ATMEL ATmega16A-PU 8-bit microcontroller with 16k bytes in-system programmable flash Datasheet

Features
• High-performance, Low-power AVR® 8-bit Microcontroller
• Advanced RISC Architecture
•
•
•
•
•
•
•
•
– 131 Powerful Instructions – Most Single-clock Cycle Execution
– 32 x 8 General Purpose Working Registers
– Fully Static Operation
– Up to 16 MIPS Throughput at 16 MHz
– On-chip 2-cycle Multiplier
High Endurance Non-volatile Memory segments
– 16K Bytes of In-System Self-programmable Flash program memory
– 512 Bytes EEPROM
– 1K Byte Internal SRAM
– 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
– 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
Peripheral Features
– Two 8-bit Timer/Counters with Separate Prescalers and Compare Modes
– One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture
Mode
– Real Time Counter with Separate Oscillator
– Four PWM Channels
– 8-channel, 10-bit ADC
• 8 Single-ended Channels
• 7 Differential Channels in TQFP Package Only
• 2 Differential Channels with Programmable Gain at 1x, 10x, or 200x
– Byte-oriented Two-wire Serial Interface
– Programmable Serial USART
– Master/Slave SPI Serial Interface
– Programmable Watchdog Timer with Separate On-chip Oscillator
– On-chip Analog Comparator
Special Microcontroller Features
– Power-on Reset and Programmable Brown-out Detection
– Internal Calibrated RC Oscillator
– External and Internal Interrupt Sources
– Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby
and Extended Standby
I/O and Packages
– 32 Programmable I/O Lines
– 40-pin PDIP, 44-lead TQFP, and 44-pad QFN/MLF
Operating Voltages
– 2.7 - 5.5V for ATmega16A
Speed Grades
– 0 - 16 MHz for ATmega16A
Power Consumption @ 1 MHz, 3V, and 25°C for ATmega16A
– Active: 0.6 mA
– Idle Mode: 0.2 mA
– Power-down Mode: < 1µA
8-bit
Microcontroller
with 16K Bytes
In-System
Programmable
Flash
ATmega16A
8154B–AVR–07/09
1. Pin Configurations
Figure 1-1.
Pinout ATmega16A
PDIP
(XCK/T0) PB0
(T1) PB1
(INT2/AIN0) PB2
(OC0/AIN1) PB3
(SS) PB4
(MOSI) PB5
(MISO) PB6
(SCK) PB7
RESET
VCC
GND
XTAL2
XTAL1
(RXD) PD0
(TXD) PD1
(INT0) PD2
(INT1) PD3
(OC1B) PD4
(OC1A) PD5
(ICP1) PD6
PA0 (ADC0)
PA1 (ADC1)
PA2 (ADC2)
PA3 (ADC3)
PA4 (ADC4)
PA5 (ADC5)
PA6 (ADC6)
PA7 (ADC7)
AREF
GND
AVCC
PC7 (TOSC2)
PC6 (TOSC1)
PC5 (TDI)
PC4 (TDO)
PC3 (TMS)
PC2 (TCK)
PC1 (SDA)
PC0 (SCL)
PD7 (OC2)
PB4 (SS)
PB3 (AIN1/OC0)
PB2 (AIN0/INT2)
PB1 (T1)
PB0 (XCK/T0)
GND
VCC
PA0 (ADC0)
PA1 (ADC1)
PA2 (ADC2)
PA3 (ADC3)
TQFP/QFN/MLF
(MOSI) PB5
(MISO) PB6
(SCK) PB7
RESET
VCC
GND
XTAL2
XTAL1
(RXD) PD0
(TXD) PD1
(INT0) PD2
2
PD3
PD4
PD5
PD6
PD7
VCC
GND
(SCL) PC0
(SDA) PC1
(TCK) PC2
(TMS) PC3
(INT1)
(OC1B)
(OC1A)
(ICP1)
(OC2)
NOTE:
Bottom pad should
be soldered to ground.
PA4 (ADC4)
PA5 (ADC5)
PA6 (ADC6)
PA7 (ADC7)
AREF
GND
AVCC
PC7 (TOSC2)
PC6 (TOSC1)
PC5 (TDI)
PC4 (TDO)
ATmega16A
8154B–AVR–07/09
ATmega16A
2. Overview
The ATmega16A 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 ATmega16A
achieves throughputs approaching 1 MIPS per MHz allowing the system designer to optimize
power consumption versus processing speed.
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2.1
Block Diagram
Figure 2-1.
Block Diagram
PA0 - PA7
PC0 - PC7
PORTA DRIVERS/BUFFERS
PORTC DRIVERS/BUFFERS
PORTA DIGITAL INTERFACE
PORTC DIGITAL INTERFACE
VCC
GND
AVCC
MUX &
ADC
ADC
INTERFACE
TWI
AREF
PROGRAM
COUNTER
STACK
POINTER
PROGRAM
FLASH
SRAM
TIMERS/
COUNTERS
OSCILLATOR
INTERNAL
OSCILLATOR
XTAL1
INSTRUCTION
REGISTER
GENERAL
PURPOSE
REGISTERS
WATCHDOG
TIMER
OSCILLATOR
XTAL2
X
INSTRUCTION
DECODER
Y
MCU CTRL.
& TIMING
RESET
Z
CONTROL
LINES
ALU
AVR CPU
STATUS
REGISTER
EEPROM
PROGRAMMING
LOGIC
SPI
USART
+
-
INTERNAL
CALIBRATED
OSCILLATOR
COMP.
INTERFACE
PORTB DIGITAL INTERFACE
PORTD DIGITAL INTERFACE
PORTB DRIVERS/BUFFERS
PORTD DRIVERS/BUFFERS
PB0 - PB7
4
INTERRUPT
UNIT
PD0 - PD7
ATmega16A
8154B–AVR–07/09
ATmega16A
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 ATmega16A provides the following features: 16K bytes of In-System Programmable Flash
Program memory with Read-While-Write capabilities, 512 bytes EEPROM, 1K byte SRAM, 32
general purpose I/O lines, 32 general purpose working registers, a JTAG interface for Boundaryscan, On-chip Debugging support and programming, three flexible Timer/Counters with compare modes, Internal and External Interrupts, a serial programmable USART, a byte oriented
Two-wire Serial Interface, an 8-channel, 10-bit ADC with optional differential input stage with
programmable gain (TQFP package only), a programmable Watchdog Timer with Internal Oscillator, an SPI serial port, and six software selectable power saving modes. The Idle mode stops
the CPU while allowing the USART, Two-wire interface, A/D Converter, 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 External Interrupt or Hardware Reset. In Power-save mode, the Asynchronous Timer continues to run,
allowing the user to maintain a timer base while the rest of the device is sleeping. The ADC
Noise Reduction mode stops the CPU and all I/O modules except Asynchronous Timer and
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. In Extended Standby mode, both the main Oscillator
and the Asynchronous Timer continue to run.
The device is manufactured using Atmel’s high density nonvolatile memory technology. The Onchip 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 Atmel ATmega16A is a powerful microcontroller that provides a highly-flexible and costeffective solution to many embedded control applications.
The ATmega16A 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.
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2.2
2.2.1
Pin Descriptions
VCC
Digital supply voltage.
2.2.2
GND
Ground.
2.2.3
Port A (PA7:PA0)
Port A serves as the analog inputs to the A/D Converter.
Port A also serves as an 8-bit bi-directional I/O port, if the A/D Converter is not used. Port pins
can provide internal pull-up resistors (selected for each bit). The Port A output buffers have symmetrical drive characteristics with both high sink and source capability. When pins PA0 to PA7
are used as inputs and are externally pulled low, they will source current if the internal pull-up
resistors are activated. The Port A pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
2.2.4
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 tri-stated when a reset condition becomes active,
even if the clock is not running.
Port B also serves the functions of various special features of the ATmega16A as listed on page
57.
2.2.5
Port C (PC7:PC0)
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 tri-stated when a reset condition becomes active,
even if the clock is not running. If the JTAG interface is enabled, the pull-up resistors on pins
PC5(TDI), PC3(TMS) and PC2(TCK) will be activated even if a reset occurs.
Port C also serves the functions of the JTAG interface and other special features of the
ATmega16A as listed on page 60.
2.2.6
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 tri-stated when a reset condition becomes active,
even if the clock is not running.
Port D also serves the functions of various special features of the ATmega16A as listed on page
62.
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ATmega16A
8154B–AVR–07/09
ATmega16A
2.2.7
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 27-2 on page
296. Shorter pulses are not guaranteed to generate a reset.
2.2.8
XTAL1
Input to the inverting Oscillator amplifier and input to the internal clock operating circuit.
2.2.9
XTAL2
Output from the inverting Oscillator amplifier.
2.2.10
AVCC
AVCC is the supply voltage pin for Port A and the A/D Converter. It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC
through a low-pass filter.
2.2.11
AREF
AREF is the analog reference pin for the A/D Converter.
3. Resources
A comprehensive set of development tools, application notes and datasheets are available for
download on http://www.atmel.com/avr.
Note:
1.
4. Data Retention
Reliability Qualification results show that the projected data retention failure rate is much less
than 1 PPM over 20 years at 85°C or 100 years at 25°C.
5. About Code Examples
This documentation contains simple code examples that briefly show how to use various parts of
the device. These code examples assume that the part specific header file is included before
compilation. Be aware that not all C Compiler vendors include bit definitions in the header files
and interrupt handling in C is compiler dependent. Please confirm with the C Compiler documentation for more details.
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6. AVR CPU
6.1
Overview
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.
Figure 6-1.
Block Diagram of the AVR MCU Architecture
Data Bus 8-bit
Flash
Program
Memory
Program
Counter
Status
and Control
32 x 8
General
Purpose
Registrers
Control Lines
Direct Addressing
Instruction
Decoder
Indirect Addressing
Instruction
Register
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 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 Z-register, described later in this section.
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ATmega16A
8154B–AVR–07/09
ATmega16A
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, $20 - $5F.
6.2
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 the “Instruction Set” section for a detailed description.
6.3
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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6.3.1
SREG – AVR Status Register
Bit
7
6
5
4
3
2
1
0
I
T
H
S
V
N
Z
C
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
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 the “Instruction Set Description” 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 the “Instruction Set Description” for detailed information.
• Bit 3 – V: Two’s Complement Overflow Flag
The Two’s Complement Overflow Flag V supports two’s complement arithmetics. See the
“Instruction Set Description” for detailed information.
• Bit 2 – N: Negative Flag
The Negative Flag N indicates a negative result in an arithmetic or logic operation. See the
“Instruction Set Description” for detailed information.
• Bit 1 – Z: Zero Flag
The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the “Instruction
Set Description” for detailed information.
• Bit 0 – C: Carry Flag
The Carry Flag C indicates a carry in an arithmetic or logic operation. See the “Instruction Set
Description” for detailed information.
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ATmega16A
8154B–AVR–07/09
ATmega16A
6.4
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
• Two 8-bit output operands and one 16-bit result input
• One 16-bit output operand and one 16-bit result input
Figure 6-2 shows the structure of the 32 general purpose working registers in the CPU.
Figure 6-2.
AVR CPU General Purpose Working Registers
7
0
Addr.
R0
$00
R1
$01
R2
$02
…
R13
$0D
General
R14
$0E
Purpose
R15
$0F
Working
R16
$10
Registers
R17
$11
…
R26
$1A
X-register Low Byte
R27
$1B
X-register High Byte
R28
$1C
Y-register Low Byte
R29
$1D
Y-register High Byte
R30
$1E
Z-register Low Byte
R31
$1F
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 6-2, 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.
6.4.1
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 6-3.
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8154B–AVR–07/09
Figure 6-3.
The X-, Y-, and Z-registers
15
XH
XL
7
X - register
0
R27 ($1B)
15
YH
YL
0
R29 ($1D)
Z - register
0
R26 ($1A)
7
Y - register
0
7
0
7
0
R28 ($1C)
15
ZH
7
0
ZL
7
R31 ($1F)
0
0
R30 ($1E)
In the different addressing modes these address registers have functions as fixed displacement,
automatic increment, and automatic decrement (see the Instruction Set Reference for details).
6.5
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. Note that the Stack is implemented as
growing from higher to lower memory locations. The Stack Pointer Register always points to the
top of the Stack. The Stack Pointer points to the data SRAM Stack area where the Subroutine
and Interrupt Stacks are located. A Stack PUSH command will decrease the Stack Pointer.
The Stack in the data SRAM must be defined by the program before any subroutine calls are
executed or interrupts are enabled. Initial Stack Pointer value equals the last address of the
internal SRAM and the Stack Pointer must be set to point above start of the SRAM, see Figure
7-2 on page 17.
See Table 6-1 for Stack Pointer details.
Table 6-1.
Stack Pointer instructions
Instruction
Stack pointer
Description
PUSH
Decremented by 1
Data is pushed onto the stack
CALL
ICALL
RCALL
Decremented by 2
Return address is pushed onto the stack with a subroutine call or
interrupt
POP
Incremented by 1
Data is popped from the stack
RET
RETI
Incremented by 2
Return address is popped from the stack with return from
subroutine or return from interrupt
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.
6.5.1
SPH and SPL – Stack Pointer High and Low Register
Bit
12
15
14
13
12
11
10
9
8
SP15
SP14
SP13
SP12
SP11
SP10
SP9
SP8
SPH
ATmega16A
8154B–AVR–07/09
ATmega16A
Read/Write
Initial Value
6.6
SP7
SP6
SP5
SP4
SP3
SP2
SP1
SP0
7
6
5
4
3
2
1
0
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
SPL
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 6-4 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 6-4.
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 6-5 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.
Figure 6-5.
Single Cycle ALU Operation
T1
T2
T3
T4
clkCPU
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
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6.7
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 264 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 44. 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 General Interrupt Control Register (GICR). Refer to “Interrupts” on page 44 for more
information. The Reset Vector can also be moved to the start of the boot Flash section by programming the BOOTRST Fuse, see “Boot Loader Support – Read-While-Write SelfProgramming” on page 250.
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.
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.
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ATmega16A
Assembly Code Example
in r16, SREG
cli
; store SREG value
; disable interrupts during timed sequence
sbi EECR, EEMWE
; start EEPROM write
sbi EECR, EEWE
out SREG, r16
; restore SREG value (I-bit)
C Code Example
char cSREG;
cSREG = SREG; /* store SREG value */
/* disable interrupts during timed sequence */
_CLI();
EECR |= (1<<EEMWE); /* start EEPROM write */
EECR |= (1<<EEWE);
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
_SEI(); /* set global interrupt enable */
_SLEEP(); /* enter sleep, waiting for interrupt */
/* note: will enter sleep before any pending interrupt(s) */
6.7.1
Interrupt Response Time
The interrupt execution response for all the enabled AVR interrupts is four clock cycles minimum. After four clock cycles the program vector address for the actual interrupt handling routine
is executed. During this four 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 four clock cycles. This increase comes in addition to the
start-up time from the selected sleep mode.
A return from an interrupt handling routine takes four clock cycles. During these four clock
cycles, the Program Counter (two bytes) is popped back from the Stack, the Stack Pointer is
incremented by two, and the I-bit in SREG is set.
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7. AVR Memories
7.1
Overview
This section describes the different memories in the ATmega16A. The AVR architecture has two
main memory spaces, the Data Memory and the Program Memory space. In addition, the
ATmega16A features an EEPROM Memory for data storage. All three memory spaces are linear
and regular.
7.2
In-System Reprogrammable Flash Program Memory
The ATmega16A contains 16K 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 8K
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 10,000 write/erase cycles. The ATmega16A
Program Counter (PC) is 13 bits wide, thus addressing the 8K program memory locations. The
operation of Boot Program section and associated Boot Lock bits for software protection are
described in detail in “Boot Loader Support – Read-While-Write Self-Programming” on page
250. “Memory Programming” on page 264 contains a detailed description on Flash data serial
downloading using the SPI pins or the JTAG interface.
Constant tables can be allocated within the entire program memory address space (see the LPM
– Load Program Memory Instruction Description).
Timing diagrams for instruction fetch and execution are presented in “Instruction Execution Timing” on page 13.
Figure 7-1.
Program Memory Map
$0000
Application Flash Section
Boot Flash Section
$1FFF
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ATmega16A
7.3
SRAM Data Memory
Figure 7-2 shows how the ATmega16A SRAM Memory is organized.
The lower 1120 Data Memory locations address the Register File, the I/O Memory, and the internal data SRAM. The first 96 locations address the Register File and I/O Memory, and the next
1024 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 Z-register.
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 1024 bytes of internal data
SRAM in the ATmega16A are all accessible through all these addressing modes. The Register
File is described in “General Purpose Register File” on page 11.
Figure 7-2.
Data Memory Map
Register File
Data Address Space
R0
R1
R2
...
$0000
$0001
$0002
...
R29
R30
R31
I/O Registers
$00
$01
$02
...
$001D
$001E
$001F
$3D
$3E
$3F
$005D
$005E
$005F
Internal SRAM
$0060
$0061
...
$0020
$0021
$0022
...
$045E
$045F
7.3.1
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 7-3.
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8154B–AVR–07/09
Figure 7-3.
On-chip Data SRAM Access Cycles
T1
T2
T3
clkCPU
Address
Compute Address
Address Valid
Write
Data
WR
Read
Data
RD
Memory Access Instruction
7.4
Next Instruction
EEPROM Data Memory
The ATmega16A contains 512 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 276, page 280, and page 267, respectively.
7.4.1
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 7-1. 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 19 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.
7.4.2
18
EEPROM Write During Power-down Sleep Mode
When entering Power-down Sleep mode while an EEPROM write operation is active, the
EEPROM write operation will continue, and will complete before the Write Access time has
passed. However, when the write operation is completed, the Oscillator continues running, and
ATmega16A
8154B–AVR–07/09
ATmega16A
as a consequence, the device does not enter Power-down entirely. It is therefore recommended
to verify that the EEPROM write operation is completed before entering Power-down.
7.4.3
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.
7.5
I/O Memory
The I/O space definition of the ATmega16A is shown in “Register Summary” on page 334.
All ATmega16A I/Os and peripherals are placed in the I/O space. The I/O locations are
accessed by the IN and OUT instructions, transferring data between the 32 general purpose
working registers and the I/O space. 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. Refer to the Instruction Set section for more details. 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.
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 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 $00 to $1F only.
The I/O and Peripherals Control Registers are explained in later sections.
7.6
7.6.1
Register Description
EEARH and EEARL – The EEPROM Address Register
Bit
Read/Write
Initial Value
15
14
13
12
11
10
9
8
–
–
–
–
–
–
–
EEAR8
EEARH
EEAR7
EEAR6
EEAR5
EEAR4
EEAR3
EEAR2
EEAR1
EEAR0
EEARL
7
6
5
4
3
2
1
0
R
R
R
R
R
R
R
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
0
0
0
0
0
0
0
X
X
X
X
X
X
X
X
X
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• Bits 15:9 – Res: Reserved Bits
These bits are reserved bits in the ATmega16A and will always read as zero.
• Bits 8:0 – EEAR8:0: EEPROM Address
The EEPROM Address Registers – EEARH and EEARL – specify the EEPROM address in the
512 bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and
511. The initial value of EEAR is undefined. A proper value must be written before the EEPROM
may be accessed.
7.6.2
EEDR – The EEPROM Data Register
Bit
7
6
5
4
3
2
1
MSB
0
LSB
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
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.
7.6.3
EECR – The EEPROM Control Register
Bit
7
6
5
4
3
2
1
0
–
–
–
–
EERIE
EEMWE
EEWE
EERE
Read/Write
R
R
R
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
X
0
EECR
• Bits 7:4 – Res: Reserved Bits
These bits are reserved bits in the ATmega16A and will always read as zero.
• 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 EEWE is cleared.
• Bit 2 – EEMWE: EEPROM Master Write Enable
The EEMWE bit determines whether setting EEWE to one causes the EEPROM to be written.
When EEMWE is set, setting EEWE within four clock cycles will write data to the EEPROM at
the selected address If EEMWE is zero, setting EEWE will have no effect. When EEMWE has
been written to one by software, hardware clears the bit to zero after four clock cycles. See the
description of the EEWE bit for an EEPROM write procedure.
• Bit 1 – EEWE: EEPROM Write Enable
The EEPROM Write Enable Signal EEWE is the write strobe to the EEPROM. When address
and data are correctly set up, the EEWE bit must be written to one to write the value into the
EEPROM. The EEMWE bit must be written to one before a logical one is written to EEWE, 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):
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ATmega16A
1. Wait until EEWE becomes zero.
2. Wait until SPMEN in SPMCR 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 EEMWE bit while writing a zero to EEWE in EECR.
6. Within four clock cycles after setting EEMWE, write a logical one to EEWE.
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 “Boot Loader
Support – Read-While-Write Self-Programming” on page 250 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 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 EEWE bit is cleared by hardware. The user software can poll this bit and wait for a zero before writing the next byte. When EEWE 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 EEWE 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. Table 7-1 lists the typical programming time for EEPROM access from the CPU.
Table 7-1.
EEPROM Programming Time
Symbol
Number of Calibrated RC Oscillator
Cycles(1)
Typ Programming Time
EEPROM write (from CPU)
8448
8.5 ms
Note:
1. Uses 1 MHz clock, independent of CKSEL Fuse setting.
The following code examples show one assembly and one C function for writing to the
EEPROM. The examples assume that interrupts are controlled (for example by disabling interrupts globally) so that no interrupts will occur during execution of these functions. The examples
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8154B–AVR–07/09
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.
Assembly Code Example
EEPROM_write:
; Wait for completion of previous write
sbic EECR,EEWE
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 EEMWE
sbi EECR,EEMWE
; Start eeprom write by setting EEWE
sbi EECR,EEWE
ret
C Code Example
void EEPROM_write(unsigned int uiAddress, unsigned char ucData)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEWE))
;
/* Set up address and data registers */
EEAR = uiAddress;
EEDR = ucData;
/* Write logical one to EEMWE */
EECR |= (1<<EEMWE);
/* Start eeprom write by setting EEWE */
EECR |= (1<<EEWE);
}
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ATmega16A
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.
Assembly Code Example
EEPROM_read:
; Wait for completion of previous write
sbic EECR,EEWE
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
unsigned char EEPROM_read(unsigned int uiAddress)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEWE))
;
/* Set up address register */
EEAR = uiAddress;
/* Start eeprom read by writing EERE */
EECR |= (1<<EERE);
/* Return data from data register */
return EEDR;
}
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8. System Clock and Clock Options
8.1
Clock Systems and their Distribution
Figure 8-1 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 32. The clock systems are detailed Figure 8-1.
Figure 8-1.
Clock Distribution
Asynchronous
Timer/Counter
General I/O
Modules
ADC
CPU Core
Flash and
EEPROM
RAM
clkADC
clkI/O
AVR Clock
Control Unit
clkASY
clkCPU
clkFLASH
Reset Logic
Source Clock
Watchdog Clock
Clock
Multiplexer
Timer/Counter
Oscillator
External RC
Oscillator
External Clock
Watchdog Timer
Watchdog
Oscillator
Crystal
Oscillator
Low-frequency
Crystal Oscillator
Calibrated RC
Oscillator
8.1.1
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.
8.1.2
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 note that address recognition in the TWI module is carried out asynchronously when clkI/O is halted, enabling TWI address reception in all sleep modes.
24
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8154B–AVR–07/09
ATmega16A
8.1.3
Flash Clock – clkFLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously with the CPU clock.
8.1.4
Asynchronous Timer Clock – clkASY
The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly
from an external 32 kHz clock crystal. The dedicated clock domain allows using this
Timer/Counter as a real-time counter even when the device is in sleep mode.
8.1.5
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.
8.2
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.
Device Clocking Options Select(1)
Table 8-1.
Device Clocking Option
CKSEL3:0
External Crystal/Ceramic Resonator
1111 - 1010
External Low-frequency Crystal
1001
External RC Oscillator
1000 - 0101
Calibrated Internal RC Oscillator
0100 - 0001
External Clock
Note:
0000
1. For all fuses “1” means unprogrammed while “0” means programmed.
The various choices for each clocking option is given in the following sections. When the CPU
wakes up from Power-down or Power-save, the selected clock source is used to time the startup, ensuring stable Oscillator operation before instruction execution starts. When the CPU starts
from Reset, there is as an additional delay allowing the power to reach a stable level before
commencing normal operation. The Watchdog Oscillator is used for timing this real-time part of
the start-up time. The number of WDT Oscillator cycles used for each time-out is shown in Table
8-2. The frequency of the Watchdog Oscillator is voltage dependent as shown in “Typical Characteristics” on page 305.
Table 8-2.
8.3
Number of Watchdog Oscillator Cycles
Typ Time-out (VCC = 5.0V)
Typ Time-out (VCC = 3.0V)
Number of Cycles
4.1 ms
4.3 ms
4K (4,096)
65 ms
69 ms
64K (65,536)
Default Clock Source
The device is shipped with CKSEL = “0001” and SUT = “10”. The default clock source setting is
therefore the 1 MHz Internal RC Oscillator with longest startup time. This default setting ensures
that all users can make their desired clock source setting using an In-System or Parallel
Programmer.
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8154B–AVR–07/09
8.4
Crystal Oscillator
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 8-2. Either a quartz crystal or a
ceramic resonator may be used. The CKOPT Fuse selects between two different Oscillator
amplifier modes. When CKOPT is programmed, the Oscillator output will oscillate will a full railto-rail swing on the output. This mode is suitable when operating in a very noisy environment or
when the output from XTAL2 drives a second clock buffer. This mode has a wide frequency
range. When CKOPT is unprogrammed, the Oscillator has a smaller output swing. This reduces
power consumption considerably. This mode has a limited frequency range and it can not be
used to drive other clock buffers.
For resonators, the maximum frequency is 8 MHz with CKOPT unprogrammed and 16 MHz with
CKOPT programmed. 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 Table 8-3. For ceramic resonators, the
capacitor values given by the manufacturer should be used.
Figure 8-2.
Crystal Oscillator Connections
C2
C1
XTAL2
XTAL1
GND
The Oscillator can operate in three different modes, each optimized for a specific frequency
range. The operating mode is selected by the fuses CKSEL3:1 as shown in Table 8-3.
Table 8-3.
CKOPT
CKSEL3:1
Frequency Range
(MHz)
Recommended Range for Capacitors C1
and C2 for Use with Crystals (pF)
1
101(1)
0.4 - 0.9
–
1
110
0.9 - 3.0
12 - 22
1
111
3.0 - 8.0
12 - 22
0
101, 110, 111
1.0 ≤
12 - 22
Note:
26
Crystal Oscillator Operating Modes
1. This option should not be used with crystals, only with ceramic resonators.
ATmega16A
8154B–AVR–07/09
ATmega16A
The CKSEL0 Fuse together with the SUT1:0 Fuses select the start-up times as shown in Table
8-4.
Table 8-4.
Start-up Times for the Crystal Oscillator Clock Selection
CKSEL0
SUT1:0
Start-up Time from
Power-down and
Power-save
0
00
258 CK(1)
4.1 ms
Ceramic resonator, fast rising
power
0
01
258 CK(1)
65 ms
Ceramic resonator, slowly
rising power
0
10
1K CK(2)
–
Ceramic resonator, BOD
enabled
0
11
1K CK(2)
4.1 ms
Ceramic resonator, fast rising
power
1
00
1K CK(2)
65 ms
Ceramic resonator, slowly
rising power
1
01
16K CK
–
1
10
16K CK
4.1 ms
Crystal Oscillator, fast rising
power
1
11
16K CK
65 ms
Crystal Oscillator, slowly
rising power
Notes:
Additional Delay
from Reset
(VCC = 5.0V)
Recommended Usage
Crystal Oscillator, BOD
enabled
1. 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.
2. 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.
8.5
Low-frequency Crystal Oscillator
To use a 32.768 kHz watch crystal as the clock source for the device, the Low-frequency Crystal
Oscillator must be selected by setting the CKSEL Fuses to “1001”. The crystal should be connected as shown in Figure 8-2. By programming the CKOPT Fuse, the user can enable internal
capacitors on XTAL1 and XTAL2, thereby removing the need for external capacitors. The internal capacitors have a nominal value of 36 pF.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 8-5.
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8154B–AVR–07/09
Table 8-5.
Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1:0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(VCC = 5.0V)
00
1K CK(1)
4.1 ms
Fast rising power or BOD enabled
01
(1)
1K CK
65 ms
Slowly rising power
10
32K CK
65 ms
Stable frequency at start-up
11
Note:
8.6
Recommended Usage
Reserved
1. These options should only be used if frequency stability at start-up is not important for the
application.
External RC Oscillator
For timing insensitive applications, the external RC configuration shown in Figure 8-3 can be
used. The frequency is roughly estimated by the equation f = 1/(3RC). C should be at least 22
pF. By programming the CKOPT Fuse, the user can enable an internal 36 pF capacitor between
XTAL1 and GND, thereby removing the need for an external capacitor. For more information on
Oscillator operation and details on how to choose R and C, refer to the External RC Oscillator
application note.
Figure 8-3.
External RC Configuration
VCC
R
NC
XTAL2
XTAL1
C
GND
The Oscillator can operate in four different modes, each optimized for a specific frequency
range. The operating mode is selected by the fuses CKSEL3:0 as shown in Table 8-6.
Table 8-6.
External RC Oscillator Operating Modes
CKSEL3:0
Frequency Range (MHz)
0101
0.1 ≤ 0.9
0110
0.9 - 3.0
0111
3.0 - 8.0
1000
8.0 - 12.0
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 8-7.
