setting of Lock bits and JTAGEN
Fuse. A programmed OCDEN Fuse enables some parts of the clock system to be running in all sleep modes.
This may increase the power consumption.
Fuse Low Byte
Fuse Low Byte
Bit Nr
Description
Default Value
ATmega16U4/32U4
ATmega16U4RC/32U4RC
CKDIV8(3)
7
Divide clock by 8
CKOUT(2)
6
Clock output
SUT1
5
Select start-up time
0 (programmed)
SUT0
4
Select start-up time
1 (unprogrammed)
CKSEL3
3
Select Clock source
1 (unprogrammed)(1)
0 (programmed)(1)
CKSEL2
2
Select Clock source
1 (unprogrammed)(1)
0 (programmed)(1)
CKSEL1
1
Select Clock source
1 (unprogrammed)(1)
1 (unprogrammed)(1)
CKSEL0
0
Select Clock source
0 (programmed)(1)
0 (programmed)(1)
Note:
1.
2.
3.
0 (programmed)
1 (unprogrammed)
The default setting of CKSEL3..0 results in Low Power Crystal Oscillator for ATmega16U4 and ATmega32U4,
and Internal RC oscillator for ATmega16U4RC and ATmega32U4RC.
The CKOUT Fuse allow the system clock to be output on PORTC7. See “CLKPR – Clock Prescaler Register”
on page 39 for details.
See “System Clock Prescaler” on page 35 for details.
The status of the Fuse bits is not affected by Chip Erase. Note that the Fuse bits are locked if Lock bit1 (LB1) is
programmed. Program the Fuse bits before programming the Lock bits.
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28.2.1 Latching of Fuses
The fuse values are latched when the device enters programming mode and changes of the fuse values will
have no effect until the part leaves Programming mode. This does not apply to the EESAVE Fuse which will
take effect once it is programmed. The fuses are also latched on Power-up in Normal mode.
28.3
Signature Bytes
All Atmel microcontrollers have a three-byte signature code which identifies the device. This code can be read in
both serial and parallel mode, also when the device is locked. The three bytes reside in a separate address
space.
ATmega16U4 Signature Bytes:
1. 0x000: 0x1E (indicates manufactured by Atmel).
2.
0x001: 0x94 (indicates 16KB Flash memory).
3.
0x002: 0x88 (indicates ATmega16U4 device).
ATmega32U4 Signature Bytes:
1. 0x000: 0x1E (indicates manufactured by Atmel).
28.4
2.
0x001: 0x95 (indicates 32KB Flash memory).
3.
0x002: 0x87 (indicates ATmega32U4 device).
Calibration Byte
The device has a byte calibration value for the internal RC Oscillator. This byte resides in the high byte of
address 0x000 in the signature address space. During reset, this byte is automatically written into the OSCCAL
Register to ensure correct frequency of the calibrated RC Oscillator.
28.5
Parallel Programming Parameters, Pin Mapping, and Commands
This section describes how to parallel program and verify Flash Program memory, EEPROM Data memory,
Memory Lock bits, and Fuse bits in the device. Pulses are assumed to be at least 250ns unless otherwise
noted.
28.5.1 Signal Names
In this section, some pins of the device are referenced by signal names describing their functionality during
parallel programming, see Figure 28-1 on page 357 and Table 28-6 on page 357. Pins not described in the
following table are referenced by pin names.
The XA1/XA0 pins determine the action executed when the XTAL1 pin is given a positive pulse. The bit coding
is shown in Table 28-9 on page 358.
When pulsing WR or OE, the command loaded determines the action executed. The different commands are
shown in Table 28-10 on page 358.
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Parallel Programming(1)
Figure 28-1.
+5V
RDY/BSY
PD1
OE
PD2
WR
PD3
BS1
PD4
XA0
PD5
XA1
PD6
PAGEL
PD7
+12 V
VCC
+5V
AVCC
PB7 - PB0
DATA
RESET
BS2
PE6
XTAL1
GND
Note:
1.
Unused Pins should be left floating.
Table 28-6.
Pin Name Mapping
Signal Name in
Programming Mode
Pin Name
I/O
Function
RDY/BSY
PD1
O
0: Device is busy programming, 1: Device is ready for new
command
OE
PD2
I
Output Enable (Active low)
WR
PD3
I
Write Pulse (Active low)
BS1
PD4
I
Byte Select 1
XA0
PD5
I
XTAL Action Bit 0
XA1
PD6
I
XTAL Action Bit 1
PAGEL
PD7
I
Program Memory and EEPROM data Page Load
BS2
PE6
I
Byte Select 2
DATA
PB7-0
I/O
Bi-directional Data bus (Output when OE is low)
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Table 28-7.
BS2 and BS1 Encoding
Flash Data
Loading /
Reading
Fuse
Programming
Reading Fuse
and Lock Bits
BS2
BS1
Flash / EEPROM
Address
0
0
Low Byte
Low Byte
Low Byte
Fuse Low Byte
0
1
High Byte
High Byte
High Byte
Lock bits
1
0
Extended High
Byte
Reserved
Extended Byte
Extended Fuse
Byte
1
1
Reserved
Reserved
Reserved
Fuse High Byte
Table 28-8.
Table 28-9.
Pin Values Used to Enter Programming Mode
Pin
Symbol
Value
PAGEL
Prog_enable[3]
0
XA1
Prog_enable[2]
0
XA0
Prog_enable[1]
0
BS1
Prog_enable[0]
0
XA1 and XA0 Enoding
XA1
XA0
0
0
Load Flash or EEPROM Address (High or low address byte determined by
BS2 and BS1)
0
1
Load Data (High or Low data byte for Flash determined by BS1)
1
0
Load Command
1
1
No Action, Idle
Table 28-10.
Action when XTAL1 is Pulsed
Command Byte Bit Encoding
Command Byte
Command Executed
1000 0000
Chip Erase
0100 0000
Write Fuse bits
0010 0000
Write Lock bits
0001 0000
Write Flash
0001 0001
Write EEPROM
0000 1000
Read Signature Bytes and Calibration byte
0000 0100
Read Fuse and Lock bits
0000 0010
Read Flash
0000 0011
Read EEPROM
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Table 28-11.
No. of Words in a Page and No. of Pages in the Flash
Device
Flash Size
Page Size
PCWORD
No. of
Pages
PCPAGE
PCMSB
ATmega16U4
8K words (16KB)
64 words
PC[5:0]
128
PC[12:6]
12
ATmega32U4
16K words (32KB)
64 words
PC[5:0]
256
PC[13:6]
13
Table 28-12.
No. of Words in a Page and No. of Pages in the EEPROM
Device
EEPROM Size
Page Size
PCWORD
No. of
Pages
PCPAGE
EEAMSB
ATmega16U4
512 bytes
4 bytes
EEA[1:0]
128
EEA[8:2]
8
ATmega32U4
1KB
4 bytes
EEA[1:0]
256
EEA[9:2]
9
28.6
Parallel Programming
28.6.1 Enter Programming Mode
The following algorithm puts the device in parallel programming mode:
1. Apply 4.5 - 5.5V between VCC and GND.
2.
Set RESET to “0” and toggle XTAL1 at least six times.
3.
Set the Prog_enable pins listed in Table 28-8 on page 358 to “0000” and wait at least 100ns.
4.
Apply 11.5 - 12.5V to RESET. Any activity on Prog_enable pins within 100ns after +12V has been applied
to RESET, will cause the device to fail entering programming mode.
5.
Wait at least 50µs before sending a new command.
28.6.2 Considerations for Efficient Programming
The loaded command and address are retained in the device during programming. For efficient programming,
the following should be considered.

The command needs only be loaded once when writing or reading multiple memory locations

Skip writing the data value 0xFF, that is the contents of the entire EEPROM (unless the EESAVE Fuse is
programmed) and Flash after a Chip Erase