28
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 8-7.
SUT1:0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(VCC = 5.0V)
00
18 CK
–
01
18 CK
4.1 ms
Fast rising power
10
18 CK
65 ms
Slowly rising power
11
(1)
4.1 ms
Fast rising power or BOD enabled
Note:
8.7
Start-up Times for the External RC Oscillator Clock Selection
6 CK
Recommended Usage
BOD enabled
1. This option should not be used when operating close to the maximum frequency of the device.
Calibrated Internal RC Oscillator
The Calibrated Internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All frequencies are nominal values at 5V and 25°C. This clock may be selected as the sys-tem clock
by programming the CKSEL Fuses as shown in Table 8-8. If selected, it will operate with no
external components. The CKOPT Fuse should always be unpro-grammed when using this
clock option. During Reset, hardware loads the calibration byte into the OSCCAL Register and
thereby automatically calibrates the RC Oscillator. At 5V, 25°C and 1.0, 2.0, 4.0 or 8.0 MHz
Oscillator frequency selected, this calibration gives a frequency within ± 3% of the nominal frequency. Using calibration methods as described in application notes available at
www.atmel.com/avr it is possible to achieve ±1% accuracy at any given VCC and Temperature.
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 266.
Table 8-8.
Internal Calibrated RC Oscillator Operating Modes
CKSEL3:0
Nominal Frequency (MHz)
(1)
1.0
0001
Note:
0010
2.0
0011
4.0
0100
8.0
1. The device is shipped with this option selected.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 8-9. XTAL1 and XTAL2 should be left unconnected (NC).
Table 8-9.
Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
SUT1:0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(VCC = 5.0V)
00
6 CK
–
01
6 CK
4.1 ms
Fast rising power
10(1)
6 CK
65 ms
Slowly rising power
11
Note:
Recommended Usage
BOD enabled
Reserved
1. The device is shipped with this option selected.
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8154B–AVR–07/09
8.8
External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
8-4. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
By programming the CKOPT Fuse, the user can enable an internal 36 pF capacitor between
XTAL1 and GND.
Figure 8-4.
External Clock Drive Configuration
EXTERNAL
CLOCK
SIGNAL
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 8-10.
Table 8-10.
Start-up Times for the External Clock Selection
SUT1:0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(VCC = 5.0V)
00
6 CK
–
01
6 CK
4.1 ms
Fast rising power
10
6 CK
65 ms
Slowly rising power
11
Recommended Usage
BOD enabled
Reserved
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. It is required to ensure that the
MCU is kept in reset during such changes in the clock frequency.
8.9
Timer/Counter Oscillator
For AVR microcontrollers with Timer/Counter Oscillator pins (TOSC1 and TOSC2), the crystal is
connected directly between the pins. No external capacitors are needed. The Oscillator is optimized for use with a 32.768 kHz watch crystal. Applying an external clock source to TOSC1 is
not recommended.
Note:
30
The Timer/Counter Oscillator uses the same type of crystal oscillator as Low-Frequency Oscillator
and the internal capacitors have the same nominal value of 36 pF.
ATmega16A
8154B–AVR–07/09
ATmega16A
8.10
8.10.1
Register Description
OSCCAL – Oscillator Calibration Register
Bit
Read/Write
7
6
5
4
3
2
1
0
CAL7
CAL6
CAL5
CAL4
CAL3
CAL2
CAL1
CAL0
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
OSCCAL
Device Specific Calibration Value
• Bits 7:0 – CAL7:0: Oscillator Calibration Value
Writing the calibration byte to this address will trim the Internal Oscillator to remove process variations from the Oscillator frequency. This is done automatically during Chip Reset. When
OSCCAL is zero, the lowest available frequency is chosen. Writing non-zero values to this register will increase the frequency of the Internal Oscillator. Writing $FF to the register gives the
highest available frequency. The calibrated Oscillator is used to time EEPROM and Flash
access. If EEPROM or Flash is written, do not calibrate to more than 10% above the nominal frequency. Otherwise, the EEPROM or Flash write may fail. Note that the Oscillator is intended for
calibration to 1.0, 2.0, 4.0, or 8.0 MHz. Tuning to other values is not guaranteed, as indicated in
Table 8-11.
Table 8-11.
Internal RC Oscillator Frequency Range.
OSCCAL Value
Min Frequency in Percentage of
Nominal Frequency (%)
Max Frequency in Percentage of
Nominal Frequency (%)
$00
50
100
$7F
75
150
$FF
100
200
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8154B–AVR–07/09
9. Power Management and Sleep Modes
9.1
Overview
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.
9.2
Sleep Modes
Figure 8-1 on page 24 presents the different clock systems in the ATmega16A, and their distribution. The figure is helpful in selecting an appropriate sleep mode. Table 9-1 shows the
different sleep modes and their wake-up sources.
Active Clock Domains and Wake Up Sources in the Different Sleep Modes
Idle
X
ADC NRM
X
X
X
X
X
X(2)
X
(2)
X
Power Down
Power Save
X(2)
Standby(1)
Extended
Standby(1)
Notes:
X(2)
X
X(2)
X
X(2)
X
X
X
X
X
(3)
X
X
X
X
(3)
X
X
X(3)
X
X(3)
X
X(3)
X
X
Other I/O
ADC
SPM / EEPROM Ready
Timer2
INT2, INT1, INT0
Timer Osc. Enabled
clkASY
Sleep Mode
Wake-up Sources
TWI Address Match
Oscillators
clkADC
clkIO
clkFLASH
clkCPU
Active Clock domains
Main Clock Source Enabled
Table 9-1.
X
X(2)
X(2)
1. External Crystal or resonator selected as clock source.
2. If AS2 bit in ASSR is set.
3. Only INT2 or level interrupt INT1 and INT0.
To enter any of the six sleep modes, the SE bit in MCUCR must be written to logic one and a
SLEEP instruction must be executed. The SM2, SM1, and SM0 bits in the MCUCR Register
select which sleep mode (Idle, ADC Noise Reduction, Power-down, Power-save, Standby, or
Extended Standby) will be activated by the SLEEP instruction. See Table 9-2 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, it 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.
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ATmega16A
9.3
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 SPI, USART, Analog Comparator, ADC, Two-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.
9.4
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, the
Two-wire Serial Interface address watch, Timer/Counter2 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.
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 Reset, a Brown-out
Reset, a Two-wire Serial Interface Address Match Interrupt, a Timer/Counter2 interrupt, an
SPM/EEPROM ready interrupt, an External level interrupt on INT0 or INT1, or an external interrupt on INT2 can wake up the MCU from ADC Noise Reduction mode.
9.5
Power-down Mode
When the SM2:0 bits are written to 010, the SLEEP instruction makes the MCU enter Powerdown mode. In this mode, the External Oscillator is stopped, while the External interrupts, the
Two-wire Serial Interface address watch, and the Watchdog continue operating (if enabled).
Only an External Reset, a Watchdog Reset, a Brown-out Reset, a Two-wire Serial Interface
address match interrupt, an External level interrupt on INT0 or INT1, or an External interrupt on
INT2 can wake up the MCU. This sleep mode basically halts all generated clocks, allowing operation of asynchronous modules only.
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 67
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 25.
9.6
Power-save Mode
When the SM2:0 bits are written to 011, the SLEEP instruction makes the MCU enter Powersave mode. This mode is identical to Power-down, with one exception:
If Timer/Counter2 is clocked asynchronously, i.e., the AS2 bit in ASSR is set, Timer/Counter2
will run during sleep. The device can wake up from either Timer Overflow or Output Compare
33
8154B–AVR–07/09
event from Timer/Counter2 if the corresponding Timer/Counter2 interrupt enable bits are set in
TIMSK, and the Global Interrupt Enable bit in SREG is set.
If the Asynchronous Timer is NOT clocked asynchronously, Power-down mode is recommended
instead of Power-save mode because the contents of the registers in the Asynchronous Timer
should be considered undefined after wake-up in Power-save mode if AS2 is 0.
This sleep mode basically halts all clocks except clkASY, allowing operation only of asynchronous
modules, including Timer/Counter2 if clocked asynchronously.
9.7
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.
9.8
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. This mode is identical to
Power-save mode with the exception that the Oscillator is kept running. From Extended Standby
mode, the device wakes up in six clock cycles.
9.9
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.
9.9.1
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” on page 207
for details on ADC operation.
9.9.2
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 the 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 204 for details on how to
configure the Analog Comparator.
9.9.3
Brown-out Detector
If the Brown-out Detector is not needed in the application, this module should be turned off. If the
Brown-out Detector is enabled by the BODEN Fuse, 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 39 for details on how to
configure the Brown-out Detector.
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ATmega16A
8154B–AVR–07/09
ATmega16A
9.9.4
Internal Voltage Reference
The Internal Voltage Reference will be enabled when needed by the Brown-out Detector, 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 40 for details on the start-up time.
9.9.5
Watchdog Timer
If the Watchdog Timer is not needed in the application, this 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 “Watchdog Timer” on page 41 for details on how to configure the Watchdog Timer.
9.9.6
Port Pins
When entering a sleep mode, all port pins should be configured to use minimum power. The
most important thing is then to ensure that no pins drive resistive loads. In sleep modes where
the 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 53 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.
9.9.7
JTAG Interface and On-chip Debug System
If the On-chip debug system is enabled by the OCDEN Fuse and the chip enter Power down or
Power save sleep mode, the main clock source remains enabled. In these sleep modes, this will
contribute significantly to the total current consumption. There are three alternative ways to
avoid this:
• Disable OCDEN Fuse.
• Disable JTAGEN Fuse.
• Write one to the JTD bit in MCUCSR.
The TDO pin is left floating when the JTAG interface is enabled while the JTAG TAP controller is
not shifting data. If the hardware connected to the TDO pin does not pull up the logic level,
power consumption will increase. Note that the TDI pin for the next device in the scan chain contains a pull-up that avoids this problem. Writing the JTD bit in the MCUCSR register to one or
leaving the JTAG fuse unprogrammed disables the JTAG interface.
35
8154B–AVR–07/09
9.10
9.10.1
Register Description
MCUCR – MCU Control Register
The MCU Control Register contains control bits for power management.
Bit
7
6
5
4
3
2
1
0
SM2
SE
SM1
SM0
ISC11
ISC10
ISC01
ISC00
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
MCUCR
• Bits 7, 5, 4 – SM2:0: Sleep Mode Select Bits 2, 1, and 0
These bits select between the six available sleep modes as shown in Table 9-2.
Table 9-2.
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)
Note:
Sleep Mode
1. Standby mode and Extended Standby mode are only available with external crystals or
resonators.
• Bit 6 – 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 programmers
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.
36
ATmega16A
8154B–AVR–07/09
ATmega16A
10. System Control and Reset
10.1
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 10-1 shows the reset
logic. “System and Reset Characteristics” on page 296 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 CKSEL Fuses. The different selections for the delay period are presented in “Clock Sources” on page 25.
10.1.1
Reset Sources
The ATmega16A 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) Boundaryscan” on page 232 for details.
37
8154B–AVR–07/09
Figure 10-1. Reset Logic
DATA BUS
PORF
BORF
EXTRF
WDRF
JTRF
MCU Control and Status
Register (MCUCSR)
Power-on
Reset Circuit
INTERNAL RESET
Brown-out
Reset Circuit
BODEN
BODLEVEL
SPIKE
FILTER
Reset Circuit
JTAG Reset
Register
Watchdog
Timer
COUNTER RESET
Pull-up Resistor
Watchdog
Oscillator
Clock
Generator
CK
Delay Counters
TIMEOUT
CKSEL[3:0]
SUT[1:0]
10.1.2
Power-on Reset
A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level
is defined in “System and Reset Characteristics” on page 296. 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 10-2. MCU Start-up, RESET Tied to VCC.
VCC
RESET
TIME-OUT
VPOT
VRST
tTOUT
INTERNAL
RESET
38
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 10-3. MCU Start-up, RESET Extended Externally
VCC
RESET
VPOT
VRST
TIME-OUT
tTOUT
INTERNAL
RESET
10.1.3
External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the
minimum pulse width (see “System and Reset Characteristics” on page 296) 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 10-4. External Reset During Operation
CC
10.1.4
Brown-out Detection
ATmega16A 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 fuse BODLEVEL to be 2.7V (BODLEVEL unprogrammed), or 4.0V (BODLEVEL
programmed). 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.
The BOD circuit can be enabled/disabled by the fuse BODEN. When the BOD is enabled
(BODEN programmed), and VCC decreases to a value below the trigger level (VBOT- in Figure 105), the Brown-out Reset is immediately activated. When VCC increases above the trigger level
(VBOT+ in Figure 10-5), 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 “System and Reset Characteristics” on page 296.
39
8154B–AVR–07/09
Figure 10-5. Brown-out Reset During Operation
VCC
VBOT-
VBOT+
RESET
tTOUT
TIME-OUT
INTERNAL
RESET
10.1.5
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, refer to “Watchdog Timer” on page 41.
Figure 10-6. Watchdog Reset During Operation
CC
CK
10.2
Internal Voltage Reference
ATmega16A 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. The 2.56V reference to the ADC is generated from the internal bandgap reference.
10.2.1
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 “System and Reset Characteristics” on page 296. 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 BODEN 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
40
ATmega16A
8154B–AVR–07/09
ATmega16A
ADC is used. To reduce power 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.
10.3
Watchdog Timer
The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at 1 MHz. This is
the typical value at VCC = 5V. See characterization data for typical values at other VCC levels. By
controlling the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as
shown in Table 10-1 on page 43. The WDR – Watchdog Reset – instruction resets the Watchdog Timer. The Watchdog Timer is also reset when it is disabled and when a Chip Reset occurs.
Eight different clock cycle periods can be selected to determine the reset period. If the reset
period expires without another Watchdog Reset, the ATmega16A resets and executes from the
Reset Vector. For timing details on the Watchdog Reset, refer to page 40.
To prevent unintentional disabling of the Watchdog, a special turn-off sequence must be followed when the Watchdog is disabled. Refer to the description of the Watchdog Timer Control
Register for details.
Figure 10-7. Watchdog Timer
WATCHDOG
OSCILLATOR
10.4
10.4.1
Register Description
MCUCSR – MCU Control and Status Register
The MCU Control and Status Register provides information on which reset source caused an
MCU Reset.
Bit
7
6
5
4
3
2
1
0
JTD
ISC2
–
JTRF
WDRF
BORF
EXTRF
PORF
Read/Write
R/W
R/W
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
MCUCSR
See Bit Description
• 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.
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8154B–AVR–07/09
• 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 MCUCSR 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.
10.4.2
WDTCR – Watchdog Timer Control Register
Bit
7
6
5
4
3
2
1
0
–
–
–
WDTOE
WDE
WDP2
WDP1
WDP0
Read/Write
R
R
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
WDTCR
• Bits 7:5 – Res: Reserved Bits
These bits are reserved bits in the ATmega16A and will always read as zero.
• Bit 4 – WDTOE: Watchdog Turn-off Enable
This bit must be set when the WDE bit is written to logic zero. Otherwise, the Watchdog will not
be disabled. Once written to one, hardware will clear this bit after four clock cycles. Refer to the
description of the WDE bit for a Watchdog disable procedure.
• Bit 3 – WDE: Watchdog Enable
When the WDE is written to logic one, the Watchdog Timer is enabled, and if the WDE is written
to logic zero, the Watchdog Timer function is disabled. WDE can only be cleared if the WDTOE
bit has logic level one. To disable an enabled Watchdog Timer, the following procedure must be
followed:
1. In the same operation, write a logic one to WDTOE and WDE. A logic one must be written to WDE even though it is set to one before the disable operation starts.
2. Within the next four clock cycles, write a logic 0 to WDE. This disables the Watchdog.
• Bits 2:0 – WDP2, WDP1, WDP0: Watchdog Timer Prescaler 2, 1, and 0
The WDP2, WDP1, and WDP0 bits determine the Watchdog Timer prescaling when the Watchdog Timer is enabled. The different prescaling values and their corresponding Timeout Periods
are shown in Table 10-1.
42
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 10-1.
Watchdog Timer Prescale Select
WDP2
WDP1
WDP0
Number of WDT
Oscillator Cycles
Typical Time-out
at VCC = 3.0V
Typical Time-out
at VCC = 5.0V
0
0
0
16K (16,384)
17.1 ms
16.3 ms
0
0
1
32K (32,768)
34.3 ms
32.5 ms
0
1
0
64K (65,536)
68.5 ms
65 ms
0
1
1
128K (131,072)
0.14 s
0.13 s
1
0
0
256K (262,144)
0.27 s
0.26 s
1
0
1
512K (524,288)
0.55 s
0.52 s
1
1
0
1,024K (1,048,576)
1.1 s
1.0 s
1
1
1
2,048K (2,097,152)
2.2 s
2.1 s
The following code example shows one assembly and one C function for turning off the WDT.
The example assumes that interrupts are controlled (for example by disabling interrupts globally)
so that no interrupts will occur during execution of these functions.
Assembly Code Example
WDT_off:
; Reset WDT
WDR
; Write logical one to WDTOE and WDE
in
r16, WDTCR
ori r16, (1<<WDTOE)|(1<<WDE)
out WDTCR, r16
; Turn off WDT
ldi r16, (0<<WDE)
out WDTCR, r16
ret
C Code Example
void WDT_off(void)
{
/* Reset WDT*/
_WDR();
/* Write logical one to WDTOE and WDE */
WDTCR |= (1<<WDTOE) | (1<<WDE);
/* Turn off WDT */
WDTCR = 0x00;
}
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8154B–AVR–07/09
11. Interrupts
11.1
Overview
This section describes the specifics of the interrupt handling as performed in ATmega16A. For a
general explanation of the AVR interrupt handling, refer to “Reset and Interrupt Handling” on
page 14.
11.2
Interrupt Vectors
Table 11-1.
Reset and Interrupt Vectors
Vector No.
Program
Address(2)
Source
Interrupt Definition
1
$000(1)
RESET
External Pin, Power-on Reset, Brown-out Reset,
Watchdog Reset, and JTAG AVR Reset
2
$002
INT0
External Interrupt Request 0
3
$004
INT1
External Interrupt Request 1
4
$006
TIMER2 COMP
5
$008
TIMER2 OVF
6
$00A
TIMER1 CAPT
7
$00C
TIMER1 COMPA
Timer/Counter1 Compare Match A
8
$00E
TIMER1 COMPB
Timer/Counter1 Compare Match B
9
$010
TIMER1 OVF
Timer/Counter1 Overflow
10
$012
TIMER0 OVF
Timer/Counter0 Overflow
11
$014
SPI, STC
Serial Transfer Complete
12
$016
USART, RXC
13
$018
USART, UDRE
14
$01A
USART, TXC
15
$01C
ADC
16
$01E
EE_RDY
17
$020
ANA_COMP
18
$022
TWI
Two-wire Serial Interface
19
$024
INT2
External Interrupt Request 2
20
$026
TIMER0 COMP
21
$028
SPM_RDY
Notes:
Timer/Counter2 Compare Match
Timer/Counter2 Overflow
Timer/Counter1 Capture Event
USART, Rx Complete
USART Data Register Empty
USART, Tx Complete
ADC Conversion Complete
EEPROM Ready
Analog Comparator
Timer/Counter0 Compare Match
Store Program Memory Ready
1. When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader address at
reset, see “Boot Loader Support – Read-While-Write Self-Programming” on page 250.
2. When the IVSEL bit in GICR 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.
Table 11-2 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
44
ATmega16A
8154B–AVR–07/09
ATmega16A
the case if the Reset Vector is in the Application section while the Interrupt Vectors are in the
Boot section or vice versa.
Table 11-2.
Reset and Interrupt Vectors Placement(1)
BOOTRST
IVSEL
1
Note:
Reset address
Interrupt Vectors Start Address
0
$0000
$0002
1
1
$0000
Boot Reset Address + $0002
0
0
Boot Reset Address
$0002
0
1
Boot Reset Address
Boot Reset Address + $0002
1. The Boot Reset Address is shown in Table 25-6 on page 262. For the BOOTRST Fuse “1”
means unprogrammed while “0” means programmed.
The most typical and general program setup for the Reset and Interrupt Vector Addresses in
ATmega16A is:
Address Labels
Code
Comments
$000
jmp
RESET
; Reset Handler
$002
jmp
EXT_INT0
; IRQ0 Handler
$004
jmp
EXT_INT1
; IRQ1 Handler
$006
jmp
TIM2_COMP
; Timer2 Compare Handler
$008
jmp
TIM2_OVF
; Timer2 Overflow Handler
$00A
jmp
TIM1_CAPT
; Timer1 Capture Handler
$00C
jmp
TIM1_COMPA
; Timer1 CompareA Handler
$00E
jmp
TIM1_COMPB
; Timer1 CompareB Handler
$010
jmp
TIM1_OVF
; Timer1 Overflow Handler
$012
jmp
TIM0_OVF
; Timer0 Overflow Handler
$014
jmp
SPI_STC
; SPI Transfer Complete Handler
$016
jmp
USART_RXC
; USART RX Complete Handler
$018
jmp
USART_UDRE
; UDR Empty Handler
$01A
jmp
USART_TXC
; USART TX Complete Handler
$01C
jmp
ADC
; ADC Conversion Complete Handler
$01E
jmp
EE_RDY
; EEPROM Ready Handler
$020
jmp
ANA_COMP
; Analog Comparator Handler
$022
jmp
TWSI
; Two-wire Serial Interface Handler
$024
jmp
EXT_INT2
; IRQ2 Handler
$026
jmp
TIM0_COMP
; Timer0 Compare Handler
$028
jmp
SPM_RDY
; Store Program Memory Ready Handler
;
$02A
ldi
r16,high(RAMEND); Main program start
$02B
out
SPH,r16
$02C
ldi
r16,low(RAMEND)
$02D
out
SPL,r16
$02E
sei
$02F
<instr>
:.
RESET:
:.
; Set Stack Pointer to top of RAM
; Enable interrupts
xxx
:.
45
8154B–AVR–07/09
When the BOOTRST Fuse is unprogrammed, the Boot section size set to 2K bytes and the
IVSEL bit in the GICR Register is set before any interrupts are enabled, the most typical and
general program setup for the Reset and Interrupt Vector Addresses is:
Address Labels
$000
RESET:
$001
$002
Code
Comments
ldi
r16,high(RAMEND); Main program start
out
SPH,r16
ldi
; Set Stack Pointer to top of RAM
r16,low(RAMEND)
$003
out
SPL,r16
$004
sei
$005
<instr>
; Enable interrupts
xxx
;
.org $1C02
$1C02
jmp
EXT_INT0
; IRQ0 Handler
$1C04
jmp
EXT_INT1
; IRQ1 Handler
SPM_RDY
; Store Program Memory Ready Handler
:.
:..
$1C28
:
jmp
;
When the BOOTRST Fuse is programmed and the Boot section size set to 2K bytes, the most
typical and general program setup for the Reset and Interrupt Vector Addresses is:
Address Labels
Code
Comments
.org $002
$002
jmp
EXT_INT0
; IRQ0 Handler
$004
jmp
EXT_INT1
; IRQ1 Handler
jmp
SPM_RDY
; Store Program Memory Ready Handler
.org $1C00
$1C00
RESET:
ldi
r16,high(RAMEND); Main program start
$1C01
out
SPH,r16
$1C02
ldi
r16,low(RAMEND)
$1C03
out
SPL,r16
$1C04
sei
$1C05
<instr>
:.
:..
$028
:
;
;
; Set Stack Pointer to top of RAM
; Enable interrupts
xxx
When the BOOTRST Fuse is programmed, the Boot section size set to 2K bytes and the IVSEL
bit in the GICR Register is set before any interrupts are enabled, the most typical and general
program setup for the Reset and Interrupt Vector Addresses is:
Address Labels
Code
.org $1C00
$1C00
$1C02
jmp
jmp
RESET
EXT_INT0
; Reset handler
; IRQ0 Handler
$1C04
jmp
EXT_INT1
; IRQ1 Handler
jmp
SPM_RDY
; Store Program Memory Ready Handler
ldi
r16,high(RAMEND); Main program start
out
SPH,r16
:.
:..
$1C28
Comments
:
;
;
$1C2A
$1C2B
46
RESET:
; Set Stack Pointer to top of RAM
ATmega16A
8154B–AVR–07/09
ATmega16A
$1C2C
ldi
r16,low(RAMEND)
$1C2D
out
SPL,r16
$1C2E
sei
$1C2F
<instr>
; Enable interrupts
xxx
11.2.1
Moving Interrupts Between Application and Boot Space
The General Interrupt Control Register controls the placement of the Interrupt Vector table.
11.2.2
GICR – General Interrupt Control Register
Bit
7
6
5
4
3
2
1
0
INT1
INT0
INT2
–
–
–
IVSEL
IVCE
Read/Write
R/W
R/W
R/W
R
R
R
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
GICR
• 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 “Boot Loader Support – Read-While-Write
Self-Programming” on page 250 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 “Boot Loader Support – Read-WhileWrite Self-Programming” on page 250 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
47
8154B–AVR–07/09
.
Assembly Code Example
Move_interrupts:
; Enable change of interrupt vectors
ldi r16, (1<<IVCE)
out GICR, r16
; Move interrupts to boot Flash section
ldi r16, (1<<IVSEL)
out GICR, r16
ret
C Code Example
void Move_interrupts(void)
{
/* Enable change of interrupt vectors */
GICR = (1<<IVCE);
/* Move interrupts to boot Flash section */
GICR = (1<<IVSEL);
}
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ATmega16A
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ATmega16A
12. I/O Ports
12.1
Overview
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 12-1. Refer to “Electrical Characteristics” on page 293 for a complete list of parameters.
Figure 12-1. I/O Pin Equivalent Schematic
Rpu
Logic
Pxn
Cpin
See Figure 23
"General Digital I/O" for
Details
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. i.e., PORTB3
for bit no. 3 in Port B, here documented generally as PORTxn. The physical I/O Registers and
bit locations are listed in “Register Description” on page 65.
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.
In addition, the Pull-up Disable – PUD bit in SFIOR 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
50. 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 54. Refer to the individual module sections for a full description of the alternate functions.
49
8154B–AVR–07/09
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.
12.2
Ports as General Digital I/O
The ports are bi-directional I/O ports with optional internal pull-ups. Figure 12-2 shows a functional description of one I/O-port pin, here generically called Pxn.
Figure 12-2. General Digital I/O(1)
PUD
Q
D
DDxn
Q CLR
RESET
WDx
Q
Pxn
D
PORTxn
Q CLR
WPx
DATA BUS
RDx
RESET
RRx
SLEEP
SYNCHRONIZER
D
Q
L
Q
D
RPx
Q
PINxn
Q
clk I/O
PUD:
SLEEP:
clkI/O:
Note:
12.2.1
PULLUP DISABLE
SLEEP CONTROL
I/O CLOCK
WDx:
RDx:
WPx:
RRx:
RPx:
WRITE DDRx
READ DDRx
WRITE PORTx
READ PORTx REGISTER
READ PORTx PIN
1. 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.
Configuring the Pin
Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in “Register
Description” on page 65, 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 a reset condition becomes
active, even if no clocks are running.
50
ATmega16A
8154B–AVR–07/09
ATmega16A
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).
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) must occur. Normally, the pull-up enabled state is fully acceptable, as a high-impedant environment will not notice the difference between a strong high driver
and a pull-up. If this is not the case, the PUD bit in the SFIOR 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 12-1 summarizes the control signals for the pin value.
Table 12-1.
12.2.2
Port Pin Configurations
DDxn
PORTxn
PUD
(in SFIOR)
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
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 12-2, 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 12-3 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.
51
8154B–AVR–07/09
Figure 12-3. Synchronization when Reading an Externally Applied Pin Value
SYSTEM CLK
INSTRUCTIONS
XXX
in r17, PINx
XXX
SYNC LATCH
PINxn
r17
0xFF
0x00
tpd, max
tpd, 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.
When reading back a software assigned pin value, a nop instruction must be inserted as indicated in Figure 12-4. 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 12-4. 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
tpd
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
52
ATmega16A
8154B–AVR–07/09
ATmega16A
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.
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(1)
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*/
_NOP();
/* Read port pins */
i = PINB;
:.
Note:
12.2.3
1. For the assembly program, two temporary registers are used to minimize the time from pullups 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.
Digital Input Enable and Sleep Modes
As shown in Figure 12-2, the digital input signal can be clamped to ground at the input of the
schmitt-trigger. The signal denoted SLEEP in the figure, is set by the MCU Sleep Controller in
Power-down mode, Power-save mode, Standby mode, and Extended 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 54.
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 modes, as the clamping in these sleep modes produces the requested
logic change.
53
8154B–AVR–07/09
12.2.4
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 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.
12.3
Alternate Port Functions
Most port pins have alternate functions in addition to being General Digital I/Os. Figure 12-5
shows how the port pin control signals from the simplified Figure 12-2 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 12-5. Alternate Port Functions(1)
PUOExn
PUOVxn
1
PUD
0
DDOExn
DDOVxn
1
Q D
DDxn
0
Q CLR
WDx
PVOExn
RESET
1
Pxn
Q
0
D
PORTxn
Q CLR
DIEOExn
WPx
DATA BUS
RDx
PVOVxn
RESET
DIEOVxn
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:
Note:
54
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
PUD:
WDx:
RDx:
RRx:
WPx:
RPx:
clkI/O:
DIxn:
AIOxn:
PULLUP DISABLE
WRITE DDRx
READ DDRx
READ PORTx REGISTER
WRITE PORTx
READ PORTx PIN
I/O CLOCK
DIGITAL INPUT PIN n ON PORTx
ANALOG INPUT/OUTPUT PIN n ON PORTx
1. 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.