Address high byte needs only be loaded before programming or reading a new 256 word window in Flash
or 256 byte EEPROM. This consideration also applies to Signature bytes reading.
28.6.3 Chip Erase
The Chip Erase will erase the Flash and EEPROM(1) memories plus Lock bits. The Lock bits are not reset until
the program memory has been completely erased. The Fuse bits are not changed. A Chip Erase must be
performed before the Flash and/or EEPROM are reprogrammed.
Note:
1.
The EEPRPOM memory is preserved during Chip Erase if the EESAVE Fuse is programmed.
Load Command “Chip Erase”:
1. Set XA1, XA0 to “10”. This enables command loading.
2.
Set BS1 to “0”.
3.
Set DATA to “1000 0000”. This is the command for Chip Erase.
4.
Give XTAL1 a positive pulse. This loads the command.
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5.
Give WR a negative pulse. This starts the Chip Erase. RDY/BSY goes low.
6.
Wait until RDY/BSY goes high before loading a new command.
28.6.4 Programming the Flash
The Flash is organized in pages, see Table 28-11 on page 359. When programming the Flash, the program data is
latched into a page buffer. This allows one page of program data to be programmed simultaneously. The
following procedure describes how to program the entire Flash memory:
A. Load Command “Write Flash”
1. Set XA1, XA0 to “10”. This enables command loading.
2.
Set BS1 to “0”.
3.
Set DATA to “0001 0000”. This is the command for Write Flash.
4.
Give XTAL1 a positive pulse. This loads the command.
B. Load Address Low byte (Address bits 7..0)
1. Set XA1, XA0 to “00”. This enables address loading.
2.
Set BS2, BS1 to “00”. This selects the address low byte.
3.
Set DATA = Address low byte (0x00 - 0xFF).
4.
Give XTAL1 a positive pulse. This loads the address low byte.
C. Load Data Low Byte
1. Set XA1, XA0 to “01”. This enables data loading.
2.
Set DATA = Data low byte (0x00 - 0xFF).
3.
Give XTAL1 a positive pulse. This loads the data byte.
D. Load Data High Byte
1. Set BS1 to “1”. This selects high data byte.
2.
Set XA1, XA0 to “01”. This enables data loading.
3.
Set DATA = Data high byte (0x00 - 0xFF).
4.
Give XTAL1 a positive pulse. This loads the data byte.
E. Latch Data
1. Set BS1 to “1”. This selects high data byte.
2.
Give PAGEL a positive pulse. This latches the data bytes. (See Figure 28-3 on page 361 for signal
waveforms)
F. Repeat B through E until the entire buffer is filled or until all data within the page is loaded.
While the lower bits in the address are mapped to words within the page, the higher bits address the pages
within the FLASH. This is illustrated in Figure 28-2 on page 361. Note that if less than eight bits are required to
address words in the page (page size < 256), the most significant bit(s) in the address low byte are used to
address the page when performing a Page Write.
G. Load Address High byte (Address bits15..8)
1. Set XA1, XA0 to “00”. This enables address loading.
2.
Set BS2, BS1 to “01”. This selects the address high byte.
3.
Set DATA = Address high byte (0x00 - 0xFF).
4.
Give XTAL1 a positive pulse. This loads the address high byte.
H. Load Address Extended High byte (Address bits 23..16)
1. Set XA1, XA0 to “00”. This enables address loading.
2.
Set BS2, BS1 to “10”. This selects the address extended high byte.
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3.
Set DATA = Address extended high byte (0x00 - 0xFF).
4.
Give XTAL1 a positive pulse. This loads the address high byte.
I. Program Page
1. Set BS2, BS1 to “00”
2.
Give WR a negative pulse. This starts programming of the entire page of data. RDY/BSY goes low.
3.
Wait until RDY/BSY goes high (See Figure 28-3 on page 361 for signal waveforms).
J. Repeat B through I until the entire Flash is programmed or until all data has been programmed
K. End Page Programming
1. 1. Set XA1, XA0 to “10”. This enables command loading.
2.
Set DATA to “0000 0000”. This is the command for No Operation.
3.
Give XTAL1 a positive pulse. This loads the command, and the internal write signals are reset.
Figure 28-2.
Addressing the Flash Which is Organized in Pages(1)
PCMSB
PROGRAM
COUNTER
PAGEMSB
PCPAGE
PCWORD
PAGE ADDRESS
WITHIN THE FLASH
WORD ADDRESS
WITHIN A PAGE
PROGRAM MEMORY
PAGE
PAGE
PCWORD[PAGEMSB:0]:
00
INSTRUCTION WORD
01
02
PAGEEND
Note:
1.
Figure 28-3.
PCPAGE and PCWORD are listed in Table 28-11 on page 359.
Programming the Flash Waveforms(1)
F
DATA
A
B
C
D
E
0x10
ADDR. LOW
DATA LOW
DATA HIGH
XX
B
ADDR. LOW
C
D
DATA LOW
DATA HIGH
E
XX
G
H
ADDR. HIGH ADDR. EXT.H
I
XX
XA1
XA0
BS1
BS2
XTAL1
WR
RDY/BSY
RESET +12V
OE
PAGEL
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Note:
1.
“XX” is don’t care. The letters refer to the programming description above.
28.6.5 Programming the EEPROM
The EEPROM is organized in pages, see Table 28-12 on page 359. When programming the EEPROM, the
program data is latched into a page buffer. This allows one page of data to be programmed simultaneously. The
programming algorithm for the EEPROM data memory is as follows (refer to “Programming the Flash” on
page 360 for details on Command, Address and Data loading):
1. A: Load Command “0001 0001”.
2.
G: Load Address High Byte (0x00 - 0xFF).
3.
B: Load Address Low Byte (0x00 - 0xFF).
4.
C: Load Data (0x00 - 0xFF).
5.
E: Latch data (give PAGEL a positive pulse).
K: Repeat 3 through 5 until the entire buffer is filled
L: Program EEPROM page
1. Set BS2, BS1 to “00”.
2.
Give WR a negative pulse. This starts programming of the EEPROM page. RDY/BSY goes low.
3.
Wait until to RDY/BSY goes high before programming the next page (See Figure 28-4 on page 362 for
signal waveforms).
Figure 28-4.
Programming the EEPROM Waveforms
K
DATA
A
G
0x11
ADDR. HIGH
B
ADDR. LOW
C
DATA
E
XX
B
ADDR. LOW
C
E
DATA
L
XX
XA1
XA0
BS1
XTAL1
WR
RDY/BSY
RESET +12V
OE
PAGEL
BS2
28.6.6 Reading the Flash
The algorithm for reading the Flash memory is as follows (refer to “Programming the Flash” on page 360 for
details on Command and Address loading):
1. A: Load Command “0000 0010”.
2.
H: Load Address Extended Byte (0x00- 0xFF).
3.
G: Load Address High Byte (0x00 - 0xFF).
4.
B: Load Address Low Byte (0x00 - 0xFF).
5.
Set OE to “0”, and BS1 to “0”. The Flash word low byte can now be read at DATA.
6.
Set BS to “1”. The Flash word high byte can now be read at DATA.
7.
Set OE to “1”.
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28.6.7 Reading the EEPROM
The algorithm for reading the EEPROM memory is as follows (refer to “Programming the Flash” on page 360 for
details on Command and Address loading):
1. A: Load Command “0000 0011”.
2.
G: Load Address High Byte (0x00 - 0xFF).
3.
B: Load Address Low Byte (0x00 - 0xFF).
4.
Set OE to “0”, and BS1 to “0”. The EEPROM Data byte can now be read at DATA.
5.
Set OE to “1”.
28.6.8 Programming the Fuse Low Bits
The algorithm for programming the Fuse Low bits is as follows (refer to “Programming the Flash” on page 360
for details on Command and Data loading):
1. A: Load Command “0100 0000”.
2.
C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3.
Give WR a negative pulse and wait for RDY/BSY to go high.
28.6.9 Programming the Fuse High Bits
The algorithm for programming the Fuse High bits is as follows (refer to “Programming the Flash” on page 360
for details on Command and Data loading):
1. A: Load Command “0100 0000”.
2.
C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3.
Set BS2, BS1 to “01”. This selects high data byte.
4.
Give WR a negative pulse and wait for RDY/BSY to go high.
5.
Set BS2, BS1 to “00”. This selects low data byte.
28.6.10 Programming the Extended Fuse Bits
The algorithm for programming the Extended Fuse bits is as follows (refer to “Programming the Flash” on
page 360 for details on Command and Data loading):
1. 1. A: Load Command “0100 0000”.
2.
2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3.
3. Set BS2, BS1 to “10”. This selects extended data byte.
4.
4. Give WR a negative pulse and wait for RDY/BSY to go high.
5.
5. Set BS2, BS1 to “00”. This selects low data byte.
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Figure 28-5.
Programming the FUSES Waveforms
Write Fuse Low byte
DATA
A
C
0x40
DATA
XX
Write Fuse high byte
A
C
0x40
DATA
XX
Write Extended Fuse byte
A
C
0x40
DATA
XX
XA1
XA0
BS1
BS2
XTAL1
WR
RDY/BSY
RESET +12V
OE
PAGEL
28.6.11 Programming the Lock Bits
The algorithm for programming the Lock bits is as follows (refer to “Programming the Flash” on page 360 for
details on Command and Data loading):
1. A: Load Command “0010 0000”.
2.
C: Load Data Low Byte. Bit n = “0” programs the Lock bit. If LB mode 3 is programmed (LB1 and LB2 is
programmed), it is not possible to program the Boot Lock bits by any External Programming mode.
3.
Give WR a negative pulse and wait for RDY/BSY to go high.
The Lock bits can only be cleared by executing Chip Erase.
28.6.12 Reading the Fuse and Lock Bits
The algorithm for reading the Fuse and Lock bits is as follows (refer to “Programming the Flash” on page 360 for
details on Command loading):
1. A: Load Command “0000 0100”.
2.
Set OE to “0”, and BS2, BS1 to “00”. The status of the Fuse Low bits can now be read at DATA (“0” means
programmed).
3.
Set OE to “0”, and BS2, BS1 to “11”. The status of the Fuse High bits can now be read at DATA (“0”
means programmed).
4.
Set OE to “0”, and BS2, BS1 to “10”. The status of the Extended Fuse bits can now be read at DATA (“0”
means programmed).
5.
Set OE to “0”, and BS2, BS1 to “01”. The status of the Lock bits can now be read at DATA (“0” means
programmed).
6.
Set OE to “1”.
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Figure 28-6.
Mapping Between BS1, BS2 and the Fuse and Lock Bits During Read
Fuse Low Byte
0
Extended Fuse Byte
1
0
DATA
BS2
0
Lock Bits
1
Fuse High Byte
BS1
1
BS2
28.6.13 Reading the Signature Bytes
The algorithm for reading the Signature bytes is as follows (refer to “Programming the Flash” on page 360 for
details on Command and Address loading):
1. A: Load Command “0000 1000”.
2.
B: Load Address Low Byte (0x00 - 0x02).
3.
Set OE to “0”, and BS to “0”. The selected Signature byte can now be read at DATA.
4.
Set OE to “1”.
28.6.14 Reading the Calibration Byte
The algorithm for reading the Calibration byte is as follows (refer to “Programming the Flash” on page 360 for
details on Command and Address loading):
1. A: Load Command “0000 1000”.
2.
B: Load Address Low Byte, 0x00.
3.
Set OE to “0”, and BS1 to “1”. The Calibration byte can now be read at DATA.
4.
Set OE to “1”.
28.6.15 Parallel Programming Characteristics
Figure 28-7.
Parallel Programming Timing, Including some General Timing Requirements
tXLWL
tXHXL
XTAL1
tDVXH
tXLDX
Data & Contol
(DATA, XA0/1, BS1, BS2)
tPLBX t BVWL
tBVPH
PAGEL
tWLBX
tPHPL
tWLWH
WR
tPLWL
WLRL
RDY/BSY
tWLRH
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Figure 28-8.
Parallel Programming Timing, Loading Sequence with Timing Requirements(1)
LOAD ADDRESS
(LOW BYTE)
LOAD DATA LOAD DATA
(HIGH BYTE)
LOAD DATA
(LOW BYTE)
tXLPH
t XLXH
LOAD ADDRESS
(LOW BYTE)
tPLXH
XTAL1
BS1
PAGEL
DATA
ADDR0 (Low Byte)
DATA (Low Byte)
DATA (High Byte)
ADDR1 (Low Byte)
XA0
XA1
Note:
1.
Figure 28-9.
The timing requirements shown in Figure 28-7 (i.e., tDVXH, tXHXL, and tXLDX) also apply to loading operation.
Parallel Programming Timing, Reading Sequence (within the Same Page) with Timing
Requirements(1)
LOAD ADDRESS
(LOW BYTE)
READ DATA
(LOW BYTE)
READ DATA
(HIGH BYTE)
LOAD ADDRESS
(LOW BYTE)
tXLOL
XTAL1
tBVDV
BS1
tOLDV
OE
DATA
tOHDZ
ADDR0 (Low Byte)
ADDR1 (Low Byte)
DATA (High Byte)
DATA (Low Byte)
XA0
XA1
Note:
1.
Table 28-13.
The timing requirements shown in Figure 28-7 (i.e., tDVXH, tXHXL, and tXLDX) also apply to reading operation.
Parallel Programming Characteristics, VCC = 5V ± 10%
Symbol
Parameter
Min.
VPP
Programming Enable Voltage
11.5
IPP
Programming Enable Current
Typ.
Max.
Units
12.5
V
250
A
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Table 28-13.
Symbol
Parameter
tDVXH
Data and Control Valid before XTAL1 High
67
tXLXH
XTAL1 Low to XTAL1 High
200
tXHXL
XTAL1 Pulse Width High
150
tXLDX
Data and Control Hold after XTAL1 Low
67
tXLWL
XTAL1 Low to WR Low
0
tXLPH
XTAL1 Low to PAGEL high
0
tPLXH
PAGEL low to XTAL1 high
150
tBVPH
BS1 Valid before PAGEL High
67
tPHPL
PAGEL Pulse Width High
150
tPLBX
BS1 Hold after PAGEL Low
67
tWLBX
BS2/1 Hold after WR Low
67
tPLWL
PAGEL Low to WR Low
67
tBVWL
BS2/1 Valid to WR Low
67
tWLWH
WR Pulse Width Low
150
tWLRL
WR Low to RDY/BSY Low
Min.
tWLRH
Typ.
Max.
1
WR Low to RDY/BSY High(1)
3.7
4.5
tWLRH_CE
WR Low to RDY/BSY High for Chip Erase(2)
7.5
9
tXLOL
XTAL1 Low to OE Low
0
tBVDV
BS1 Valid to DATA valid
0
tOLDV
OE Low to DATA Valid
250
tOHDZ
OE High to DATA Tri-stated
250
1.
2.
Units
ns
0
Notes:
28.7
Parallel Programming Characteristics, VCC = 5V ± 10%
250
s
ms
ns
tWLRH is valid for the Write Flash, Write EEPROM, Write Fuse bits and Write Lock bits commands.
tWLRH_CE is valid for the Chip Erase command.
Serial Downloading
Both the Flash and EEPROM memory arrays can be programmed using a serial programming bus while RESET
is pulled to GND. The serial programming interface consists of pins SCK, PDI (input) and PDO (output). After
RESET is set low, the Programming Enable instruction needs to be executed first before program/erase
operations can be executed. NOTE, in Table 28-14 on page 368, the pin mapping for serial programming is listed.
Not all packages use the SPI pins dedicated for the internal Serial Peripheral Interface - SPI.
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28.8
Serial Programming Pin Mapping
Table 28-14.
Pin Mapping Serial Programming
Symbol
Pins (TQFP-64)
I/O
Description
PDI
PB2
I
Serial Data in
PDO
PB3
O
Serial Data out
SCK
PB1
I
Serial Clock
Figure 28-10. Serial Programming and Verify(1)
+1.8 - 5.5V
VCC
+1.8 - 5.5V(2)
PDI
AVCC
PDO
SCK
XTAL1
RESET
GND
Notes:
1.
2.
If the device is clocked by the internal Oscillator, it is no need to connect a clock source to the XTAL1 pin.
VCC - 0.3V < AVCC < VCC + 0.3V, however, AVCC should always be within 1.8 - 5.5V
When programming the EEPROM, an auto-erase cycle is built into the self-timed programming operation (in the
Serial mode ONLY) and there is no need to first execute the Chip Erase instruction. The Chip Erase operation
turns the content of every memory location in both the Program and EEPROM arrays into 0xFF.
Depending on CKSEL Fuses, a valid clock must be present. The minimum low and high periods for the serial
clock (SCK) input are defined as follows:
Low:> two CPU clock cycles for fck < 12MHz, three CPU clock cycles for fck >= 12MHz
High:> two CPU clock cycles for fck < 12MHz, three CPU clock cycles for fck >= 12MHz
28.8.1 Serial Programming Algorithm
When writing serial data to the device, data is clocked on the rising edge of SCK.
When reading data from the device, data is clocked on the falling edge of SCK. See Figure 28-11 on page 369
for timing details.
To program and verify the device in the serial programming mode, the following sequence is recommended (see
four byte instruction formats in Table 28-16 on page 370):
1. Power-up sequence:
Apply power between VCC and GND while RESET and SCK are set to “0”. In some systems, the programmer can not guarantee that SCK is held low during power-up. In this case, RESET must be given
a positive pulse of at least two CPU clock cycles duration after SCK has been set to “0”.
2.
Wait for at least 20ms and enable serial programming by sending the Programming Enable serial
instruction to pin PDI.
3.
The serial programming instructions will not work if the communication is out of synchronization. When in
sync. the second byte (0x53), will echo back when issuing the third byte of the Programming Enable
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instruction. Whether the echo is correct or not, all four bytes of the instruction must be transmitted. If the
0x53 did not echo back, give RESET a positive pulse and issue a new Programming Enable command.
4.
The Flash is programmed one page at a time. The memory page is loaded one byte at a time by supplying
the seven LSB of the address and data together with the Load Program Memory Page instruction. To
ensure correct loading of the page, the data low byte must be loaded before data high byte is applied for a
given address. The Program Memory Page is stored by loading the Write Program Memory Page
instruction with the address lines 15..8. Before issuing this command, make sure the instruction Load
Extended Address Byte has been used to define the MSB of the address. The extended address byte is
stored until the command is re-issued, i.e., the command needs only be issued for the first page, and
when crossing the 64KWord boundary. If polling (RDY/BSY) is not used, the user must wait at least
tWD_FLASH before issuing the next page. (See Table 28-15 on page 369.) Accessing the serial programming
interface before the Flash write operation completes can result in incorrect programming.
5.
The EEPROM array is programmed one byte at a time by supplying the address and data together with
the appropriate Write instruction. An EEPROM memory location is first automatically erased before new
data is written. If polling is not used, the user must wait at least tWD_EEPROM before issuing the next byte.
(See Table 28-15 on page 369.) In a chip erased device, no 0xFFs in the data file(s) need to be
programmed.
6.
Any memory location can be verified by using the Read instruction which returns the content at the
selected address at serial output PDO. When reading the Flash memory, use the instruction Load
Extended Address Byte to define the upper address byte, which is not included in the Read Program
Memory instruction. The extended address byte is stored until the command is re-issued, i.e., the
command needs only be issued for the first page, and when crossing the 64KWord boundary.
7.
At the end of the programming session, RESET can be set high to commence normal operation.
8.
Power-off sequence (if needed):
Set RESET to “1”.
Turn VCC power off.
Table 28-15.
Minimum Wait Delay Before Writing the Next Flash or EEPROM
Symbol
Minimum Wait Delay
tWD_FLASH
4.5ms
tWD_EEPROM
9.0ms
tWD_ERASE
9.0ms
Figure 28-11. Serial Programming Waveforms
SERIAL DATA INPUT
(MOSI)
MSB
LSB
SERIAL DATA OUTPUT
(MISO)
MSB
LSB
SERIAL CLOCK INPUT
(SCK)
SAMPLE
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Table 28-16.
Serial Programming Instruction Set
Instruction Format
Instruction
Byte 1
Byte 2
Byte 3
Byte4
Programming Enable
1010 1100
0101 0011
xxxx xxxx
xxxx xxxx
Enable Serial Programming after
RESET goes low.
Chip Erase
1010 1100
100x xxxx
xxxx xxxx
xxxx xxxx
Chip Erase EEPROM and Flash.
0100 1101
0000 0000
cccc cccc
xxxx xxxx
Defines Extended Address Byte for
Read Program Memory and Write
Program Memory Page.
0010 H000
aaaa aaaa
bbbb bbbb
oooo oooo
Read H (high or low) data o from
Program memory at word address
c:a:b.
0100 H000
xxxx xxxx
xxbb bbbb
iiii iiii
Write H (high or low) data i to Program
Memory page at word address b. Data
low byte must be loaded before Data
high byte is applied within the same
address.
Write Program Memory Page
0100 1100
aaaa aaaa
bbxx xxxx
xxxx xxxx
Write Program Memory Page at
address c:a:b.
Read EEPROM Memory
1010 0000
0000 aaaa
bbbb bbbb
oooo oooo
Read data o from EEPROM memory at
address a:b.
Write EEPROM Memory
1100 0000
0000 aaaa
bbbb bbbb
iiii iiii
Write data i to EEPROM memory at
address a:b.
Load EEPROM Memory
Page (page access)
1100 0001
0000 0000
0000 00bb
iiii iiii
Load data i to EEPROM memory page
buffer. After data is loaded, program
EEPROM page.
Write EEPROM Memory
Page (page access)
1100 0010
0000 aaaa
bbbb bb00
xxxx xxxx
Write EEPROM page at address a:b.
0101 1000
0000 0000
xxxx xxxx
xxoo oooo
Read Lock bits. “0” = programmed, “1”
= unprogrammed. See Table 28-1 on
page 353 for details.
1010 1100
111x xxxx
xxxx xxxx
11ii iiii
Write Lock bits. Set bits = “0” to
program Lock bits. See Table 28-1 on
page 353 for details.
Read Signature Byte
0011 0000
000x xxxx
xxxx xxbb
oooo oooo
Read Signature Byte o at address b.
Write Fuse bits
1010 1100
1010 0000
xxxx xxxx
iiii iiii
Set bits = “0” to program, “1” to
unprogram.
Write Fuse High bits
1010 1100
1010 1000
xxxx xxxx
iiii iiii
Set bits = “0” to program, “1” to
unprogram.
1010 1100
1010 0100
xxxx xxxx
iiii iiii
Set bits = “0” to program, “1” to
unprogram. See Table 28-3 on
page 354 for details.
0101 0000
0000 0000
xxxx xxxx
oooo oooo
Read Fuse bits. “0” = programmed, “1”
= unprogrammed.
Load Extended Address Byte
Read Program Memory
Load Program Memory Page
Read Lock bits
Write Lock bits
Write Extended Fuse Bits
Read Fuse bits
Operation
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Table 28-16.
Serial Programming Instruction Set
Instruction Format
Instruction
Read Fuse High bits
Byte 1
Byte 2
Byte 3
Byte4
0101 1000
0000 1000
xxxx xxxx
oooo oooo
Read Fuse High bits. “0” = programmed, “1” = unprogrammed.
0101 0000
0000 1000
xxxx xxxx
oooo oooo
Read Extended Fuse bits. “0” = programmed, “1” = unprogrammed. See
Table 28-3 on page 354 for details.
0011 1000
000x xxxx
0000 0000
oooo oooo
Read Calibration Byte
1111 0000
0000 0000
xxxx xxxx
xxxx xxxo
If o = “1”, a programming operation is
still busy. Wait until this bit returns to
“0” before applying another command.
Read Extended Fuse Bits
Read Calibration Byte
Poll RDY/BSY
Operation
Note: a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in,
x = don’t care.
28.8.2 Serial Programming Characteristics
For characteristics of the Serial Programming module see “SPI Timing Characteristics” on page 388.
28.9
Programming via the JTAG Interface
Programming through the JTAG interface requires control of the four JTAG specific pins: TCK, TMS, TDI, and
TDO. Control of the reset and clock pins is not required.
To be able to use the JTAG interface, the JTAGEN Fuse must be programmed. The device is default shipped
with the fuse programmed. In addition, the JTD bit in MCUCSR must be cleared. Alternatively, if the JTD bit is
set, the external reset can be forced low. Then, the JTD bit will be cleared after two chip clocks, and the JTAG
pins are available for programming. This provides a means of using the JTAG pins as normal port pins in
Running mode while still allowing In-System Programming via the JTAG interface. Note that this technique can
not be used when using the JTAG pins for Boundary-scan or On-chip Debug. In these cases the JTAG pins
must be dedicated for this purpose.
During programming the clock frequency of the TCK Input must be less than the maximum frequency of the
chip. The System Clock Prescaler can not be used to divide the TCK Clock Input into a sufficiently low
frequency.
As a definition in this datasheet, the LSB is shifted in and out first of all Shift Registers.
28.9.1 Programming Specific JTAG Instructions
The Instruction Register is 4-bit wide, supporting up to 16 instructions. The JTAG instructions useful for
programming are listed below.
The OPCODE for each instruction is shown behind the instruction name in hex format. The text describes which
Data Register is selected as path between TDI and TDO for each instruction.
The Run-Test/Idle state of the TAP controller is used to generate internal clocks. It can also be used as an idle
state between JTAG sequences. The state machine sequence for changing the instruction word is shown in
Figure 28-12 on page 372.
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Figure 28-12. State Machine Sequence for Changing the Instruction Word
1
Test-Logic-Reset
0
0
Run-Test/Idle
1
Select-DR Scan
1
Select-IR Scan
0
1
0
1
Capture-DR
Capture-IR
0
0
0
Shift-DR
1
1
Exit1-DR
1
Exit1-IR
0
0
0
Pause-DR
0
Pause-IR
1
1
0
Exit2-DR
Exit2-IR
1
1
Update-DR
1
0
Shift-IR
1
0
1
Update-IR
0
1
0
28.9.2 AVR_RESET (0xC)
The AVR specific public JTAG instruction for setting the AVR device in the Reset mode or taking the device out
from the Reset mode. The TAP controller is not reset by this instruction. The one bit Reset Register is selected
as Data Register. Note that the reset will be active as long as there is a logic “one” in the Reset Chain. The
output from this chain is not latched.
The active states are:

Shift-DR: The Reset Register is shifted by the TCK input.
28.9.3 PROG_ENABLE (0x4)
The AVR specific public JTAG instruction for enabling programming via the JTAG port. The 16-bit Programming
Enable Register is selected as Data Register. The active states are the following:

Shift-DR: The programming enable signature is shifted into the Data Register

Update-DR: The programming enable signature is compared to the correct value, and Programming
mode is entered if the signature is valid
28.9.4 PROG_COMMANDS (0x5)
The AVR specific public JTAG instruction for entering programming commands via the JTAG port. The 15-bit
Programming Command Register is selected as Data Register. The active states are the following:
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
Capture-DR: The result of the previous command is loaded into the Data Register

Shift-DR: The Data Register is shifted by the TCK input, shifting out the result of the previous command
and shifting in the new command

Update-DR: The programming command is applied to the Flash inputs

Run-Test/Idle: One clock cycle is generated, executing the applied command
28.9.5 PROG_PAGELOAD (0x6)
The AVR specific public JTAG instruction to directly load the Flash data page via the JTAG port. An 8-bit Flash
Data Byte Register is selected as the Data Register. This is physically the 8 LSBs of the Programming
Command Register. The active states are the following:

Shift-DR: The Flash Data Byte Register is shifted by the TCK input

Update-DR: The content of the Flash Data Byte Register is copied into a temporary register. A write
sequence is initiated that within 11 TCK cycles loads the content of the temporary register into the Flash
page buffer. The AVR automatically alternates between writing the low and the high byte for each new
Update-DR state, starting with the low byte for the first Update-DR encountered after entering the
PROG_PAGELOAD command. The Program Counter is pre-incremented before writing the low byte,
except for the first written byte. This ensures that the first data is written to the address set up by
PROG_COMMANDS, and loading the last location in the page buffer does not make the program counter
increment into the next page.
28.9.6 PROG_PAGEREAD (0x7)
The AVR specific public JTAG instruction to directly capture the Flash content via the JTAG port. An 8-bit Flash
Data Byte Register is selected as the Data Register. This is physically the 8 LSBs of the Programming
Command Register. The active states are the following:

Capture-DR: The content of the selected Flash byte is captured into the Flash Data Byte Register. The
AVR automatically alternates between reading the low and the high byte for each new Capture-DR state,
starting with the low byte for the first Capture-DR encountered after entering the PROG_PAGEREAD
command. The Program Counter is post-incremented after reading each high byte, including the first read
byte. This ensures that the first data is captured from the first address set up by PROG_COMMANDS,
and reading the last location in the page makes the program counter increment into the next page.

Shift-DR: The Flash Data Byte Register is shifted by the TCK input
28.9.7 Data Registers
The Data Registers are selected by the JTAG instruction registers described in section “Programming Specific
JTAG Instructions” on page 371. The Data Registers relevant for programming operations are:

Reset Register

Programming Enable Register

Programming Command Register

Flash Data Byte Register
28.9.8 Reset Register
The Reset Register is a Test Data Register used to reset the part during programming. It is required to reset the
part before entering Programming mode.
A high value in the Reset Register corresponds to pulling the external reset low. The part is reset as long as
there is a high value present in the Reset Register. Depending on the Fuse settings for the clock options, the
part will remain reset for a Reset Time-out period (refer to “Clock Sources” on page 28) after releasing the Reset
Register. The output from this Data Register is not latched, so the reset will take place immediately, as shown in
Figure 8-1 on page 51.
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28.9.9 Programming Enable Register
The Programming Enable Register is a 16-bit register. The contents of this register is compared to the
programming enable signature, binary code 0b1010_0011_0111_0000. When the contents of the register is
equal to the programming enable signature, programming via the JTAG port is enabled. The register is reset to
0 on Power-on Reset, and should always be reset when leaving Programming mode.
Figure 28-13. Programming Enable Register
TDI
D
A
T
A
0xA370
=
D
Q
Programming Enable
ClockDR & PROG_ENABLE
TDO
28.9.10 Programming Command Register
The Programming Command Register is a 15-bit register. This register is used to serially shift in programming
commands, and to serially shift out the result of the previous command, if any. The JTAG Programming
Instruction Set is shown in Figure 28-15 on page 378. The state sequence when shifting in the programming
commands is illustrated in Figure 28-15 on page 378.
Figure 28-14. Programming Command Register
TDI
S
T
R
O
B
E
S
A
D
D
R
E
S
S
/
D
A
T
A
Flash
EEPROM
Fuses
Lock Bits
TDO
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Table 28-17.
JTAG Programming Instruction
Set a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out,
i = data in, x = don’t care
Instruction
TDI Sequence
TDO Sequence
0100011_10000000
xxxxxxx_xxxxxxxx
0110001_10000000
xxxxxxx_xxxxxxxx
0110011_10000000
xxxxxxx_xxxxxxxx
0110011_10000000
xxxxxxx_xxxxxxxx
1b. Poll for Chip Erase Complete
0110011_10000000
xxxxxox_xxxxxxxx
2a. Enter Flash Write
0100011_00010000
xxxxxxx_xxxxxxxx
2b. Load Address Extended High Byte
0001011_cccccccc
xxxxxxx_xxxxxxxx
2c. Load Address High Byte
0000111_aaaaaaaa
xxxxxxx_xxxxxxxx
2d. Load Address Low Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
2e. Load Data Low Byte
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
2f. Load Data High Byte
0010111_iiiiiiii
xxxxxxx_xxxxxxxx
0110111_00000000
xxxxxxx_xxxxxxxx
1110111_00000000
xxxxxxx_xxxxxxxx
0110111_00000000
xxxxxxx_xxxxxxxx
0110111_00000000
xxxxxxx_xxxxxxxx
0110101_00000000
xxxxxxx_xxxxxxxx
0110111_00000000
xxxxxxx_xxxxxxxx
0110111_00000000
xxxxxxx_xxxxxxxx
2i. Poll for Page Write Complete
0110111_00000000
xxxxxox_xxxxxxxx
3a. Enter Flash Read
0100011_00000010
xxxxxxx_xxxxxxxx
3b. Load Address Extended High Byte
0001011_cccccccc
xxxxxxx_xxxxxxxx
3c. Load Address High Byte
0000111_aaaaaaaa
xxxxxxx_xxxxxxxx
3d. Load Address Low Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
0110010_00000000
xxxxxxx_xxxxxxxx
0110110_00000000
xxxxxxx_oooooooo
Low byte
0110111_00000000
xxxxxxx_oooooooo
High byte
4a. Enter EEPROM Write
0100011_00010001
xxxxxxx_xxxxxxxx
4b. Load Address High Byte
0000111_aaaaaaaa
xxxxxxx_xxxxxxxx
4c. Load Address Low Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
4d. Load Data Byte
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
0110111_00000000
xxxxxxx_xxxxxxxx
1110111_00000000
xxxxxxx_xxxxxxxx
0110111_00000000
xxxxxxx_xxxxxxxx
1a. Chip Erase
2g. Latch Data
2h. Write Flash Page
3e. Read Data Low and High Byte
4e. Latch Data
Notes
(2)
(10)
(1)
(1)
(2)
(10)
(10)
(1)
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Table 28-17.
JTAG Programming Instruction
Set a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out,
i = data in, x = don’t care
Instruction
TDI Sequence
TDO Sequence
0110011_00000000
xxxxxxx_xxxxxxxx
0110001_00000000
xxxxxxx_xxxxxxxx
0110011_00000000
xxxxxxx_xxxxxxxx
0110011_00000000
xxxxxxx_xxxxxxxx
4g. Poll for Page Write Complete
0110011_00000000
xxxxxox_xxxxxxxx
5a. Enter EEPROM Read
0100011_00000011
xxxxxxx_xxxxxxxx
5b. Load Address High Byte
0000111_aaaaaaaa
xxxxxxx_xxxxxxxx
5c. Load Address Low Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
0110011_bbbbbbbb
xxxxxxx_xxxxxxxx
0110010_00000000
xxxxxxx_xxxxxxxx
0110011_00000000
xxxxxxx_oooooooo
0100011_01000000
xxxxxxx_xxxxxxxx
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
0111011_00000000
xxxxxxx_xxxxxxxx
0111001_00000000
xxxxxxx_xxxxxxxx
0111011_00000000
xxxxxxx_xxxxxxxx
0111011_00000000
xxxxxxx_xxxxxxxx
6d. Poll for Fuse Write Complete
0110111_00000000
xxxxxox_xxxxxxxx
(2)
6e. Load Data Low Byte(7)
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
(3)
0110111_00000000
xxxxxxx_xxxxxxxx
0110101_00000000
xxxxxxx_xxxxxxxx
0110111_00000000
xxxxxxx_xxxxxxxx
0110111_00000000
xxxxxxx_xxxxxxxx
0110111_00000000
xxxxxox_xxxxxxxx
(2)
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
(3)
0110011_00000000
xxxxxxx_xxxxxxxx
0110001_00000000
xxxxxxx_xxxxxxxx
0110011_00000000
xxxxxxx_xxxxxxxx
0110011_00000000
xxxxxxx_xxxxxxxx
6j. Poll for Fuse Write Complete
0110011_00000000
xxxxxox_xxxxxxxx
7a. Enter Lock Bit Write
0100011_00100000
xxxxxxx_xxxxxxxx
7b. Load Data Byte(9)
0010011_11iiiiii
xxxxxxx_xxxxxxxx
4f. Write EEPROM Page
5d. Read Data Byte
6a. Enter Fuse Write
(6)
6b. Load Data Low Byte
6c. Write Fuse Extended Byte
6f. Write Fuse High Byte
6g. Poll for Fuse Write Complete
(7)
6h. Load Data Low Byte
6i. Write Fuse Low Byte
Notes
(1)
(2)
(10)
(3)
(1)
(1)
(1)
(2)
(4)
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Table 28-17.
JTAG Programming Instruction
Set a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out,
i = data in, x = don’t care
Instruction
TDI Sequence
TDO Sequence
0110011_00000000
xxxxxxx_xxxxxxxx
0110001_00000000
xxxxxxx_xxxxxxxx
0110011_00000000
xxxxxxx_xxxxxxxx
0110011_00000000
xxxxxxx_xxxxxxxx
7d. Poll for Lock Bit Write complete
0110011_00000000
xxxxxox_xxxxxxxx
8a. Enter Fuse/Lock Bit Read
0100011_00000100
xxxxxxx_xxxxxxxx
0111010_00000000
xxxxxxx_xxxxxxxx
0111011_00000000
xxxxxxx_oooooooo
0111110_00000000
xxxxxxx_xxxxxxxx
0111111_00000000
xxxxxxx_oooooooo
0110010_00000000
xxxxxxx_xxxxxxxx
0110011_00000000
xxxxxxx_oooooooo
0110110_00000000
xxxxxxx_xxxxxxxx
0110111_00000000
xxxxxxx_xxoooooo
0111010_00000000
xxxxxxx_xxxxxxxx
(5)
0111110_00000000
xxxxxxx_oooooooo
Fuse Ext. byte
0110010_00000000
xxxxxxx_oooooooo
Fuse High byte
0110110_00000000
xxxxxxx_oooooooo
Fuse Low byte
0110111_00000000
xxxxxxx_oooooooo
Lock bits
9a. Enter Signature Byte Read
0100011_00001000
xxxxxxx_xxxxxxxx
9b. Load Address Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
0110010_00000000
xxxxxxx_xxxxxxxx
0110011_00000000
xxxxxxx_oooooooo
10a. Enter Calibration Byte Read
0100011_00001000
xxxxxxx_xxxxxxxx
10b. Load Address Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
0110110_00000000
xxxxxxx_xxxxxxxx
0110111_00000000
xxxxxxx_oooooooo
0100011_00000000
xxxxxxx_xxxxxxxx
0110011_00000000
xxxxxxx_xxxxxxxx
7c. Write Lock Bits
8b. Read Extended Fuse Byte(6)
8c. Read Fuse High Byte(7)
8d. Read Fuse Low Byte(8)
8e. Read Lock Bits(9)
8f. Read Fuses and Lock Bits
9c. Read Signature Byte
10c. Read Calibration Byte
11a. Load No Operation Command
Notes:
Notes
(1)
(2)
(5)
1. This command sequence is not required if the seven MSB are correctly set by the previous command sequence (which is
normally the case).
2. Repeat until o = “1”.
3. Set bits to “0” to program the corresponding Fuse, “1” to unprogram the Fuse.
4. Set bits to “0” to program the corresponding Lock bit, “1” to leave the Lock bit unchanged.
5. “0” = programmed, “1” = unprogrammed.
6. The bit mapping for Fuses Extended byte is listed in Table 28-3 on page 354.
7. The bit mapping for Fuses High byte is listed in Table 28-4 on page 355.
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8. The bit mapping for Fuses Low byte is listed in Table 28-5 on page 355.
9. The bit mapping for Lock bits byte is listed in Table 28-1 on page 353.
10. Address bits exceeding PCMSB and EEAMSB (Table 28-11 on page 359 and Table 28-12 on page 359) are don’t care.
11. All TDI and TDO sequences are represented by binary digits (0b...).
Figure 28-15. State Machine Sequence for Changing/Reading the Data Word
1
Test-Logic-Reset
0
0
Run-Test/Idle
1
Select-DR Scan
1
Select-IR Scan
0
1
0
1
Capture-DR
Capture-IR
0
0
Shift-DR
Shift-IR
0
1
Exit1-DR
1
Exit1-IR
0
0
Pause-DR
0
0
Pause-IR
1
1
0
Exit2-DR
Exit2-IR
1
1
Update-DR
1
0
1
1
0
1
Update-IR
0
1
0
28.9.11 Flash Data Byte Register
The Flash Data Byte Register provides an efficient way to load the entire Flash page buffer before executing
Page Write, or to read out/verify the content of the Flash. A state machine sets up the control signals to the
Flash and senses the strobe signals from the Flash, thus only the data words need to be shifted in/out.
The Flash Data Byte Register actually consists of the 8-bit scan chain and a 8-bit temporary register. During
page load, the Update-DR state copies the content of the scan chain over to the temporary register and initiates
a write sequence that within 11 TCK cycles loads the content of the temporary register into the Flash page
buffer. The AVR automatically alternates between writing the low and the high byte for each new Update-DR
state, starting with the low byte for the first Update-DR encountered after entering the PROG_PAGELOAD
command. The Program Counter is pre-incremented before writing the low byte, except for the first written byte.
This ensures that the first data is written to the address set up by PROG_COMMANDS, and loading the last
location in the page buffer does not make the Program Counter increment into the next page.
During Page Read, the content of the selected Flash byte is captured into the Flash Data Byte Register during
the Capture-DR state. The AVR automatically alternates between reading the low and the high byte for each
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new Capture-DR state, starting with the low byte for the first Capture-DR encountered after entering the
PROG_PAGEREAD command. The Program Counter is post-incremented after reading each high byte,
including the first read byte. This ensures that the first data is captured from the first address set up by
PROG_COMMANDS, and reading the last location in the page makes the program counter increment into the
next page.
Figure 28-16. Flash Data Byte Register
STROBES
TDI
State
Machine
ADDRESS
Flash
EEPROM
Fuses
Lock Bits
D
A
T
A
TDO
The state machine controlling the Flash Data Byte Register is clocked by TCK. During normal operation in which
eight bits are shifted for each Flash byte, the clock cycles needed to navigate through the TAP controller
automatically feeds the state machine for the Flash Data Byte Register with sufficient number of clock pulses to
complete its operation transparently for the user. However, if too few bits are shifted between each Update-DR
state during page load, the TAP controller should stay in the Run-Test/Idle state for some TCK cycles to ensure
that there are at least 11 TCK cycles between each Update-DR state.
28.9.12 Programming Algorithm
All references below of type “1a”, “1b”, and so on, refer to Table 28-17 on page 375.
28.9.13 Entering Programming Mode
1. Enter JTAG instruction AVR_RESET and shift 1 in the Reset Register.
2.
Enter instruction PROG_ENABLE and shift 0b1010_0011_0111_0000 in the Programming Enable
Register.
28.9.14 Leaving Programming Mode
1. Enter JTAG instruction PROG_COMMANDS.
2.
Disable all programming instructions by using no operation instruction 11a.
3.
Enter instruction PROG_ENABLE and shift 0b0000_0000_0000_0000 in the programming Enable
Register.
4.
Enter JTAG instruction AVR_RESET and shift 0 in the Reset Register.
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28.9.15 Performing Chip Erase
1. Enter JTAG instruction PROG_COMMANDS.
2.
Start Chip Erase using programming instruction 1a.
3.
Poll for Chip Erase complete using programming instruction 1b, or wait for tWLRH_CE (refer to Table 28-13
on page 366).
28.9.16 Programming the Flash
Before programming the Flash a Chip Erase must be performed, see “Performing Chip Erase” on page 380.
1. Enter JTAG instruction PROG_COMMANDS.
2.
Enable Flash write using programming instruction 2a.
3.
Load address Extended High byte using programming instruction 2b.
4.
Load address High byte using programming instruction 2c.
5.
Load address Low byte using programming instruction 2d.
6.
Load data using programming instructions 2e, 2f, and 2g.
7.
Repeat steps 5 and 6 for all instruction words in the page.
8.
Write the page using programming instruction 2h.
9.
Poll for Flash write complete using programming instruction 2i, or wait for tWLRH (refer to Table 28-13 on
page 366).
10. Repeat steps 3 to 9 until all data have been programmed.
A more efficient data transfer can be achieved using the PROG_PAGELOAD instruction:
1. Enter JTAG instruction PROG_COMMANDS.
2.
Enable Flash write using programming instruction 2a.
3.
Load the page address using programming instructions 2b, 2c, and 2d. PCWORD (refer to Table 28-11
on page 359) is used to address within one page and must be written as 0.
4.
Enter JTAG instruction PROG_PAGELOAD.
5.
Load the entire page by shifting in all instruction words in the page byte-by-byte, starting with the LSB of
the first instruction in the page and ending with the MSB of the last instruction in the page. Use UpdateDR to copy the contents of the Flash Data Byte Register into the Flash page location and to autoincrement the Program Counter before each new word.
6.
Enter JTAG instruction PROG_COMMANDS.
7.
Write the page using programming instruction 2h.
8.
Poll for Flash write complete using programming instruction 2i, or wait for tWLRH (refer to Table 28-13 on
page 366).
9.
Repeat steps 3 to 8 until all data have been programmed.
28.9.17 Reading the Flash
1. Enter JTAG instruction PROG_COMMANDS.
2.
Enable Flash read using programming instruction 3a.
3.
Load address using programming instructions 3b, 3c and 3d.
4.
Read data using programming instruction 3e.
5.
Repeat steps 3 and 4 until all data have been read.
A more efficient data transfer can be achieved using the PROG_PAGEREAD instruction:
1. Enter JTAG instruction PROG_COMMANDS.
2.
Enable Flash read using programming instruction 3a.
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3.
Load the page address using programming instructions 3b, 3c, and 3d. PCWORD (refer to Table 28-11
on page 359) is used to address within one page and must be written as 0.
4.
Enter JTAG instruction PROG_PAGEREAD.
5.
Read the entire page (or Flash) by shifting out all instruction words in the page (or Flash), starting with the
LSB of the first instruction in the page (Flash) and ending with the MSB of the last instruction in the page
(Flash). The Capture-DR state both captures the data from the Flash, and also auto-increments the
program counter after each word is read. Note that Capture-DR comes before the shift-DR state. Hence,
the first byte which is shifted out contains valid data.
6.
Enter JTAG instruction PROG_COMMANDS.
7.
Repeat steps 3 to 6 until all data have been read.
28.9.18 Programming the EEPROM
Before programming the EEPROM a Chip Erase must be performed, see “Performing Chip Erase” on page 380.
1. Enter JTAG instruction PROG_COMMANDS.
2.
Enable EEPROM write using programming instruction 4a.
3.
Load address High byte using programming instruction 4b.
4.
Load address Low byte using programming instruction 4c.
5.
Load data using programming instructions 4d and 4e.
6.
Repeat steps 4 and 5 for all data bytes in the page.
7.
Write the data using programming instruction 4f.
8.
Poll for EEPROM write complete using programming instruction 4g, or wait for tWLRH (refer to Table 28-13
on page 366).
9.
Repeat steps 3 to 8 until all data have been programmed.
Note that the PROG_PAGELOAD instruction can not be used when programming the EEPROM.
28.9.19 Reading the EEPROM
1. Enter JTAG instruction PROG_COMMANDS.
2.
Enable EEPROM read using programming instruction 5a.
3.
Load address using programming instructions 5b and 5c.
4.
Read data using programming instruction 5d.
5.
Repeat steps 3 and 4 until all data have been read.
Note that the PROG_PAGEREAD instruction can not be used when reading the EEPROM.
28.9.20 Programming the Fuses
1. Enter JTAG instruction PROG_COMMANDS.
2.
Enable Fuse write using programming instruction 6a.
3.
Load data high byte using programming instructions 6b. A bit value of “0” will program the corresponding
fuse, a “1” will unprogram the fuse.
4.
Write Fuse High byte using programming instruction 6c.
5.
Poll for Fuse write complete using programming instruction 6d, or wait for tWLRH (refer to Table 28-13 on
page 366).
6.
Load data low byte using programming instructions 6e. A “0” will program the fuse, a “1” will unprogram
the fuse.
7.
Write Fuse low byte using programming instruction 6f.
8.
Poll for Fuse write complete using programming instruction 6g, or wait for tWLRH (refer to Table 28-13 on
page 366).
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28.9.21 Programming the Lock Bits
1. Enter JTAG instruction PROG_COMMANDS.
2.
Enable Lock bit write using programming instruction 7a.
3.
Load data using programming instructions 7b. A bit value of “0” will program the corresponding lock bit, a
“1” will leave the lock bit unchanged.
4.
Write Lock bits using programming instruction 7c.
5.
Poll for Lock bit write complete using programming instruction 7d, or wait for tWLRH (refer to Table 28-13
on page 366).
28.9.22 Reading the Fuses and Lock Bits
1. Enter JTAG instruction PROG_COMMANDS.
2.
Enable Fuse/Lock bit read using programming instruction 8a.
3.
To read all Fuses and Lock bits, use programming instruction 8e.
To only read Fuse High byte, use programming instruction 8b.
To only read Fuse Low byte, use programming instruction 8c.
To only read Lock bits, use programming instruction 8d.
28.9.23 Reading the Signature Bytes
1. Enter JTAG instruction PROG_COMMANDS.
2.
Enable Signature byte read using programming instruction 9a.
3.
Load address 0x00 using programming instruction 9b.
4.
Read first signature byte using programming instruction 9c.
5.
Repeat steps 3 and 4 with address 0x01 and address 0x02 to read the second and third signature bytes,
respectively.
28.9.24 Reading the Calibration Byte
1. Enter JTAG instruction PROG_COMMANDS.
2.
Enable Calibration byte read using programming instruction 10a.
3.
Load address 0x00 using programming instruction 10b.
4.
Read the calibration byte using programming instruction 10c.
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29.
Electrical Characteristics
29.1
Absolute Maximum Ratings*
Operating Temperature. . . . . . . . . . . . -40C to +85C
*NOTICE:
Stresses beyond those listed under “Absolute
Maximum Ratings” may cause permanent damage to the device. This is a stress rating only
and functional operation of the device at these
or other conditions beyond those indicated in
the operational sections of this specification is
not implied. Exposure to absolute maximum rating conditions for extended periods may affect
device reliability.
Storage Temperature . . . . . . . . . . . . -65°C to +150°C
Voltage on any Pin except RESET and VBUS
with respect to Ground(8) . . . . . . . . -0.5V to VCC+0.5V
Voltage on RESET with respect to Ground-0.5V to +13.0V
Voltage on VBUS with respect to Ground-0.5V to +6.0V
Maximum Operating Voltage . . . . . . . . . . . . . . . . 6.0V
DC Current per I/O Pin . . . . . . . . . . . . . . . . . . 40.0mA
DC Current VCC and GND Pins . . . . . . . . . . 200.0mA
29.2
DC Characteristics
Table 29-1. DC Characteristic, TA = -40°C to 85°C, VCC = 2.7V to 5.5V (unless otherwise noted)
Min.(5)
Parameter
Condition
VIL
Input Low Voltage,
Except XTAL1 and
Reset pin
VCC = 2.7V - 5.5V
VIL1
Input Low Voltage,
XTAL1 pin
VCC = 2.7V - 5.5V
-0.5
0.1VCC(1)
VIL2
Input Low Voltage,
RESET pin
VCC = 2.7V - 5.5V
-0.5
0.1VCC(1)
VIH
Input High Voltage,
Except XTAL1 and
RESET pins
VCC = 2.7V - 5.5V
0.2VCC+0.9
V(2)
VCC + 0.5
VIH1
Input High Voltage,
XTAL1 pin
VCC = 2.7V - 5.5V
0.7VCC(2)
VCC + 0.5
VIH2
Input High Voltage,
RESET pin
VCC = 2.7V - 5.5V
0.9VCC(2)
VCC + 0.5
VOL
Output Low Voltage(3),
IOL = 10mA, VCC = 5V
IOL = 5mA, VCC = 3V
VOH
Output High Voltage(4),
IOH = -10mA, VCC = 5V
IOH = -5mA, VCC = 3V
IIL
Input Leakage
Current I/O Pin
VCC = 5.5V, pin low
(absolute value)
-0.5
Typ.
Max.(5)
Symbol
Units
0.2VCC0.1V(1)
(LVTTL)
(LVTTL)
V
0.7
0.5
4.2
2.3
1
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Table 29-1. DC Characteristic, TA = -40°C to 85°C, VCC = 2.7V to 5.5V (unless otherwise noted)
Min.(5)
Typ.
Max.(5)
Units
1
µA
Symbol
Parameter
Condition
IIH
Input Leakage
Current I/O Pin
VCC = 5.5V, pin high
(absolute value)
RRST
Reset Pull-up Resistor
30
60
RPU
I/O Pin Pull-up Resistor
20
50
Active 4MHz, VCC = 3V
(ATmega16U4/ATmega32U4)
5
Active 8MHz, VCC = 5V
(ATmega16U4/ATmega32U4)
Power Supply Current(6)
ICC
Power-down mode
10
15
Active 16MHz, VCC = 5V
(ATmega16U4/ATmega32U4)
27
Idle 4MHz, VCC = 3V
(ATmega16U4/ATmega32U4)
2
Idle 8MHz, VCC = 5V
(ATmega16U4/ATmega32U4)
6
WDT enabled, VCC = 3V,
Regulator Disabled
<10
Analog Comparator
Input Offset Voltage
VCC = 5V
Vin = VCC/2
IACLK
Analog Comparator
Input Leakage Current
VCC = 5V
Vin = VCC/2
tACID
Analog Comparator
Propagation Delay
VCC = 2.7V
VCC = 4.0V
Rusb
USB Series resistor
(external)
mA
12
µA
WDT disabled, VCC =
3V,Regulator Disabled
VACIO
k
1
5
<10
40
mV
50
nA
-50
750
500
ns
22±5%