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 12-2 summarizes the function of the overriding signals. The pin and port indexes from Figure 12-5 are not shown in the succeeding tables. The overriding signals are generated internally
in the modules having the alternate function.
Table 12-2.
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.
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
modes).
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 modes).
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
bi-directionally.
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.
12.3.1
Alternate Functions of Port A
Port A has an alternate function as analog input for the ADC as shown in Table 12-3. If some
Port A pins are configured as outputs, it is essential that these do not switch when a conversion
is in progress. This might corrupt the result of the conversion.
55
8154B–AVR–07/09
Table 12-3.
Port Pin
Port A Pins Alternate Functions
Alternate Function
PA7
ADC7 (ADC input channel 7)
PA6
ADC6 (ADC input channel 6)
PA5
ADC5 (ADC input channel 5)
PA4
ADC4 (ADC input channel 4)
PA3
ADC3 (ADC input channel 3)
PA2
ADC2 (ADC input channel 2)
PA1
ADC1 (ADC input channel 1)
PA0
ADC0 (ADC input channel 0)
Table 12-4 and Table 12-5 relate the alternate functions of Port A to the overriding signals
shown in Figure 12-5 on page 54.
Table 12-4.
Signal Name
PA7/ADC7
PA6/ADC6
PA5/ADC5
PA4/ADC4
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
0
0
0
0
DDOV
0
0
0
0
PVOE
0
0
0
0
PVOV
0
0
0
0
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
–
–
–
–
ADC7 INPUT
ADC6 INPUT
ADC5 INPUT
ADC4 INPUT
AIO
56
Overriding Signals for Alternate Functions in PA7:PA4
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 12-5.
Signal Name
PA3/ADC3
PA2/ADC2
PA1/ADC1
PA0/ADC0
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
0
0
0
0
DDOV
0
0
0
0
PVOE
0
0
0
0
PVOV
0
0
0
0
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
–
–
–
–
ADC3 INPUT
ADC2 INPUT
ADC1 INPUT
ADC0 INPUT
AIO
12.3.2
Overriding Signals for Alternate Functions in PA3:PA0
Alternate Functions of Port B
The Port B pins with alternate functions are shown in Table 12-6.
Table 12-6.
Port Pin
Port B Pins Alternate Functions
Alternate Functions
PB7
SCK (SPI Bus Serial Clock)
PB6
MISO (SPI Bus Master Input/Slave Output)
PB5
MOSI (SPI Bus Master Output/Slave Input)
PB4
SS (SPI Slave Select Input)
PB3
AIN1 (Analog Comparator Negative Input)
OC0 (Timer/Counter0 Output Compare Match Output)
PB2
AIN0 (Analog Comparator Positive Input)
INT2 (External Interrupt 2 Input)
PB1
T1 (Timer/Counter1 External Counter Input)
PB0
T0 (Timer/Counter0 External Counter Input)
XCK (USART External Clock Input/Output)
The alternate pin configuration is as follows:
• SCK – Port B, Bit 7
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 DDB7. When the SPI is
enabled as a Master, the data direction of this pin is controlled by DDB7. When the pin is forced
by the SPI to be an input, the pull-up can still be controlled by the PORTB7 bit.
• MISO – Port B, Bit 6
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 DDB6. When the SPI is
enabled as a Slave, the data direction of this pin is controlled by DDB6. When the pin is forced
by the SPI to be an input, the pull-up can still be controlled by the PORTB6 bit.
57
8154B–AVR–07/09
• MOSI – Port B, Bit 5
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 DDB5. When the SPI is
enabled as a Master, the data direction of this pin is controlled by DDB5. When the pin is forced
by the SPI to be an input, the pull-up can still be controlled by the PORTB5 bit.
• SS – Port B, Bit 4
SS: Slave Select input. When the SPI is enabled as a Slave, this pin is configured as an input
regardless of the setting of DDB4. 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 DDB4. When
the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB4 bit.
• AIN1/OC0 – Port B, Bit 3
AIN1, Analog Comparator Negative Input. Configure the port pin as input with the internal pull-up
switched off to avoid the digital port function from interfering with the function of the analog
comparator.
OC0, Output Compare Match output: The PB3 pin can serve as an external output for the
Timer/Counter0 Compare Match. The PB3 pin has to be configured as an output (DDB3 set
(one)) to serve this function. The OC0 pin is also the output pin for the PWM mode timer
function.
• AIN0/INT2 – Port B, Bit 2
AIN0, Analog Comparator Positive input. Configure the port pin as input with the internal pull-up
switched off to avoid the digital port function from interfering with the function of the Analog
Comparator.
INT2, External Interrupt Source 2: The PB2 pin can serve as an external interrupt source to the
MCU.
• T1 – Port B, Bit 1
T1, Timer/Counter1 Counter Source.
• T0/XCK – Port B, Bit 0
T0, Timer/Counter0 Counter Source.
XCK, USART External Clock. The Data Direction Register (DDB0) controls whether the clock is
output (DDB0 set) or input (DDB0 cleared). The XCK pin is active only when the USART operates in Synchronous mode.
Table 12-7 and Table 12-8 relate the alternate functions of Port B to the overriding signals
shown in Figure 12-5 on page 54. SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the
MISO signal, while MOSI is divided into SPI MSTR OUTPUT and SPI SLAVE INPUT.
58
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 12-7.
Overriding Signals for Alternate Functions in PB7:PB4
Signal
Name
PB7/SCK
PB6/MISO
PB5/MOSI
PB4/SS
PUOE
SPE • MSTR
SPE • MSTR
SPE • MSTR
SPE • MSTR
PUOV
PORTB7 • PUD
PORTB6 • PUD
PORTB5 • PUD
PORTB4 • PUD
DDOE
SPE • MSTR
SPE • MSTR
SPE • MSTR
SPE • MSTR
DDOV
0
0
0
0
PVOE
SPE • MSTR
SPE • MSTR
SPE • MSTR
0
PVOV
SCK OUTPUT
SPI SLAVE OUTPUT
SPI MSTR OUTPUT
0
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
SCK INPUT
SPI MSTR INPUT
SPI SLAVE INPUT
SPI SS
AIO
–
–
–
–
Table 12-8.
Overriding Signals for Alternate Functions in PB3:PB0
Signal
Name
PB3/OC0/AIN1
PB2/INT2/AIN0
PB1/T1
PB0/T0/XCK
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
0
0
0
0
DDOV
0
0
0
0
PVOE
OC0 ENABLE
0
0
UMSEL
PVOV
OC0
0
0
XCK OUTPUT
DIEOE
0
INT2 ENABLE
0
0
DIEOV
0
1
0
0
DI
–
INT2 INPUT
T1 INPUT
XCK INPUT/T0 INPUT
AIO
AIN1 INPUT
AIN0 INPUT
–
–
59
8154B–AVR–07/09
12.3.3
Alternate Functions of Port C
The Port C pins with alternate functions are shown in Table 12-9. If the JTAG interface is
enabled, the pull-up resistors on pins PC5(TDI), PC3(TMS) and PC2(TCK) will be activated
even if a reset occurs.
Table 12-9.
Port Pin
Port C Pins Alternate Functions
Alternate Function
PC7
TOSC2 (Timer Oscillator Pin 2)
PC6
TOSC1 (Timer Oscillator Pin 1)
PC5
TDI (JTAG Test Data In)
PC4
TDO (JTAG Test Data Out)
PC3
TMS (JTAG Test Mode Select)
PC2
TCK (JTAG Test Clock)
PC1
SDA (Two-wire Serial Bus Data Input/Output Line)
PC0
SCL (Two-wire Serial Bus Clock Line)
The alternate pin configuration is as follows:
• TOSC2 – Port C, Bit 7
TOSC2, Timer Oscillator pin 2: When the AS2 bit in ASSR is set (one) to enable asynchronous
clocking of Timer/Counter2, pin PC7 is disconnected from the port, and becomes the inverting
output of the Oscillator amplifier. In this mode, a Crystal Oscillator is connected to this pin, and
the pin can not be used as an I/O pin.
• TOSC1 – Port C, Bit 6
TOSC1, Timer Oscillator pin 1: When the AS2 bit in ASSR is set (one) to enable asynchronous
clocking of Timer/Counter2, pin PC6 is disconnected from the port, and becomes the input of the
inverting Oscillator amplifier. In this mode, a Crystal Oscillator is connected to this pin, and the
pin can not be used as an I/O pin.
• TDI – Port C, Bit 5
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 – Port C, Bit 4
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 TD0 pin is tri-stated unless TAP states that shifts out data are entered.
• TMS – Port C, Bit 3
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 – Port C, Bit 2
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.
60
ATmega16A
8154B–AVR–07/09
ATmega16A
• SDA – Port C, Bit 1
SDA, Two-wire Serial Interface Data: When the TWEN bit in TWCR is set (one) to enable the
Two-wire Serial Interface, pin PC1 is disconnected from the port and becomes the Serial Data
I/O pin for the Two-wire Serial Interface. In this mode, there is a spike filter on the pin to suppress spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver
with slew-rate limitation. When this pin is used by the Two-wire Serial Interface, the pull-up can
still be controlled by the PORTC1 bit.
• SCL – Port C, Bit 0
SCL, Two-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to enable the
Two-wire Serial Interface, pin PC0 is disconnected from the port and becomes the Serial Clock
I/O pin for the Two-wire Serial Interface. In this mode, there is a spike filter on the pin to suppress spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver
with slew-rate limitation. When this pin is used by the Two-wire Serial Interface, the pull-up can
still be controlled by the PORTC0 bit.
Table 12-10 and Table 12-11 relate the alternate functions of Port C to the overriding signals
shown in Figure 12-5 on page 54.
Table 12-10. Overriding Signals for Alternate Functions in PC7:PC4
Signal
Name
PC7/TOSC2
PC6/TOSC1
PC5/TDI
PC4/TDO
PUOE
AS2
AS2
JTAGEN
JTAGEN
PUOV
0
0
1
0
DDOE
AS2
AS2
JTAGEN
JTAGEN
DDOV
0
0
0
SHIFT_IR + SHIFT_DR
PVOE
0
0
0
JTAGEN
PVOV
0
0
0
TDO
DIEOE
AS2
AS2
JTAGEN
JTAGEN
DIEOV
0
0
0
0
DI
–
–
–
–
AIO
T/C2 OSC OUTPUT
T/C2 OSC INPUT
TDI
–
61
8154B–AVR–07/09
Table 12-11. Overriding Signals for Alternate Functions in PC3:PC0(1)
Signal
Name
PC3/TMS
PC2/TCK
PC1/SDA
PC0/SCL
PUOE
JTAGEN
JTAGEN
TWEN
TWEN
PUOV
1
1
PORTC1 • PUD
PORTC0 • PUD
DDOE
JTAGEN
JTAGEN
TWEN
TWEN
DDOV
0
0
SDA_OUT
SCL_OUT
PVOE
0
0
TWEN
TWEN
PVOV
0
0
0
0
DIEOE
JTAGEN
JTAGEN
0
0
DIEOV
0
0
0
0
DI
–
–
–
–
AIO
TMS
TCK
SDA INPUT
SCL INPUT
Note:
12.3.4
1. When enabled, the Two-wire Serial Interface enables slew-rate controls on the output pins
PC0 and PC1. This is not shown in the figure. In addition, spike filters are connected between
the AIO outputs shown in the port figure and the digital logic of the TWI module.
Alternate Functions of Port D
The Port D pins with alternate functions are shown in Table 12-12.
Table 12-12. Port D Pins Alternate Functions
Port Pin
Alternate Function
PD7
OC2 (Timer/Counter2 Output Compare Match Output)
PD6
ICP1 (Timer/Counter1 Input Capture Pin)
PD5
OC1A (Timer/Counter1 Output Compare A Match Output)
PD4
OC1B (Timer/Counter1 Output Compare B Match Output)
PD3
INT1 (External Interrupt 1 Input)
PD2
INT0 (External Interrupt 0 Input)
PD1
TXD (USART Output Pin)
PD0
RXD (USART Input Pin)
The alternate pin configuration is as follows:
• OC2 – Port D, Bit 7
OC2, Timer/Counter2 Output Compare Match output: The PD7 pin can serve as an external output for the Timer/Counter2 Output Compare. The pin has to be configured as an output (DDD7
set (one)) to serve this function. The OC2 pin is also the output pin for the PWM mode timer
function.
• ICP1 – Port D, Bit 6
ICP1 – Input Capture Pin: The PD6 pin can act as an Input Capture pin for Timer/Counter1.
• OC1A – Port D, Bit 5
62
ATmega16A
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ATmega16A
OC1A, Output Compare Match A output: The PD5 pin can serve as an external output for the
Timer/Counter1 Output Compare A. The pin has to be configured as an output (DDD5 set (one))
to serve this function. The OC1A pin is also the output pin for the PWM mode timer function.
• OC1B – Port D, Bit 4
OC1B, Output Compare Match B output: The PD4 pin can serve as an external output for the
Timer/Counter1 Output Compare B. The pin has to be configured as an output (DDD4 set (one))
to serve this function. The OC1B pin is also the output pin for the PWM mode timer function.
• INT1 – Port D, Bit 3
INT1, External Interrupt Source 1: The PD3 pin can serve as an external interrupt source.
• INT0 – Port D, Bit 2
INT0, External Interrupt Source 0: The PD2 pin can serve as an external interrupt source.
• TXD – Port D, Bit 1
TXD, Transmit Data (Data output pin for the USART). When the USART Transmitter is enabled,
this pin is configured as an output regardless of the value of DDD1.
• RXD – Port D, Bit 0
RXD, Receive Data (Data input pin for the USART). When the USART Receiver is enabled this
pin is configured as an input regardless of the value of DDD0. When the USART forces this pin
to be an input, the pull-up can still be controlled by the PORTD0 bit.
Table 12-13 and Table 12-14 relate the alternate functions of Port D to the overriding signals
shown in Figure 12-5 on page 54.
Table 12-13. Overiding Signals for Alternate Functions PD7:PD4
Signal Name
PD7/OC2
PD6/ICP1
PD5/OC1A
PD4/OC1B
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
0
0
0
0
DDOV
0
0
0
0
PVOE
OC2 ENABLE
0
OC1A ENABLE
OC1B ENABLE
PVOV
OC2
0
OC1A
OC1B
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
–
ICP1 INPUT
–
–
AIO
–
–
–
–
63
8154B–AVR–07/09
Table 12-14. Overriding Signals for Alternate Functions in PD3:PD0
64
Signal Name
PD3/INT1
PD2/INT0
PD1/TXD
PD0/RXD
PUOE
0
0
TXEN
RXEN
PUOV
0
0
0
PORTD0 • PUD
DDOE
0
0
TXEN
RXEN
DDOV
0
0
1
0
PVOE
0
0
TXEN
0
PVOV
0
0
TXD
0
DIEOE
INT1 ENABLE
INT0 ENABLE
0
0
DIEOV
1
1
0
0
DI
INT1 INPUT
INT0 INPUT
–
RXD
AIO
–
–
–
–
ATmega16A
8154B–AVR–07/09
ATmega16A
12.4
12.4.1
Register Description
SFIOR – Special Function I/O Register
Bit
7
6
5
4
3
2
1
0
ADTS2
ADTS1
ADTS0
–
ACME
PUD
PSR2
PSR10
Read/Write
R/W
R/W
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
SFIOR
• Bit 2 – 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 50 for more details about this feature.
12.4.2
PORTA – Port A Data Register
Bit
12.4.3
7
6
5
4
3
2
1
0
PORTA7
PORTA6
PORTA5
PORTA4
PORTA3
PORTA2
PORTA1
PORTA0
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
DDRA – Port A Data Direction Register
Bit
12.4.4
7
6
5
4
3
2
1
0
DDA7
DDA6
DDA5
DDA4
DDA3
DDA2
DDA1
DDA0
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
7
6
5
4
3
2
1
0
PINA7
PINA6
PINA5
PINA4
PINA3
PINA2
PINA1
PINA0
Read/Write
R
R
R
R
R
R
R
R
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PINA
PORTB – Port B Data Register
Bit
12.4.6
DDRA
PINA – Port A Input Pins Address
Bit
12.4.5
PORTA
7
6
5
4
3
2
1
0
PORTB7
PORTB6
PORTB5
PORTB4
PORTB3
PORTB2
PORTB1
PORTB0
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
PORTB
DDRB – Port B Data Direction Register
Bit
7
6
5
4
3
2
1
0
DDB7
DDB6
DDB5
DDB4
DDB3
DDB2
DDB1
DDB0
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
DDRB
65
8154B–AVR–07/09
12.4.7
PINB – Port B Input Pins Address
Bit
12.4.8
7
6
5
4
3
2
1
0
PINB7
PINB6
PINB5
PINB4
PINB3
PINB2
PINB1
PINB0
Read/Write
R
R
R
R
R
R
R
R
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PORTC – Port C Data Register
Bit
12.4.9
7
6
5
4
3
2
1
0
PORTC7
PORTC6
PORTC5
PORTC4
PORTC3
PORTC2
PORTC1
PORTC0
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
7
6
5
4
3
2
1
0
DDC7
DDC6
DDC5
DDC4
DDC3
DDC2
DDC1
DDC0
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
7
6
5
4
3
2
1
0
PINC7
PINC6
PINC5
PINC4
PINC3
PINC2
PINC1
PINC0
Read/Write
R
R
R
R
R
R
R
R
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
7
6
5
4
3
2
1
0
PORTD7
PORTD6
PORTD5
PORTD4
PORTD3
PORTD2
PORTD1
PORTD0
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
7
6
5
4
3
2
1
0
DDD7
DDD6
DDD5
DDD4
DDD3
DDD2
DDD1
DDD0
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
DDRD
PIND – Port D Input Pins Address
Bit
66
PORTD
DDRD – Port D Data Direction Register
Bit
12.4.13
PINC
PORTD – Port D Data Register
Bit
12.4.12
DDRC
PINC – Port C Input Pins Address
Bit
12.4.11
PORTC
DDRC – Port C Data Direction Register
Bit
12.4.10
PINB
7
6
5
4
3
2
1
0
PIND7
PIND6
PIND5
PIND4
PIND3
PIND2
PIND1
PIND0
Read/Write
R
R
R
R
R
R
R
R
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PIND
ATmega16A
8154B–AVR–07/09
ATmega16A
13. External Interrupts
The External Interrupts are triggered by the INT0, INT1, and INT2 pins. Observe that, if enabled,
the interrupts will trigger even if the INT0:2 pins are configured as outputs. This feature provides
a way of generating a software interrupt. The external interrupts can be triggered by a falling or
rising edge or a low level (INT2 is only an edge triggered interrupt). This is set up as indicated in
the specification for the MCU Control Register – MCUCR – and MCU Control and Status Register – MCUCSR. When the external interrupt is enabled and is configured as level triggered (only
INT0/INT1), the interrupt will trigger as long as the pin is held low. Note that recognition of falling
or rising edge interrupts on INT0 and INT1 requires the presence of an I/O clock, described in
“Clock Systems and their Distribution” on page 24. Low level interrupts on INT0/INT1 and the
edge interrupt on INT2 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 mode, the changed
level must be held for some time to wake up the MCU. This makes the MCU less sensitive to
noise. The changed level is sampled twice by the Watchdog Oscillator clock. The period of the
Watchdog Oscillator is 1 µs (nominal) at 5.0V and 25°C. The frequency of the Watchdog Oscillator is voltage dependent as shown in “Electrical Characteristics” on page 293. The MCU will
wake up if the input has the required level during this sampling or if it is held until the end of the
start-up time. The start-up time is defined by the SUT Fuses as described in “System Clock and
Clock Options” on page 24. If the level is sampled twice by the Watchdog Oscillator clock but
disappears before the end of the start-up time, the MCU will still wake up, but no interrupt will be
generated. The required level must be held long enough for the MCU to complete the wake up to
trigger the level interrupt.
13.1
13.1.1
Register Description
MCUCR – MCU Control Register
The MCU Control Register contains control bits for interrupt sense control and general MCU
functions.
Bit
7
6
5
4
3
2
1
0
SM2
SE
SM1
SM0
ISC11
ISC10
ISC01
ISC00
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
MCUCR
• Bit 3, 2 – ISC11, ISC10: Interrupt Sense Control 1 Bit 1 and Bit 0
The External Interrupt 1 is activated by the external pin INT1 if the SREG I-bit and the corresponding interrupt mask in the GICR are set. The level and edges on the external INT1 pin that
activate the interrupt are defined in Table 13-1. The value on the INT1 pin is sampled before
detecting edges. If edge or toggle interrupt is selected, pulses that last longer than one clock
period will generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt. If
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low level interrupt is selected, the low level must be held until the completion of the currently
executing instruction to generate an interrupt.
Table 13-1.
Interrupt 1 Sense Control
ISC11
ISC10
Description
0
0
The low level of INT1 generates an interrupt request.
0
1
Any logical change on INT1 generates an interrupt request.
1
0
The falling edge of INT1 generates an interrupt request.
1
1
The rising edge of INT1 generates an interrupt request.
• Bit 1, 0 – ISC01, ISC00: Interrupt Sense Control 0 Bit 1 and Bit 0
The External Interrupt 0 is activated by the external pin INT0 if the SREG I-flag and the corresponding interrupt mask are set. The level and edges on the external INT0 pin that activate the
interrupt are defined in Table 13-2. The value on the INT0 pin is sampled before detecting
edges. If edge or toggle interrupt is selected, pulses that last longer than one clock period 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.
Table 13-2.
13.1.2
Interrupt 0 Sense Control
ISC01
ISC00
Description
0
0
The low level of INT0 generates an interrupt request.
0
1
Any logical change on INT0 generates an interrupt request.
1
0
The falling edge of INT0 generates an interrupt request.
1
1
The rising edge of INT0 generates an interrupt request.
MCUCSR – MCU Control and Status Register
Bit
7
6
5
4
3
2
1
0
JTD
ISC2
–
JTRF
WDRF
BORF
EXTRF
PORF
Read/Write
R/W
R/W
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
MCUCSR
See Bit Description
• Bit 6 – ISC2: Interrupt Sense Control 2
The Asynchronous External Interrupt 2 is activated by the external pin INT2 if the SREG I-bit and
the corresponding interrupt mask in GICR are set. If ISC2 is written to zero, a falling edge on
INT2 activates the interrupt. If ISC2 is written to one, a rising edge on INT2 activates the interrupt. Edges on INT2 are registered asynchronously. Pulses on INT2 wider than the minimum
pulse width given in “External Interrupts Characteristics” on page 297 will generate an interrupt.
Shorter pulses are not guaranteed to generate an interrupt. When changing the ISC2 bit, an
interrupt can occur. Therefore, it is recommended to first disable INT2 by clearing its Interrupt
Enable bit in the GICR Register. Then, the ISC2 bit can be changed. Finally, the INT2 Interrupt
Flag should be cleared by writing a logical one to its Interrupt Flag bit (INTF2) in the GIFR Register before the interrupt is re-enabled.
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ATmega16A
13.1.3
GICR – General Interrupt Control Register
Bit
7
6
5
4
3
2
1
0
INT1
INT0
INT2
–
–
–
IVSEL
IVCE
Read/Write
R/W
R/W
R/W
R
R
R
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
GICR
• Bit 7 – INT1: External Interrupt Request 1 Enable
When the INT1 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the external pin interrupt is enabled. The Interrupt Sense Control1 bits 1/0 (ISC11 and ISC10) in the MCU
General Control Register (MCUCR) define whether the External Interrupt is activated on rising
and/or falling edge of the INT1 pin or level sensed. Activity on the pin will cause an interrupt
request even if INT1 is configured as an output. The corresponding interrupt of External Interrupt
Request 1 is executed from the INT1 interrupt Vector.
• Bit 6 – INT0: External Interrupt Request 0 Enable
When the INT0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the external pin interrupt is enabled. The Interrupt Sense Control0 bits 1/0 (ISC01 and ISC00) in the MCU
General Control Register (MCUCR) define whether the External Interrupt is activated on rising
and/or falling edge of the INT0 pin or level sensed. Activity on the pin will cause an interrupt
request even if INT0 is configured as an output. The corresponding interrupt of External Interrupt
Request 0 is executed from the INT0 interrupt vector.
• Bit 5 – INT2: External Interrupt Request 2 Enable
When the INT2 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the external pin interrupt is enabled. The Interrupt Sense Control2 bit (ISC2) in the MCU Control and
Status Register (MCUCSR) defines whether the External Interrupt is activated on rising or falling
edge of the INT2 pin. Activity on the pin will cause an interrupt request even if INT2 is configured
as an output. The corresponding interrupt of External Interrupt Request 2 is executed from the
INT2 Interrupt Vector.
13.1.4
GIFR – General Interrupt Flag Register
Bit
7
6
5
4
3
2
1
INTF1
INTF0
INTF2
–
–
–
–
0
–
Read/Write
R/W
R/W
R/W
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
GIFR
• Bit 7 – INTF1: External Interrupt Flag 1
When an edge or logic change on the INT1 pin triggers an interrupt request, INTF1 becomes set
(one). If the I-bit in SREG and the INT1 bit in GICR 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. This flag is always cleared
when INT1 is configured as a level interrupt.
• Bit 6 – INTF0: External Interrupt Flag 0
When an edge or logic change on the INT0 pin triggers an interrupt request, INTF0 becomes set
(one). If the I-bit in SREG and the INT0 bit in GICR 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. This flag is always cleared
when INT0 is configured as a level interrupt.
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• Bit 5 – INTF2: External Interrupt Flag 2
When an event on the INT2 pin triggers an interrupt request, INTF2 becomes set (one). If the Ibit in SREG and the INT2 bit in GICR 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. Note that when entering some sleep modes with the
INT2 interrupt disabled, the input buffer on this pin will be disabled. This may cause a logic
change in internal signals which will set the INTF2 Flag. See “Digital Input Enable and Sleep
Modes” on page 53 for more information.
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ATmega16A
14. 8-bit Timer/Counter0 with PWM
14.1
Features
•
•
•
•
•
•
•
14.2
Single Compare Unit Counter
Clear Timer on Compare Match (Auto Reload)
Glitch-free, Phase Correct Pulse Width Modulator (PWM)
Frequency Generator
External Event Counter
10-bit Clock Prescaler
Overflow and Compare Match Interrupt Sources (TOV0 and OCF0)
Overview
Timer/Counter0 is a general purpose, single compare unit, 8-bit Timer/Counter module. A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 14-1. For the actual placement
of I/O pins, refer to “Pinout ATmega16A” on page 2. 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 “Register Description” on page 82.
Figure 14-1. 8-bit Timer/Counter Block Diagram
TCCRn
count
TOVn
(Int.Req.)
clear
Control Logic
direction
clk Tn
Clock Select
Edge
Detector
DATABUS
BOTTOM
Tn
TOP
( From Prescaler )
Timer/Counter
TCNTn
=
=0
= 0xFF
OCn
(Int.Req.)
Waveform
Generation
OCn
OCRn
14.2.1
Registers
The Timer/Counter (TCNT0) and Output Compare Register (OCR0) are 8-bit registers. Interrupt
request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag
Register (TIFR). All interrupts are individually masked with the Timer Interrupt Mask Register
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(TIMSK). TIFR and TIMSK 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 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).
The double buffered Output Compare Register (OCR0) is 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 Pin (OC0). See “Output Compare
Unit” on page 73. for details. The compare match event will also set the Compare Flag (OCF0)
which can be used to generate an output compare interrupt request.
14.2.2
Definitions
Many register and bit references in this document are written in general form. A lower case “n”
replaces the Timer/Counter number, in this case 0. However, when using the register or bit
defines in a program, the precise form must be used i.e., TCNT0 for accessing Timer/Counter0
counter value and so on.
The definitions in Table 14-1 are also used extensively throughout the document.
Table 14-1.
14.3
Definitions
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 OCR0 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 (TCCR0). For details on clock sources and prescaler, see
“Timer/Counter0 and Timer/Counter1 Prescalers” on page 86.
14.4
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
14-2 shows a block diagram of the counter and its surroundings.
Figure 14-2. Counter Unit Block Diagram
TOVn
(Int. Req.)
DATA BUS
Clock Select
count
TCNTn
clear
Control Logic
clkTn
Edge
Detector
Tn
direction
( From Prescaler )
BOTTOM
72
TOP
ATmega16A
8154B–AVR–07/09
ATmega16A
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.
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 (TCCR0). There are close connections between how the
counter behaves (counts) and how waveforms are generated on the Output Compare output
OC0. For more details about advanced counting sequences and waveform generation, see
“Modes of Operation” on page 76.
The Timer/Counter Overflow (TOV0) Flag is set according to the mode of operation selected by
the WGM01:0 bits. TOV0 can be used for generating a CPU interrupt.
14.5
Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Register
(OCR0). Whenever TCNT0 equals OCR0, the comparator signals a match. A match will set the
Output Compare Flag (OCF0) at the next timer clock cycle. If enabled (OCIE0 = 1 and Global
Interrupt Flag in SREG is set), the Output Compare Flag generates an output compare interrupt.