CUCAP = 1µF ±20%,
Vreg
Regulator Output
Voltage
UVcc  4.0V, I80mA(7),
or
3.0
3.3
3.6
V
UVcc  3.4V, I55mA(7)
Note:
1. "Max" means the highest value where the pin is guaranteed to be read as low
2. "Min" means the lowest value where the pin is guaranteed to be read as high
3. Although each I/O port can sink more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state
conditions (non-transient), the following must be observed:
ATmega16U4/ATmega32U4:
1.)The sum of all IOL, for ports A0-A7, G2, C4-C7 should not exceed 100mA.
2.)The sum of all IOL, for ports C0-C3, G0-G1, D0-D7 should not exceed 100mA.
3.)The sum of all IOL, for ports G3-G5, B0-B7, E0-E7 should not exceed 100mA.
4.)The sum of all IOL, for ports F0-F7 should not exceed 100mA.
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current
greater than the listed test condition.
4. Although each I/O port can source more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady
state conditions (non-transient), the following must be observed:
ATmega16U4/ATmega32U4:
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5.
6.
7.
8.
29.3
1)The sum of all IOH, for ports A0-A7, G2, C4-C7 should not exceed 100mA.
2)The sum of all IOH, for ports C0-C3, G0-G1, D0-D7 should not exceed 100mA.
3)The sum of all IOH, for ports G3-G5, B0-B7, E0-E7 should not exceed 100mA.
4)The sum of all IOH, for ports F0-F7 should not exceed 100mA.
All DC Characteristics contained in this datasheet are based on simulation and characterization of other AVR
microcontrollers manufactured in the same process technology. These values are preliminary values representing design
targets, and will be updated after characterization of actual silicon
Values with “Power Reduction Register 1 - PRR1” disabled (0x00).
Maximum regulator output current should be reduced by the USB buffer current required when USB is active (about
25mA). The remaining regulator output current can be used for the external application.
As specified on the USB Electrical chapter, the D+/D- pads can withstand voltages down to -1V applied through a 39
resistor
External Clock Drive Waveforms
Figure 29-1.
External Clock Drive Waveforms
V IH1
V IL1
29.4
External Clock Drive
Table 29-2.
External Clock Drive
VCC=2.7-5.5V
Symbol
1/tCLCL
Parameter
Oscillator Frequency
VCC=4.5-5.5V
Min.
Max.
Min.
Max.
Units
0
8
0
16
MHz
tCLCL
Clock Period
125
62.5
ns
tCHCX
High Time
50
25
ns
tCLCX
Low Time
50
25
ns
tCLCH
Rise Time
1.6
0.5
s
tCHCL
Fall Time
1.6
0.5
s
tCLCL
Change in period from one clock
cycle to the next
2
2
%
Note:
All DC Characteristics contained in this datasheet are based on simulation and characterization of other AVR
microcontrollers manufactured in the same process technology.
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29.5
System and Reset Characteristics
Table 29-3. Reset and Brown-out Detection Characteristics
Symbol
VPOT
Parameter
Condition
Min.
Typ.
Max.
Units
Power-on Reset Threshold Voltage (rising)
1.4
2.3
V
Power-on Reset Threshold Voltage (falling)()
1.3
2.3
V
+0.1
V
VPOR
VCC Start Voltage to ensure internal Power-on Reset signal
-0.1
VCCRR
VCC Rise Rate to ensure internal Power_on Reset signal
0.3
VRST
RESET Pin Threshold Voltage
tRST
Minimum pulse width on RESET Pin
V/ms
0.2VCC
0.85VCC
5V, 25°C
400
V
ns
The Power-on Reset will not work unless the supply voltage has been below VPOT (falling).
29.6
Maximum speed vs. VCC
Maximum frequency is depending on VCC. As shown in Figure 29-2 on page 386, the Maximum Frequency vs.
VCC curve is linear between 2.7V < VCC < 5.5V.
Figure 29-2.
Maximum Frequency vs. VCC
16 MHz
8 MHz
Safe Operating Area
2.7V
29.7
4.5V
5.5V
2-wire Serial Interface Characteristics
The following table describes the requirements for devices connected to the 2-wire Serial Bus. The device 2-wire Serial
Interface meets or exceeds these requirements under the noted conditions.
Timing symbols refer to Figure 29-3 on page 388.
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Table 29-4.
2-wire Serial Bus Requirements
Symbol
Parameter
VIL
VIH
Vhys
(1)
(1)
VOL
tr
(1)
Min.
Max.
Input Low-voltage
-0.5
0.3 VCC
Input High-voltage
0.7VCC
VCC + 0.5
0.05VCC(2)
–
0
0.4
Hysteresis of Schmitt Trigger Inputs
Output Low-voltage
Output Fall Time from VIHmin to VILmax
(1)
tSP
Spikes Suppressed by Input Filter
Ii
Input Current each I/O Pin
Ci(1)
Capacitance for each I/O Pin
fSCL
SCL Clock Frequency
tHD;STA
Value of Pull-up resistor
Hold Time (repeated) START Condition
tLOW
Low Period of the SCL Clock
tHIGH
High period of the SCL clock
tSU;STA
Set-up time for a repeated START condition
tHD;DAT
Data hold time
tSU;DAT
Data setup time
tSU;STO
Setup time for STOP condition
tBUF
Bus free time between a STOP and START
condition
Notes:
3mA sink current
Units
V
0.1Cb(3)(2)
300
20 + 0.1Cb(3)(2)
250
0
50(2)
-10
10
µA
–
10
pF
fCK(4) > max(16fSCL, 250kHz)(5)
0
400
kHz
fSCL  100kHz
V CC – 0.4V
---------------------------3mA
1000ns
------------------Cb
fSCL > 100kHz
V CC – 0.4V
---------------------------3mA
300ns
---------------Cb
fSCL  100kHz
4.0
–
fSCL > 100kHz
Rise Time for both SDA and SCL
(1)
tof
Rp
Condition
20 +
10pF < Cb < 400pF(3)
0.1VCC < Vi < 0.9VCC
0.6
–
(6)
fSCL  100kHz
4.7
–
fSCL > 100kHz(7)
1.3
–
fSCL  100kHz
4.0
–
fSCL > 100kHz
0.6
–
fSCL  100kHz
4.7
–
fSCL > 100kHz
0.6
–
fSCL  100kHz
0
3.45
fSCL > 100kHz
0
0.9
fSCL  100kHz
250
–
fSCL > 100kHz
100
–
fSCL  100kHz
4.0
–
fSCL > 100kHz
0.6
–
fSCL  100kHz
4.7
–
fSCL > 100kHz
1.3
–
ns