The OCF0 Flag is automatically cleared when the interrupt is executed. Alternatively, the OCF0
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
WGM01:0 bits and Compare Output mode (COM01:0) 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 (See “Modes of Operation” on page 76.).
Figure 14-3 shows a block diagram of the output compare unit.
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Figure 14-3. Output Compare Unit, Block Diagram
DATA BUS
OCRn
TCNTn
= (8-bit Comparator )
OCFn (Int.Req.)
top
bottom
Waveform Generator
OCn
FOCn
WGMn1:0
COMn1:0
The OCR0 Register is 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 OCR0 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 OCR0 Register access may seem complex, but this is not case. When the double buffering
is enabled, the CPU has access to the OCR0 Buffer Register, and if double buffering is disabled
the CPU will access the OCR0 directly.
14.5.1
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 (FOC0) bit. Forcing compare match will not set the
OCF0 Flag or reload/clear the timer, but the OC0 pin will be updated as if a real compare match
had occurred (the COM01:0 bits settings define whether the OC0 pin is set, cleared or toggled).
14.5.2
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 OCR0 to be initialized
to the same value as TCNT0 without triggering an interrupt when the Timer/Counter clock is
enabled.
14.5.3
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
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ATmega16A
the OCR0 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 downcounting.
The setup of the OC0 should be performed before setting the Data Direction Register for the port
pin to output. The easiest way of setting the OC0 value is to use the Force Output Compare
(FOC0) strobe bits in Normal mode. The OC0 Register keeps its value even when changing
between waveform generation modes.
Be aware that the COM01:0 bits are not double buffered together with the compare value.
Changing the COM01:0 bits will take effect immediately.
14.6
Compare Match Output Unit
The Compare Output mode (COM01:0) bits have two functions. The Waveform Generator uses
the COM01:0 bits for defining the Output Compare (OC0) state at the next compare match. Also,
the COM01:0 bits control the OC0 pin output source. Figure 14-4 shows a simplified schematic
of the logic affected by the COM01:0 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 COM01:0 bits are shown. When referring to the OC0 state, the
reference is for the internal OC0 Register, not the OC0 pin. If a System Reset occur, the OC0
Register is reset to “0”.
Figure 14-4. Compare Match Output Unit, Schematic
COMn1
COMn0
FOCn
Waveform
Generator
D
Q
1
OCn
DATA BUS
D
0
OCn
Pin
Q
PORT
D
Q
DDR
clk I/O
The general I/O port function is overridden by the Output Compare (OC0) from the Waveform
Generator if either of the COM01:0 bits are set. However, the OC0 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 OC0 pin (DDR_OC0) must be set as output before the OC0 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 OC0 state before the output is enabled. Note that some COM01:0 bit settings are reserved for certain modes of
operation. See “Register Description” on page 82.
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14.6.1
Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM01:0 bits differently in normal, CTC, and PWM modes.
For all modes, setting the COM01:0 = 0 tells the waveform generator that no action on the OC0
Register is to be performed on the next compare match. For compare output actions in the nonPWM modes refer to Table 14-3 on page 83. For fast PWM mode, refer to Table 14-4 on page
83, and for phase correct PWM refer to Table 14-5 on page 84.
A change of the COM01:0 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
FOC0 strobe bits.
14.7
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 (WGM01:0) and Compare Output
mode (COM01:0) bits. The Compare Output mode bits do not affect the counting sequence,
while the Waveform Generation mode bits do. The COM01:0 bits control whether the PWM output generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes
the COM01:0 bits control whether the output should be set, cleared, or toggled at a compare
match (See “Compare Match Output Unit” on page 75.).
For detailed timing information refer to Figure 14-8, Figure 14-9, Figure 14-10 and Figure 14-11
in “Timer/Counter Timing Diagrams” on page 80.
14.7.1
Normal Mode
The simplest mode of operation is the normal mode (WGM01:0 = 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 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.
14.7.2
Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM01:0 = 2), the OCR0 Register is used to manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value
(TCNT0) matches the OCR0. The OCR0 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 14-5. The counter value (TCNT0)
increases until a compare match occurs between TCNT0 and OCR0, and then counter (TCNT0)
is cleared.
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ATmega16A
Figure 14-5. CTC Mode, Timing Diagram
OCn Interrupt Flag Set
TCNTn
OCn
(Toggle)
Period
(COMn1:0 = 1)
1
2
3
4
An interrupt can be generated each time the counter value reaches the TOP value by using the
OCF0 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 OCR0 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 OC0 output can be set to toggle its logical
level on each compare match by setting the Compare Output mode bits to toggle mode
(COM01:0 = 1). The OC0 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 OCR0 is set to zero (0x00). The waveform frequency is defined by the following equation:
f clk_I/O
f OCn = ----------------------------------------------2 ⋅ N ⋅ ( 1 + OCRn )
The N variable represents the prescale 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.
14.7.3
Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM01:0 = 3) 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 MAX then restarts from BOTTOM. In
non-inverting Compare Output mode, the Output Compare (OC0) is cleared on the compare
match between TCNT0 and OCR0, 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 MAX value.
The counter is then cleared at the following timer clock cycle. The timing diagram for the fast
PWM mode is shown in Figure 14-6. The TCNT0 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The diagram includes non-inverted and
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inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent compare
matches between OCR0 and TCNT0.
Figure 14-6. Fast PWM Mode, Timing Diagram
OCRn Interrupt Flag Set
OCRn Update and
TOVn Interrupt Flag Set
TCNTn
OCn
(COMn1:0 = 2)
OCn
(COMn1:0 = 3)
Period
1
2
3
4
5
6
7
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches MAX. 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 OC0 pin. Setting the COM01:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can
be generated by setting the COM01:0 to 3 (See Table 14-4 on page 83). The actual OC0 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 OC0 Register at the compare match between
OCR0 and TCNT0, and clearing (or setting) the OC0 Register at the timer clock cycle the counter is cleared (changes from MAX to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
f clk_I/O
f OCnPWM = -----------------N ⋅ 256
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0 Register represents special cases when generating a PWM
waveform output in the fast PWM mode. If the OCR0 is set equal to BOTTOM, the output will be
a narrow spike for each MAX+1 timer clock cycle. Setting the OCR0 equal to MAX will result in a
constantly high or low output (depending on the polarity of the output set by the COM01:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC0 to toggle its logical level on each compare match (COM01:0 = 1). The waveform
generated will have a maximum frequency of fOC0 = fclk_I/O/2 when OCR0 is set to zero. This feature is similar to the OC0 toggle in CTC mode, except the double buffer feature of the output
compare unit is enabled in the fast PWM mode.
14.7.4
78
Phase Correct PWM Mode
The phase correct PWM mode (WGM01:0 = 1) 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 MAX and then from MAX to BOTTOM. In non-
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ATmega16A
inverting Compare Output mode, the Output Compare (OC0) is cleared on the compare match
between TCNT0 and OCR0 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 is fixed to eight bits. In phase correct
PWM mode the counter is incremented until the counter value matches MAX. When the counter
reaches MAX, it changes the count direction. The TCNT0 value will be equal to MAX for one
timer clock cycle. The timing diagram for the phase correct PWM mode is shown on Figure 14-7.
The TCNT0 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 TCNT0 slopes represent compare matches between OCR0 and TCNT0.
Figure 14-7. Phase Correct PWM Mode, Timing Diagram
OCn Interrupt Flag Set
OCRn Update
TOVn Interrupt Flag Set
TCNTn
OCn
(COMn1:0 = 2)
OCn
(COMn1:0 = 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
OC0 pin. Setting the COM01:0 bits to 2 will produce a non-inverted PWM. An inverted PWM output can be generated by setting the COM01:0 to 3 (see Table 14-5 on page 84). The actual OC0
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 OC0 Register at the compare match
between OCR0 and TCNT0 when the counter increments, and setting (or clearing) the OC0
Register at compare match between OCR0 and TCNT0 when the counter decrements. The
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PWM frequency for the output when using phase correct PWM can be calculated by the following equation:
f clk_I/O
f OCnPCPWM = -----------------N ⋅ 510
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0 Register represent special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR0 is set equal to BOTTOM, the output will be continuously low and if set equal to MAX the output will be continuously high for noninverted PWM mode. For inverted PWM the output will have the opposite logic values.
At the very start of Period 2 in Figure 14-7 OCn 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:
• OCR0A changes its value from MAX, like in Figure 14-7. 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 be correspond to the result of an upcounting 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.
14.8
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 14-8 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 14-8. Timer/Counter Timing Diagram, no Prescaling
clkI/O
clkTn
(clkI/O /1)
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
TOVn
Figure 14-9 shows the same timing data, but with the prescaler enabled.
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Figure 14-9. 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 14-10 shows the setting of OCF0 in all modes except CTC mode.
Figure 14-10. Timer/Counter Timing Diagram, Setting of OCF0, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
OCRn
OCRn - 1
OCRn
OCRn + 1
OCRn + 2
OCRn Value
OCFn
Figure 14-11 shows the setting of OCF0 and the clearing of TCNT0 in CTC mode.
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Figure 14-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
(CTC)
TOP - 1
TOP
BOTTOM
OCRn
BOTTOM + 1
TOP
OCFn
14.9
14.9.1
Register Description
TCCR0 – Timer/Counter Control Register
Bit
7
6
5
4
3
2
1
0
FOC0
WGM00
COM01
COM00
WGM01
CS02
CS01
CS00
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
TCCR0
• Bit 7 – FOC0: Force Output Compare
The FOC0 bit is only active when the WGM00 bit specifies a non-PWM mode. However, for
ensuring compatibility with future devices, this bit must be set to zero when TCCR0 is written
when operating in PWM mode. When writing a logical one to the FOC0 bit, an immediate compare match is forced on the Waveform Generation unit. The OC0 output is changed according to
its COM01:0 bits setting. Note that the FOC0 bit is implemented as a strobe. Therefore it is the
value present in the COM01:0 bits that determines the effect of the forced compare.
A FOC0 strobe will not generate any interrupt, nor will it clear the timer in CTC mode using
OCR0 as TOP.
The FOC0 bit is always read as zero.
• Bit 3, 6 – WGM0[1:0]: Waveform Generation Mode
These bits control the counting sequence of the counter, the source for the maximum (TOP)
counter value, and what type of Waveform Generation to be used. Modes of operation supported by the Timer/Counter unit are: Normal mode, Clear Timer on Compare Match (CTC)
mode, and two types of Pulse Width Modulation (PWM) modes. See Table 14-2 and “Modes of
Operation” on page 76.
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Table 14-2.
Waveform Generation Mode Bit Description(1)
Mode
WGM01
(CTC0)
WGM00
(PWM0)
Timer/Counter Mode of
Operation
TOP
Update of
OCR0
TOV0 Flag
Set-on
0
0
0
Normal
0xFF
Immediate
MAX
1
0
1
PWM, Phase Correct
0xFF
TOP
BOTTOM
2
1
0
CTC
OCR0
Immediate
MAX
3
1
1
Fast PWM
0xFF
BOTTOM
MAX
Note:
1. The CTC0 and PWM0 bit definition names are now obsolete. Use the WGM01:0 definitions.
However, the functionality and location of these bits are compatible with previous versions of
the timer.
• Bit 5:4 – COM01:0: Compare Match Output Mode
These bits control the Output Compare pin (OC0) behavior. If one or both of the COM01:0 bits
are set, the OC0 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 OC0 pin must be
set in order to enable the output driver.
When OC0 is connected to the pin, the function of the COM01:0 bits depends on the WGM01:0
bit setting. Table 14-3 shows the COM01:0 bit functionality when the WGM01:0 bits are set to a
normal or CTC mode (non-PWM).
Table 14-3.
Compare Output Mode, non-PWM Mode
COM01
COM00
Description
0
0
Normal port operation, OC0 disconnected.
0
1
Toggle OC0 on compare match
1
0
Clear OC0 on compare match
1
1
Set OC0 on compare match
Table 14-4 shows the COM01:0 bit functionality when the WGM01:0 bits are set to fast PWM
mode.
Table 14-4.
Compare Output Mode, Fast PWM Mode(1)
COM01
COM00
0
0
Normal port operation, OC0 disconnected.
0
1
Reserved
1
0
Clear OC0 on compare match, set OC0 at BOTTOM,
(non-inverting mode)
1
1
Set OC0 on compare match, clear OC0 at BOTTOM,
(inverting mode)
Note:
Description
1. A special case occurs when OCR0 equals TOP and COM01 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 77 for
more details.
Table 14-5 shows the COM01:0 bit functionality when the WGM01:0 bits are set to phase correct PWM mode.
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Table 14-5.
Compare Output Mode, Phase Correct PWM Mode(1)
COM01
COM00
0
0
Normal port operation, OC0 disconnected.
0
1
Reserved
1
0
Clear OC0 on compare match when up-counting. Set OC0 on compare match
when downcounting.
1
1
Set OC0 on compare match when up-counting. Clear OC0 on compare match
when downcounting.
Note:
Description
1. A special case occurs when OCR0 equals TOP and COM01 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
78 for more details.
• Bit 2:0 – CS02:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter.
Table 14-6.
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.
14.9.2
TCNT0 – Timer/Counter Register
Bit
7
6
5
4
3
2
1
0
TCNT0[7:0]
TCNT0
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
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 OCR0 Register.
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14.9.3
OCR0 – Output Compare Register
Bit
7
6
5
4
3
2
1
0
OCR0[7:0]
OCR0
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
The Output Compare Register 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 OC0 pin.
14.9.4
TIMSK – Timer/Counter Interrupt Mask Register
Bit
7
6
5
4
3
2
1
0
OCIE2
TOIE2
TICIE1
OCIE1A
OCIE1B
TOIE1
OCIE0
TOIE0
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
TIMSK
• Bit 1 – OCIE0: Timer/Counter0 Output Compare Match Interrupt Enable
When the OCIE0 bit is written to one, and the I-bit in the Status Register is set (one), the
Timer/Counter0 Compare Match interrupt is enabled. The corresponding interrupt is executed if
a compare match in Timer/Counter0 occurs, i.e., when the OCF0 bit is set in the Timer/Counter
Interrupt Flag Register – TIFR.
• 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 (one), 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 Interrupt
Flag Register – TIFR.
14.9.5
TIFR qP Timer/Counter Interrupt Flag Register
Bit
7
6
5
4
3
2
1
0
OCF2
TOV2
ICF1
OCF1A
OCF1B
TOV1
OCF0
TOV0
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
TIFR
• Bit 1 – OCF0: Output Compare Flag 0
The OCF0 bit is set (one) when a compare match occurs between the Timer/Counter0 and the
data in OCR0 – Output Compare Register0. OCF0 is cleared by hardware when executing the
corresponding interrupt handling vector. Alternatively, OCF0 is cleared by writing a logic one to
the flag. When the I-bit in SREG, OCIE0 (Timer/Counter0 Compare Match Interrupt Enable), and
OCF0 are set (one), the Timer/Counter0 Compare Match Interrupt is executed.
• Bit 0 – TOV0: Timer/Counter0 Overflow Flag
The bit TOV0 is set (one) 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 (one), the Timer/Counter0 Overflow interrupt is executed. In
phase correct PWM mode, this bit is set when Timer/Counter0 changes counting direction at
$00.
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15. Timer/Counter0 and Timer/Counter1 Prescalers
15.1
Overview
Timer/Counter1 and Timer/Counter0 share the same prescaler module, but the Timer/Counters
can have different prescaler settings. The description below applies to both Timer/Counter1 and
Timer/Counter0.
15.2
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.
15.3
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 Timer/Counter1 and Timer/Counter0. 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.
15.4
External Clock Source
An external clock source applied to the T1/T0 pin can be used as Timer/Counter clock
(clkT1/clkT0). The T1/T0 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 15-1
shows a functional equivalent block diagram of the T1/T0 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 clkT1/clkT0 pulse for each positive (CSn2:0 = 7) or negative
(CSn2:0 = 6) edge it detects.
Figure 15-1. T1/T0 Pin Sampling
Tn
D
Q
D
Q
D
Tn_sync
(To Clock
Select Logic)
Q
LE
clk I/O
Synchronization
86
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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 T1/T0 pin to the counter is updated.
Enabling and disabling of the clock input must be done when T1/T0 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 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 15-2. Prescaler for Timer/Counter0 and Timer/Counter1(1)
clk I/O
Clear
PSR10
T0
Synchronization
T1
Synchronization
clkT1
Note:
15.5
15.5.1
clkT0
1. The synchronization logic on the input pins (T1/T0) is shown in Figure 15-1.
Register Description
SFIOR – Special Function IO Register
Bit
7
6
5
4
3
2
1
0
ADTS2
ADTS1
ADTS0
–
ACME
PUD
PSR2
PSR10
Read/Write
R/W
R/W
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
SFIOR
• Bit 0 – PSR10: Prescaler Reset Timer/Counter1 and Timer/Counter0
When this bit is written to one, the Timer/Counter1 and Timer/Counter0 prescaler 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. Note that Timer/Counter1 and Timer/Counter0 share the same prescaler and a
reset of this prescaler will affect both timers. This bit will always be read as zero.
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16. 16-bit Timer/Counter1
16.1
Features
•
•
•
•
•
•
•
•
•
•
•
16.2
True 16-bit Design (i.e., Allows 16-bit PWM)
Two 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
Four Independent Interrupt Sources (TOV1, OCF1A, OCF1B, and ICF1)
Overview
The 16-bit Timer/Counter unit allows accurate program execution timing (event management),
wave generation, and signal timing measurement. 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. 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 16-1. For the actual
placement of I/O pins, refer to Figure 1-1 on page 2. 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 “Register Description” on page 109.
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Figure 16-1. 16-bit Timer/Counter Block Diagram(1)
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
DATABUS
OCRnA
OCnB
(Int.Req.)
Fixed
TOP
Values
Waveform
Generation
=
OCRnB
OCnB
( From Analog
Comparator Ouput )
ICFn (Int.Req.)
Edge
Detector
ICRn
Noise
Canceler
ICPn
TCCRnA
Note:
16.2.1
TCCRnB
1. Refer to Figure 1-1 on page 2, Table 12-6 on page 57, and Table 12-12 on page 62 for
Timer/Counter1 pin placement and description.
Registers
The Timer/Counter (TCNT1), Output Compare Registers (OCR1A/B), and Input Capture Register (ICR1) are all 16-bit registers. Special procedures must be followed when accessing the 16bit registers. These procedures are described in the section “Accessing 16-bit Registers” on
page 91. The Timer/Counter Control Registers (TCCR1A/B) are 8-bit registers and have no CPU
access restrictions. Interrupt requests (abbreviated to Int.Req. in the figure) signals are all visible
in the Timer Interrupt Flag Register (TIFR). All interrupts are individually masked with the Timer
Interrupt Mask Register (TIMSK). TIFR and TIMSK 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 T1 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 (clkT1).
The double buffered Output Compare Registers (OCR1A/B) 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 (OC1A/B). See “Out-
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put Compare Units” on page 97. The compare match event will also set the Compare Match
Flag (OCF1A/B) which can be used to generate an output compare interrupt request.
The Input Capture Register can capture the Timer/Counter value at a given external (edge triggered) event on either the Input Capture Pin (ICP1) or on the Analog Comparator pins (See
“Analog Comparator” on page 204.) 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 OCR1A Register, the ICR1 Register, or by a set of fixed values. When using
OCR1A as TOP value in a PWM mode, the OCR1A 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 ICR1 Register can be used
as an alternative, freeing the OCR1A to be used as PWM output.
16.2.2
Definitions
The following definitions are used extensively throughout the document:
Table 16-1.
16.2.3
Definitions
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 OCR1A or ICR1 Register. The assignment is dependent of the mode of operation.
Compatibility
The 16-bit Timer/Counter has been updated and improved from previous versions of the 16-bit
AVR Timer/Counter. This 16-bit Timer/Counter is fully compatible with the earlier version
regarding:
• All 16-bit Timer/Counter related I/O Register address locations, including Timer Interrupt
Registers.
• Bit locations inside all 16-bit Timer/Counter Registers, including Timer Interrupt Registers.
• Interrupt Vectors.
The following control bits have changed name, but have same functionality and register location:
• PWM10 is changed to WGM10.
• PWM11 is changed to WGM11.
• CTC1 is changed to WGM12.
The following bits are added to the 16-bit Timer/Counter Control Registers:
• FOC1A and FOC1B are added to TCCR1A.
• WGM13 is added to TCCR1B.
The 16-bit Timer/Counter has improvements that will affect the compatibility in some special
cases.
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16.3
Accessing 16-bit Registers
The TCNT1, OCR1A/B, and ICR1 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 OCR1A/B 16bit 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 OCR1A/B and ICR1 Registers. Note that when using “C”, the compiler handles the 16-bit
access.
Assembly Code Example(1)
:.
; Set TCNT1 to 0x01FF
ldi r17,0x01
ldi r16,0xFF
out TCNT1H,r17
out TCNT1L,r16
; Read TCNT1 into r17:r16
in r16,TCNT1L
in r17,TCNT1H
:.
C Code Example(1)
unsigned int i;
:.
/* Set TCNT1 to 0x01FF */
TCNT1 = 0x1FF;
/* Read TCNT1 into i */
i = TCNT1;
:.
Note:
1. See “About Code Examples” on page 7.
The assembly code example returns the TCNT1 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
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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 TCNT1 Register contents.
Reading any of the OCR1A/B or ICR1 Registers can be done by using the same principle.
Assembly Code Example(1)
TIM16_ReadTCNT1:
; Save global interrupt flag
in r18,SREG
; Disable interrupts
cli
; Read TCNT1 into r17:r16
in r16,TCNT1L
in r17,TCNT1H
; Restore global interrupt flag
out SREG,r18
ret
C Code Example(1)
unsigned int TIM16_ReadTCNT1( void )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
_CLI();
/* Read TCNT1 into i */
i = TCNT1;
/* Restore global interrupt flag */
SREG = sreg;
return i;
}
Note:
1. See “About Code Examples” on page 7.
The assembly code example returns the TCNT1 value in the r17:r16 register pair.
The following code examples show how to do an atomic write of the TCNT1 Register contents.
Writing any of the OCR1A/B or ICR1 Registers can be done by using the same principle.
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Assembly Code Example(1)
TIM16_WriteTCNT1:
; Save global interrupt flag
in r18,SREG
; Disable interrupts
cli
; Set TCNT1 to r17:r16
out TCNT1H,r17
out TCNT1L,r16
; Restore global interrupt flag
out SREG,r18
ret
C Code Example(1)
void TIM16_WriteTCNT1 ( unsigned int i )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
_CLI();
/* Set TCNT1 to i */
TCNT1 = i;
/* Restore global interrupt flag */
SREG = sreg;
}
Note:
1. See “About Code Examples” on page 7.
The assembly code example requires that the r17:r16 register pair contains the value to be written to TCNT1.
16.3.1
16.4
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.
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 (CS12:0) bits
located in the Timer/Counter Control Register B (TCCR1B). For details on clock sources and
prescaler, see “Timer/Counter0 and Timer/Counter1 Prescalers” on page 86.
16.5
Counter Unit
The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit.
Figure 16-2 shows a block diagram of the counter and its surroundings.
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Figure 16-2. Counter Unit Block Diagram
DATA BUS
(8-bit)
TOVn
(Int.Req.)
TEMP (8-bit)
Clock Select
Count
TCNTnH (8-bit)
TCNTnL (8-bit)
Clear
Direction
TCNTn (16-bit Counter)
Control Logic
clkTn
Edge
Detector
Tn
( From Prescaler )
TOP
BOTTOM
Signal description (internal signals):
Count
Increment or decrement TCNT1 by 1.
Direction
Select between increment and decrement.
Clear
Clear TCNT1 (set all bits to zero).
clkT1
Timer/Counter clock.
TOP
Signalize that TCNT1 has reached maximum value.
BOTTOM
Signalize that TCNT1 has reached minimum value (zero).
The 16-bit counter is mapped into two 8-bit I/O memory locations: Counter High (TCNT1H) containing the upper eight bits of the counter, and Counter Low (TCNT1L) containing the lower 8
bits. The TCNT1H Register can only be indirectly accessed by the CPU. When the CPU does an
access to the TCNT1H I/O location, the CPU accesses the High byte temporary register
(TEMP). The temporary register is updated with the TCNT1H value when the TCNT1L is read,
and TCNT1H is updated with the temporary register value when TCNT1L 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 TCNT1 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 (clkT1). The clkT1 can be generated from an external or internal clock source,
selected by the Clock Select bits (CS12:0). When no clock source is selected (CS12:0 = 0) the
timer is stopped. However, the TCNT1 value can be accessed by the CPU, independent of
whether clkT1 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
(WGM13:0) located in the Timer/Counter Control Registers A and B (TCCR1A and TCCR1B).
There are close connections between how the counter behaves (counts) and how waveforms
are generated on the Output Compare outputs OC1x. For more details about advanced counting
sequences and waveform generation, see “Modes of Operation” on page 99.
The Timer/Counter Overflow (TOV1) Flag is set according to the mode of operation selected by
the WGM13:0 bits. TOV1 can be used for generating a CPU interrupt.
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16.6
Input Capture Unit
The Timer/Counter incorporates an Input Capture unit that can capture external events and give
them a time-stamp indicating time of occurrence. The external signal indicating an event, or multiple events, can be applied via the ICP1 pin or alternatively, 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.
The Input Capture unit is illustrated by the block diagram shown in Figure 16-3. 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 16-3. 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
When a change of the logic level (an event) occurs on the Input Capture pin (ICP1), 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
(TCNT1) is written to the Input Capture Register (ICR1). The Input Capture Flag (ICF1) is set at
the same system clock as the TCNT1 value is copied into ICR1 Register. If enabled (TICIE1 =
1), the Input Capture Flag generates an Input Capture Interrupt. The ICF1 Flag is automatically
cleared when the interrupt is executed. Alternatively the ICF1 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 (ICR1) is done by first reading the Low
byte (ICR1L) and then the High byte (ICR1H). When the Low byte is read the High byte is copied
into the High byte temporary register (TEMP). When the CPU reads the ICR1H I/O location it will
access the TEMP Register.
The ICR1 Register can only be written when using a Waveform Generation mode that utilizes
the ICR1 Register for defining the counter’s TOP value. In these cases the Waveform Generation mode (WGM13:0) bits must be set before the TOP value can be written to the ICR1
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Register. When writing the ICR1 Register the High byte must be written to the ICR1H I/O location before the Low byte is written to ICR1L.
For more information on how to access the 16-bit registers refer to “Accessing 16-bit Registers”
on page 91.
16.6.1
Input Capture Pin Source
The main trigger source for the Input Capture unit is the Input Capture pin (ICP1).
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.
Both the Input Capture pin (ICP1) and the Analog Comparator output (ACO) inputs are sampled
using the same technique as for the T1 pin (Figure 15-1 on page 86). 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 ICR1 to define TOP.
An Input Capture can be triggered by software by controlling the port of the ICP1 pin.
16.6.2
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 (ICNC1) bit in
Timer/Counter Control Register B (TCCR1B). 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
ICR1 Register. The noise canceler uses the system clock and is therefore not affected by the
prescaler.
16.6.3
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 ICR1 Register before the next event occurs, the ICR1 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 ICR1 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 ICR1
Register has been read. After a change of the edge, the Input Capture Flag (ICF1) must be
cleared by software (writing a logical one to the I/O bit location). For measuring frequency only,
the clearing of the ICF1 Flag is not required (if an interrupt handler is used).
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16.7
Output Compare Units
The 16-bit comparator continuously compares TCNT1 with the Output Compare Register
(OCR1x). If TCNT equals OCR1x the comparator signals a match. A match will set the Output
Compare Flag (OCF1x) at the next timer clock cycle. If enabled (OCIE1x = 1), the Output Compare Flag generates an output compare interrupt. The OCF1x Flag is automatically cleared
when the interrupt is executed. Alternatively the OCF1x 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
(WGM13:0) bits and Compare Output mode (COM1x1:0) 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 99.)
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.
Figure 16-4 shows a block diagram of the output compare unit. The small “n” in the register and
bit names indicates the device number (n = 1 for Timer/Counter1), and the “x” indicates output
compare unit (A/B). The elements of the block diagram that are not directly a part of the output
compare unit are gray shaded.
Figure 16-4. 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 OCR1x 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 OCR1x 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 OCR1x Register access may seem complex, but this is not case. When the double buffering
is enabled, the CPU has access to the OCR1x Buffer Register, and if double buffering is dis-
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abled the CPU will access the OCR1x 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 OCR1x Registers must be done via the TEMP Register since the compare of all 16 bits is done continuously. The High byte (OCR1xH) 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 (OCR1xL) is written to the lower eight
bits, the High byte will be copied into the upper 8-bits of either the OCR1x buffer or OCR1x 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 91.
16.7.1
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 (FOC1x) bit. Forcing compare match will not set the
OCF1x Flag or reload/clear the timer, but the OC1x pin will be updated as if a real compare
match had occurred (the COM1x1:0 bits settings define whether the OC1x pin is set, cleared or
toggled).
16.7.2
Compare Match Blocking by TCNT1 Write
All CPU writes to the TCNT1 Register will block any compare match that occurs in the next timer
clock cycle, even when the timer is stopped. This feature allows OCR1x to be initialized to the
same value as TCNT1 without triggering an interrupt when the Timer/Counter clock is enabled.