µs
µs
ns
µs
1. In ATmega16U4/ATmega32U4, this parameter is characterized and not 100% tested.
2. Required only for fSCL > 100kHz.
3. Cb = capacitance of one bus line in pF.
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4. fCK = CPU clock frequency.
5. This requirement applies to all ATmega16U4/ATmega32U4 2-wire Serial Interface operation. Other devices connected to
the 2-wire Serial Bus need only obey the general fSCL requirement.
6. The actual low period generated by the ATmega16U4/ATmega32U4 2-wire Serial Interface is (1/fSCL - 2/fCK), thus fCK
must be greater than 6MHz for the low time requirement to be strictly met at fSCL = 100kHz.
7. The actual low period generated by the ATmega16U4/ATmega32U4 2-wire Serial Interface is (1/fSCL - 2/fCK), thus the low
time requirement will not be strictly met for fSCL > 308kHz when fCK = 8MHz. Still, ATmega16U4/ATmega32U4 devices
connected to the bus may communicate at full speed (400kHz) with other ATmega16U4/ATmega32U4 devices, as well
as any other device with a proper tLOW acceptance margin.
Figure 29-3.
2-wire Serial Bus Timing
tof
tHIGH
tr
tLOW
tLOW
SCL
tSU;STA
tHD;STA
tHD;DAT
tSU;DAT
SDA
tSU;STO
tBUF
29.8
SPI Timing Characteristics
See Figure 29-4 and Figure 29-5 on page 389 for details.
Table 29-5.
SPI Timing Parameters
Description
Mode
1
SCK period
Master
See Table 17-2
2
SCK high/low
Master
50% duty cycle
3
Rise/Fall time
Master
4
Setup
Master
10
5
Hold
Master
10
6
Out to SCK
Master
0.5 • tsck
7
SCK to out
Master
10
8
SCK to out high
Master
10
9
SS low to out
Slave
15
10
SCK period
Slave
4 • tck
11
SCK high/low(1)
Slave
2 • tck
12
Rise/Fall time
Slave
13
Setup
Slave
10
14
Hold
Slave
tck
15
SCK to out
Slave
16
SCK to SS high
Slave
17
SS high to tri-state
Slave
18
SS low to SCK
Slave
Note:
1.
Min.
Typ.
Max.
1600
ns
1600
15
20
10
20
In SPI Programming mode the minimum SCK high/low period is:
- 2 tCLCL for fCK < 12MHz
- 3 tCLCL for fCK > 12MHz
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Figure 29-4.
SPI Interface Timing Requirements (Master Mode)
SS
6
1
SCK
(CPOL = 0)
2
2
SCK
(CPOL = 1)
4
MISO
(Data Input)
5
3
MSB
...
LSB
8
7
MOSI
(Data Output)
Figure 29-5.
MSB
...
LSB
SPI Interface Timing Requirements (Slave Mode)
SS
10
9
16
SCK
(CPOL = 0)
11
11
SCK
(CPOL = 1)
13
MOSI
(Data Input)
14
12
MSB
...
LSB
15
MISO
(Data Output)
29.9
17
MSB
...
LSB
X
Hardware Boot Entrance Timing Characteristics
Figure 29-6.
Hardware Boot Timing Requirements
RESET
tSHRH
tHHRH
ALE/HWB
Table 29-6.
Hardware Boot Timings
Symbol
Parameter
tSHRH
HWB low Setup before Reset High
tHHRH
HWB low Hold after Reset High
Min.
Max.
0
StartUpTime(SUT) +
Time Out Delay(TOUT)
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Table 29-7.
Symbol
ADC Characteristics
Parameter
Resolution
TUE
INL
DNL
Absolute accuracy
Integral Non-Linearity
Differential Non-Linearity
Gain Error
Offset Error
VREF
Reference Voltage
AVCC
Analog Supply Voltage
VIN
Input Voltage
Input Bandwidth
Condition
Min.
Typ.
Single Ended Conversion
10
Differential conversion, gain = 1x/10x/40x
8
Differential conversion, gain = 200x
8
Max.
Bits
VREF = 4V, VCC = 4V, ADC clock = 200kHz
2.0
3.0
Gain = 1x/10x/40x, VREF = 4V, VCC = 5V,
ADC clock = 200 kHz
2.0
3.0
Gain = 200x, VREF = 4V, VCC = 5V,
ADC clock = 200kHz
2.0
4.0
VREF = 4V, VCC = 4V, ADC clock = 200kHz
0.5
1.5
Gain = 1x/10x/40x, VREF = 4V, VCC = 5V,
ADC clock = 200kHz
0.3
1.5
Gain = 200x, VREF = 4V, VCC = 5V,
ADC clock = 200kHz
0.5
1.5
VREF = 4V, VCC = 4V, ADC clock = 200kHz
0.4
0.7
Gain = 1x/10x/40x, VREF = 4V, VCC = 5V,
ADC clock = 200kHz
0.3
1.0
Gain = 200x, VREF = 4V, VCC = 5V,
ADC clock = 200kHz
0.6
1.0
VREF = 4V, VCC = 4V, ADC clock = 200kHz
-2.5
-1.0
2.5
Gain = 1x/10x/40x, VREF = 4V, VCC = 5V,
ADC clock = 200kHz
0.0
-1.5
-2.5
Gain = 200x, VREF = 4V, VCC = 5V,
ADC clock = 200kHz
0.0
-1.8
-3.0
VREF = 4V, VCC = 4V, ADC clock = 200kHz
-2.5
1.5
2.5
VREF= 4V, VCC = 5V, ADC clock = 200kHz,
Differential mode
-2.0
0.0
2.0
Single Ended Conversion
2.56
AVCC
Differential Conversion
2.56
AVCC - 0.5
VCC 0.3
VCC + 0.3
Single ended channels
GND
VREF
Differential Conversion
0
AVCC
Single Ended Channels
Differential Channels
Units
38.5
4
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LSB
LSB
LSB
LSB
LSB
V
V
V
kHz
390
Table 29-7.
ADC Characteristics
Symbol
Parameter
Condition
Min.
Typ.
Max.
Units
VINT
Internal Voltage
Reference
2.56V
2.4
2.56
2.8
V
RREF
Reference Input
Resistance
32
k
RAIN
Analog Input Resistance
100
M
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30.
Typical Characteristics
The following charts show typical behavior. These figures are not tested during manufacturing. All current consumption measurements are performed with all I/O pins configured as inputs and with internal pull-ups enabled.
A sine wave generator with rail-to-rail output is used as clock source.
All Active- and Idle current consumption measurements are done with all bits in the PRR registers set and thus,
the corresponding I/O modules are turned off. Also the Analog Comparator is disabled during these measurements. See “Power Reduction Register” on page 45 for details.
The power consumption in Power-down mode is independent of clock selection.
The current consumption is a function of several factors such as: operating voltage, operating frequency, loading of I/O pins, switching rate of I/O pins, code executed and ambient temperature. The dominating factors are
operating voltage and frequency.
The current drawn from capacitive loaded pins may be estimated (for one pin) as CL*VCC*f where CL = load
capacitance, VCC = operating voltage and f = average switching frequency of I/O pin.
The parts are characterized at frequencies higher than test limits. Parts are not guaranteed to function properly
at frequencies higher than the ordering code indicates.
The difference between current consumption in Power-down mode with Watchdog Timer enabled and Powerdown mode with Watchdog Timer disabled represents the differential current drawn by the Watchdog Timer.
Active Supply Current
Figure 30-1.
Active Supply Current vs. Low Frequency (1MHz) and T= 25°C
1.6
1.4
5.5V
1.2
5.0V
4.5V
1
I CC (mA)
30.1
0.8
4.0V
3.6V
0.6
2.7V
0.4
0.2
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Frequency (MHz)
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Figure 30-2.
Active Supply Current vs. Low Frequency (1MHz) and T= 85°C
2.3
5.5V
I CC (mA)
2.1
1.9
5.0V
1.7
4.5V
1.5
4.0V
3.6V
1.3
2.7V
1.1
0.9
0.7
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Frequency (MHz)
Active Supply Current vs. Frequency (1 - 16MHz) and T= -40°C
18
ICC (mA)
Figure 30-3.
16
5.5V
14
5.0V
12
4.5V
10
4.0V
8
3.6V
6
2.7V
4
2
0
2
4
6
8
10
12
14
16
Frequency (MHz)
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Figure 30-4.
Active Supply Current vs. Frequency (1 - 16MHz) and T = 25°C
16
5.5V
14
5.0V
12
4.5V
ICC (mA)
10
4.0V
8
3.6V
6
4
2.7V
2
0
2
4
6
8
10
12
14
16
Frequency (MHz)
Active Supply Current vs. Frequency (1 - 16MHz) and T = 85°C
16
5.5V
14
5.0V
12
4.5V
10
ICC (mA)
Figure 30-5.
4.0V
8
3.6V
6
4
2.7V
2
0
2
4
6
8
10
12
14
16
Frequency (MHz)
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Idle Supply Current
Figure 30-6.
Idle Supply Current vs. Low Frequency (1MHz) and T = 25°C
I CC (mA)
0.5
0.45
5.5V
0.4
5.0V
0.35
4.5V
0.3
4.0V
3.6V
0.25
0.2
2.7V
0.15
0.1
0.05
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Frequency (MHz)
Figure 30-7.
Idle Supply Current vs. Low Frequency (1MHz) and T = 85°C
0.5
5.5V
0.45
I CC (mA)
30.2
0.4
5.0V
0.35
4.5V
0.3
4.0V
3.6V
0.25
2.7V
0.2
0.15
0.1
0.05
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Frequency (MHz)
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Figure 30-8.
Idle Supply Current vs. Frequency (1 - 16MHz) T = 25°C
7
5.5V
6
5.0V
ICC (mA)
5
4.5V
4
4.0V
3
3.6V
2
3.3V
2.7V
1
0
2
4
6
8
10
12
14
16
Frequency (MHz)
Idle Supply Current vs. Frequency (1 - 16MHz) T = 85°C
7
5.5V
6
5.0V
5
ICC (mA)
Figure 30-9.
4.5V
4
4.0V
3
3.6V
2
2.7V
1
0
2
4
6
8
10
12
14
16
Frequency (MHz)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
396
Power-down Supply Current
Figure 30-10. Power-Down Supply Current vs. VCC (WDT Disabled)
3.5
85°C
3
I CC (µA)
2.5
2
1.5
1
25°C
-40°C
0.5
0
2.5
2.8
3.1
3.4
3.7
4
4.3
4.6
4.9
5.2
5.5
V CC (V)
Figure 30-11. Power-Down Supply Current vs. VCC (WDT Enabled)
24
85°C
22
20
18
16
I CC (µA)
30.3
25°C
14
12
-40°C
10
8
6
4
2.5
2.8
3.1
3.4
3.7
4
4.3
4.6
4.9
5.2
5.5
V CC (V)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
397
Figure 30-12. Power-down Supply Current vs. VCC (WDT Enabled, BOD EN)
48
85°C
45
42
I CC (µA)
39
25°C
36
-40°C
33
30
27
24
21
2.5
2.8
3.1
3.4
3.7
4
4.3
4.6
4.9
5.2
5.5
V CC (V)
Power-save Supply Current
Figure 30-13. Power-save Supply Current vs. VCC (WDT Disabled)
200
-40°C
25°C
85°C
185
170
155
140
I CC (µA)
30.4
125
110
95
80
65
50
2.5
2.8
3.1
3.4
3.7
4
4.3
4.6
4.9
5.2
5.5
V CC (V)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
398
Pin Pull-Up
Figure 30-14. I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 2.7V)
80
70
60
I OP (µA)
50
40
30
20
25°C
-40°C
85°C
10
0
0
0.5
1
1.5
2
2.5
3
V OP (V)
Figure 30-15. I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 5V)
140
120
100
I OP (µA)
30.5
80
60
40
25°C
85°C
-40°C
20
0
0
1
2
3
4
5
V OP (V)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
399
Figure 30-16. Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC= 5V)
120
100
I RESET (µA)
80
60
40
25°C
85°C
-40°C
20
0
0
1
2
3
4
5
V RESET (V)
Pin Driver Strength
Figure 30-17. I/O Pin Output Voltage vs. Sink Current (VCC = 3V)
4
85°C
3.5
3
2.5
V OL (V)
30.6
2
1.5
25°C
1
-40°C
0.5
0
0
2
4
6
8
10
12
14
16
18
20
I OL (mA)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
400
Figure 30-18. I/O Pin Output Voltage vs. Sink Current (VCC = 5V)
1
85°C
0.9
0.8
25°C
0.7
V OL (V)
0.6
-40°C
0.5
0.4
0.3
0.2
0.1
0
0
2
4
6
8
10
12
14
16
18
20
I OL (mA)
Figure 30-19. I/O Pin Output Voltage vs. Source Current (VCC = 3V)
3.5
3
V OH (V)
2.5
2
-40°C
1.5
25°C
1
0.5
85°C
0
0
2
4
6
8
10
12
14
16
18
20
I OH (mA)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
401
Figure 30-20. I/O Pin Output Voltage vs. Source Current (VCC = 5V)
5.1
4.9
V OH (V)
4.7
4.5
-40°C
4.3
25°C
85°C
4.1
3.9
0
2
4
6
8
10
12
14
16
18
20
I OH (mA)
Figure 30-21. USB DP LO Pull-Up Resistor Current vs. USB Pin Voltage
2800
2400
I USB (µA)
2000
1600
1200
800
85°C
400
25°C
-40°C
0
0
0.5
1
1.5
2
2.5
3
3.5
V USB (V)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
402
Pin Threshold and Hysteresis
Figure 30-22. I/O Pin Input Threshold Voltage vs. VCC (VIH, IO Pin read as ‘1’)
1.8
-40°C
25°C
85°C
1.6
Threshold (V)
1.4
1.2
1
0.8
0.6
2.5
2.8
3.1
3.4
3.7
4
4.3
4.6
4.9
5.2
5.5
V CC (V)
Figure 30-23. I/O Pin Input Threshold Voltage vs. VCC (VIL, IO Pin read as ‘0’)
1.8
-40°C
25°C
85°C
1.6
1.4
Threshold (V)
30.7
1.2
1
0.8
0.6
2.5
2.8
3.1
3.4
3.7
4
4.3
4.6
4.9
5.2
5.5
V CC (V)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
403
Figure 30-24. USB Pin Input Threshold Voltage vs. VCC (VIH, IO Pin read as ‘1’)
2
85°C
25°C
1.9
Threshold (V)
1.8
-40°C
1.7
1.6
1.5
1.4
1.3
1.2
2.7
2.8
2.9
3
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
V CC (V)
Figure 30-25. USB Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’)
1.6
-40°C
85°C
25°C
1.5
Threshold (V)
1.4
1.3
1.2
1.1
1
0.9
2.7
2.8
2.9
3
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
V CC (V)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
404
Figure 30-26. Vbus Pin Input Threshold Voltage vs. VCC (VIH, IO Pin read as ‘1’)
4.6
4.58
85°C
4.56
25°C
Threshold (V)
4.54
4.52
4.5
4.48
4.46
4.44
4.42
4.4
-40°C
4.38
2.5
2.8
3.1
3.4
3.7
4
4.3
4.6
4.9
5.2
5.5
V CC (V)
Figure 30-27. Vbus Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’)
1.52
85°C
25°C
1.51
Threshold (V)
1.5
1.49
1.48
1.47
-40°C
1.46
2.5
2.8
3.1
3.4
3.7
4
4.3
4.6
4.9
5.2
5.5
V CC (V)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
405
BOD Threshold
Figure 30-28. BOD Thresholds vs. Temperature (BODLEVEL is 2.6V)
2.8
Rising Vcc
2.78
Threshold (V)
2.76
2.74
Falling Vcc
2.72
2.7
2.68
2.66
2.64
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
Temperature (°C)
Figure 30-29. BOD Thresholds vs. Temperature (BODLEVEL is 3.5V)
3.73
Rising Vcc
3.69
Falling Vcc
3.65
Threshold (V)
30.8
3.61
3.57
3.53
3.49
3.45
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
Temperature (°C)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
406
Figure 30-30. BOD Thresholds vs. Temperature (BODLEVEL is 4.3V)
4.6
4.55
Falling Vcc
Rising Vcc
4.5
Threshold (V)
4.45
4.4
4.35
4.3
4.25
4.2
4.15
4.1
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
Temperature (°C)
Figure 30-31. Bandgap Voltage vs. VCC
1.11
Bandgap Voltage (V)
1.1
1.09
85°C
25°C
1.08
1.07
1.06
-40°C
1.05
1.5
2
2.5
3
3.5
4
4.5
5
5.5
Vcc (V)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
407
Figure 30-32. Bandgap Voltage vs. Temperature
1.11
1.9V
3.0V
4.5V
5.0V
5.5V
Bandgap Voltage (V)
1.1
1.09
1.08
1.07
1.06
1.05
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
Temperature (°C)
Internal Oscillator Speed
Figure 30-33. Watchdog Oscillator Frequency vs. Temperature
124
122
120
F RC (kHz)
30.9
118
1.9V
116
3.0V
4.0V
4.5V
5.5V
114
112
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
Temperature (°C)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
408
Figure 30-34. Watchdog Oscillator Frequency vs. VCC
124
122
-40°C
F RC (kHz)
120
25°C
118
116
114
85°C
112
110
1.5
2
2.5
3
3.5
4
4.5
5
5.5
V CC (V)
Figure 30-35. Calibrated 8MHz RC Oscillator Frequency vs. Osccal Value
14
85°C
25°C
-40°C
12
F RC (MHz)
10
8
6
4
2
0
0
16
32
48
64
80
96
112 128 144 160 176 192 208 224 240 256
OSCCAL (X1)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
409
Figure 30-36. Calibrated 8MHz RC Oscillator Frequency vs. Temperature
8.4
2.7V
4.0V
5.5V
8.3
F RC (MHz)
8.2
8.1
8
7.9
7.8
7.7
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
Temperature (°C)
Figure 30-37. Calibrated 8MHz RC Oscillator Frequency vs. Operating Voltage
8.4
8.3
85°C
8.2
F RC (MHz)
8.1
8
25°C
7.9
7.8
-40°C
7.7
7.6
2.5
2.8
3.1
3.4
3.7
4
4.3
4.6
4.9
5.2
5.5
V CC (V)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
410
Figure 30-38. OSCCAL VALUE STEP SIZE IN% (Base frequency = 0.0MHz)
1.4
1.2
F RC change (%)
1
0.8
0.6
0.4
85°C
0.2
-40°C
25°C
0
0
16
32
48
64
80
96
112 128 144 160 176 192 208 224 240 256
OSCCAL (X1)
30.10 Current Consumption of Peripheral Units
Figure 30-39. USB Regulator Level vs. VCC
3.5
3.4
-40°C
85°C
3.3
25°C
I CC (µA)
3.2
3.1
3
2.9
2.8
2.7
2.7
2.9
3.1
3.3
3.5
3.7
3.9
4.1
4.3
4.5
4.7
4.9
5.1
5.3
5.5
V CC (V)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
411
Figure 30-40. USB Regulator Level with load 75 vs. VCC
3.4
-40°C
85°C
25°C
3.3
3.2
Current (µA)
3.1
3
2.9
2.8
2.7
2.6
2.5
2.8
3.1
3.4
3.7
4
4.3
4.6
4.9
5.2
5.5
V CC (V)
Figure 30-41. ADC Internal VREF vs. VCC
2.54
2.53
Voltage VIntRef (V)
2.52
2.51
85°C
25°C
2.5
2.49
2.48
2.47
2.46
-40°C
2.45
2.6
2.9
3.2
3.5
3.8
4.1
4.4
4.7
5
5.3
5.6
Voltage (V)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
412
Figure 30-42. Internal Reference Voltage vs. Sink Current
2.52
85°C
25°C
2.51
Voltage VIntRef (V)
2.5
2.49
2.48
2.47
2.46
-40°C
2.45
2.44
2.43
-7
-6
-5
-4
-3
-2
-1
0
Sink current (mA)
30.11 Current Consumption in Reset and Reset Pulse Width
Figure 30-43. Reset Supply Current vs. Frequency (1 - 20MHz)
ICC (mA)
3.5
5.5V
3
5.0V
2.5
4.5V
2
4.0V
3.6V
1.5
1
2.7V
0.5
0
2
4
6
8
10
12
14
16
Frequency (MHz)
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
413
31.
Register Summary
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
(0xFF)
Reserved
-
-
-
-
-
-
-
-
(0xFE)
Reserved
-
-
-
-
-
-
-
-
(0xFD)
Reserved
-
-
-
-
-
-
-
-
(0xFC)
Reserved
-
-
-
-
-
-
-
-
(0xFB)
Reserved
-
-
-
-
-
-
-
-
(0xFA)
Reserved
-
-
-
-
-
-
-
-
(0xF9)
Reserved
-
-
-
-
(0xF8)
Reserved
-
-
-
-
-
-
-
-
(0xF7)
Reserved
-
-
-
-
-
-
-
-
(0xF6)
Reserved
-
-
-
-
-
-
-
-
(0xF5)
Reserved
-
-
-
-
-
-
-
-
(0xF4)
UEINT
-
(0xF3)
UEBCHX
-
(0xF2)
UEBCLX
(0xF1)
UEDATX
(0xF0)
UEIENX
FLERRE
NAKINE
-
NAKOUTE
RXSTPE
RXOUTE
(0xEF)
UESTA1X
-
-
-
-
-
CTRLDIR
CFGOK
OVERFI
UNDERFI
-
EPINT6:0
-
-
-
-
BYCT10:8
BYCT7:0
DAT7:0
(0xEE)
UESTA0X
(0xED)
UECFG1X
(0xEC)
UECFG0X
(0xEB)
UECONX
-
(0xEA)
UERST
-
EPTYPE1:0
-
STALLEDE
NBUSYBK1:0
EPBK1:0
-
-
STALLRQ
STALLRQC
TXINE
CURRBK1:0
DTSEQ1:0
EPSIZE2:0
ALLOC
-
-
-
-
EPDIR
RSTDT
-
-
EPEN
STALLEDI
TXINI
-
-
EPRST6:0
(0xE9)
UENUM
-
-
-
-
-
(0xE8)
UEINTX
FIFOCON
NAKINI
RWAL
NAKOUTI
RXSTPI
RXOUTI
(0xE7)
Reserved
-
-
-
-
EPNUM2:0
-
(0xE6)
UDMFN
-
-
-
FNCERR
-
(0xE5)
UDFNUMH
-
-
-
-
-
(0xE4)
UDFNUML
(0xE3)
UDADDR
ADDEN
(0xE2)
UDIEN
-
UPRSME
EORSME
WAKEUPE
EORSTE
SOFE
MSOFE
(0xE1)
UDINT
-
UPRSMI
EORSMI
WAKEUPI
EORSTI
SOFI
MSOFI
SUSPI
(0xE0)
UDCON
-
-
-
-
RSTCPU
LSM
RMWKUP
DETACH
VBUSTI
(0xDF)
Reserved
(0xDE)
Reserved
(0xDD)
Reserved
(0xDC)
Reserved
(0xDB)
Reserved
FNUM10:8
FNUM7:0
UADD6:0
SUSPE
(0xDA)
USBINT
-
-
-
-
-
-
-
(0xD9)
USBSTA
-
-
-
-
-
-
ID
VBUS
(0xD8)
USBCON
USBE
-
FRZCLK
OTGPADE
-
-
-
VBUSTE
(0xD7)
UHWCON
-
-
-
-
-
-
-
UVREGE
(0xD6)
Reserved
(0xD5)
Reserved
DT4H3
DT4H2
DT4H1
DT4H0
DT4L3
DT4L2
DT4L1
DT4L0
(0xD4)
DT4
(0xD3)
Reserved
(0xD2)
OCR4D
Timer/Counter4 - Output Compare Register D
(0xD1)
OCR4C
Timer/Counter4 - Output Compare Register C
(0xD0)
OCR4B
Timer/Counter4 - Output Compare Register B
(0xCF)
OCR4A
Timer/Counter4 - Output Compare Register A
(0xCE)
UDR1
(0xCD)
UBRR1H
(0xCC)
UBRR1L
Page
USART1 I/O Data Register
-
-
-
-
USART1 Baud Rate Register High Byte
USART1 Baud Rate Register Low Byte
(0xCB)
UCSR1D
-
-
-
-
-
-
CTSEN
RTSEN
(0xCA)
UCSR1C
UMSEL11
UMSEL10
UPM11
UPM10
USBS1
UCSZ11
UCSZ10
UCPOL1
(0xC9)
UCSR1B
RXCIE1
TXCIE1
UDRIE1
RXEN1
TXEN1
UCSZ12
RXB81
TXB81
(0xC8)
UCSR1A
RXC1
TXC1
UDRE1
FE1
DOR1
PE1
U2X1
MPCM1
(0xC7)
CLKSTA
-
-
-
-
-
-
RCON
EXTON
(0xC6)
CLKSEL1
RCCKSEL3
RCCKSEL2
RCCKSEL1
RCCKSEL0
EXCKSEL3
EXCKSEL2
EXCKSEL1
EXCKSEL0
(0xC5)
CLKSEL0
RCSUT1
RCSUT0
EXSUT1
EXSUT0
RCE
EXTE
-
CLKS
(0xC4)
TCCR4E
TLOCK4
ENHC4
OC4OE5
OC4OE4
OC4OE3
OC4OE2
OC4OE1
OC4OE0
(0xC3)
TCCR4D
FPIE4
FPEN4
FPNC4
FPES4
FPAC4
FPF4
WGM41
WGM40
(0xC2)
TCCR4C
COM4A1S
COM4A0S
COM4B1S
COM4B0S
COM4D1S
COM4D0S
FOC4D
PWM4D
(0xC1)
TCCR4B
PWM4X
PSR4
DTPS41
DTPS40
CS43
CS42
CS41
CS40
(0xC0)
TCCR4A
COM4A1
COM4A0
COM4B1
COM4B0
FOC4A
FOC4B
PWM4A
PWM4B
(0xBF)
TC4H
-
-
-
-
-
Timer/Counter4 High Byte
ATmega16U4/32U4 [DATASHEET SUMMARY]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
414
Address
Name
(0xBE)
TCNT4
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
(0xBD)
TWAMR
TWAM6
TWAM5
TWAM4
TWAM3
TWAM2
TWAM1
TWAM0
-
(0xBC)
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
-
TWIE
(0xBB)
TWDR
(0xBA)
TWAR
TWA6
TWA5
TWA4
TWA3
TWA2
TWA1
TWA0
TWGCE
(0xB9)
TWSR
TWS7
TWS6
TWS5
TWS4
TWS3
-
TWPS1
TWPS0
(0xB8)
TWBR
(0xB7)
Reserved
-
(0xB6)
Reserved
-
(0xB5)
Reserved
(0xB4)
Reserved
(0xB3)
Page
Timer/Counter4 - Counter Register Low Byte
2-wire Serial Interface Data Register
2-wire Serial Interface Bit Rate Register
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Reserved
-
-
-
-
-
-
-
-
(0xB2)
Reserved
-
-
-
-
-
-
-
-
(0xB1)
Reserved
-
-
-
-
-
-
-
-
(0xB0)
Reserved
-
-
-
-
-
-
-
-
(0xAF)
Reserved
-
-
-
-
-
-
-
-
(0xAE)
Reserved
-
-
-
-
-
-
-
-
(0xAD)
Reserved
-
-
-
-
-
-
-
-
(0xAC)
Reserved
-
-
-
-
-
-
-
-
(0xAB)
Reserved
-
-
-
-
-
-
-
-
(0xAA)
Reserved
-
-
-
-
-
-
-
-
(0xA9)
Reserved
-
-
-
-
-
-
-
-
(0xA8)
Reserved
-
-
-
-
-
-
-
-
(0xA7)
Reserved
-
-
-
-
-
-
-
-
(0xA6)
Reserved
-
-
-
-
-
-
-
-
(0xA5)
Reserved
-
-
-
-
-
-
-
-
(0xA4)
Reserved
-
-
-
-
-
-
-
-
(0xA3)
Reserved
-
-
-
-
-
-
-
-
(0xA2)
Reserved
-
-
-
-
-
-
-
-
(0xA1)
Reserved
-
-
-
-
-
-
-
-
(0xA0)
Reserved
-
-
-
-
-
-
-
-
(0x9F)
Reserved
-
-
-
-
-
-
-
-
(0x9E)
Reserved
-
-
-
-
-
-
-
-
(0x9D)
OCR3CH
-
Timer/Counter3 - Output Compare Register C High Byte
(0x9C)
OCR3CL
Timer/Counter3 - Output Compare Register C Low Byte
(0x9B)
OCR3BH
Timer/Counter3 - Output Compare Register B High Byte
(0x9A)
OCR3BL
Timer/Counter3 - Output Compare Register B Low Byte
(0x99)
OCR3AH
Timer/Counter3 - Output Compare Register A High Byte
(0x98)
OCR3AL
Timer/Counter3 - Output Compare Register A Low Byte
(0x97)
ICR3H
Timer/Counter3 - Input Capture Register High Byte
(0x96)
ICR3L
Timer/Counter3 - Input Capture Register Low Byte
(0x95)
TCNT3H
Timer/Counter3 - Counter Register High Byte
(0x94)
TCNT3L
(0x93)
Reserved
-
-
-
-
-
-
-
(0x92)
TCCR3C
FOC3A
-
-
-
-
-
-
-
(0x91)
TCCR3B
ICNC3
ICES3
-
WGM33
WGM32
CS32
CS31
CS30
Timer/Counter3 - Counter Register Low Byte
(0x90)
TCCR3A
COM3A1
COM3A0
COM3B1
COM3B0
COM3C1
COM3C0
WGM31
WGM30
(0x8F)
Reserved
-
-
-
-
-
-
-
-
(0x8E)
Reserved
-
-
-
-
-
-
-
-
(0x8D)
OCR1CH
-
Timer/Counter1 - Output Compare Register C High Byte
(0x8C)
OCR1CL
Timer/Counter1 - Output Compare Register C Low Byte
(0x8B)
OCR1BH
Timer/Counter1 - Output Compare Register B High Byte
(0x8A)
OCR1BL
Timer/Counter1 - Output Compare Register B Low Byte
(0x89)
OCR1AH
Timer/Counter1 - Output Compare Register A High Byte
(0x88)
OCR1AL
Timer/Counter1 - Output Compare Register A Low Byte
(0x87)
ICR1H
Timer/Counter1 - Input Capture Register High Byte
(0x86)
ICR1L
Timer/Counter1 - Input Capture Register Low Byte
(0x85)
TCNT1H
Timer/Counter1 - Counter Register High Byte
(0x84)
TCNT1L
(0x83)
Reserved
-
-
-
-
-
-
-
(0x82)
TCCR1C
FOC1A
FOC1B
FOC1C
-
-
-
-
-
(0x81)
TCCR1B
ICNC1
ICES1
-
WGM13
WGM12
CS12
CS11
CS10
WGM10
Timer/Counter1 - Counter Register Low Byte
(0x80)
TCCR1A
COM1A1
COM1A0
COM1B1
COM1B0
COM1C1
COM1C0
WGM11
(0x7F)
DIDR1
-
-
-
-
-
-
-
AIN0D
(0x7E)
DIDR0
ADC7D
ADC6D
ADC5D
ADC4D
-
-
ADC1D
ADC0D
ADC8D
(0x7D)
DIDR2
-
-
ADC13D
ADC12D
ADC11D
ADC10D
ADC9D
(0x7C)
ADMUX
REFS1
REFS0
ADLAR
MUX4
MUX3
MUX2
MUX1
MUX0
(0x7B)
ADCSRB
ADHSM
ACME
MUX5
-
ADTS3
ADTS2
ADTS1
ADTS0
ATmega16U4/32U4 [DATASHEET SUMMARY]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
415
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
(0x7A)
ADCSRA
ADEN
ADSC
ADATE
ADIF
ADIE
ADPS2
ADPS1
ADPS0
(0x79)
ADCH
(0x78)
ADCL
(0x77)
Reserved
-
-
-
-
-
-
-
-
(0x76)
Reserved
-
-
-
-
-
-
-
-
(0x75)
Reserved
-
-
-
-
-
-
-
-
(0x74)
Reserved
-
-
-
-
-
-
-
-
(0x73)
Reserved
-
-
-
-
-
-
-
-
(0x72)
TIMSK4
OCIE4D
OCIE4A
OCIE4B
-
-
TOIE4
-
-
(0x71)
TIMSK3
-
-
ICIE3
-
OCIE3C
OCIE3B
OCIE3A
TOIE3
(0x70)
Reserved
-
-
-
-
-
-
-
-
(0x6F)
TIMSK1
-
-
ICIE1
-
OCIE1C
OCIE1B
OCIE1A
TOIE1
Page
ADC Data Register High byte
ADC Data Register Low byte
(0x6E)
TIMSK0
-
-
-
-
-
OCIE0B
OCIE0A
TOIE0
(0x6D)
Reserved
-
-
-
-
-
-
-
-
(0x6C)
Reserved
-
-
-
-
-
-
-
-
(0x6B)
PCMSK0
PCINT7
PCINT6
PCINT5
PCINT4
PCINT3
PCINT2
PCINT1
PCINT0
(0x6A)
EICRB
-
-
ISC61
ISC60
-
-
-
-
(0x69)
EICRA
ISC31
ISC30
ISC21
ISC20
ISC11
ISC10
ISC01
ISC00
(0x68)
PCICR
-
-
-
-
-
-
-
PCIE0
(0x67)
RCCTRL
-
-
-
-
-
-
-
RCFREQ
(0x66)
OSCCAL
(0x65)
PRR1
PRUSB
-
-
PRTIM4
PRTIM3
-
-
PRUSART1
(0x64)
PRR0
PRTWI
-
PRTIM0
-
PRTIM1
PRSPI
-
PRADC
(0x63)
Reserved
-
-
-
-
-
-
-
-
(0x62)
Reserved
-
-
-
-
-
-
-
-
(0x61)
CLKPR
CLKPCE
-
-
-
CLKPS3
CLKPS2
CLKPS1
CLKPS0
WDP0
RC Oscillator Calibration Register
(0x60)
WDTCSR
WDIF
WDIE
WDP3
WDCE
WDE
WDP2
WDP1
0x3F (0x5F)
SREG
I
T
H
S
V
N
Z
C
0x3E (0x5E)
SPH
SP15
SP14
SP13
SP12
SP11
SP10
SP9
SP8
0x3D (0x5D)
SPL
SP7
SP6
SP5
SP4
SP3
SP2
SP1
SP0
0x3C (0x5C)
Reserved
-
-
-
-
-
-
-
-
0x3B (0x5B)
RAMPZ
-
-
-
-
-
-
RAMPZ1
RAMPZ0
0x3A (0x5A)
Reserved
-
-
-
-
-
-
-
-
0x39 (0x59)
Reserved
-
-
-
-
-
-
-
-
0x38 (0x58)
Reserved
-
-
-
-
-
-
-
-
0x37 (0x57)
SPMCSR
SPMIE
RWWSB
SIGRD
RWWSRE
BLBSET
PGWRT
PGERS
SPMEN
0x36 (0x56)
Reserved
-
-
-
-
-
-
-
-
0x35 (0x55)
MCUCR
JTD
-
-
PUD
-
-
IVSEL
IVCE
PORF
0x34 (0x54)
MCUSR
-
-
USBRF
JTRF
WDRF
BORF
EXTRF
0x33 (0x53)
SMCR
-
-
-
-
SM2
SM1
SM0
SE
0x32 (0x52)
PLLFRQ
PINMUX
PLLUSB
PLLTM1
PLLTM0
PDIV3
PDIV2
PDIV1
PDIV0
0x31 (0x51)
OCDR/
MONDR
OCDR7
OCDR6
OCDR5
OCDR4
OCDR3
OCDR2
OCDR1
OCDR0
Monitor Data Register
0x30 (0x50)
ACSR
ACD
ACBG
ACO
ACI
ACIE
ACIC
ACIS1
ACIS0
0x2F (0x4F)
Reserved
-
-
-
-
-
-
-
-
0x2E (0x4E)
SPDR
SPI Data Register
0x2D (0x4D)
SPSR
SPIF
WCOL
-
-
-
-
-
SPI2X
0x2C (0x4C)
SPCR
SPIE
SPE
DORD
MSTR
CPOL
CPHA
SPR1
SPR0
0x2B (0x4B)
GPIOR2
-
PLLE
PLOCK
General Purpose I/O Register 2
0x2A (0x4A)
GPIOR1
0x29 (0x49)
PLLCSR
General Purpose I/O Register 1
0x28 (0x48)
OCR0B
Timer/Counter0 Output Compare Register B
0x27 (0x47)
OCR0A
Timer/Counter0 Output Compare Register A
0x26 (0x46)
TCNT0
0x25 (0x45)
TCCR0B
FOC0A
FOC0B
-
-
WGM02
CS02
CS01
CS00
0x24 (0x44)
TCCR0A
COM0A1
COM0A0
COM0B1
COM0B0
-
-
WGM01
WGM00
0x23 (0x43)
GTCCR
TSM
-
-
-
-
-
PSRASY
PSRSYNC
0x22 (0x42)
EEARH
-
-
-
-
0x21 (0x41)
EEARL
0x20 (0x40)
EEDR
0x1F (0x3F)
EECR
-
-
EEPM1
0x1E (0x3E)
GPIOR0
0x1D (0x3D)
EIMSK
-
INT6
-
-
INT3
INT2
INT1
INT0
0x1C (0x3C)
EIFR
-
INTF6
-
-
INTF3
INTF2
INTF1
INTF0
0x1B (0x3B)
PCIFR
-
-
-
-
-
-
-
PCIF0
0x1A (0x3A)
Reserved
-
-
-
-
-
-
-
-
0x19 (0x39)
TIFR4
OCF4D
OCF4A
OCF4B
-
-
TOV4
-
-
0x18 (0x38)
TIFR3
-
-
ICF3
-
OCF3C
OCF3B
OCF3A
TOV3
-
-
-
PINDIV
-
Timer/Counter0 (8 Bit)
EEPROM Address Register High Byte
EEPROM Address Register Low Byte
EEPROM Data Register
EEPM0
EERIE
EEMPE
EEPE
EERE
General Purpose I/O Register 0
ATmega16U4/32U4 [DATASHEET SUMMARY]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
416
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
0x17 (0x37)
Reserved
-
-
-
-
-
-
-
-
0x16 (0x36)
TIFR1
-
-
ICF1
-
OCF1C
OCF1B
OCF1A
TOV1
0x15 (0x35)
TIFR0
-
-
-
-
-
OCF0B
OCF0A
TOV0
0x14 (0x34)
Reserved
-
-
-
-
-
-
-
-
0x13 (0x33)
Reserved
-
-
-
-
-
-
-
-
0x12 (0x32)
Reserved
-
-
-
-
-
-
-
-
0x11 (0x31)
PORTF
PORTF7
PORTF6
PORTF5
PORTF4
-
-
PORTF1
PORTF0
0x10 (0x30)
DDRF
DDF7
DDF6
DDF5
DDF4
-
-
DDF1
DDF0
0x0F (0x2F)
PINF
PINF7
PINF6
PINF5
PINF4
-
-
PINF1
PINF0
0x0E (0x2E)
PORTE
-
PORTE6
-
-
-
PORTE2
-
-
0x0D (0x2D)
DDRE
-
DDE6
-
-
-
DDE2
-
-
0x0C (0x2C)
PINE
-
PINE6
-
-
-
PINE2
-
-
0x0B (0x2B)
PORTD
PORTD7
PORTD6
PORTD5
PORTD4
PORTD3
PORTD2
PORTD1
PORTD0
0x0A (0x2A)
DDRD
DDD7
DDD6
DDD5
DDD4
DDD3
DDD2
DDD1
DDD0
0x09 (0x29)
PIND
PIND7
PIND6
PIND5
PIND4
PIND3
PIND2
PIND1
PIND0
0x08 (0x28)
PORTC
PORTC7
PORTC6
-
-
-
-
-
-
0x07 (0x27)
DDRC
DDC7
DDC6
-
-
-
-
-
-
0x06 (0x26)
PINC
PINC7
PINC6
-
-
-
-
-
-
0x05 (0x25)
PORTB
PORTB7
PORTB6
PORTB5
PORTB4
PORTB3
PORTB2
PORTB1
PORTB0
0x04 (0x24)
DDRB
DDB7
DDB6
DDB5
DDB4
DDB3
DDB2
DDB1
DDB0
0x03 (0x23)
PINB
PINB7
PINB6
PINB5
PINB4
PINB3
PINB2
PINB1
PINB0
0x02 (0x22)
Reserved
-
-
-
-
-
-
-
-
0x01 (0x21)
Reserved
-
-
-
-
-
-
-
-
0x00 (0x20)
Reserved
-
-
-
-
-
-
-
-
Note:
Page
1. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory
addresses should never be written.
2. I/O registers within the address range $00 - $1F are directly bit-accessible using the SBI and CBI instructions. In these
registers, the value of single bits can be checked by using the SBIS and SBIC instructions.
3. Some of the status flags are cleared by writing a logical one to them. Note that the CBI and SBI instructions will operate
on all bits in the I/O register, writing a one back into any flag read as set, thus clearing the flag. The CBI and SBI instructions work with registers 0x00 to 0x1F only.
4. When using the I/O specific commands IN and OUT, the I/O addresses $00 - $3F must be used. When addressing I/O
registers as data space using LD and ST instructions, $20 must be added to these addresses. The
ATmega16U4/ATmega32U4 is a complex microcontroller with more peripheral units than can be supported within the 64
location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space from $60 - $1FF in SRAM, only
the ST/STS/STD and LD/LDS/LDD instructions can be used.
ATmega16U4/32U4 [DATASHEET SUMMARY]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
417
32.
Instruction Set Summary
Mnemonics
Operands
Description
Operation
Flags
#Clocks
1
ARITHMETIC AND LOGIC INSTRUCTIONS
ADD
Rd, Rr
Add two Registers
Rd  Rd + Rr
Z,C,N,V,H
ADC
Rd, Rr
Add with Carry two Registers
Rd  Rd + Rr + C
Z,C,N,V,H
1
ADIW
Rdl,K
Add Immediate to Word
Rdh:Rdl  Rdh:Rdl + K
Z,C,N,V,S
2
SUB
Rd, Rr
Subtract two Registers
Rd  Rd - Rr
Z,C,N,V,H
1
SUBI
Rd, K
Subtract Constant from Register
Rd  Rd - K
Z,C,N,V,H
1
SBC
Rd, Rr
Subtract with Carry two Registers
Rd  Rd - Rr - C
Z,C,N,V,H
1
SBCI
Rd, K
Subtract with Carry Constant from Reg.
Rd  Rd - K - C
Z,C,N,V,H
1
SBIW
Rdl,K
Subtract Immediate from Word
Rdh:Rdl  Rdh:Rdl - K
Z,C,N,V,S
2
AND
Rd, Rr
Logical AND Registers
Rd Rd  Rr
Z,N,V
1
ANDI
Rd, K
Logical AND Register and Constant
Rd  Rd K
Z,N,V
1
OR
Rd, Rr
Logical OR Registers
Rd  Rd v Rr
Z,N,V
1
ORI
Rd, K
Logical OR Register and Constant
Rd Rd v K
Z,N,V
1
EOR
Rd, Rr
Exclusive OR Registers
Rd  Rd  Rr
Z,N,V
1
COM
Rd
One’s Complement
Rd  0xFF  Rd
Z,C,N,V
1
1
NEG
Rd
Two’s Complement
Rd  0x00  Rd
Z,C,N,V,H
SBR
Rd,K
Set Bit(s) in Register
Rd  Rd v K
Z,N,V
1
CBR
Rd,K
Clear Bit(s) in Register
Rd  Rd  (0xFF - K)
Z,N,V
1
INC
Rd
Increment
Rd  Rd + 1
Z,N,V
1
DEC
Rd
Decrement
Rd  Rd  1
Z,N,V
1
TST
Rd
Test for Zero or Minus
Rd  Rd  Rd
Z,N,V
1
CLR
Rd
Clear Register
Rd  Rd  Rd
Z,N,V
1
SER
Rd
Set Register
Rd  0xFF
None
1
MUL
Rd, Rr
Multiply Unsigned
R1:R0  Rd x Rr
Z,C
2
MULS
Rd, Rr
Multiply Signed
R1:R0  Rd x Rr
Z,C
2
MULSU
Rd, Rr
Multiply Signed with Unsigned
R1:R0  Rd x Rr
Z,C
2
FMUL
Rd, Rr
Fractional Multiply Unsigned
R1:R0  (Rd x Rr) << 1
Z,C
2
FMULS
Rd, Rr
Fractional Multiply Signed
R1:R0  (Rd x Rr) << 1
Z,C
2
FMULSU
Rd, Rr
Fractional Multiply Signed with Unsigned
R1:R0  (Rd x Rr) << 1
Z,C
2
2
BRANCH INSTRUCTIONS
Relative Jump
PC PC + k + 1
None
IJMP
Indirect Jump to (Z)
None
2
EIJMP
Extended Indirect Jump to (Z)
PC  Z
PC (EIND:Z)
None
2
RJMP
k
JMP
k
Direct Jump
PC k
None
3
RCALL
k
Relative Subroutine Call
PC  PC + k + 1
None
4
ICALL
Indirect Call to (Z)
4
Extended Indirect Call to (Z)
PC  Z
PC (EIND:Z)
None
EICALL
None
4
Direct Subroutine Call
PC  k
None
5
RET
Subroutine Return
PC  STACK
None
5
RETI
Interrupt Return
PC  STACK
I
5
Compare, Skip if Equal
if (Rd = Rr) PC PC + 2 or 3
None
1/2/3
CALL
k
CPSE
Rd,Rr
CP
Rd,Rr
Compare
Rd  Rr
Z, N,V,C,H
1
CPC
Rd,Rr
Compare with Carry
Rd  Rr  C
Z, N,V,C,H
1
CPI
Rd,K
Compare Register with Immediate
Rd  K
Z, N,V,C,H
1
SBRC
Rr, b
Skip if Bit in Register Cleared
if (Rr(b)=0) PC  PC + 2 or 3
None
1/2/3
SBRS
Rr, b
Skip if Bit in Register is Set
if (Rr(b)=1) PC  PC + 2 or 3
None
1/2/3
SBIC
P, b
Skip if Bit in I/O Register Cleared
if (P(b)=0) PC  PC + 2 or 3
None
1/2/3
SBIS
P, b
Skip if Bit in I/O Register is Set
if (P(b)=1) PC  PC + 2 or 3
None
1/2/3
BRBS
s, k
Branch if Status Flag Set
if (SREG(s) = 1) then PCPC+k + 1
None
1/2
BRBC
s, k
Branch if Status Flag Cleared
if (SREG(s) = 0) then PCPC+k + 1
None
1/2
BREQ
k
Branch if Equal
if (Z = 1) then PC  PC + k + 1
None
1/2
BRNE
k
Branch if Not Equal
if (Z = 0) then PC  PC + k + 1
None
1/2
BRCS
k
Branch if Carry Set
if (C = 1) then PC  PC + k + 1
None
1/2
BRCC
k
Branch if Carry Cleared
if (C = 0) then PC  PC + k + 1
None
1/2
BRSH
k
Branch if Same or Higher
if (C = 0) then PC  PC + k + 1
None
1/2
BRLO
k
Branch if Lower
if (C = 1) then PC  PC + k + 1
None
1/2
BRMI
k
Branch if Minus
if (N = 1) then PC  PC + k + 1
None
1/2
BRPL
k
Branch if Plus
if (N = 0) then PC  PC + k + 1
None
1/2
BRGE
k
Branch if Greater or Equal, Signed
if (N  V= 0) then PC  PC + k + 1
None
1/2
BRLT
k
Branch if Less Than Zero, Signed
if (N  V= 1) then PC  PC + k + 1
None
1/2
BRHS
k
Branch if Half Carry Flag Set
if (H = 1) then PC  PC + k + 1
None
1/2
BRHC
k
Branch if Half Carry Flag Cleared
if (H = 0) then PC  PC + k + 1
None
1/2
BRTS
k
Branch if T Flag Set
if (T = 1) then PC  PC + k + 1
None
1/2
BRTC
k
Branch if T Flag Cleared
if (T = 0) then PC  PC + k + 1
None
1/2
BRVS
k
Branch if Overflow Flag is Set
if (V = 1) then PC  PC + k + 1
None
1/2
ATmega16U4/32U4 [DATASHEET SUMMARY]
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Mnemonics
Operands
Description
Operation
Flags
#Clocks
BRVC
k
Branch if Overflow Flag is Cleared
if (V = 0) then PC  PC + k + 1
None
1/2
BRIE
k
Branch if Interrupt Enabled
if ( I = 1) then PC  PC + k + 1
None
1/2
BRID
k
Branch if Interrupt Disabled
if ( I = 0) then PC  PC + k + 1
None
1/2
BIT AND BIT-TEST INSTRUCTIONS
SBI
P,b
Set Bit in I/O Register
I/O(P,b)  1
None
2
CBI
P,b
Clear Bit in I/O Register
I/O(P,b)  0
None
2
LSL
Rd
Logical Shift Left
Rd(n+1)  Rd(n), Rd(0)  0
Z,C,N,V
1
LSR
Rd
Logical Shift Right
Rd(n)  Rd(n+1), Rd(7)  0
Z,C,N,V
1
ROL
Rd
Rotate Left Through Carry
Rd(0)C,Rd(n+1) Rd(n),CRd(7)
Z,C,N,V
1
ROR
Rd
Rotate Right Through Carry
Rd(7)C,Rd(n) Rd(n+1),CRd(0)
Z,C,N,V
1
ASR
Rd
Arithmetic Shift Right
Rd(n)  Rd(n+1), n=0..6
Z,C,N,V
1
SWAP
Rd
Swap Nibbles
Rd(3..0)Rd(7..4),Rd(7..4)Rd(3..0)
None
1
BSET
s
Flag Set
SREG(s)  1
SREG(s)
1
BCLR
s
Flag Clear
SREG(s)  0
SREG(s)
1
BST
Rr, b
Bit Store from Register to T
T  Rr(b)
T
1
BLD
Rd, b
Bit load from T to Register
Rd(b)  T
None
1
1
SEC
Set Carry
C1
C
CLC
Clear Carry
C0
C
1
SEN
Set Negative Flag
N1
N
1
CLN
Clear Negative Flag
N0
N
1
SEZ
Set Zero Flag
Z1
Z
1
CLZ
Clear Zero Flag
Z0
Z
1
SEI
Global Interrupt Enable
I1
I
1
CLI
Global Interrupt Disable
I 0
I
1
SES
Set Signed Test Flag
S1
S
1
CLS
Clear Signed Test Flag
S0
S
1
SEV
Set Twos Complement Overflow.
V1
V
1
CLV
Clear Twos Complement Overflow
V0
V
1
SET
Set T in SREG
T1
T
1
CLT
Clear T in SREG
T0
T
1
SEH
CLH
Set Half Carry Flag in SREG
Clear Half Carry Flag in SREG
H1
H0
H
H
1
1
Rd  Rr
Rd+1:Rd  Rr+1:Rr
None
1
None
1
DATA TRANSFER INSTRUCTIONS
MOV
Rd, Rr
Move Between Registers
MOVW
Rd, Rr
Copy Register Word
LDI
Rd, K
Load Immediate
Rd  K
None
1
LD
Rd, X
Load Indirect
Rd  (X)
None
2
LD
Rd, X+
Load Indirect and Post-Inc.
Rd  (X), X  X + 1
None
2
LD
Rd, - X
Load Indirect and Pre-Dec.
X  X - 1, Rd  (X)
None
2
2
LD
Rd, Y
Load Indirect
Rd  (Y)
None
LD
Rd, Y+
Load Indirect and Post-Inc.
Rd  (Y), Y  Y + 1
None
2
LD
Rd, - Y
Load Indirect and Pre-Dec.
Y  Y - 1, Rd  (Y)
None
2
LDD
Rd,Y+q