16.7.3
Using the Output Compare Unit
Since writing TCNT1 in any mode of operation will block all compare matches for one timer clock
cycle, there are risks involved when changing TCNT1 when using any of the output compare
units, independent of whether the Timer/Counter is running or not. If the value written to TCNT1
equals the OCR1x value, the compare match will be missed, resulting in incorrect waveform
generation. Do not write the TCNT1 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 TCNT1 value equal to BOTTOM when the counter is downcounting.
The setup of the OC1x should be performed before setting the Data Direction Register for the
port pin to output. The easiest way of setting the OC1x value is to use the force output compare
(FOC1x) strobe bits in Normal mode. The OC1x Register keeps its value even when changing
between waveform generation modes.
Be aware that the COM1x1:0 bits are not double buffered together with the compare value.
Changing the COM1x1:0 bits will take effect immediately.
16.8
Compare Match Output Unit
The Compare Output mode (COM1x1:0) bits have two functions. The Waveform Generator uses
the COM1x1:0 bits for defining the Output Compare (OC1x) state at the next compare match.
Secondly the COM1x1:0 bits control the OC1x pin output source. Figure 16-5 shows a simplified
schematic of the logic affected by the COM1x1:0 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 COM1x1:0 bits are shown. When referring to the
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OC1x state, the reference is for the internal OC1x Register, not the OC1x pin. If a System Reset
occur, the OC1x Register is reset to “0”.
Figure 16-5. Compare Match Output Unit, Schematic
COMnx1
COMnx0
FOCnx
Waveform
Generator
D
Q
1
OCnx
DATABUS
D
0
OCnx
Pin
Q
PORT
D
Q
DDR
clk I/O
The general I/O port function is overridden by the Output Compare (OC1x) from the Waveform
Generator if either of the COM1x1:0 bits are set. However, the OC1x 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 OC1x pin (DDR_OC1x) must be set as output before the OC1x 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 16-2, Table 16-3 and Table 16-4 for
details.
The design of the output compare pin logic allows initialization of the OC1x state before the output is enabled. Note that some COM1x1:0 bit settings are reserved for certain modes of
operation. See “Register Description” on page 109.
The COM1x1:0 bits have no effect on the Input Capture unit.
16.8.1
Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM1x1:0 bits differently in normal, CTC, and PWM modes.
For all modes, setting the COM1x1:0 = 0 tells the Waveform Generator that no action on the
OC1x Register is to be performed on the next compare match. For compare output actions in the
non-PWM modes refer to Table 16-2 on page 110. For fast PWM mode refer to Table 16-3 on
page 111, and for phase correct and phase and frequency correct PWM refer to Table 16-4 on
page 111.
A change of the COM1x1:0 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
FOC1x strobe bits.
16.9
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 (WGM13:0) and Compare Output
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mode (COM1x1:0) bits. The Compare Output mode bits do not affect the counting sequence,
while the Waveform Generation mode bits do. The COM1x1:0 bits control whether the PWM output generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes
the COM1x1:0 bits control whether the output should be set, cleared or toggle at a compare
match (See “Compare Match Output Unit” on page 98.)
For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 107.
16.9.1
Normal Mode
The simplest mode of operation is the Normal mode (WGM13:0 = 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 (TOV1) will be set in
the same timer clock cycle as the TCNT1 becomes zero. The TOV1 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 TOV1 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.
16.9.2
Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM13:0 = 4 or 12), the OCR1A or ICR1 Register
are used to manipulate the counter resolution. In CTC mode the counter is cleared to zero when
the counter value (TCNT1) matches either the OCR1A (WGM13:0 = 4) or the ICR1 (WGM13:0 =
12). The OCR1A or ICR1 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 16-6. The counter value (TCNT1)
increases until a compare match occurs with either OCR1A or ICR1, and then counter (TCNT1)
is cleared.
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Figure 16-6. 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 OCF1A or ICF1 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 OCR1A or ICR1 is lower than the current value of
TCNT1, 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 OCR1A for defining TOP (WGM13:0 = 15) since the OCR1A then will be double buffered.
For generating a waveform output in CTC mode, the OC1A output can be set to toggle its logical
level on each compare match by setting the compare output mode bits to toggle mode
(COM1A1:0 = 1). The OC1A value will not be visible on the port pin unless the data direction for
the pin is set to output (DDR_OC1A = 1). The waveform generated will have a maximum frequency of fOC1A = fclk_I/O/2 when OCR1A 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 TOV1 Flag is set in the same timer clock cycle that the
counter counts from MAX to 0x0000.
16.9.3
Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM13:0 = 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 (OC1x) is cleared
on the compare match between TCNT1 and OCR1x, and set at BOTTOM. In inverting Compare
Output mode 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 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
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applications. High frequency allows physically small sized external components (coils, capacitors), hence reduces total system cost.
The PWM resolution for fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either ICR1 or
OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and the maximum resolution is 16-bit (ICR1 or OCR1A 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 (WGM13:0 = 5, 6, or 7), the value in ICR1 (WGM13:0 =
14), or the value in OCR1A (WGM13:0 = 15). The counter is then cleared at the following timer
clock cycle. The timing diagram for the fast PWM mode is shown in Figure 16-7. The figure
shows fast PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1 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 TCNT1
slopes represent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will
be set when a compare match occurs.
Figure 16-7. Fast PWM Mode, Timing Diagram
OCRnx / TOP Update and
TOVn Interrupt Flag Set and
OCnA Interrupt Flag Set
OCnA 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 (TOV1) is set each time the counter reaches TOP. In addition
the OC1A or ICF1 Flag is set at the same timer clock cycle as TOV1 is set when either OCR1A
or ICR1 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 TCNT1 and the OCR1x.
Note that when using fixed TOP values the unused bits are masked to zero when any of the
OCR1x Registers are written.
The procedure for updating ICR1 differs from updating OCR1A when used for defining the TOP
value. The ICR1 Register is not double buffered. This means that if ICR1 is changed to a low
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value when the counter is running with none or a low prescaler value, there is a risk that the new
ICR1 value written is lower than the current value of TCNT1. 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 OCR1A Register however, is double buffered. This feature allows the OCR1A I/O location
to be written anytime. When the OCR1A I/O location is written the value written will be put into
the OCR1A Buffer Register. The OCR1A Compare Register will then be updated with the value
in the Buffer Register at the next timer clock cycle the TCNT1 matches TOP. The update is done
at the same timer clock cycle as the TCNT1 is cleared and the TOV1 Flag is set.
Using the ICR1 Register for defining TOP works well when using fixed TOP values. By using
ICR1, the OCR1A Register is free to be used for generating a PWM output on OC1A. However,
if the base PWM frequency is actively changed (by changing the TOP value), using the OCR1A
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 OC1x pins.
Setting the COM1x1:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output
can be generated by setting the COM1x1:0 to 3 (See Table 16-2 on page 110). The actual OC1x
value will only be visible on the port pin if the data direction for the port pin is set as output
(DDR_OC1x). The PWM waveform is generated by setting (or clearing) the OC1x Register at
the compare match between OCR1x and TCNT1, and clearing (or setting) the OC1x 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 OCR1x Register represents special cases when generating a PWM
waveform output in the fast PWM mode. If the OCR1x is set equal to BOTTOM (0x0000) the output will be a narrow spike for each TOP+1 timer clock cycle. Setting the OCR1x equal to TOP
will result in a constant high or low output (depending on the polarity of the output set by the
COM1x1:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC1A to toggle its logical level on each compare match (COM1A1:0 = 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 fOC1A = fclk_I/O/2 when OCR1A is set to zero (0x0000). This feature is
similar to the OC1A toggle in CTC mode, except the double buffer feature of the output compare
unit is enabled in the fast PWM mode.
16.9.4
Phase Correct PWM Mode
The phase correct Pulse Width Modulation or phase correct PWM mode (WGM13:0 = 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 (OC1x) is cleared on
the compare match between TCNT1 and OCR1x 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.
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The PWM resolution for the phase correct PWM mode can be fixed to 8-, 9-, or 10-bit, or defined
by either ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to
0x0003), and the maximum resolution is 16-bit (ICR1 or OCR1A 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 (WGM13:0 = 1, 2, or 3), the value in ICR1
(WGM13:0 = 10), or the value in OCR1A (WGM13:0 = 11). The counter has then reached the
TOP and changes the count direction. The TCNT1 value will be equal to TOP for one timer clock
cycle. The timing diagram for the phase correct PWM mode is shown on Figure 16-8. The figure
shows phase correct PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1
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 TCNT1 slopes represent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set when a compare match occurs.
Figure 16-8. 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 (TOV1) is set each time the counter reaches BOTTOM. When
either OCR1A or ICR1 is used for defining the TOP value, the OC1A or ICF1 Flag is set accordingly at the same timer clock cycle as the OCR1x 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 TCNT1 and the OCR1x.
Note that when using fixed TOP values, the unused bits are masked to zero when any of the
OCR1x Registers are written. As the third period shown in Figure 16-8 illustrates, changing the
TOP actively while the Timer/Counter is running in the phase correct mode can result in an
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unsymmetrical output. The reason for this can be found in the time of update of the OCR1x Register. Since the OCR1x 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
OC1x pins. Setting the COM1x1:0 bits to 2 will produce a non-inverted PWM and an inverted
PWM output can be generated by setting the COM1x1:0 to 3 (See Table 16-2 on page 110). The
actual OC1x value will only be visible on the port pin if the data direction for the port pin is set as
output (DDR_OC1x). The PWM waveform is generated by setting (or clearing) the OC1x Register at the compare match between OCR1x and TCNT1 when the counter increments, and
clearing (or setting) the OC1x Register at compare match between OCR1x and TCNT1 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 OCR1x Register represent special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR1x 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.
16.9.5
Phase and Frequency Correct PWM Mode
The phase and frequency correct Pulse Width Modulation, or phase and frequency correct PWM
mode (WGM13:0 = 8 or 9) provides a high resolution phase and frequency correct PWM waveform generation option. The phase and frequency correct PWM mode is, like the phase 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 (OC1x) is cleared on the compare match between TCNT1 and OCR1x while
upcounting, and set on the compare match while downcounting. In inverting Compare Output
mode, the operation is inverted. The dual-slope operation gives a lower maximum operation frequency compared to the 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 OCR1x Register is updated by the OCR1x Buffer Register, (see Figure 168 and Figure 16-9).
The PWM resolution for the phase and frequency correct PWM mode can be defined by either
ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and
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the maximum resolution is 16-bit (ICR1 or OCR1A 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 ICR1 (WGM13:0 = 8), or the value in OCR1A (WGM13:0 = 9). The
counter has then reached the TOP and changes the count direction. The TCNT1 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 16-9. The figure shows phase and frequency correct
PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1 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 TCNT1 slopes represent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set when a
compare match occurs.
Figure 16-9. Phase and Frequency Correct PWM Mode, Timing Diagram
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
OCRnx / TOP Update
and
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 (TOV1) is set at the same timer clock cycle as the OCR1x
Registers are updated with the double buffer value (at BOTTOM). When either OCR1A or ICR1
is used for defining the TOP value, the OC1A or ICF1 Flag set when TCNT1 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 TCNT1 and the OCR1x.
As Figure 16-9 shows the output generated is, in contrast to the phase correct mode, symmetrical in all periods. Since the OCR1x 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.
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Using the ICR1 Register for defining TOP works well when using fixed TOP values. By using
ICR1, the OCR1A Register is free to be used for generating a PWM output on OC1A. However,
if the base PWM frequency is actively changed by changing the TOP value, using the OCR1A 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 OC1x pins. Setting the COM1x1:0 bits to 2 will produce a non-inverted PWM and
an inverted PWM output can be generated by setting the COM1x1:0 to 3 (See Table on page
111). The actual OC1x value will only be visible on the port pin if the data direction for the port
pin is set as output (DDR_OC1x). The PWM waveform is generated by setting (or clearing) the
OC1x Register at the compare match between OCR1x and TCNT1 when the counter increments, and clearing (or setting) the OC1x Register at compare match between OCR1x and
TCNT1 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 OCR1x Register represents special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR1x 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 noninverted PWM mode. For inverted PWM the 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.
16.10 Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkT1) 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 OCR1x Register is updated with the OCR1x buffer value (only for
modes utilizing double buffering). Figure 16-10 shows a timing diagram for the setting of OCF1x.
Figure 16-10. Timer/Counter Timing Diagram, Setting of OCF1x, No Prescaling
clkI/O
clkTn
(clkI/O /1)
TCNTn
OCRnx
OCRnx - 1
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
OCFnx
Figure 16-11 shows the same timing data, but with the prescaler enabled.
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Figure 16-11. Timer/Counter Timing Diagram, Setting of OCF1x, 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 16-12 shows the count sequence close to TOP in various modes. When using phase and
frequency correct PWM mode the OCR1x 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 TOV1 Flag at BOTTOM.
Figure 16-12. 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)
Old OCRnx Value
New OCRnx Value
Figure 16-13 shows the same timing data, but with the prescaler enabled.
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Figure 16-13. 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 ICFn (if used
as TOP)
OCRnx
Old OCRnx Value
(Update at TOP)
New OCRnx Value
16.11 Register Description
16.11.1
TCCR1A – Timer/Counter1 Control Register A
Bit
7
6
5
4
3
2
1
0
COM1A1
COM1A0
COM1B1
COM1B0
FOC1A
FOC1B
WGM11
WGM10
Read/Write
R/W
R/W
R/W
R/W
W
W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TCCR1A
• Bit 7:6 – COM1A1:0: Compare Output Mode for Channel A
• Bit 5:4 – COM1B1:0: Compare Output Mode for Channel B
The COM1A1:0 and COM1B1:0 control the Output Compare pins (OC1A and OC1B respectively) behavior. If one or both of the COM1A1:0 bits are written to one, the OC1A output
overrides the normal port functionality of the I/O pin it is connected to. If one or both of the
COM1B1:0 bit are written to one, the OC1B 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 OC1A or OC1B pin must be set in order to enable the output driver.
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When the OC1A or OC1B is connected to the pin, the function of the COM1x1:0 bits is dependent of the WGM13:0 bits setting. Table 16-2 shows the COM1x1:0 bit functionality when the
WGM13:0 bits are set to a normal or a CTC mode (non-PWM).
Table 16-2.
Compare Output Mode, non-PWM
COM1A1/COM1B1
COM1A0/COM1B0
Description
0
0
Normal port operation, OC1A/OC1B disconnected.
0
1
Toggle OC1A/OC1B on compare match
1
0
Clear OC1A/OC1B on compare match (Set output
to low level)
1
1
Set OC1A/OC1B on compare match (Set output to
high level)
Table 16-3 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the fast
PWM mode.
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Table 16-3.
Compare Output Mode, Fast PWM(1)
COM1A1/COM1B1
Note:
COM1A0/COM1B0
Description
0
0
Normal port operation, OC1A/OC1B disconnected.
0
1
WGM13:0 = 15: Toggle OC1A on Compare Match,
OC1B disconnected (normal port operation).
For all other WGM13:0 settings, normal port
operation, OCnA/OCnB disconnected.
1
0
Clear OC1A/OC1B on compare match, set
OC1A/OC1B at BOTTOM,
(non-inverting mode)
1
1
Set OC1A/OC1B on compare match, clear
OC1A/OC1B at BOTTOM,
(inverting mode)
1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. In
this case the compare match is ignored, but the set or clear is done at BOTTOM. See “Fast
PWM Mode” on page 101. for more details.
Table 16-4 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the phase
correct or the phase and frequency correct, PWM mode.
Table 16-4.
Compare Output Mode, Phase Correct and Phase and Frequency Correct PWM
(1)
COM1A1/COM1B1
COM1A0/COM1B0
0
0
Normal port operation, OC1A/OC1B disconnected.
0
1
WGM13:0 = 9 or 14: Toggle OCnA on Compare
Match, OCnB disconnected (normal port
operation).
For all other WGM13:0 settings, normal port
operation, OC1A/OC1B disconnected.
1
0
Clear OC1A/OC1B on compare match when upcounting. Set OC1A/OC1B on compare match
when downcounting.
1
1
Set OC1A/OC1B on compare match when upcounting. Clear OC1A/OC1B on compare match
when downcounting.
Note:
Description
1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. See
“Phase Correct PWM Mode” on page 103. for more details.
• Bit 3 – FOC1A: Force Output Compare for Channel A
• Bit 2 – FOC1B: Force Output Compare for Channel B
The FOC1A/FOC1B bits are only active when the WGM13:0 bits specifies a non-PWM mode.
However, for ensuring compatibility with future devices, these bits must be set to zero when
TCCR1A is written when operating in a PWM mode. When writing a logical one to the
FOC1A/FOC1B bit, an immediate compare match is forced on the Waveform Generation unit.
The OC1A/OC1B output is changed according to its COM1x1:0 bits setting. Note that the
FOC1A/FOC1B bits are implemented as strobes. Therefore it is the value present in the
COM1x1:0 bits that determine the effect of the forced compare.
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A FOC1A/FOC1B strobe will not generate any interrupt nor will it clear the timer in Clear Timer
on Compare match (CTC) mode using OCR1A as TOP.
The FOC1A/FOC1B bits are always read as zero.
• Bit 1:0 – WGM11:0: Waveform Generation Mode
Combined with the WGM13:2 bits found in the TCCR1B 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 Table 16-5. 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. (See “Modes of Operation” on page 99.)
Waveform Generation Mode Bit Description(1)
Table 16-5.
Mode
WGM13
WGM12
(CTC1)
WGM11
(PWM11)
WGM10
(PWM10)
0
0
0
0
1
0
0
2
0
3
Timer/Counter Mode of Operation
TOP
Update of
OCR1x
TOV1 Flag Set
on
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
OCR1A
Immediate
MAX
5
0
1
0
1
Fast PWM, 8-bit
0x00FF
BOTTOM
TOP
6
0
1
1
0
Fast PWM, 9-bit
0x01FF
BOTTOM
TOP
7
0
1
1
1
Fast PWM, 10-bit
0x03FF
BOTTOM
TOP
8
1
0
0
0
PWM, Phase and Frequency Correct
ICR1
BOTTOM
BOTTOM
9
1
0
0
1
PWM, Phase and Frequency Correct
OCR1A
BOTTOM
BOTTOM
10
1
0
1
0
PWM, Phase Correct
ICR1
TOP
BOTTOM
11
1
0
1
1
PWM, Phase Correct
OCR1A
TOP
BOTTOM
12
1
1
0
0
CTC
ICR1
Immediate
MAX
13
1
1
0
1
Reserved
–
–
–
14
1
1
1
0
Fast PWM
ICR1
BOTTOM
TOP
15
1
1
1
1
Fast PWM
OCR1A
BOTTOM
TOP
Note:
16.11.2
1. The CTC1 and PWM11:0 bit definition names are obsolete. Use the WGM12:0 definitions. However, the functionality and
location of these bits are compatible with previous versions of the timer.
TCCR1B – Timer/Counter1 Control Register B
Bit
7
6
5
4
3
2
1
0
ICNC1
ICES1
–
WGM13
WGM12
CS12
CS11
CS10
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
TCCR1B
• Bit 7 – ICNC1: 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 (ICP1) is filtered. The filter function requires four
successive equal valued samples of the ICP1 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 – ICES1: Input Capture Edge Select
This bit selects which edge on the Input Capture Pin (ICP1) that is used to trigger a capture
event. When the ICES1 bit is written to zero, a falling (negative) edge is used as trigger, and
when the ICES1 bit is written to one, a rising (positive) edge will trigger the capture.
When a capture is triggered according to the ICES1 setting, the counter value is copied into the
Input Capture Register (ICR1). The event will also set the Input Capture Flag (ICF1), and this
can be used to cause an Input Capture Interrupt, if this interrupt is enabled.
When the ICR1 is used as TOP value (see description of the WGM13:0 bits located in the
TCCR1A and the TCCR1B Register), the ICP1 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 TCCR1B is written.
• Bit 4:3 – WGM13:2: Waveform Generation Mode
See TCCR1A Register description.
• Bit 2:0 – CS12:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter, see Figure
16-10 and Figure 16-11.
Table 16-6.
CS12
Clock Select Bit Description
CS11
CS10
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 T1 pin. Clock on falling edge.
1
1
1
External clock source on T1 pin. Clock on rising edge.
If external pin modes are used for the Timer/Counter1, transitions on the T1 pin will clock the
counter even if the pin is configured as an output. This feature allows software control of the
counting.
16.11.3
TCNT1H and TCNT1L –Timer/Counter1 High and Low Register
Bit
7
6
5
4
3
2
1
0
TCNT1[15:8]
TCNT1H
TCNT1[7:0]
TCNT1L
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
The two Timer/Counter I/O locations (TCNT1H and TCNT1L, combined TCNT1) give direct
access, both for read and for write operations, to the Timer/Counter unit 16-bit counter. To
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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 91.
Modifying the counter (TCNT1) while the counter is running introduces a risk of missing a compare match between TCNT1 and one of the OCR1x Registers.
Writing to the TCNT1 Register blocks (removes) the compare match on the following timer clock
for all compare units.
16.11.4
OCR1AH and OCR1AL – Output Compare Register 1 A
Bit
7
6
5
4
3
2
1
0
OCR1A[15:8]
OCR1AH
OCR1A[7:0]
16.11.5
OCR1AL
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
4
3
2
1
0
OCR1BH and OCR1BL – Output Compare Register 1 B
Bit
7
6
5
OCR1B[15:8]
OCR1BH
OCR1B[7:0]
OCR1BL
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
The Output Compare Registers contain a 16-bit value that is continuously compared with the
counter value (TCNT1). A match can be used to generate an output compare interrupt, or to
generate a waveform output on the OC1x pin.
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 91.
16.11.6
ICR1H and ICR1L – Input Capture Register 1
Bit
7
6
5
4
3
2
1
0
ICR1[15:8]
ICR1H
ICR1[7:0]
ICR1L
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
The Input Capture is updated with the counter (TCNT1) value each time an event occurs on the
ICP1 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 91.
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16.11.7
TIMSK – Timer/Counter Interrupt Mask Register(1)
Bit
7
6
5
4
3
2
1
0
OCIE2
TOIE2
TICIE1
OCIE1A
OCIE1B
TOIE1
OCIE0
TOIE0
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
Note:
TIMSK
1. This register contains interrupt control bits for several Timer/Counters, but only Timer1 bits are
described in this section. The remaining bits are described in their respective timer sections.
• Bit 5 – TICIE1: Timer/Counter1, 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/Counter1 Input Capture Interrupt is enabled. The corresponding Interrupt
Vector (See “Interrupts” on page 44.) is executed when the ICF1 Flag, located in TIFR, is set.
• Bit 4 – OCIE1A: Timer/Counter1, 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/Counter1 Output Compare A match interrupt is enabled. The corresponding
Interrupt Vector (See “Interrupts” on page 44.) is executed when the OCF1A Flag, located in
TIFR, is set.
• Bit 3 – OCIE1B: Timer/Counter1, 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/Counter1 Output Compare B match interrupt is enabled. The corresponding
Interrupt Vector (See “Interrupts” on page 44.) is executed when the OCF1B Flag, located in
TIFR, is set.
• Bit 2 – TOIE1: Timer/Counter1, 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/Counter1 Overflow Interrupt is enabled. The corresponding Interrupt Vector
(See “Interrupts” on page 44.) is executed when the TOV1 Flag, located in TIFR, is set.
16.11.8
TIFR – Timer/Counter Interrupt Flag Register
Bit
7
6
5
4
3
2
1
0
OCF2
TOV2
ICF1
OCF1A
OCF1B
TOV1
OCF0
TOV0
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
Note:
TIFR
This register contains flag bits for several Timer/Counters, but only Timer1 bits are described in
this section. The remaining bits are described in their respective timer sections.
• Bit 5 – ICF1: Timer/Counter1, Input Capture Flag
This flag is set when a capture event occurs on the ICP1 pin. When the Input Capture Register
(ICR1) is set by the WGM13:0 to be used as the TOP value, the ICF1 Flag is set when the counter reaches the TOP value.
ICF1 is automatically cleared when the Input Capture Interrupt Vector is executed. Alternatively,
ICF1 can be cleared by writing a logic one to its bit location.
• Bit 4 – OCF1A: Timer/Counter1, Output Compare A Match Flag
This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output
Compare Register A (OCR1A).
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Note that a Forced Output Compare (FOC1A) strobe will not set the OCF1A Flag.
OCF1A is automatically cleared when the Output Compare Match A Interrupt Vector is executed. Alternatively, OCF1A can be cleared by writing a logic one to its bit location.
• Bit 3 – OCF1B: Timer/Counter1, Output Compare B Match Flag
This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output
Compare Register B (OCR1B).
Note that a forced output compare (FOC1B) strobe will not set the OCF1B Flag.
OCF1B is automatically cleared when the Output Compare Match B Interrupt Vector is executed. Alternatively, OCF1B can be cleared by writing a logic one to its bit location.
• Bit 2 – TOV1: Timer/Counter1, Overflow Flag
The setting of this flag is dependent of the WGM13:0 bits setting. In normal and CTC modes, the
TOV1 Flag is set when the timer overflows. Refer to Table 16-5 on page 112 for the TOV1 Flag
behavior when using another WGM13:0 bit setting.
TOV1 is automatically cleared when the Timer/Counter1 Overflow interrupt vector is executed.
Alternatively, TOV1 can be cleared by writing a logic one to its bit location.
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ATmega16A
17. 8-bit Timer/Counter2 with PWM and Asynchronous Operation
17.1
Features
•
•
•
•
•
•
•
17.2
Single Compare unit Counter
Clear Timer on Compare Match (Auto Reload)
Glitch-free, Phase Correct Pulse Width Modulator (PWM)
Frequency Generator
10-bit Clock Prescaler
Overflow and Compare Match Interrupt Sources (TOV2 and OCF2)
Allows clocking from External 32 kHz Watch Crystal Independent of the I/O Clock
Overview
Timer/Counter2 is a general purpose, single compare unit, 8-bit Timer/Counter module. A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 17-1. For the actual placement
of I/O pins, refer to “Pinout ATmega16A” on page 2. 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 “Register Description” on page 130.
Figure 17-1. 8-bit Timer/Counter Block Diagram
TCCRn
count
TOVn
(Int.Req.)
clear
Control Logic
direction
clkTn
TOSC1
BOTTOM
TOP
Prescaler
T/C
Oscillator
TOSC2
Timer/Counter
TCNTn
=0
= 0xFF
DATABUS
OCn
(Int.Req.)
Waveform
Generation
=
clkI/O
OCn
OCRn
Synchronized Status flags
clkI/O
Synchronization Unit
clkASY
Status flags
ASSRn
asynchronous mode
select (ASn)
17.2.1
Registers
The Timer/Counter (TCNT2) and Output Compare Register (OCR2) are 8-bit registers. Interrupt
request (shorten as Int.Req.) signals are all visible in the Timer Interrupt Flag Register (TIFR).
All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK). TIFR and
TIMSK are not shown in the figure since these registers are shared by other timer units.
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The Timer/Counter can be clocked internally, via the prescaler, or asynchronously clocked from
the TOSC1/2 pins, as detailed later in this section. The asynchronous operation is controlled by
the Asynchronous Status Register (ASSR). The Clock Select logic block controls which clock
source 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 (clkT2).
The double buffered Output Compare Register (OCR2) is 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 Pin (OC2). See “Output Compare
Unit” on page 119. for details. The compare match event will also set the Compare Flag (OCF2)
which can be used to generate an output compare interrupt request.
17.2.2
Definitions
Many register and bit references in this document are written in general form. A lower case “n”
replaces the Timer/Counter number, in this case 2. However, when using the register or bit
defines in a program, the precise form must be used (i.e., TCNT2 for accessing Timer/Counter2
counter value and so on). The definitions in Table 17-1 are also used extensively throughout the
document.
Table 17-1.
17.3
Definitions
BOTTOM
The counter reaches the BOTTOM when it becomes zero (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 OCR2 Register. The assignment is dependent
on the mode of operation.
Timer/Counter Clock Sources
The Timer/Counter can be clocked by an internal synchronous or an external asynchronous
clock source. The clock source clkT2 is by default equal to the MCU clock, clkI/O. When the AS2
bit in the ASSR Register is written to logic one, the clock source is taken from the Timer/Counter
Oscillator connected to TOSC1 and TOSC2. For details on asynchronous operation, see “ASSR
– Asynchronous Status Register” on page 133. For details on clock sources and prescaler, see
“Timer/Counter Prescaler” on page 130.
17.4
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
17-2 shows a block diagram of the counter and its surrounding environment.
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ATmega16A
Figure 17-2. Counter Unit Block Diagram
TOVn
(Int.Req.)
DATA BUS
TOSC1
count
TCNTn
clear
clk Tn
Control Logic
Prescaler
T/C
Oscillator
direction
bottom
TOSC2
top
clkI/O
Signal description (internal signals):
count
Increment or decrement TCNT2 by 1.
direction
Selects between increment and decrement.
clear
Clear TCNT2 (set all bits to zero).
clkT2
Timer/Counter clock.
top
Signalizes that TCNT2 has reached maximum value.
bottom
Signalizes that TCNT2 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or decremented
at each timer clock (clkT2). clkT2 can be generated from an external or internal clock source,
selected by the Clock Select bits (CS22:0). When no clock source is selected (CS22:0 = 0) the
timer is stopped. However, the TCNT2 value can be accessed by the CPU, regardless of
whether clkT2 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 WGM21 and WGM20 bits located in
the Timer/Counter Control Register (TCCR2). There are close connections between how the
counter behaves (counts) and how waveforms are generated on the Output Compare output
OC2. For more details about advanced counting sequences and waveform generation, see
“Modes of Operation” on page 122.
The Timer/Counter Overflow (TOV2) Flag is set according to the mode of operation selected by
the WGM21:0 bits. TOV2 can be used for generating a CPU interrupt.
17.5
Output Compare Unit
The 8-bit comparator continuously compares TCNT2 with the Output Compare Register
(OCR2). Whenever TCNT2 equals OCR2, the comparator signals a match. A match will set the
Output Compare Flag (OCF2) at the next timer clock cycle. If enabled (OCIE2 = 1), the Output
Compare Flag generates an output compare interrupt. The OCF2 Flag is automatically cleared
when the interrupt is executed. Alternatively, the OCF2 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 WGM21:0 bits and Compare Output
mode (COM21:0) 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 122). Figure 17-3 shows a block diagram of the output compare unit.
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Figure 17-3. Output Compare Unit, Block Diagram
DATA BUS
OCRn
TCNTn
= (8-bit Comparator )
OCFn (Int.Req.)
top
bottom
Waveform Generator
OCxy
FOCn
WGMn1:0
COMn1:0
The OCR2 Register is 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 OCR2 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 OCR2 Register access may seem complex, but this is not case. When the double buffering
is enabled, the CPU has access to the OCR2 Buffer Register, and if double buffering is disabled
the CPU will access the OCR2 directly.
17.5.1
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 (FOC2) bit. Forcing compare match will not set the
OCF2 Flag or reload/clear the timer, but the OC2 pin will be updated as if a real compare match
had occurred (the COM21:0 bits settings define whether the OC2 pin is set, cleared or toggled).
17.5.2
Compare Match Blocking by TCNT2 Write
All CPU write operations to the TCNT2 Register will block any compare match that occurs in the
next timer clock cycle, even when the timer is stopped. This feature allows OCR2 to be initialized
to the same value as TCNT2 without triggering an interrupt when the Timer/Counter clock is
enabled.
17.5.3
Using the Output Compare Unit
Since writing TCNT2 in any mode of operation will block all compare matches for one timer clock
cycle, there are risks involved when changing TCNT2 when using the output compare unit, independently of whether the Timer/Counter is running or not. If the value written to TCNT2 equals
the OCR2 value, the compare match will be missed, resulting in incorrect waveform generation.
Similarly, do not write the TCNT2 value equal to BOTTOM when the counter is downcounting.
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ATmega16A
The setup of the OC2 should be performed before setting the Data Direction Register for the port
pin to output. The easiest way of setting the OC2 value is to use the Force Output Compare
(FOC2) strobe bit in Normal mode. The OC2 Register keeps its value even when changing
between Waveform Generation modes.
Be aware that the COM21:0 bits are not double buffered together with the compare value.
Changing the COM21:0 bits will take effect immediately.
17.6
Compare Match Output Unit
The Compare Output mode (COM21:0) bits have two functions. The Waveform Generator uses
the COM21:0 bits for defining the Output Compare (OC2) state at the next compare match. Also,
the COM21:0 bits control the OC2 pin output source. Figure 17-4 shows a simplified schematic
of the logic affected by the COM21:0 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 COM21:0 bits are shown. When referring to the OC2 state, the
reference is for the internal OC2 Register, not the OC2 pin.
Figure 17-4. Compare Match Output Unit, Schematic
COMn1
COMn0
FOCn
Waveform
Generator
D
Q
1
OCn
DATA BUS
D
0
OCn
Pin
Q
PORT
D
Q
DDR
clk I/O
The general I/O port function is overridden by the Output Compare (OC2) from the waveform
generator if either of the COM21:0 bits are set. However, the OC2 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 OC2 pin (DDR_OC2) must be set as output before the OC2 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 OC2 state before the output is enabled. Note that some COM21:0 bit settings are reserved for certain modes of
operation. See “Register Description” on page 130.
17.6.1
Compare Output Mode and Waveform Generation
The waveform generator uses the COM21:0 bits differently in Normal, CTC, and PWM modes.
For all modes, setting the COM21:0 = 0 tells the Waveform Generator that no action on the OC2
Register is to be performed on the next compare match. For compare output actions in the nonPWM modes refer to Table 17-3 on page 131. For fast PWM mode, refer to Table 17-4 on page
132, and for phase correct PWM refer to Table 17-5 on page 132.
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A change of the COM21:0 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
FOC2 strobe bits.
17.7
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 (WGM21:0) and Compare Output
mode (COM21:0) bits. The Compare Output mode bits do not affect the counting sequence,
while the Waveform Generation mode bits do. The COM21:0 bits control whether the PWM output generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes
the COM21:0 bits control whether the output should be set, cleared, or toggled at a compare
match (See “Compare Match Output Unit” on page 121.).
For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 126.
17.7.1
Normal Mode
The simplest mode of operation is the Normal mode (WGM21:0 = 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 8-bit value (TOP = 0xFF) and then restarts from the bottom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV2) will be set in the same
timer clock cycle as the TCNT2 becomes zero. The TOV2 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 TOV2 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.
17.7.2
Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM21:0 = 2), the OCR2 Register is used to manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value
(TCNT2) matches the OCR2. The OCR2 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 17-5. The counter value (TCNT2)
increases until a compare match occurs between TCNT2 and OCR2, and then counter (TCNT2)
is cleared.
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Figure 17-5. CTC Mode, Timing Diagram
OCn Interrupt Flag Set
TCNTn
OCn
(Toggle)
Period
(COMn1:0 = 1)
1
2
3
4
An interrupt can be generated each time the counter value reaches the TOP value by using the
OCF2 Flag. 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 OCR2 is lower than the current
value of TCNT2, 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 OC2 output can be set to toggle its logical
level on each compare match by setting the Compare Output mode bits to toggle mode
(COM21:0 = 1). The OC2 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 fOC2 = fclk_I/O/2
when OCR2 is set to zero (0x00). The waveform frequency is defined by the following equation:
f clk_I/O
f OCn = ----------------------------------------------2 ⋅ N ⋅ ( 1 + OCRn )
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
As for the Normal mode of operation, the TOV2 Flag is set in the same timer clock cycle that the
counter counts from MAX to 0x00.
17.7.3
Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM21:0 = 3) 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 MAX then restarts from BOTTOM. In
non-inverting Compare Output mode, the Output Compare (OC2) is cleared on the compare
match between TCNT2 and OCR2, 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 uses 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 MAX value.
The counter is then cleared at the following timer clock cycle. The timing diagram for the fast
PWM mode is shown in Figure 17-6. The TCNT2 value is in the timing diagram shown as a his123
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togram for illustrating the single-slope operation. The diagram includes non-inverted and
inverted PWM outputs. The small horizontal line marks on the TCNT2 slopes represent compare
matches between OCR2 and TCNT2.
Figure 17-6. Fast PWM Mode, Timing Diagram
OCRn Interrupt Flag Set
OCRn Update and
TOVn Interrupt Flag Set
TCNTn
OCn
(COMn1:0 = 2)
OCn
(COMn1:0 = 3)
Period
1
2
3
4
5
6
7
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches MAX. 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 OC2 pin. Setting the COM21:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can
be generated by setting the COM21:0 to 3 (see Table 17-4 on page 132). The actual OC2 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 OC2 Register at the compare match between
OCR2 and TCNT2, and clearing (or setting) the OC2 Register at the timer clock cycle the counter is cleared (changes from MAX to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
f clk_I/O
f OCnPWM = -----------------N ⋅ 256
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2 Register represent special cases when generating a PWM
waveform output in the fast PWM mode. If the OCR2 is set equal to BOTTOM, the output will be
a narrow spike for each MAX+1 timer clock cycle. Setting the OCR2 equal to MAX will result in a
constantly high or low output (depending on the polarity of the output set by the COM21:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC2 to toggle its logical level on each compare match (COM21:0 = 1). The waveform
generated will have a maximum frequency of foc2 = fclk_I/O/2 when OCR2 is set to zero. This feature is similar to the OC2 toggle in CTC mode, except the double buffer feature of the output
compare unit is enabled in the fast PWM mode.
17.7.4
124
Phase Correct PWM Mode
The phase correct PWM mode (WGM21:0 = 1) provides a high resolution phase correct PWM
waveform generation option. The phase correct PWM mode is based on a dual-slope operation.
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ATmega16A
The counter counts repeatedly from BOTTOM to MAX and then from MAX to BOTTOM. In noninverting Compare Output mode, the Output Compare (OC2) is cleared on the compare match
between TCNT2 and OCR2 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 is fixed to 8 bits. In phase correct PWM
mode the counter is incremented until the counter value matches MAX. When the counter
reaches MAX, it changes the count direction. The TCNT2 value will be equal to MAX for one
timer clock cycle. The timing diagram for the phase correct PWM mode is shown on Figure 17-7.
The TCNT2 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 TCNT2 slopes represent compare matches between OCR2 and TCNT2.
Figure 17-7. Phase Correct PWM Mode, Timing Diagram
OCn Interrupt Flag Set
OCRn Update
TOVn Interrupt Flag Set
TCNTn
OCn
(COMn1:0 = 2)
OCn
(COMn1:0 = 3)
Period
1
2
3
The Timer/Counter Overflow Flag (TOV2) 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
OC2 pin. Setting the COM21:0 bits to 2 will produce a non-inverted PWM. An inverted PWM output can be generated by setting the COM21:0 to 3 (see Table 17-5 on page 132). The actual
OC2 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 OC2 Register at the compare
match between OCR2 and TCNT2 when the counter increments, and setting (or clearing) the
OC2 Register at compare match between OCR2 and TCNT2 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 OCnPCPWM = -----------------N ⋅ 510
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The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2 Register represent special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR2 is set equal to BOTTOM, the output will be continuously low and if set equal to MAX the output will be continuously high for noninverted PWM mode. For inverted PWM the output will have the opposite logic values.
At the very start of Period 2 in Figure 17-7 OCn has a transition from high to l ow even though
there is no Compare Match. The point of this transition is to guarantee symmetry around BOTTOM. There are two cases that will give transition without Compare Match:
• OCR2A changes its value from Max, like in Figure 17-7. When the OCR2A 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 be correspond the the result of an
up-counting Compare Match.
• The Timer starts counting from a value higher than the one in OCR2A, and for that reason
misses the Compare Match and hence the OCn that would have happened on the way up.
17.8
Timer/Counter Timing Diagrams
The following figures show the Timer/Counter in Synchronous mode, and the timer clock (clkT2)
is therefore shown as a clock enable signal. In Asynchronous mode, clkI/O should be replaced by
the Timer/Counter Oscillator clock. The figures include information on when Interrupt Flags are
set. Figure 17-8 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 17-8. Timer/Counter Timing Diagram, no Prescaling
clkI/O
clkTn
(clkI/O /1)
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
TOVn
Figure 17-9 shows the same timing data, but with the prescaler enabled.
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Figure 17-9. 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 17-10 shows the setting of OCF2 in all modes except CTC mode.
Figure 17-10. Timer/Counter Timing Diagram, Setting of OCF2, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
OCRn
OCRn - 1
OCRn
OCRn + 1
OCRn + 2
OCRn Value
OCFn
Figure 17-11 shows the setting of OCF2 and the clearing of TCNT2 in CTC mode.
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Figure 17-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
(CTC)
TOP - 1
TOP
OCRn
BOTTOM
BOTTOM + 1
TOP
OCFn
17.9
Asynchronous Operation of the Timer/Counter2
When Timer/Counter2 operates asynchronously, some considerations must be taken.
• Warning: When switching between asynchronous and synchronous clocking of
Timer/Counter2, the Timer Registers TCNT2, OCR2, and TCCR2 might be corrupted. A safe
procedure for switching clock source is:
1. Disable the Timer/Counter2 interrupts by clearing OCIE2 and TOIE2.
2. Select clock source by setting AS2 as appropriate.
3. Write new values to TCNT2, OCR2, and TCCR2.
4. To switch to asynchronous operation: Wait for TCN2UB, OCR2UB, and TCR2UB.
5. Clear the Timer/Counter2 Interrupt Flags.
6. Enable interrupts, if needed.
• The Oscillator is optimized for use with a 32.768 kHz watch crystal. Applying an external
clock to the TOSC1 pin may result in incorrect Timer/Counter2 operation. The CPU main
clock frequency must be more than four times the Oscillator frequency.
• When writing to one of the registers TCNT2, OCR2, or TCCR2, the value is transferred to a
temporary register, and latched after two positive edges on TOSC1. The user should not
write a new value before the contents of the temporary register have been transferred to its
destination. Each of the three mentioned registers have their individual temporary register,
which means for example that writing to TCNT2 does not disturb an OCR2 write in progress.
To detect that a transfer to the destination register has taken place, the Asynchronous Status
Register – ASSR has been implemented.
• When entering Power-save or Extended Standby mode after having written to TCNT2,
OCR2, or TCCR2, the user must wait until the written register has been updated if
Timer/Counter2 is used to wake up the device. Otherwise, the MCU will enter sleep mode
before the changes are effective. This is particularly important if the Output Compare2
interrupt is used to wake up the device, since the output compare function is disabled during
writing to OCR2 or TCNT2. If the write cycle is not finished, and the MCU enters sleep mode
before the OCR2UB bit returns to zero, the device will never receive a compare match
interrupt, and the MCU will not wake up.
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• If Timer/Counter2 is used to wake the device up from Power-save or Extended Standby
mode, precautions must be taken if the user wants to re-enter one of these modes: The
interrupt logic needs one TOSC1 cycle to be reset. If the time between wake-up and reentering sleep mode is less than one TOSC1 cycle, the interrupt will not occur, and the
device will fail to wake up. If the user is in doubt whether the time before re-entering Powersave or Extended Standby mode is sufficient, the following algorithm can be used to ensure
that one TOSC1 cycle has elapsed:
1. Write a value to TCCR2, TCNT2, or OCR2.
2. Wait until the corresponding Update Busy Flag in ASSR returns to zero.
3. Enter Power-save or Extended Standby mode.
• When the asynchronous operation is selected, the 32.768 kHz Oscillator for Timer/Counter2
is always running, except in Power-down and Standby modes. After a Power-up Reset or
wake-up from Power-down or Standby mode, the user should be aware of the fact that this
Oscillator might take as long as one second to stabilize. The user is advised to wait for at
least one second before using Timer/Counter2 after power-up or wake-up from Power-down
or Standby mode. The contents of all Timer/Counter2 Registers must be considered lost after
a wake-up from Power-down or Standby mode due to unstable clock signal upon start-up, no
matter whether the Oscillator is in use or a clock signal is applied to the TOSC1 pin.
• Description of wake up from Power-save or Extended Standby mode when the timer is
clocked asynchronously: When the interrupt condition is met, the wake up process is started
on the following cycle of the timer clock, that is, the timer is always advanced by at least one
before the processor can read the counter value. After wake-up, the MCU is halted for four
cycles, it executes the interrupt routine, and resumes execution from the instruction following
SLEEP.
• Reading of the TCNT2 Register shortly after wake-up from Power-save may give an incorrect
result. Since TCNT2 is clocked on the asynchronous TOSC clock, reading TCNT2 must be
done through a register synchronized to the internal I/O clock domain. Synchronization takes
place for every rising TOSC1 edge. When waking up from Power-save mode, and the I/O
clock (clkI/O) again becomes active, TCNT2 will read as the previous value (before entering
sleep) until the next rising TOSC1 edge. The phase of the TOSC clock after waking up from
Power-save mode is essentially unpredictable, as it depends on the wake-up time. The
recommended procedure for reading TCNT2 is thus as follows:
1. Write any value to either of the registers OCR2 or TCCR2.
2. Wait for the corresponding Update Busy Flag to be cleared.
3. Read TCNT2.
• During asynchronous operation, the synchronization of the Interrupt Flags for the
asynchronous timer takes three processor cycles plus one timer cycle. The timer is therefore
advanced by at least one before the processor can read the timer value causing the setting of
the Interrupt Flag. The output compare pin is changed on the timer clock and is not
synchronized to the processor clock.
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17.10 Timer/Counter Prescaler
Figure 17-12. Prescaler for Timer/Counter2
clkT2S
PSR2
clkT2S/1024
clkT2S/256
clkT2S/128
clkT2S/8
AS2
clkT2S/64
10-BIT T/C PRESCALER
Clear
TOSC1
clkT2S/32
clkI/O
0
CS20
CS21
CS22
TIMER/COUNTER2 CLOCK SOURCE
clkT2
The clock source for Timer/Counter2 is named clkT2S. clkT2S is by default connected to the main
system I/O clock clk IO. By setting the AS2 bit in ASSR, Timer/Counter2 is asynchronously
clocked from the TOSC1 pin. This enables use of Timer/Counter2 as a Real Time Counter
(RTC). When AS2 is set, pins TOSC1 and TOSC2 are disconnected from Port C. A crystal can
then be connected between the TOSC1 and TOSC2 pins to serve as an independent clock
source for Timer/Counter2. The Oscillator is optimized for use with a 32.768 kHz crystal. Applying an external clock source to TOSC1 is not recommended.
For Timer/Counter2, the possible prescaled selections are: clk T2S /8, clk T2S /32, clk T2S /64,
clkT2S/128, clkT2S/256, and clkT2S/1024. Additionally, clkT2S as well as 0 (stop) may be selected.
Setting the PSR2 bit in “SFIOR – Special Function IO Register” on page 134, resets the prescaler. This allows the user to operate with a predictable prescaler.
17.11 Register Description
17.11.1
TCCR2 – Timer/Counter Control Register
Bit
7
6
5
4
3
2
1
0
FOC2
WGM20
COM21
COM20
WGM21
CS22
CS21
CS20
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
TCCR2
• Bit 7 – FOC2: Force Output Compare
The FOC2 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 TCCR2 is written when
operating in PWM mode. When writing a logical one to the FOC2 bit, an immediate compare
match is forced on the waveform generation unit. The OC2 output is changed according to its
COM21:0 bits setting. Note that the FOC2 bit is implemented as a strobe. Therefore it is the
value present in the COM21:0 bits that determines the effect of the forced compare.
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A FOC2 strobe will not generate any interrupt, nor will it clear the timer in CTC mode using
OCR2 as TOP.
The FOC2 bit is always read as zero.
• Bit 3, 6 – WGM2[1:0]: Waveform Generation Mode
These bits control the counting sequence of the counter, the source for the maximum (TOP)
counter value, and what type of waveform generation to be used. Modes of operation supported
by the Timer/Counter unit are: Normal mode, Clear Timer on Compare match (CTC) mode, and
two types of Pulse Width Modulation (PWM) modes. See Table 17-2 and “Modes of Operation”
on page 122.
Table 17-2.
Waveform Generation Mode Bit Description(1)
Mode
WGM21
(CTC2)
WGM20
(PWM2)
Timer/Counter Mode of
Operation
TOP
Update of
OCR2
TOV2 Flag
Set on
0
0
0
Normal
0xFF
Immediate
MAX
1
0
1
PWM, Phase Correct
0xFF
TOP
BOTTOM
2
1
0
CTC
OCR2
Immediate
MAX
3
1
1
Fast PWM
0xFF
BOTTOM
MAX
Note:
1. The CTC2 and PWM2 bit definition names are now obsolete. Use the WGM21:0 definitions.
However, the functionality and location of these bits are compatible with previous versions of
the timer.
• Bit 5:4 – COM21:0: Compare Match Output Mode
These bits control the Output Compare pin (OC2) behavior. If one or both of the COM21:0 bits
are set, the OC2 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 OC2 pin must be set
in order to enable the output driver.
When OC2 is connected to the pin, the function of the COM21:0 bits depends on the WGM21:0
bit setting. Table 17-3 shows the COM21:0 bit functionality when the WGM21:0 bits are set to a
normal or CTC mode (non-PWM).
Table 17-3.
Compare Output Mode, non-PWM Mode
COM21
COM20
Description
0
0
Normal port operation, OC2 disconnected.
0
1
Toggle OC2 on compare match
1
0
Clear OC2 on compare match
1
1
Set OC2 on compare match
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Table 17-4 shows the COM21:0 bit functionality when the WGM21:0 bits are set to fast PWM
mode.
Table 17-4.
Compare Output Mode, Fast PWM Mode(1)
COM21
COM20
0
0
Normal port operation, OC2 disconnected.
0
1
Reserved
1
0
Clear OC2 on compare match, set OC2 at BOTTOM,
(non-inverting mode)
1
1
Set OC2 on compare match, clear OC2 at BOTTOM,
(inverting mode)
Note:
Description
1. A special case occurs when OCR2 equals TOP and COM21 is set. In this case, the compare
match is ignored, but the set or clear is done at BOTTOM. See “Fast PWM Mode” on page 123
for more details.
Table 17-5 shows the COM21:0 bit functionality when the WGM21:0 bits are set to phase correct PWM mode
.
Table 17-5.
Compare Output Mode, Phase Correct PWM Mode(1)
COM21
COM20
0
0
Normal port operation, OC2 disconnected.
0
1
Reserved
1
0
Clear OC2 on compare match when up-counting. Set OC2 on compare match
when downcounting.
1
1
Set OC2 on compare match when up-counting. Clear OC2 on compare match
when downcounting.
Note:
Description
1. A special case occurs when OCR2 equals TOP and COM21 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
124 for more details.
• Bit 2:0 – CS22:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter, see Table
17-6.
Table 17-6.
132
Clock Select Bit Description
CS22
CS21
CS20
Description
0
0
0
No clock source (Timer/Counter stopped).
0
0
1
clkT2S/(No prescaling)
0
1
0
clkT2S/8 (From prescaler)
0
1
1
clkT2S/32 (From prescaler)
1
0
0
clkT2S/64 (From prescaler)
1
0
1
clkT2S/128 (From prescaler)
1
1
0
clkT2S/256 (From prescaler)
1
1
1
clkT2S/1024 (From prescaler)
ATmega16A
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ATmega16A
17.11.2
TCNT2 – Timer/Counter Register
Bit
7
6
5
4
3
2
1
0
TCNT2[7:0]
TCNT2
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
The Timer/Counter Register gives direct access, both for read and write operations, to the
Timer/Counter unit 8-bit counter. Writing to the TCNT2 Register blocks (removes) the compare
match on the following timer clock. Modifying the counter (TCNT2) while the counter is running,
introduces a risk of missing a compare match between TCNT2 and the OCR2 Register.
17.11.3
OCR2 – Output Compare Register
Bit
7
6
5
4
3
2
1
0
OCR2[7:0]
OCR2
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
The Output Compare Register contains an 8-bit value that is continuously compared with the
counter value (TCNT2). A match can be used to generate an output compare interrupt, or to
generate a waveform output on the OC2 pin.
17.11.4
ASSR – Asynchronous Status Register
Bit
7
6
5
4
3
2
1
0
–
–
–
–
AS2
TCN2UB
OCR2UB
TCR2UB
Read/Write
R
R
R
R
R/W
R
R
R
Initial Value
0
0
0
0
0
0
0
0
ASSR
• Bit 3 – AS2: Asynchronous Timer/Counter2
When AS2 is written to zero, Timer/Counter 2 is clocked from the I/O clock, clkI/O. When AS2 is
written to one, Timer/Counter2 is clocked from a Crystal Oscillator connected to the Timer Oscillator 1 (TOSC1) pin. When the value of AS2 is changed, the contents of TCNT2, OCR2, and
TCCR2 might be corrupted.
• Bit 2 – TCN2UB: Timer/Counter2 Update Busy
When Timer/Counter2 operates asynchronously and TCNT2 is written, this bit becomes set.
When TCNT2 has been updated from the temporary storage register, this bit is cleared by hardware. A logical zero in this bit indicates that TCNT2 is ready to be updated with a new value.
• Bit 1 – OCR2UB: Output Compare Register2 Update Busy
When Timer/Counter2 operates asynchronously and OCR2 is written, this bit becomes set.
When OCR2 has been updated from the temporary storage register, this bit is cleared by hardware. A logical zero in this bit indicates that OCR2 is ready to be updated with a new value.
• Bit 0 – TCR2UB: Timer/Counter Control Register2 Update Busy
When Timer/Counter2 operates asynchronously and TCCR2 is written, this bit becomes set.
When TCCR2 has been updated from the temporary storage register, this bit is cleared by hardware. A logical zero in this bit indicates that TCCR2 is ready to be updated with a new value.
If a write is performed to any of the three Timer/Counter2 Registers while its update busy flag is
set, the updated value might get corrupted and cause an unintentional interrupt to occur.
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The mechanisms for reading TCNT2, OCR2, and TCCR2 are different. When reading TCNT2,
the actual timer value is read. When reading OCR2 or TCCR2, the value in the temporary storage register is read.
17.11.5
TIMSK – Timer/Counter Interrupt Mask Register
Bit
7
6
5
4
3
2
1
0
OCIE2
TOIE2
TICIE1
OCIE1A
OCIE1B
TOIE1
OCIE0
TOIE0
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
TIMSK
• Bit 7 – OCIE2: Timer/Counter2 Output Compare Match Interrupt Enable
When the OCIE2 bit is written to one and the I-bit in the Status Register is set (one), the
Timer/Counter2 Compare Match interrupt is enabled. The corresponding interrupt is executed if
a compare match in Timer/Counter2 occurs, i.e., when the OCF2 bit is set in the Timer/Counter
Interrupt Flag Register – TIFR.
• Bit 6 – TOIE2: Timer/Counter2 Overflow Interrupt Enable
When the TOIE2 bit is written to one and the I-bit in the Status Register is set (one), the
Timer/Counter2 Overflow interrupt is enabled. The corresponding interrupt is executed if an
overflow in Timer/Counter2 occurs, i.e., when the TOV2 bit is set in the Timer/Counter Interrupt
Flag Register – TIFR.
17.11.6
TIFR – Timer/Counter Interrupt Flag Register
Bit
7
6
5
4
3
2
1
0
OCF2
TOV2
ICF1
OCF1A
OCF1B
TOV1
OCF0
TOV0
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
TIFR
• Bit 7 – OCF2: Output Compare Flag 2
The OCF2 bit is set (one) when a compare match occurs between the Timer/Counter2 and the
data in OCR2 – Output Compare Register2. OCF2 is cleared by hardware when executing the
corresponding interrupt handling vector. Alternatively, OCF2 is cleared by writing a logic one to
the flag. When the I-bit in SREG, OCIE2 (Timer/Counter2 Compare match Interrupt Enable), and
OCF2 are set (one), the Timer/Counter2 Compare match Interrupt is executed.
• Bit 6 – TOV2: Timer/Counter2 Overflow Flag
The TOV2 bit is set (one) when an overflow occurs in Timer/Counter2. TOV2 is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, TOV2 is cleared
by writing a logic one to the flag. When the SREG I-bit, TOIE2 (Timer/Counter2 Overflow Interrupt Enable), and TOV2 are set (one), the Timer/Counter2 Overflow interrupt is executed. In
PWM mode, this bit is set when Timer/Counter2 changes counting direction at $00.
17.11.7
SFIOR – Special Function IO Register
Bit
7
6
5
4
3
2
1
0
ADTS2
ADTS1
ADTS0
–
ACME
PUD
PSR2
PSR10
Read/Write
R/W
R/W
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
SFIOR
• Bit 1 – PSR2: Prescaler Reset Timer/Counter2
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When this bit is written to one, the Timer/Counter2 prescaler 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 be read as zero if Timer/Counter2 is clocked by the internal CPU clock. If this bit is
written when Timer/Counter2 is operating in asynchronous mode, the bit will remain one until the
prescaler has been reset.
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18. SPI – Serial Peripheral Interface
18.1
Features
•
•
•
•
•
•
•
•
18.2
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
Overview
The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the
ATmega16A and peripheral devices or between several AVR devices. The ATmega16A SPI
iSPI Block Diagram(1)
SPI2X
SPI2X
DIVIDER
/2/4/8/16/32/64/128
Note:
1. Refer to Figure 1-1 on page 2, and Table 12-6 on page 57 for SPI pin placement.
The interconnection between Master and Slave CPUs with SPI is shown in Figure 18-1. 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
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Slave prepare the data to be sent in their respective Shift Registers, and the Master generates
the required clock pulses on the 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 18-1. SPI Master-Slave Interconnection
MSB
MASTER
LSB
MISO
MISO
8 BIT SHIFT REGISTER
SPI
CLOCK GENERATOR
MSB
SLAVE
LSB
8 BIT SHIFT REGISTER
MOSI
MOSI
SCK
SCK
SS
SHIFT
ENABLE
SS
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 minimum low and high periods should be:
Low periods: Longer than 2 CPU clock cycles.
High periods: Longer than 2 CPU clock cycles.
When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden
according to Table 18-1 on page 138. For more details on automatic port overrides, refer to
“Alternate Port Functions” on page 54.
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Table 18-1.
SPI Pin Overrides
Pin
Direction, Master SPI
Direction, Slave SPI
MOSI
User Defined
Input
MISO
Input
User Defined
SCK
User Defined
Input
SS
User Defined
Input
Note:
See “Alternate Functions of Port B” on page 57 for a detailed description of how to define the
direction of the user defined SPI pins.
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. For example if MOSI is placed on pin PB5, replace
DD_MOSI with DDB5 and DDR_SPI with DDRB.
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ATmega16A
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 “About Code Examples” on page 7.
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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, all others input
ldi
r17,(1<<DD_MISO)
out
DDR_SPI,r17
; Enable SPI
ldi
r17,(1<<SPE)
out
SPCR,r17
ret
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:
140
1. See “About Code Examples” on page 7.
ATmega16A
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ATmega16A
18.3
18.3.1
SS Pin Functionality
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 except MISO which can be user
configured as an output, 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.