Load Indirect with Displacement
Rd  (Y + q)
None
2
LD
Rd, Z
Load Indirect
Rd  (Z)
None
2
LD
Rd, Z+
Load Indirect and Post-Inc.
Rd  (Z), Z  Z+1
None
2
LD
Rd, -Z
Load Indirect and Pre-Dec.
Z  Z - 1, Rd  (Z)
None
2
LDD
Rd, Z+q
Load Indirect with Displacement
Rd  (Z + q)
None
2
LDS
Rd, k
Load Direct from SRAM
Rd  (k)
None
2
ST
X, Rr
Store Indirect
(X) Rr
None
2
ST
X+, Rr
Store Indirect and Post-Inc.
(X) Rr, X  X + 1
None
2
ST
- X, Rr
Store Indirect and Pre-Dec.
X  X - 1, (X)  Rr
None
2
ST
Y, Rr
Store Indirect
(Y)  Rr
None
2
2
ST
Y+, Rr
Store Indirect and Post-Inc.
(Y)  Rr, Y  Y + 1
None
ST
- Y, Rr
Store Indirect and Pre-Dec.
Y  Y - 1, (Y)  Rr
None
2
STD
Y+q,Rr
Store Indirect with Displacement
(Y + q)  Rr
None
2
2
ST
Z, Rr
Store Indirect
(Z)  Rr
None
ST
Z+, Rr
Store Indirect and Post-Inc.
(Z)  Rr, Z  Z + 1
None
2
ST
-Z, Rr
Store Indirect and Pre-Dec.
Z  Z - 1, (Z)  Rr
None
2
STD
Z+q,Rr
Store Indirect with Displacement
(Z + q)  Rr
None
2
STS
k, Rr
Store Direct to SRAM
(k)  Rr
None
2
Load Program Memory
R0  (Z)
None
3
LPM
Rd, Z
Load Program Memory
Rd  (Z)
None
3
LPM
Rd, Z+
Load Program Memory and Post-Inc
Rd  (Z), Z  Z+1
None
3
Extended Load Program Memory
R0  (RAMPZ:Z)
None
3
LPM
ELPM
ELPM
Rd, Z
Extended Load Program Memory
Rd  (Z)
None
3
ELPM
Rd, Z+
Extended Load Program Memory
Rd  (RAMPZ:Z), RAMPZ:Z RAMPZ:Z+1
None
3
Store Program Memory
(Z)  R1:R0
None
-
In Port
Rd  P
None
1
SPM
IN
Rd, P
ATmega16U4/32U4 [DATASHEET SUMMARY]
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Mnemonics
Operands
Description
Operation
Flags
OUT
P, Rr
Out Port
P  Rr
None
#Clocks
1
PUSH
Rr
Push Register on Stack
STACK  Rr
None
2
POP
Rd
Pop Register from Stack
Rd  STACK
None
2
MCU CONTROL INSTRUCTIONS
NOP
No Operation
None
1
SLEEP
Sleep
(see specific description for Sleep function)
None
1
WDR
BREAK
Watchdog Reset
Break
(see specific description for WDR/timer)
For On-chip Debug Only
None
None
1
N/A
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33.
Ordering Information
33.1
ATmega16U4
Speed [MHz]
16
Power Supply
2.7 - 5.5V
Ordering Code
Default Oscillator
ATmega16U4-AU
External XTAL
ATmega16U4RC-AU
Internal Calib. RC
ATmega16U4-MU
ATmega16U4RC-MU
Notes:
1.
2.
3.
Operation Range
44ML
Industrial (-40° to +85°C)
External XTAL
(1)(2)(3)
(1)(2)(3)
Package
44PW
Internal Calib. RC
For more information on running the USB from internal RC oscillator consult application note AVR291: 8MHz Internal Oscillator Calibration for USB Low
Speed on Atmel ATmega32U4RC.
USB operation from internal RC oscillator is only guaranteed for 0°C to 40°C.
These parts are shipped with no USB bootloader pre-programmed.
Package Type
44ML
ML, 44 - Lead, 10 x 10mm Body Size, 1.0mm Body Thickness
0.8mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP)
44PW
PW, 44 - Lead 7.0 x 7.0mm Body, 0.50mm Pitch
Quad Flat No Lead Package (QFN)
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33.2
ATmega32U4
Speed [MHz]
16
Power Supply
2.7 - 5.5V
Ordering Code
Default Oscillator
ATmega32U4-AU
External XTAL
ATmega32U4RC-AU
Internal Calib. RC
ATmega32U4-MU(1)(2)(3)
External XTAL
ATmega32U4RC-MU(1)
(2) (3)
Notes:
1.
2.
3.
Package
Internal Calib. RC
Operation Range
44ML
Industrial (-40° to +85°C)
44PW
For more information on running the USB from internal RC oscillator consult application note AVR291: 8MHz Internal Oscillator Calibration for USB Low
Speed on Atmel ATmega32U4RC.
USB operation from internal RC oscillator is only guaranteed for 0°C to 40°C.
These parts are shipped with no USB bootloader pre-programmed.
Package Type
44ML
ML, 44 - Lead, 10 x 10mm Body Size, 1.0mm Body Thickness
0.8mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP)
44PW
PW, 44 - Lead 7.0 x 7.0mm Body, 0.50mm Pitch
Quad Flat No Lead Package (QFN)
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34.
Packaging Information
34.1
TQFP44
0.17
0.60
0.37
02/06/2014
J
ATmega16U4/32U4 [DATASHEET]
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34.2
QFN44
ATmega16U4/32U4 [DATASHEET]
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ATmega16U4/32U4 [DATASHEET]
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35.
Errata
The revision letter in this section refers to the revision of the ATmega16U4/ATmega32U4 device.
35.1
ATmega16U4/ATmega32U4 Rev E
• Spike on TWI pins when TWI is enabled
• High current consumption in sleep mode
• MSB of OCR4A/B/D is write only in 11-bits enhanced PWM mode
1. Spike on TWI pins when TWI is enabled
100 ns negative spike occurs on SDA and SCL pins when TWI is enabled.
Problem Fix/work around
Enable ATmega16U4/ATmega32U4 TWI before the other nodes of the TWI network.
2. High current consumption in sleep mode
If a pending interrupt cannot wake the part up from the selected mode, the current consumption will
increase during sleep when executing the SLEEP instruction directly after a SEI instruction.
Problem Fix/work around
Before entering sleep, interrupts not used to wake up the part from the sleep mode should be disabled.
3. MSB of OCR4A/B/D is write only in 11-bits enhanced PWM mode
In the 11-bits enhanced PWM mode the MSB of OCR4A/B/D is write only. A read of OCR4A/B/D will
always return zero in the MSB position.
Problem Fix/work around
None.
35.2
ATmega16U4/ATmega32U4 Rev D
• Spike on TWI pins when TWI is enabled
• High current consumption in sleep mode
• Timer 4 11-bits enhanced PWM mode
1. Spike on TWI pins when TWI is enabled
100 ns negative spike occurs on SDA and SCL pins when TWI is enabled.
Problem Fix/work around
Enable ATmega16U4/ATmega32U4 TWI before the other nodes of the TWI network.
2. High current consumption in sleep mode
If a pending interrupt cannot wake the part up from the selected mode, the current consumption will
increase during sleep when executing the SLEEP instruction directly after a SEI instruction.
Problem Fix/work around
Before entering sleep, interrupts not used to wake up the part from the sleep mode should be disabled.
3. Timer 4 11-bits enhanced PWM mode
Timer 4 11-bits enhanced mode is not functional.
Problem Fix/work around
None.
ATmega16U4/32U4 [DATASHEET]
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35.3
ATmega16U4/ATmega32U4 Rev C
Not sampled
35.4
ATmega16U4/ATmega32U4 Rev B
•
•
•
•
Spike on TWI pins when TWI is enabled
High current consumption in sleep mode
Incorrect execution of VBUSTI interrupt
Timer 4 11-bits enhanced PWM mode
1. Spike on TWI pins when TWI is enabled
100 ns negative spike occurs on SDA and SCL pins when TWI is enabled.
Problem Fix/work around
Enable ATmega16U4/ATmega32U4 TWI before the other nodes of the TWI network.
2. High current consumption in sleep mode
If a pending interrupt cannot wake the part up from the selected mode, the current consumption will
increase during sleep when executing the SLEEP instruction directly after a SEI instruction.
Problem Fix/work around
Before entering sleep, interrupts not used to wake up the part from the sleep mode should be disabled.
3. Incorrect execution of VBUSTI interrupt
The CPU may incorrectly execute the interrupt vector related to the VBUSTI interrupt flag.
Problem fix/work around
Do not enable this interrupt. Firmware must process this USB event by polling VBUSTI.
4. Timer 4 11-bits enhanced PWM mode
Timer 4 11-bits enhanced mode is not functional.
Problem Fix/work around
None.
35.5
ATmega16U4/ATmega32U4 Rev A
•
•
•
•
•
•
•
Spike on TWI pins when TWI is enabled
High current consumption in sleep mode
Increased power consumption in power-down mode
Internal RC oscillator start up may fail
Internal RC oscillator calibration
Incorrect execution of VBUSTI interrupt
Timer 4 enhanced mode issue
1. Spike on TWI pins when TWI is enabled
100 ns negative spike occurs on SDA and SCL pins when TWI is enabled.
Problem Fix/work around
Enable ATmega16U4/ATmega32U4 TWI before the other nodes of the TWI network.
2. High current consumption in sleep mode
ATmega16U4/32U4 [DATASHEET]
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If a pending interrupt cannot wake the part up from the selected mode, the current consumption will
increase during sleep when executing the SLEEP instruction directly after a SEI instruction.
Problem Fix/work around
Before entering sleep, interrupts not used to wake up the part from the sleep mode should be disabled.
3. Increased power consumption in power-down mode
The typical power consumption is increased by about 30 µA in power-down mode.
Problem Fix/work around
None.
4. Internal RC oscillator start up may fail
When the part is configured to start on internal RC oscillator, the oscillator may not start properly after
power-on.
Problem Fix/work around
Do not configure the part to start on internal RC oscillator.
5. Internal RC oscillator calibration
8 MHz frequency can be impossible to reach with internal RC even when using maximal OSCAL value.
Problem Fix/work around
None.
6. Incorrect execution of VBUSTI interrupt
The CPU may incorrectly execute the interrupt vector related to the VBUSTI interrupt flag.
Problem fix/work around
Do not enable this interrupt. Firmware must process this USB event by polling VBUSTI.
7. Timer 4 11-bits enhanced PWM mode
Timer 4 11-bits enhanced mode is not functional.
Problem Fix/work around
None.
ATmega16U4/32U4 [DATASHEET]
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36.
Datasheet Revision History for ATmega16U4/ATmega32U4
Note that the referring page numbers in this section are referred to this document. The referring revision in this
section are referring to the document revision.
36.1
Rev. 7766J – 04/2016
1.
36.2
36.3
“Memory Programming” on page 353: Updated number of words in a page and number of
pages in the Flash and EEPROM for ATmega16U4 and ATmega32U4. Refer to Table 28-11
and Table 28-12 on page 359.
Rev. 7766I – 07/2015
1.
Applied Atmel brands throughout the contents and reorganized the contents.
2.
Updated “Power Management and Sleep Modes” on page 43. Part of contents was missing.
Rev. 7766H – 06/2014
1.
The first section in “Phase and Frequency Correct PWM Mode” on page 154 has been
corrected.
2.
Several corrections are made according to the new template.
3.
Trademarks are added to the last page.
4
Removed preliminary on the front page
5
Updated with new datasheet template from 05-2014
6.
Updated description of parts pre-programed with a default USB bootloader in Features on
page 2.
7.
Added three footnotes for the RC part numbers in Section 33., “Ordering Information” on page
421.
8.
Removed footnote on Frequency range inTable 6-3 on page 30 and Table 6-7 on page 32.
9.
Updated values and removed footnote in Table 8-3 on page 55.
10.
Removed column VCC=1.5 - 5.5V in Table 29-2 on page 385.
11.
Changed footnote for Table 29-2 on page 385.
12.
Added max value for Rise/Fall time in Table 29-4 on page 387.
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36.4
36.5
36.6
Rev. 7766G – 02/2014
1.
Updated the “Description” on page 177 of the “Output Compare Modulator (OCM1C0A)” .
Specified when the logical AND and the logical OR will be performed based on the PORTB7.
2.
Updated “USART Control and Status Register n D– UCSRnD” on page 213. “Bits 7:2 Reserved” are Read only.
3.
Updated “Crystal-less Operation” on page 259. The temperature range changed to “within the
0C and +40C.
4.
MUX bit in “ADC Control and Status Register B – ADCSRB” on page 294 changed to R/W.
5.
Updated Table 24-6 on page 318. Trigger Source: Timer/Counter0 Compare Match updated
to Timer/Counter0 Compare Match A.
6.
Updated “DC Characteristics” on page 383. Added Active 16MHz, VCC = 5V, max. 27mA, in
“Icc / Power supply current”.
7.
Updated “Register Summary” on page 414. Added UCSRnD at the address CBh.
8.
Replaced the “TQFP44” on page 423 and “QFN44” on page 424 by updated package
drawings.
9.
Updated the last page according to Atmel new Brand Style Guide (new logo).
Rev. 7766F – 11/10
1.
Replaced the “QFN44” on page 424 by an updated drawing.
2.
Updated “ADC Control and Status Register B – ADCSRB” on page 294. Defined the
ADCSRB register as in “ADC Control and Status Register B – ADCSRB” on page 317.
3.
Updated the last page according to Atmel new Brand Style Guide.
Rev. 7766E – 04/10
1.
Updated “Features” on page 1.
2.
Updated “Features” on page 256.
3.
Updated Figure 21-9 on page 261.
4.
Updated Section 21.8 on page 263.
5.
Updated “Features” on page 297.
6.
Updated “Boundary-scan Order” on page 332.
7.
Updated “Program And Data Memory Lock Bits” on page 353.
8.
Updated Table 28-5 on page 355.
9.
Updated “Electrical Characteristics” on page 383.
10.
Updated Figure 29-2 on page 386.
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36.7
36.8
11.
Added “Typical Characteristics” on page 392.
12.
Updated “Ordering Information” on page 421.
13.
Updated “Errata” on page 426.
Rev. 7766D – 01/09
1.
Updated Memory section in “Features” on page 1.
2.
Added section “Resources” on page 8.
3.
Added section “Data Retention” on page 8.
4.
Updated “Ordering Information” on page 421.
Rev. 7766C – 11/08
1.
36.9
Updated Memory section in “Features” on page 1.
Rev. 7766B – 11/08
1.
Added ATmega16U4 device.
2.
Created errata section and added ATmega16U4.
3.
Updated High Speed Timer, asynchronous description Section 15. on page 139
36.10 Rev. 7766A – 07/08
1.
Initial revision
ATmega16U4/32U4 [DATASHEET]
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Table of Contents
1.
Pin Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1
2.2
3.
About . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1
3.2
3.3
3.4
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Architectural Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
ALU – Arithmetic Logic Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Status Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
General Purpose Register File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Stack Pointer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Instruction Execution Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Reset and Interrupt Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
In-System Reprogrammable Flash Program Memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SRAM Data Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
EEPROM Data Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I/O Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
19
20
25
System Clock and Clock Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10
6.11
7.
8
8
8
8
AVR Memories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.1
5.2
5.3
5.4
6.
Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Code Examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data Retention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AVR CPU Core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
5.
Block Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Pin Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Clock Systems and their Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Clock Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Low Power Crystal Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Low Frequency Crystal Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Calibrated Internal RC Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
External Clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Clock Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Clock Output Buffer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PLL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Clock switch Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
28
29
31
32
33
34
35
36
36
38
Power Management and Sleep Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
Idle Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ADC Noise Reduction Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power-down Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power-save Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Standby Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Extended Standby Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power Reduction Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Minimizing Power Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
43
43
44
44
44
45
45
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i
7.9
8.
System Control and Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
8.10
8.11
9.
Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Resetting the AVR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reset Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power-on Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
External Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Brown-out Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Watchdog Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