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.
18.3.2
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.
18.4
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
18-2 and Figure 18-3. 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 18-3 and Table 18-4, as done below:
Table 18-2.
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 18-2. 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
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
Figure 18-3. 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)
142
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
ATmega16A
8154B–AVR–07/09
ATmega16A
18.5
18.5.1
Register Description
SPCR – SPI Control Register
Bit
7
6
5
4
3
2
1
0
SPIE
SPE
DORD
MSTR
CPOL
CPHA
SPR1
SPR0
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
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 18-2 and Figure 18-3 for an example. The CPOL functionality is summarized below:
Table 18-3.
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 18-2 and Figure 18-3 for an example. The CPHA
functionality is summarized below:
Table 18-4.
CPHA Functionality
CPHA
Leading Edge
Trailing Edge
0
Sample
Setup
1
Setup
Sample
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• 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 in the following table:
Table 18-5.
18.5.2
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
SPSR – SPI Status Register
Bit
7
6
5
4
3
2
1
0
SPIF
WCOL
–
–
–
–
–
SPI2X
Read/Write
R
R
R
R
R
R
R
R/W
Initial Value
0
0
0
0
0
0
0
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 bits in the ATmega16A 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 18-5). 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 ATmega16A is also used for program memory and EEPROM downloading or uploading. See page 276 for SPI Serial Programming and Verification.
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ATmega16A
18.5.3
SPDR – SPI Data Register
Bit
7
6
5
4
3
2
1
MSB
0
LSB
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
X
X
X
X
X
X
X
X
SPDR
Undefined
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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19. USART
19.1
Features
•
•
•
•
•
•
•
•
•
•
•
•
19.2
Full Duplex Operation (Independent Serial Receive and Transmit Registers)
Asynchronous or Synchronous Operation
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
Overview
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a
highly flexible serial communication device. A simplified block diagram of the USART transmitter
is shown in Figure 19-1. CPU accessible I/O Registers and I/O pins are shown in bold.
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ATmega16A
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ATmega16A
Figure 19-1. USART Block Diagram(1)
Clock Generator
UBRR[H:L]
OSC
BAUD RATE GENERATOR
SYNC LOGIC
PIN
CONTROL
XCK
Transmitter
TX
CONTROL
UDR (Transmit)
DATABUS
PARITY
GENERATOR
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
1. Refer to Figure 1-1 on page 2, Table 12-14 on page 64, and Table 12-8 on page 59 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 XCK (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 (UDR). The receiver supports the same frame formats as the transmitter, and can
detect frame error, data overrun and parity errors.
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19.2.1
AVR USART vs. AVR UART – Compatibility
The USART is fully compatible with the AVR UART regarding:
• Bit locations inside all USART Registers
• Baud Rate Generation
• Transmitter Operation
• Transmit Buffer Functionality
• Receiver Operation
However, the receive buffering has two improvements that will affect the compatibility in some
special cases:
• A second Buffer Register has been added. The two Buffer Registers operate as a circular
FIFO buffer. Therefore the UDR must only be read once for each incoming data! More
important is the fact that the Error Flags (FE and DOR) and the 9th data bit (RXB8) are
buffered with the data in the receive buffer. Therefore the status bits must always be read
before the UDR Register is read. Otherwise the error status will be lost since the buffer state
is lost.
• The receiver Shift Register can now act as a third buffer level. This is done by allowing the
received data to remain in the serial Shift Register (see Figure 19-1) if the Buffer Registers
are full, until a new start bit is detected. The USART is therefore more resistant to Data
OverRun (DOR) error conditions.
The following control bits have changed name, but have same functionality and register location:
• CHR9 is changed to UCSZ2
• OR is changed to DOR
19.3
Clock Generation
The clock generation logic generates the base clock for the Transmitter and Receiver. The
USART supports four modes of clock operation: Normal Asynchronous, Double Speed Asynchronous, Master Synchronous and Slave Synchronous mode. The UMSEL bit in USART
Control and Status Register C (UCSRC) selects between asynchronous and synchronous operation. Double Speed (Asynchronous mode only) is controlled by the U2X found in the UCSRA
Register. When using Synchronous mode (UMSEL = 1), the Data Direction Register for the XCK
pin (DDR_XCK) controls whether the clock source is internal (Master mode) or external (Slave
mode). The XCK pin is only active when using Synchronous mode.
Figure 19-2 shows a block diagram of the clock generation logic.
148
ATmega16A
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ATmega16A
Figure 19-2. 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
0
UMSEL
1
xcko
DDR_XCK
txclk
1
UCPOL
1
rxclk
0
Signal description:
txclk
Transmitter clock (Internal Signal).
rxclk
Receiver base clock (Internal Signal).
xcki
operation.
19.3.1
Input from XCK pin (Internal Signal). Used for synchronous Slave
xcko
Clock output to XCK pin (Internal Signal). Used for synchronous Master
operation.
fosc
XTAL pin frequency (System Clock).
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 19-2.
The USART Baud Rate Register (UBRR) 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 UBRR value each time the counter has counted down to zero or when
the UBRRL Register is written. A clock is generated each time the counter reaches zero. This
clock is the baud rate generator clock output (= fosc/(UBRR+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
UMSEL, U2X and DDR_XCK bits.
Table 19-1 contains equations for calculating the baud rate (in bits per second) and for calculating the UBRR value for each mode of operation using an internally generated clock source.
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Table 19-1.
Equations for Calculating Baud Rate Register Setting
Equation for Calculating
Baud Rate(1)
Equation for Calculating
UBRR Value
f OSC
BAUD = --------------------------------------16 ( UBRR + 1 )
f OSC
UBRR = ------------------------ – 1
16BAUD
Asynchronous Double Speed Mode (U2X
= 1)
f OSC
BAUD = ----------------------------------8 ( UBRR + 1 )
f OSC
UBRR = -------------------- – 1
8BAUD
Synchronous Master Mode
f OSC
BAUD = ----------------------------------2 ( UBRR + 1 )
f OSC
UBRR = -------------------- – 1
2BAUD
Operating Mode
Asynchronous Normal Mode
(U2X = 0)
Note:
1. 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
UBRR
Contents of the UBRRH and UBRRL Registers, (0 - 4095)
Some examples of UBRR values for some system clock frequencies are found in Table 19-9
(see page 171).
19.3.2
Double Speed Operation (U2X)
The transfer rate can be doubled by setting the U2X bit in UCSRA. 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.
19.3.3
External Clock
External clocking is used by the synchronous Slave modes of operation. The description in this
section refers to Figure 19-2 for details.
External clock input from the XCK pin is sampled by a synchronization register to minimize the
chance of meta-stability. 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 XCK 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.
19.3.4
150
Synchronous Clock Operation
When Synchronous mode is used (UMSEL = 1), the XCK 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 basic principle is that data input (on RxD) is sampled at the
opposite XCK clock edge of the edge the data output (TxD) is changed.
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 19-3. Synchronous Mode XCK Timing.
UCPOL = 1
XCK
RxD / TxD
Sample
UCPOL = 0
XCK
RxD / TxD
Sample
The UCPOL bit UCRSC selects which XCK clock edge is used for data sampling and which is
used for data change. As Figure 19-3 shows, when UCPOL is zero the data will be changed at
rising XCK edge and sampled at falling XCK edge. If UCPOL is set, the data will be changed at
falling XCK edge and sampled at rising XCK edge.
19.4
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.
Figure 19-4 illustrates the possible combinations of the frame formats. Bits inside brackets are
optional.
Figure 19-4. Frame Formats
FRAME
(IDLE)
St
0
1
2
3
4
[5]
[6]
[7]
[8]
[P] Sp1 [Sp2]
(St / IDLE)
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 (RxD or TxD). An IDLE line must
be
high.
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The frame format used by the USART is set by the UCSZ2:0, UPM1:0, and USBS bits in
UCSRB and UCSRC. 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 (UCSZ2:0) bits select the number of data bits in the frame. The
USART Parity mode (UPM1:0) 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 (USBS) 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.
19.4.1
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
Parity bit using even parity
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.
19.5
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 TXC 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 TXC Flag must be
cleared before each transmission (before UDR 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 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. When the function writes to the UCSRC Register, the URSEL bit (MSB) must be set due to
the sharing of I/O location by UBRRH and UCSRC.
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ATmega16A
Assembly Code Example(1)
USART_Init:
; Set baud rate
out
UBRRH, r17
out
UBRRL, r16
; Enable receiver and transmitter
ldi
r16, (1<<RXEN)|(1<<TXEN)
out
UCSRB,r16
; Set frame format: 8data, 2stop bit
ldi
r16, (1<<URSEL)|(1<<USBS)|(3<<UCSZ0)
out
UCSRC,r16
ret
C Code Example(1)
#define FOSC 1843200// Clock Speed
#define BAUD 9600
#define MYUBRR FOSC/16/BAUD-1
void main( void )
{
:.
USART_Init ( MYUBRR );
:.
}
void USART_Init( unsigned int ubrr)
{
/* Set baud rate */
UBRRH = (unsigned char)(ubrr>>8);
UBRRL = (unsigned char)ubrr;
/* Enable receiver and transmitter */
UCSRB = (1<<RXEN)|(1<<TXEN);
/* Set frame format: 8data, 2stop bit */
UCSRC = (1<<URSEL)|(1<<USBS)|(3<<UCSZ0);
}
Note:
1. See “About Code Examples” on page 7.
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.
19.6
Data Transmission – The USART Transmitter
The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRB
Register. When the Transmitter is enabled, the normal port operation of the TxD 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 XCK pin will be overridden and used as transmission
clock.
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19.6.1
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 UDR 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,
U2X bit or by XCK depending on mode of operation.
The following code examples show a simple USART transmit function based on polling of the
Data Register Empty (UDRE) Flag. When using frames with less than eight bits, the most significant bits written to the UDR 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 UCSRA,UDRE
rjmp USART_Transmit
; Put data (r16) into buffer, sends the data
out
UDR,r16
ret
C Code Example(1)
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRA & (1<<UDRE)) )
;
/* Put data into buffer, sends the data */
UDR = data;
}
Note:
1. See “About Code Examples” on page 7.
The function simply waits for the transmit buffer to be empty by checking the UDRE 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.
19.6.2
154
Sending Frames with 9 Data Bit
If 9-bit characters are used (UCSZ = 7), the ninth bit must be written to the TXB8 bit in UCSRB
before the Low byte of the character is written to UDR. The following code examples show a
transmit function that handles 9-bit 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(Note:)
USART_Transmit:
; Wait for empty transmit buffer
sbis UCSRA,UDRE
rjmp USART_Transmit
; Copy 9th bit from r17 to TXB8
cbi
UCSRB,TXB8
sbrc r17,0
sbi
UCSRB,TXB8
; Put LSB data (r16) into buffer, sends the data
out
UDR,r16
ret
C Code Example(Note:)
void USART_Transmit( unsigned int data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRA & (1<<UDRE))) )
;
/* Copy 9th bit to TXB8 */
UCSRB &= ~(1<<TXB8);
if ( data & 0x0100 )
UCSRB |= (1<<TXB8);
/* Put data into buffer, sends the data */
UDR = data;
}
Note:
These transmit functions are written to be general functions. They can be optimized if the contents
of the UCSRB is static. (i.e., only the TXB8 bit of the UCSRB Register is used after initialization).
The ninth bit can be used for indicating an address frame when using multi processor communication mode or for other protocol handling as for example synchronization.
19.6.3
Transmitter Flags and Interrupts
The USART transmitter has two flags that indicate its state: USART Data Register Empty
(UDRE) and Transmit Complete (TXC). Both flags can be used for generating interrupts.
The Data Register Empty (UDRE) 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 UCSRA Register.
When the Data Register empty Interrupt Enable (UDRIE) bit in UCSRB is written to one, the
USART Data Register Empty Interrupt will be executed as long as UDRE is set (provided that
global interrupts are enabled). UDRE is cleared by writing UDR. When interrupt-driven data
transmission is used, the Data Register Empty Interrupt routine must either write new data to
UDR in order to clear UDRE or disable the Data Register empty Interrupt, otherwise a new interrupt will occur once the interrupt routine terminates.
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The Transmit Complete (TXC) 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 TXC 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 TXC Flag is useful in half-duplex
communication interfaces (like the RS485 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 (TXCIE) bit in UCSRB is set, the USART Transmit
Complete Interrupt will be executed when the TXC 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 TXC Flag, this is done automatically when the interrupt is executed.
19.6.4
Parity Generator
The parity generator calculates the parity bit for the serial frame data. When parity bit is enabled
(UPM1 = 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.
19.6.5
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 TxD pin.
19.7
Data Reception – The USART Receiver
The USART Receiver is enabled by writing the Receive Enable (RXEN) bit in the UCSRB Register to one. When the receiver is enabled, the normal pin operation of the RxD 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 XCK pin will be used as transfer clock.
19.7.1
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 XCK 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 UDR I/O location.
The following code example shows a simple USART receive function based on polling of the
Receive Complete (RXC) Flag. When using frames with less than eight bits the most significant
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bits of the data read from the UDR will be masked to zero. The USART has to be initialized
before the function can be used.
Assembly Code Example(1)
USART_Receive:
; Wait for data to be received
sbis UCSRA, RXC
rjmp USART_Receive
; Get and return received data from buffer
in
r16, UDR
ret
C Code Example(1)
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSRA & (1<<RXC)) )
;
/* Get and return received data from buffer */
return UDR;
}
Note:
1. See “About Code Examples” on page 7.
The function simply waits for data to be present in the receive buffer by checking the RXC Flag,
before reading the buffer and returning the value.
19.7.2
Receiving Frames with 9 Databits
If 9 bit characters are used (UCSZ=7) the ninth bit must be read from the RXB8 bit in UCSRB
before reading the low bits from the UDR. This rule applies to the FE, DOR and PE status Flags
as well. Read status from UCSRA, then data from UDR. Reading the UDR I/O location will
change the state of the receive buffer FIFO and consequently the TXB8, FE, DOR and PE bits,
which all are stored in the FIFO, will change.
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The following code example shows a simple USART receive function that handles both 9-bit
characters and the status bits.
Assembly Code Example(1)
USART_Receive:
; Wait for data to be received
sbis UCSRA, RXC
rjmp USART_Receive
; Get status and 9th bit, then data from buffer
in
r18, UCSRA
in
r17, UCSRB
in
r16, UDR
; If error, return -1
andi r18,(1<<FE)|(1<<DOR)|(1<<PE)
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 ( !(UCSRA & (1<<RXC)) )
;
/* Get status and 9th bit, then data */
/* from buffer */
status = UCSRA;
resh = UCSRB;
resl = UDR;
/* If error, return -1 */
if ( status & (1<<FE)|(1<<DOR)|(1<<PE) )
return -1;
/* Filter the 9th bit, then return */
resh = (resh >> 1) & 0x01;
return ((resh << 8) | resl);
}
Note:
1. See “About Code Examples” on page 7.
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.
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19.7.3
Receive Compete Flag and Interrupt
The USART Receiver has one flag that indicates the receiver state.
The Receive Complete (RXC) 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 contain any unread data). If the receiver is disabled (RXEN = 0),
the receive buffer will be flushed and consequently the RXC bit will become zero.
When the Receive Complete Interrupt Enable (RXCIE) in UCSRB is set, the USART Receive
Complete Interrupt will be executed as long as the RXC 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 UDR in order to clear the RXC Flag, otherwise a new interrupt
will occur once the interrupt routine terminates.
19.7.4
Receiver Error Flags
The USART Receiver has three Error Flags: Frame Error (FE), Data OverRun (DOR) and Parity
Error (PE). All can be accessed by reading UCSRA. 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 UCSRA must be read before the receive buffer (UDR),
since reading the UDR 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 UCSRA is written for upward compatibility of future
USART implementations. None of the Error Flags can generate interrupts.
The Frame Error (FE) Flag indicates the state of the first stop bit of the next readable frame
stored in the receive buffer. The FE Flag is zero when the stop bit was correctly read (as one),
and the FE 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 FE Flag
is not affected by the setting of the USBS bit in UCSRC 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
UCSRA.
The Data OverRun (DOR) 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 DOR Flag is set there was one or
more serial frame lost between the frame last read from UDR, and the next frame read from
UDR. For compatibility with future devices, always write this bit to zero when writing to UCSRA.
The DOR Flag is cleared when the frame received was successfully moved from the Shift Register to the receive buffer.
The Parity Error (PE) Flag indicates that the next frame in the receive buffer had a parity error
when received. If parity check is not enabled the PE bit will always be read zero. For compatibility with future devices, always set this bit to zero when writing to UCSRA. For more details see
“Parity Bit Calculation” on page 152 and “Parity Checker” on page 159.
19.7.5
Parity Checker
The Parity Checker is active when the high USART Parity mode (UPM1) bit is set. Type of parity
check to be performed (odd or even) is selected by the UPM0 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 (PE) Flag can then be read by software to
check if the frame had a parity error.
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The PE 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 (UPM1 = 1). This bit is valid
until the receive buffer (UDR) is read.
19.7.6
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 RXEN is set to zero) the Receiver will
no longer override the normal function of the RxD port pin. The receiver buffer FIFO will be
flushed when the receiver is disabled. Remaining data in the buffer will be lost
19.7.7
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 UDR I/O location until the RXC Flag is
cleared. The following code example shows how to flush the receive buffer.
Assembly Code Example(1)
USART_Flush:
sbis UCSRA, RXC
ret
in
r16, UDR
rjmp USART_Flush
C Code Example(1)
void USART_Flush( void )
{
unsigned char dummy;
while ( UCSRA & (1<<RXC) ) dummy = UDR;
}
Note:
19.8
1. See “About Code Examples” on page 7.
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 RxD 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.
19.8.1
160
Asynchronous Clock Recovery
The clock recovery logic synchronizes internal clock to the incoming serial frames. Figure 19-5
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 8 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 (U2X = 1) of operation. Samples denoted zero
are samples done when the RxD line is idle (i.e., no communication activity).
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Figure 19-5. 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 RxD line, the
start bit detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in
the figure. The clock 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.
19.8.2
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 8 states
for each bit in Double Speed mode. Figure 19-6 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 19-6. 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 RxD 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 19-7 shows the sampling of the stop bit and the earliest possible beginning of the start bit
of the next frame.
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Figure 19-7. 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 (FE) 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 19-7. 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.
19.8.3
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 19-2) 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
162
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.
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Table 19-2 and Table 19-3 list the maximum receiver baud rate error that can be tolerated. Note
that Normal Speed mode has higher toleration of baud rate variations.
Table 19-2.
Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode
(U2X = 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 19-3.
Recommended Maximum Receiver Baud Rate Error for Double Speed Mode
(U2X = 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.
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.
19.9
Multi-processor Communication Mode
Setting the Multi-processor Communication mode (MPCM) bit in UCSRA 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 MPCM
setting, but has to be used differently when it is a part of a system utilizing the Multi-processor
Communication mode.
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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 (RXB8) 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.
19.9.1
Using MPCM
For an MCU to act as a Master MCU, it can use a 9-bit character frame format (UCSZ = 7). The
ninth bit (TXB8) must be set when an address frame (TXB8 = 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 (MPCM in UCSRA is set).
2. The Master MCU sends an address frame, and all Slaves receive and read this frame.
In the Slave MCUs, the RXC Flag in UCSRA will be set as normal.
3. Each Slave MCU reads the UDR Register and determines if it has been selected. If so,
it clears the MPCM bit in UCSRA, otherwise it waits for the next address byte and
keeps the MPCM 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 MPCM bit set, will ignore the data frames.
5. When the last data frame is received by the addressed MCU, the addressed MCU sets
the MPCM 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 (USBS = 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 MPCM bit. The
MPCM bit shares the same I/O location as the TXC Flag and this might accidentally be cleared
when using SBI or CBI instructions.
19.10 Accessing UBRRH/ UCSRC Registers
The UBRRH Register shares the same I/O location as the UCSRC Register. Therefore some
special consideration must be taken when accessing this I/O location.
19.10.1
Write Access
When doing a write access of this I/O location, the high bit of the value written, the USART Register Select (URSEL) bit, controls which one of the two registers that will be written. If URSEL is
zero during a write operation, the UBRRH value will be updated. If URSEL is one, the UCSRC
setting will be updated.
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The following code examples show how to access the two registers.
Assembly Code Example(1)
:.
; Set UBRRH to 2
ldi r16,0x02
out UBRRH,r16
:.
; Set the USBS and the UCSZ1 bit to one, and
; the remaining bits to zero.
ldi r16,(1<<URSEL)|(1<<USBS)|(1<<UCSZ1)
out UCSRC,r16
:.
C Code Example(1)
:.
/* Set UBRRH to 2 */
UBRRH = 0x02;
:.
/* Set the USBS and the UCSZ1 bit to one, and */
/* the remaining bits to zero. */
UCSRC = (1<<URSEL)|(1<<USBS)|(1<<UCSZ1);
:.
Note:
1. See “About Code Examples” on page 7.
As the code examples illustrate, write accesses of the two registers are relatively unaffected of
the sharing of I/O location.
19.10.2
Read Access
Doing a read access to the UBRRH or the UCSRC Register is a more complex operation. However, in most applications, it is rarely necessary to read any of these registers.
The read access is controlled by a timed sequence. Reading the I/O location once returns the
UBRRH Register contents. If the register location was read in previous system clock cycle, reading the register in the current clock cycle will return the UCSRC contents. Note that the timed
sequence for reading the UCSRC is an atomic operation. Interrupts must therefore be controlled
(for example by disabling interrupts globally) during the read operation.
The following code example shows how to read the UCSRC Register contents.
Assembly Code Example(1)
165
8154B–AVR–07/09
USART_ReadUCSRC:
; Read UCSRC
in r16,UBRRH
in r16,UCSRC
ret
C Code Example(1)
unsigned char USART_ReadUCSRC( void )
{
unsigned char ucsrc;
/* Read UCSRC */
ucsrc = UBRRH;
ucsrc = UCSRC;
return ucsrc;
}
Note:
1. See “About Code Examples” on page 7.
The assembly code example returns the UCSRC value in r16.
Reading the UBRRH contents is not an atomic operation and therefore it can be read as an ordinary register, as long as the previous instruction did not access the register location.
19.10.3
Register Description
19.10.4
UDR – USART I/O Data Register
Bit
7
6
5
4
3
2
1
0
RXB[7:0]
UDR (Read)
TXB[7:0]
UDR (Write)
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
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 UDR. The Transmit Data Buffer Register (TXB) will be the destination for data written to the UDR Register location. Reading the
UDR 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 UDRE Flag in the UCSRA Register is set. Data
written to UDR when the UDRE 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 TxD 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.
166
ATmega16A
8154B–AVR–07/09
ATmega16A
19.10.5
UCSRA – USART Control and Status Register A
Bit
7
6
5
4
3
2
1
0
RXC
TXC
UDRE
FE
DOR
PE
U2X
MPCM
Read/Write
R
R/W
R
R
R
R
R/W
R/W
Initial Value
0
0
1
0
0
0
0
0
UCSRA
• Bit 7 – RXC: 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 RXC bit will become zero. The RXC Flag can be
used to generate a Receive Complete interrupt (see description of the RXCIE bit).
• Bit 6 – TXC: 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 (UDR). The TXC 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 TXC Flag can generate a Transmit Complete interrupt (see description of the TXCIE bit).
• Bit 5 – UDRE: USART Data Register Empty
The UDRE Flag indicates if the transmit buffer (UDR) is ready to receive new data. If UDRE is
one, the buffer is empty, and therefore ready to be written. The UDRE Flag can generate a Data
Register empty Interrupt (see description of the UDRIE bit).
UDRE is set after a reset to indicate that the transmitter is ready.
• Bit 4 – FE: 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 (UDR) is read. The FE bit is zero when the stop bit of received data is one. Always
set this bit to zero when writing to UCSRA.
• Bit 3 – DOR: 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 (UDR) is read. Always set this bit
to zero when writing to UCSRA.
• Bit 2 – PE: 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 (UPM1 = 1). This bit is valid until the receive buffer
(UDR) is read. Always set this bit to zero when writing to UCSRA.
• Bit 1 – U2X: 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.
167
8154B–AVR–07/09
• Bit 0 – MPCM: Multi-processor Communication Mode
This bit enables the Multi-processor Communication mode. When the MPCM 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 MPCM setting. For more detailed
information see “Multi-processor Communication Mode” on page 163.
19.10.6
UCSRB – USART Control and Status Register B
Bit
7
6
5
4
3
2
1
0
RXCIE
TXCIE
UDRIE
RXEN
TXEN
UCSZ2
RXB8
TXB8
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R
R/W
Initial Value
0
0
0
0
0
0
0
0
UCSRB
• Bit 7 – RXCIE: RX Complete Interrupt Enable
Writing this bit to one enables interrupt on the RXC Flag. A USART Receive Complete Interrupt
will be generated only if the RXCIE bit is written to one, the Global Interrupt Flag in SREG is written to one and the RXC bit in UCSRA is set.
• Bit 6 – TXCIE: TX Complete Interrupt Enable
Writing this bit to one enables interrupt on the TXC Flag. A USART Transmit Complete Interrupt
will be generated only if the TXCIE bit is written to one, the Global Interrupt Flag in SREG is written to one and the TXC bit in UCSRA is set.
• Bit 5 – UDRIE: USART Data Register Empty Interrupt Enable
Writing this bit to one enables interrupt on the UDRE 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 UDRE bit in UCSRA is set.
• Bit 4 – RXEN: Receiver Enable
Writing this bit to one enables the USART Receiver. The Receiver will override normal port operation for the RxD pin when enabled. Disabling the Receiver will flush the receive buffer
invalidating the FE, DOR, and PE Flags.
• Bit 3 – TXEN: Transmitter Enable
Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port
operation for the TxD pin when enabled. The disabling of the Transmitter (writing 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 TxD port.
• Bit 2 – UCSZ2: Character Size
The UCSZ2 bits combined with the UCSZ1:0 bit in UCSRC sets the number of data bits (Character Size) in a frame the receiver and transmitter use.
• Bit 1 – RXB8: Receive Data Bit 8
RXB8 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 UDR.
168
ATmega16A
8154B–AVR–07/09
ATmega16A
• Bit 0 – TXB8: Transmit Data Bit 8
TXB8 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 UDR.
19.10.7
UCSRC – USART Control and Status Register C
Bit
7
6
5
4
3
2
1
0
URSEL
UMSEL
UPM1
UPM0
USBS
UCSZ1
UCSZ0
UCPOL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
1
0
0
0
0
1
1
0
UCSRC
The UCSRC Register shares the same I/O location as the UBRRH Register. See the “Accessing
UBRRH/ UCSRC Registers” on page 164 section which describes how to access this register.
• Bit 7 – URSEL: Register Select
This bit selects between accessing the UCSRC or the UBRRH Register. It is read as one when
reading UCSRC. The URSEL must be one when writing the UCSRC.
• Bit 6 – UMSEL: USART Mode Select
This bit selects between Asynchronous and Synchronous mode of operation.
Table 19-4.
UMSEL Bit Settings
UMSEL
Mode
0
Asynchronous Operation
1
Synchronous Operation
• Bit 5:4 – UPM1:0: 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 UPM0 setting.
If a mismatch is detected, the PE Flag in UCSRA will be set.
Table 19-5.
UPM Bits Settings
UPM1
UPM0
Parity Mode
0
0
Disabled
0
1
Reserved
1
0
Enabled, Even Parity
1
1
Enabled, Odd Parity
• Bit 3 – USBS: Stop Bit Select
This bit selects the number of Stop Bits to be inserted by the Transmitter. The Receiver ignores
this setting.
Table 19-6.
USBS Bit Settings
USBS
Stop Bit(s)
0
1-bit
1
2-bit
169
8154B–AVR–07/09
• Bit 2:1 – UCSZ1:0: Character Size
The UCSZ1:0 bits combined with the UCSZ2 bit in UCSRB sets the number of data bits (Character Size) in a frame the Receiver and Transmitter use.
Table 19-7.
UCSZ Bits Settings
UCSZ2
UCSZ1
UCSZ0
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 – UCPOL: Clock Polarity
This bit is used for Synchronous mode only. Write this bit to zero when Asynchronous mode is
used. The UCPOL bit sets the relationship between data output change and data input sample,
and the synchronous clock (XCK).
Table 19-8.
Transmitted Data Changed (Output of TxD
Pin)
Received Data Sampled (Input on RxD
Pin)
0
Rising XCK Edge
Falling XCK Edge
1
Falling XCK Edge
Rising XCK Edge
UCPOL
19.10.8
UCPOL Bit Settings
UBRRL and UBRRH – USART Baud Rate Registers
Bit
15
14
13
12
URSEL
–
–
–
11
10
9
8
UBRR[11:8]
UBRRH
UBRR[7:0]
7
Read/Write
Initial Value
6
5
UBRRL
4
3
2
1
0
R/W
R
R
R
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
The UBRRH Register shares the same I/O location as the UCSRC Register. See the “Accessing
UBRRH/ UCSRC Registers” on page 164 section which describes how to access this register.
• Bit 15 – URSEL: Register Select
This bit selects between accessing the UBRRH or the UCSRC Register. It is read as zero when
reading UBRRH. The URSEL must be zero when writing the UBRRH.
• Bit 14:12 – 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.
170
ATmega16A
8154B–AVR–07/09
ATmega16A
• 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 8 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.