USB Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Internal Voltage Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Watchdog Timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MCU Status Register – MCUSR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
50
51
52
52
53
54
54
55
59
59
Interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
9.1
9.2
Interrupt Vectors in ATmega16U4/ATmega32U4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
10. I/O-Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
10.1
10.2
10.3
10.4
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ports as General Digital I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alternate Port Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Register Description for I/O-Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
67
67
72
84
11. External Interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
11.1
Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
12. Timer/Counter0, Timer/Counter1, and Timer/Counter3 Prescalers . . . . . . . . . . . . . . . 92
12.1
12.2
12.3
12.4
Internal Clock Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Prescaler Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
External Clock Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
92
92
92
93
13. 8-bit Timer/Counter0 with PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
13.1
13.2
13.3
13.4
13.5
13.6
13.7
13.8
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Timer/Counter Clock Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Counter Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Output Compare Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
Compare Match Output Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Modes of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Timer/Counter Timing Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
8-bit Timer/Counter Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
14. 16-bit Timers/Counters (Timer/Counter1 and Timer/Counter3) . . . . . . . . . . . . . . . . . . 111
14.1
14.2
14.3
14.4
14.5
14.6
14.7
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Accessing 16-bit Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Timer/Counter Clock Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Counter Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Input Capture Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Compare Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Compare Match Output Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
111
113
116
116
117
119
121
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14.8 Modes of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
14.9 Timer/Counter Timing Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
14.10 16-bit Timer/Counter Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
15. 10-bit High Speed Timer/Counter4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
15.1
15.2
15.3
15.4
15.5
15.6
15.7
15.8
15.9
15.10
15.11
15.12
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Counter Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Compare Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dead Time Generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Compare Match Output Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Synchronous update . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Modes of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Timer/Counter Timing Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fault Protection Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Accessing 10-bit Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
140
140
144
145
147
148
151
151
158
159
160
164
16. Output Compare Modulator (OCM1C0A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
16.1
16.2
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
17. Serial Peripheral Interface – SPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
17.1
17.2
SS Pin Functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
18. USART . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188
18.1
18.2
18.3
18.4
18.5
18.6
18.7
18.8
18.9
18.10
18.11
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Clock Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frame Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
USART Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data Transmission – The USART Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data Reception – The USART Receiver. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Asynchronous Data Reception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Multi-processor Communication Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hardware Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Examples of Baud Rate Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
USART Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
188
189
192
193
194
197
201
204
205
206
209
19. USART in SPI Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
19.1
19.2
19.3
19.4
19.5
19.6
19.7
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Clock Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SPI Data Modes and Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frame Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AVR USART MSPIM vs. AVR SPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
USART MSPIM Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
214
214
214
215
218
220
221
20. 2-wire Serial Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
20.1
20.2
20.3
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
2-wire Serial Interface Bus Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
Data Transfer and Frame Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
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20.4
20.5
20.6
20.7
20.8
20.9
Multi-master Bus Systems, Arbitration and Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Overview of the TWI Module. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using the TWI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Transmission Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Multi-master Systems and Arbitration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TWI Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
228
230
232
236
250
252
21. USB Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
21.1
21.2
21.3
21.4
21.5
21.6
21.7
21.8
21.9
21.10
21.11
21.12
21.13
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Block Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Typical Application Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Crystal-less Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Design Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
General Operating Modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Speed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Memory Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PAD Suspend . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Plug-in Detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
USB Software Operating Modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Registers Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
256
256
257
259
259
260
262
263
263
264
265
266
267
22. USB Device Operating Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
22.1
22.2
22.3
22.4
22.5
22.6
22.7
22.8
22.9
22.10
22.11
22.12
22.13
22.14
22.15
22.16
22.17
22.18
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power-on and Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Endpoint Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
USB Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Endpoint Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Endpoint Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Address Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Suspend, Wake-up and Resume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Detach. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Remote Wake-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
STALL Request. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CONTROL Endpoint Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OUT Endpoint Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IN endpoint management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Isochronous Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Overflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
270
270
270
271
271
271
272
272
272
273
273
274
275
276
278
278
278
281
23. Analog Comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293
23.1
23.2
Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
Analog Comparator Multiplexed Input. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295
24. Analog to Digital Converter - ADC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
24.1
24.2
24.3
24.4
24.5
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Starting a Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Prescaling and Conversion Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Changing Channel or Reference Selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
297
299
299
300
303
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24.6
24.7
24.8
24.9
Temperature Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ADC Noise Canceler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ADC Conversion Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ADC Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
304
305
309
313
25. JTAG Interface and On-chip Debug System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
25.1
25.2
25.3
25.4
25.5
25.6
25.7
25.8
25.9
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Test Access Port – TAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TAP Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using the Boundary-scan Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using the On-chip Debug System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
On-chip Debug Specific JTAG Instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
On-chip Debug Related Register in I/O Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using the JTAG Programming Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
319
319
321
322
322
323
323
324
324
26. IEEE 1149.1 (JTAG) Boundary-scan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
26.1
26.2
26.3
26.4
26.5
26.6
26.7
26.8
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Boundary-scan Specific JTAG Instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Boundary-scan Related Register in I/O Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Boundary-scan Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Boundary-scan Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Boundary-scan Description Language Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
325
325
325
327
328
329
332
335
27. Boot Loader Support – Read-While-Write Self-Programming . . . . . . . . . . . . . . . . . . . 336
27.1
27.2
27.3
27.4
27.5
27.6
27.7
Boot Loader Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Application and Boot Loader Flash Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Read-While-Write and No Read-While-Write Flash Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Boot Loader Lock Bits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Entering the Boot Loader Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Addressing the Flash During Self-Programming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self-Programming the Flash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
336
336
336
339
340
343
344
28. Memory Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353
28.1
28.2
28.3
28.4
28.5
28.6
28.7
28.8
28.9
Program And Data Memory Lock Bits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fuse Bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Signature Bytes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Calibration Byte. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parallel Programming Parameters, Pin Mapping, and Commands. . . . . . . . . . . . . . . . . . . . . . . . .
Parallel Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Serial Downloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Serial Programming Pin Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Programming via the JTAG Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
353
354
356
356
356
359
367
368
371
29. Electrical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
29.1
29.2
29.3
29.4
29.5
Absolute Maximum Ratings* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DC Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
External Clock Drive Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
External Clock Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System and Reset Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
383
383
385
385
386
ATmega16U4/32U4 [DATASHEET]
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v
29.6
29.7
29.8
29.9
Maximum speed vs. VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-wire Serial Interface Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SPI Timing Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hardware Boot Entrance Timing Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
386
386
388
389
30. Typical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392
30.1
30.2
30.3
30.4
30.5
30.6
30.7
30.8
30.9
30.10
30.11
Active Supply Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Idle Supply Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power-down Supply Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power-save Supply Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pin Pull-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pin Driver Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pin Threshold and Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
BOD Threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Internal Oscillator Speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Current Consumption of Peripheral Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Current Consumption in Reset and Reset Pulse Width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
392
395
397
398
399
400
403
406
408
411
413
31. Register Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414
32. Instruction Set Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418
33. Ordering Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421
33.1
33.2
ATmega16U4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421
ATmega32U4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422
34. Packaging Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423
34.1
34.2
TQFP44 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423
QFN44 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
35. Errata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426
35.1
35.2
35.3
35.4
35.5
ATmega16U4/ATmega32U4 Rev E
ATmega16U4/ATmega32U4 Rev D
ATmega16U4/ATmega32U4 Rev C
ATmega16U4/ATmega32U4 Rev B
ATmega16U4/ATmega32U4 Rev A
................................................
................................................
................................................
................................................
................................................
426
426
427
427
427
36. Datasheet Revision History for ATmega16U4/ATmega32U4 . . . . . . . . . . . . . . . . . . . . 429
36.1
36.2
36.3
36.4
36.5
36.6
36.7
36.8
36.9
36.10
Rev. 7766J – 04/2016 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rev. 7766I – 07/2015 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rev. 7766H – 06/2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rev. 7766G – 02/2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rev. 7766F – 11/10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rev. 7766E – 04/10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rev. 7766D – 01/09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rev. 7766C – 11/08 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rev. 7766B – 11/08 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rev. 7766A – 07/08 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
429
429
429
430
430
430
431
431
431
431
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
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XXXXXX
Atmel Corporation
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© 2015 Atmel Corporation. / Rev.: Atmel-7766I-USB-ATmega16U4-32U4-Datasheet_07/2015.
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