19.11 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 19-9. 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 162). The error values are calculated using the following equation:
BaudRate Closest Match
Error[%] = ⎛ -------------------------------------------------------- – 1⎞ • 100%
⎝
⎠
BaudRate
Table 19-9.
Examples of UBRR Settings for Commonly Used Oscillator Frequencies
fosc = 1.0000 MHz
fosc = 1.8432 MHz
fosc = 2.0000 MHz
Baud
Rate
(bps)
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
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.
U2X = 0
(1)
U2X = 1
62.5 kbps
125 kbps
U2X = 0
U2X = 1
115.2 kbps
U2X = 0
230.4 kbps
125 kbps
U2X = 1
250 kbps
UBRR = 0, Error = 0.0%
171
8154B–AVR–07/09
Table 19-10. Examples of UBRR Settings for Commonly Used Oscillator Frequencies
fosc = 3.6864 MHz
fosc = 4.0000 MHz
fosc = 7.3728 MHz
Baud
Rate
(bps)
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
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.
172
(1)
U2X = 0
U2X = 1
230.4 kbps
U2X = 0
460.8 kbps
U2X = 1
250 kbps
U2X = 0
0.5 Mbps
U2X = 1
460.8 kbps
921.6 kbps
UBRR = 0, Error = 0.0%
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 19-11. Examples of UBRR Settings for Commonly Used Oscillator Frequencies
fosc = 11.0592 MHz
fosc = 8.0000 MHz
fosc = 14.7456 MHz
Baud
Rate
(bps)
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
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.
U2X = 0
(1)
U2X = 1
0.5 Mbps
1 Mbps
U2X = 0
U2X = 1
691.2 kbps
U2X = 0
1.3824 Mbps
921.6 kbps
U2X = 1
1.8432 Mbps
UBRR = 0, Error = 0.0%
173
8154B–AVR–07/09
Table 19-12. Examples of UBRR Settings for Commonly Used Oscillator Frequencies
fosc = 16.0000 MHz
U2X = 0
Baud Rate (bps)
UBRR
Error
UBRR
Error
2400
416
-0.1%
832
0.0%
4800
207
0.2%
416
-0.1%
9600
103
0.2%
207
0.2%
14.4k
68
0.6%
138
-0.1%
19.2k
51
0.2%
103
0.2%
28.8k
34
-0.8%
68
0.6%
38.4k
25
0.2%
51
0.2%
57.6k
16
2.1%
34
-0.8%
76.8k
12
0.2%
25
0.2%
115.2k
8
-3.5%
16
2.1%
230.4k
3
8.5%
8
-3.5%
250k
3
0.0%
7
0.0%
0.5M
1
0.0%
3
0.0%
1M
0
0.0%
1
0.0%
Max
(1)
1.
174
U2X = 1
1 Mbps
2 Mbps
UBRR = 0, Error = 0.0%
ATmega16A
8154B–AVR–07/09
ATmega16A
20. Two-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 400 kHz 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
Two-wire Serial Interface Bus Definition
The Two-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
175
8154B–AVR–07/09
20.2.1
TWI Terminology
The following definitions are frequently encountered in this section.
Table 20-1.
20.2.2
TWI Terminology
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.
Electrical Interconnection
As depicted in Figure 20-1, 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 tri-state 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 400 pF and the 7-bit Slave address space. A detailed specification of the electrical characteristics of the TWI is given in “Two-wire Serial Interface Characteristics” on page 297. Two
different sets of specifications are presented there, one relevant for bus speeds below 100 kHz,
and one valid for bus speeds up to 400 kHz.
20.3
20.3.1
Data Transfer and Frame Format
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
176
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
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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 releasing 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.
Figure 20-3. START, REPEATED START, and STOP Conditions
SDA
SCL
START
20.3.3
STOP START
REPEATED START
STOP
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.
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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 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
receiverR
SCL from
Master
1
SLA+R/W
20.3.5
2
7
Data Byte
STOP, REPEATED
START or Next
Data Byte
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.
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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
2
7
Data Byte
SLA+R/W
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 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
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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.
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.
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20.5
Overview of the TWI Module
The TWI module is comprised of several submodules, as shown in Figure 20-9. All registers
drawn in a thick line are accessible through the AVR data bus.
Figure 20-9. 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)
Bit Rate Generator
Prescaler
Bit Rate Register
(TWBR)
Ack
Control Unit
Status Register
(TWSR)
Control Register
(TWCR)
TWI Unit
Address Comparator
State Machine and
Status control
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 50 ns. 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:
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CPU Clock frequency
SCL frequency = ----------------------------------------------------------TWPS
16 + 2(TWBR) ⋅ 4
• TWBR = Value of the TWI Bit Rate Register
• TWPS = Value of the prescaler bits in the TWI Status Register
Note:
20.5.3
Note: Pull-up resistor values should be selected according to the SCL frequency and the capacitive bus line load. See Table 27-4 on page 297 for value of pull-up resistor.
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 7-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.
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:
• After the TWI has transmitted a START/REPEATED START condition
• After the TWI has transmitted SLA+R/W
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• 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
20.6
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 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 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.
Figure 20-10. Interfacing the Application to the TWI in a Typical Transmission
Application
Action
1. Application
writes to TWCR to
initiate
transmission of
START
TWI bus
TWI
Hardware
Action
3. Check TWSR to see if START was
sent.
Application loads SLA+W into TWDR, and
loads appropriate control signalsinto
TWCR, making sure that TWINT is written
to one, and TWSTA is written to zero
START
SLA+W
2. TWINT set.
Status code indicates
START condition sent
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
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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.
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.
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• 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.
Assembly code example
1
ldi
r16, (1<<TWINT)|(1<<TWSTA)|
(1<<TWEN)
out
2
Comments
TWCR = (1<<TWINT)|(1<<TWSTA)|
Send START condition
(1<<TWEN)
TWCR, r16
wait1:
in
C example
r16,TWCR
while (!(TWCR & (1<<TWINT)))
;
sbrs r16,TWINT
Wait for TWINT Flag set. This indicates
that the START condition has been
transmitted
rjmp wait1
3
in
r16,TWSR
andi r16, 0xF8
cpi
if ((TWSR & 0xF8) != START)
ERROR();
r16, START
Check value of TWI Status Register. Mask
prescaler bits. If status different from
START go to ERROR
brne ERROR
4
ldi
r16, SLA_W
TWDR = SLA_W;
out
TWDR, r16
TWCR = (1<<TWINT) | (1<<TWEN);
ldi
r16, (1<<TWINT) | (1<<TWEN)
out
TWCR, r16
wait2:
in
r16,TWCR
while (!(TWCR & (1<<TWINT)))
;
sbrs r16,TWINT
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.
rjmp wait2
5
in
r16,TWSR
andi r16, 0xF8
cpi
if ((TWSR & 0xF8) != MT_SLA_ACK)
ERROR();
r16, MT_SLA_ACK
Check value of TWI Status Register. Mask
prescaler bits. If status different from
MT_SLA_ACK go to ERROR
brne ERROR
6
ldi
r16, DATA
TWDR = DATA;
out
TWDR, r16
TWCR = (1<<TWINT) | (1<<TWEN);
ldi
r16, (1<<TWINT) | (1<<TWEN)
out
TWCR, r16
wait3:
in
r16,TWCR
while (!(TWCR & (1<<TWINT)))
;
sbrs r16,TWINT
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.
rjmp wait3
7
in
r16,TWSR
andi r16, 0xF8
cpi
if ((TWSR & 0xF8) != MT_DATA_ACK) Check value of TWI Status Register. Mask
ERROR();
prescaler bits. If status different from
MT_DATA_ACK go to ERROR
r16, MT_DATA_ACK
brne ERROR
ldi
r16, (1<<TWINT)|(1<<TWEN)|
(1<<TWSTO)
out
TWCR = (1<<TWINT)|(1<<TWEN)|
Transmit STOP condition
(1<<TWSTO);
TWCR, r16
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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
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 to Figure 20-18, 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 20-2 to Table 20-5. Note that the prescaler bits are masked to zero in
these tables.
20.7.1
186
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.
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Figure 20-11. Data Transfer in Master Transmitter Mode
VCC
Device 1
Device 2
MASTER
TRANSMITTER
SLAVE
RECEIVER
Device 3
........
R1
Device n
R2
SDA
SCL
A START condition is sent by writing the following value to TWCR:
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
1
0
X
1
0
X
TWEN must be set to enable the Two-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 Two-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 $08 (See Table 20-2). In order to enter MT mode,
SLA+W must be transmitted. This is done by writing SLA+W 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
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
0
0
X
1
0
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 $18, $20, or $38. The appropriate action to be taken for each of these status codes is
detailed in Table 20-2.
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
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
0
0
X
1
0
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
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
0
1
X
1
0
X
A REPEATED START condition is generated by writing the following value to TWCR:
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
1
0
X
1
0
X
187
8154B–AVR–07/09
After a repeated START condition (state $10) the Two-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 20-2.
Status Code
(TWSR)
Prescaler Bits
are 0
Status Codes for Master Transmitter Mode
Application Software Response
Status of the Two-wire Serial
Bus and Two-wire Serial Interface Hardware
To TWCR
To/from TWDR
STA
STO
TWINT
TWEA
Next Action Taken by TWI Hardware
$08
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
$10
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
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
$18
$20
$28
$30
$38
188
SLA+W has been transmitted;
ACK has been received
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
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
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
Two-wire Serial Bus will be released and not addressed
Slave mode entered
A START condition will be transmitted when the bus becomes free
ATmega16A
8154B–AVR–07/09
ATmega16A
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
20.7.2
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
Master Receiver Mode
In the Master Receiver mode, a number of data bytes are received from a Slave Transmitter
(see Figure 20-13). 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.
189
8154B–AVR–07/09
Figure 20-13. Data Transfer in Master Receiver Mode
VCC
Device 1
Device 2
MASTER
RECEIVER
SLAVE
TRANSMITTER
Device 3
........
R1
Device n
R2
SDA
SCL
A START condition is sent by writing the following value to TWCR:
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
1
0
X
1
0
X
TWEN must be written to one to enable the Two-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 Two-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 $08 (See Table 20-2). 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
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
0
0
X
1
0
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 $38, $40, or $48. The appropriate action to be taken for each of these status codes is
detailed in Table 20-3. 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
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
0
1
X
1
0
X
A REPEATED START condition is generated by writing the following value to TWCR:
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
1
0
X
1
0
X
After a repeated START condition (state $10) the Two-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.
190
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 20-3.
Status Code
(TWSR)
Prescaler Bits
are 0
Status Codes for Master Receiver Mode
Application Software Response
Status of the Two-wire Serial
Bus and Two-wire Serial Interface Hardware
To TWCR
To/from TWDR
TWINT
TWEA
$08
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
$10
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
$38
$40
$48
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
$50
Data byte has been received;
ACK has been returned
$58
Data byte has been received;
NOT ACK has been returned
STA
STO
Next Action Taken by TWI Hardware
Two-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
191
8154B–AVR–07/09
Figure 20-14. Formats and States in the Master Receiver Mode
MR
Successfull
reception
from a slave
receiver
S
SLA
R
$08
A
DATA
A
$40
DATA
$50
A
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
A or A
Other master
continues
A
$38
Arbitration lost and
addressed as slave
$68
$78
DATA
From slave to master
20.7.3
$38
Other master
continues
A
From master to slave
Other master
continues
To corresponding
states in slave mode
$B0
Any number of data bytes
and their associated acknowledge bits
A
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
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
........
R1
Device n
R2
SDA
SCL
To initiate the Slave Receiver mode, TWAR and TWCR must be initialized as follows:
TWAR
Value
192
TWA6
TWA5
TWA4
TWA3
TWA2
TWA1
TWA0
TWGCE
Device’s Own Slave Address
ATmega16A
8154B–AVR–07/09
ATmega16A
The upper seven bits are the address to which the Two-wire Serial Interface will respond when
addressed by a Master. If the LSB is set, the TWI will respond to the general call address ($00),
otherwise it will ignore the general call address.
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
0
1
0
0
0
1
0
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 20-4.
The Slave Receiver mode may also be entered if arbitration is lost while the TWI is in the Master
mode (see states $68 and $78).
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 Two-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 Two-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 Two-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 Two-wire Serial Interface Data Register – TWDR does not reflect the last byte
present on the bus when waking up from these sleep modes.
193
8154B–AVR–07/09
Table 20-4.
Status Code
(TWSR)
Prescaler Bits
are 0
Status Codes for Slave Receiver Mode
Application Software Response
Status of the Two-wire Serial Bus
and Two-wire Serial Interface
Hardware
To TWCR
To/from TWDR
STA
STO
TWINT
TWEA
No TWDR action or
X
0
1
0
$60
Own SLA+W has been received;
ACK has been returned
No TWDR action
X
0
1
1
$68
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
$70
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
$78
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
$80
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
$88
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
$90
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
$98
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
$A0
194
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
ATmega16A
8154B–AVR–07/09
ATmega16A
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
A
P or S
$80
$A0
A
P or S
Last data byte received
is not acknowledged
$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
A
P or S
$90
$A0
A
P or S
Last data byte received is
not acknowledged
$98
Arbitration lost as master and
addressed as slave by general call
A
$78
DATA
From master to slave
From slave to master
20.7.4
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
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:
195
8154B–AVR–07/09
TWAR
TWA6
TWA5
Value
TWA4
TWA3
TWA2
TWA1
TWA0
TWGCE
Device’s Own Slave Address
The upper seven bits are the address to which the Two-wire Serial Interface will respond when
addressed by a Master. If the LSB is set, the TWI will respond to the general call address ($00),
otherwise it will ignore the general call address.
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
0
1
0
0
0
1
0
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 20-5.
The Slave Transmitter mode may also be entered if arbitration is lost while the TWI is in the
Master mode (see state $B0).
If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the transfer. State $C0 or state $C8 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 $C8 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 Twowire 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 Twowire 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 Two-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 start-up time, the
SCL line may be held low for a long time, blocking other data transmissions.
Note that the Two-wire Serial Interface Data Register – TWDR does not reflect the last byte
present on the bus when waking up from these sleep modes.
196
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 20-5.
Status Code
(TWSR)
Prescaler Bits
are 0
$A8
$B0
$B8
$C0
$C8
Status Codes for Slave Transmitter Mode
Application Software Response
Status of the Two-wire Serial Bus
and Two-wire Serial Interface
Hardware
To TWCR
To/from TWDR
STA
STO
TWINT
TWEA
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
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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
DATA
From master to slave
From slave to master
20.7.5
Any number of data bytes
and their associated acknowledge bits
A
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
Miscellaneous States
There are two status codes that do not correspond to a defined TWI state, see Table 20-6.
Status $F8 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 $00 indicates that a bus error has occurred during a Two-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 20-6.
Miscellaneous States
Status Code
(TWSR)
Prescaler Bits
are 0
Status of the Two-wire Serial
Bus and Two-wire Serial Interface Hardware
Application Software Response
To TWCR
To/from TWDR
$F8
No relevant state information
available; TWINT = “0”
No TWDR action
$00
Bus error due to an illegal
START or STOP condition
No TWDR action
20.7.6
STA
STO
TWINT
TWEA
0
1
1
Next Action Taken by TWI Hardware
Wait or proceed current transfer
No TWCR action
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.
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
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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
A
Rs
SLA+R
A
DATA
Rs = REPEATED START
Transmitted from Master to Slave
20.8
Master Receiver
A
P
P = STOP
Transmitted from Slave to Master
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.
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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.
Figure 20-21. Possible Status Codes Caused by Arbitration
START
SLA
Data
Arbitration lost in SLA
Own
Address / General Call
received
STOP
Arbitration lost in Data
No
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
20.9
20.9.1
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
Register Description
TWBR – TWI Bit Rate Register
Bit
7
6
5
4
3
2
1
0
TWBR7
TWBR6
TWBR5
TWBR4
TWBR3
TWBR2
TWBR1
TWBR0
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
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 181 for calculating bit rates.
20.9.2
TWCR – TWI Control Register
Bit
7
6
5
4
3
2
1
0
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Read/Write
R/W
R/W
R/W
R/W
R
R/W
R
R/W
Initial Value
0
0
0
0
0
0
0
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
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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 Two-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 Twowire 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.
• Bit 4 – TWSTO: TWI STOP Condition Bit
Writing the TWSTO bit to one in Master mode will generate a STOP condition on the Two-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.
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• 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
TWSR – TWI Status Register
Bit
7
6
5
4
3
2
1
0
TWS7
TWS6
TWS5
TWS4
TWS3
–
TWPS1
TWPS0
Read/Write
R
R
R
R
R
R
R/W
R/W
Initial Value
1
1
1
1
1
0
0
0
TWSR
• Bits 7:3 – TWS: TWI Status
These five bits reflect the status of the TWI logic and the Two-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.
Table 20-7.
TWI Bit Rate Prescaler
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 181. The value of TWPS1:0 is used
in the equation.
20.9.4
TWDR – TWI Data Register
Bit
7
6
5
4
3
2
1
0
TWD7
TWD6
TWD5
TWD4
TWD3
TWD2
TWD1
TWD0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
1
1
1
1
1
1
1
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.
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• Bits 7:0 – TWD: TWI Data Register
These eight bits contain the next data byte to be transmitted, or the latest data byte received on
the Two-wire Serial Bus.
20.9.5
TWA R– TWI (Slave) Address Register
Bit
7
6
5
4
3
2
1
0
TWA6
TWA5
TWA4
TWA3
TWA2
TWA1
TWA0
TWGCE
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
1
1
1
1
1
1
1
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. 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 ($00). 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[6:0]: TWI (Slave) Address Register
These seven bits constitute the Slave address of the TWI unit.
• Bit 0 – TWGCE: TWI General Call Recognition Enable Bit
If set, this bit enables the recognition of a General Call given over the Two-wire Serial Bus.
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21. Analog Comparator
The Analog Comparator compares the input values on the positive pin AIN0 and negative pin
AIN1. When the voltage on the positive pin AIN0 is higher than the voltage on the negative pin
AIN1, 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 21-1.
Figure 21-1. Analog Comparator Block Diagram(2)
BANDGAP
REFERENCE
ACBG
ACME
ADEN
ADC MULTIPLEXER
OUTPUT (1)
Notes:
1. See Table 1 on page 205.
2. Refer to Figure 1-1 on page 2 and Table 12-6 on page 57 for Analog Comparator pin
placement.
21.1
Analog Comparator Multiplexed Input
It is possible to select any of the ADC7:0 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
SFIOR) is set and the ADC is switched off (ADEN in ADCSRA is zero), MUX2:0 in ADMUX
select the input pin to replace the negative input to the Analog Comparator, as shown in Table 1.
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If ACME is cleared or ADEN is set, AIN1 is applied to the negative input to the Analog
Comparator.
Table 1. Analog Comparator Multiplexed Input
21.2
21.2.1
ACME
ADEN
MUX2:0
Analog Comparator Negative Input
0
x
xxx
AIN1
1
1
xxx
AIN1
1
0
000
ADC0
1
0
001
ADC1
1
0
010
ADC2
1
0
011
ADC3
1
0
100
ADC4
1
0
101
ADC5
1
0
110
ADC6
1
0
111
ADC7
Register Description
SFIOR – Special Function IO Register
Bit
7
6
5
4
3
2
1
0
ADTS2
ADTS1
ADTS0
–
ACME
PUD
PSR2
PSR10
Read/Write
R/W
R/W
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
SFIOR
• Bit 3 – 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 selects the negative input to the Analog Comparator. When this bit is written
logic zero, AIN1 is applied to the negative input of the Analog Comparator. For a detailed
description of this bit, see “Analog Comparator Multiplexed Input” on page 204.
21.2.2
ACSR – Analog Comparator Control and Status Register
Bit
7
6
5
4
3
2
1
0
ACD
ACBG
ACO
ACI
ACIE
ACIC
ACIS1
ACIS0
Read/Write
R/W
R/W
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
N/A
0
0
0
0
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.
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• 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 40.
• 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.
• 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 TICIE1 bit in the Timer Interrupt Mask
Register (TIMSK) 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 Table 21-1.
Table 21-1.
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.
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22. Analog to Digital Converter
22.1
Features
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
10-bit Resolution
0.5 LSB Integral Non-linearity
±2 LSB Absolute Accuracy
13 - 260 µs Conversion Time
Up to 15 kSPS at Maximum Resolution
8 Multiplexed Single Ended Input Channels
7 Differential Input Channels
2 Differential Input Channels with Optional Gain of 10x and 200x(1)
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
Note:
22.2
1. The differential input channels are not tested for devices in PDIP Package. This feature is only
guaranteed to work for devices in TQFP and QFN/MLF Packages
Overview
The ATmega16A features a 10-bit successive approximation ADC. The ADC is connected to an
8-channel Analog Multiplexer which allows 8 single-ended voltage inputs constructed from the
pins of Port A. The single-ended voltage inputs refer to 0V (GND).
The device also supports 16 differential voltage input combinations. Two of the differential inputs
(ADC1, ADC0 and ADC3, ADC2) are equipped with a programmable gain stage, providing
amplification steps of 0 dB (1x), 20 dB (10x), or 46 dB (200x) on the differential input voltage
before the A/D conversion. Seven differential analog input channels share a common negative
terminal (ADC1), while any other ADC input can be selected as the positive input terminal. If 1x
or 10x 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 22-1.
The ADC has a separate analog supply voltage pin, AVCC. AVCC must not differ more than
±0.3 V from VCC. See the paragraph “ADC Noise Canceler” on page 215 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 22-1. Analog to Digital Converter Block Schematic
ADC CONVERSION
COMPLETE IRQ
INTERRUPT
FLAGS
ADTS[2:0]
TRIGGER
SELECT
ADC[9:0]
ADPS1
0
ADC DATA REGISTER
(ADCH/ADCL)
ADPS0
ADPS2
ADIF
ADATE
ADEN
ADSC
MUX1
15
ADC CTRL. & STATUS
REGISTER (ADCSRA)
MUX0
MUX3
MUX2
MUX4
ADLAR
REFS0
REFS1
ADC MULTIPLEXER
SELECT (ADMUX)
ADIE
ADIF
8-BIT DATA BUS
AVCC
PRESCALER
START
GAIN SELECTION
CHANNEL SELECTION
MUX DECODER
CONVERSION LOGIC
INTERNAL 2.56V
REFERENCE
SAMPLE & HOLD
COMPARATOR
AREF
10-BIT DAC
+
GND
BANDGAP
REFERENCE
ADC7
SINGLE ENDED / DIFFERENTIAL SELECTION
ADC6
ADC5
POS.
INPUT
MUX
ADC MULTIPLEXER
OUTPUT
ADC4
ADC3
+
ADC2
GAIN
AMPLIFIER
ADC1
ADC0
NEG.
INPUT
MUX
22.3
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 gain amplifier.
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If differential channels are selected, the differential gain stage amplifies the voltage difference
between the selected input channel pair by the selected gain factor. This amplified value then
becomes the analog input to the ADC. If single ended channels are used, the gain amplifier is
bypassed altogether.
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. When ADC
access to the Data Registers is prohibited between reading of ADCH and ADCL, the interrupt
will trigger even if the result is lost.
22.4
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 SFIOR (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 still is 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 22-2. ADC Auto Trigger Logic
ADTS[2:0]
PRESCALER
START
ADIF
CLKADC
ADATE
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.
22.5
Prescaling and Conversion Timing
Figure 22-3. ADC Prescaler
ADEN
START
Reset
7-BIT ADC PRESCALER
CK/64
CK/128
CK/32
CK/8
CK/16
CK/4
CK/2
CK
ADPS0
ADPS1
ADPS2
ADC CLOCK SOURCE
By default, the successive approximation circuitry requires an input clock frequency between 50
kHz and 200 kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the
input clock frequency to the ADC can be higher than 200 kHz to get a higher sample rate.
The ADC module contains a prescaler, which generates an acceptable ADC clock frequency
from any CPU frequency above 100 kHz. 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
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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 Gain Channels” on
page 213 for details on differential conversion timing.
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 a 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 2 ADC clock cycles after the rising edge on the trigger source signal. Three additional CPU clock cycles are used for synchronization logic. When using Differential mode, along
with Auto triggering from a source other than the ADC Conversion Complete, each conversion
will require 25 ADC clocks. This is because the ADC must be disabled and re-enabled after
every conversion.
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 Table 22-1.
Figure 22-4. 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
ADCH
MSB of Result
ADCL
LSB of Result
MUX and REFS
Update
Sample & Hold
Conversion
Complete
MUX and REFS
Update
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Figure 22-5. ADC Timing Diagram, Single Conversion
One Conversion
Cycle Number
2
1
3
4
5
6
7
8
Next Conversion
10
9
11
12
13
1
2
3
ADC Clock
ADSC
ADIF
ADCH
MSB of Result
ADCL
LSB of Result
Sample & Hold
MUX and REFS
Update
Conversion
Complete
MUX and REFS
Update
Figure 22-6. ADC Timing Diagram, Auto Triggered Conversion
One Conversion
Cycle Number
1
2
3
4
5
6
7
8
9
Next Conversion
10
11
12
13
1
2
ADC Clock
Trigger
Source
ADATE
ADIF
ADCH
MSB of Result
ADCL
LSB of Result
Prescaler
Reset
Sample & Hold
Conversion
Complete
Prescaler
Reset
MUX and REFS
Update
Figure 22-7. ADC Timing Diagram, Free Running Conversion
One Conversion
Cycle Number
11
12
Next Conversion
13
1
2
3
4
ADC Clock
ADSC
ADIF
ADCH
MSB of Result
ADCL
LSB of Result
Conversion
Complete
212
Sample & Hold
MUX and REFS
Update
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Table 22-1.
ADC Conversion Time
Condition
Conversion Time (Cycles)
First conversion
13.5
25
Normal conversions, single ended
1.5
13
2
13.5
1.5/2.5
13/14
Auto Triggered conversions
Normal conversions, differential
22.5.1
Sample & Hold (Cycles
from Start of Conversion)
Differential Gain Channels
When using differential gain 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. 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.
The gain stage is optimized for a bandwidth of 4 kHz 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. For example, the ADC
clock period may be 6 µs, allowing a channel to be sampled at 12 kSPS, regardless of the bandwidth of this channel.
If differential gain 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 gain 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” then to “1”), only extended conversions are performed. The result from the extended conversions will be valid. See “Prescaling
and Conversion Timing” on page 210 for timing details.
22.6
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.
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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:
1. When ADATE or ADEN is cleared.
2. During conversion, minimum one ADC clock cycle after the trigger event.
3. 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 gain stage may take as much as 125 µs to stabilize to the new value.
Thus conversions should not be started within the first 125 µs 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).
22.6.1
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.
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.
22.6.2
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 impedant voltmeter. Note that VREF is a high
impedant 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
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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 27-6 on page 301.
22.7
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:
1. 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.
2. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion once
the CPU has been halted.
3. 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.
22.7.1
Analog Input Circuitry
The Analog Input Circuitry for single ended channels is illustrated in Figure 22-8. 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).
The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ 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 impedant 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 kΩ 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.
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Figure 22-8. Analog Input Circuitry
IIH
ADCn
1..100 kΩ
CS/H= 14 pF
IIL
VCC/2
22.7.2
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:
1. Keep analog signal paths as short as possible. Keep them well away from high-speed
switching digital tracks.
2. The AVCC pin on the device should be connected to the digital VCC supply voltage via
an LC network as shown in Figure 22-9.
3. Use the ADC noise canceler function to reduce induced noise from the CPU.
4. 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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PA3 (ADC3)
PA2 (ADC2)
PA1 (ADC1)
PA0 (ADC0)
VCC
GND
Figure 22-9. ADC Power Connections
PA4 (ADC4)
PA5 (ADC5)
PA6 (ADC6)
GND
AVCC
100nF
AREF
10μH
PA7 (ADC7)
PC7
22.7.3
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.
22.7.4
ADC Accuracy Definitions
An n-bit single-ended ADC converts a voltage linearly between GND and V REF in 2 n 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 22-10. 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 22-11. 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 22-12. 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 22-13. Differential Non-linearity (DNL)
Output Code
0x3FF
1 LSB
DNL
0x000
0
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.
22.8
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
where VIN is the voltage on the selected input pin and VREF the selected voltage reference (see
Table 22-3 on page 221 and Table 22-4 on page 222). 0x000 represents ground, and 0x3FF
represents the selected reference voltage minus one LSB.
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V IN ⋅ 1024
ADC = -------------------------V REF
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 results, it is sufficient to read the MSB of the result
(ADC9 in ADCH). If this bit is one, the result is negative, and if this bit is zero, the result is positive. Figure 22-14 shows the decoding of the differential input range.
Table 22-2 shows the resulting output codes if the differential input channel pair (ADCn - ADCm)
is selected with a gain of GAIN and a reference voltage of VREF.
Figure 22-14. Differential Measurement Range
Output Code
0x1FF
0x000
- V REF/GAIN
0x3FF
0
VREF/GAIN
Differential Input
Voltage (Volts)
0x200
Table 22-2.
220
Correlation between Input Voltage and Output Codes
VADCn
Read code
Corresponding Decimal Value
VADCm + VREF/GAIN
0x1FF
511
VADCm + 511/512 VREF/GAIN
0x1FF
511
VADCm + 510/512 VREF/GAIN
0x1FE
510
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Table 22-2.
Correlation between Input Voltage and Output Codes (Continued)
VADCn
Read code
Corresponding Decimal Value
:.
:.
:.
VADCm + 1/512 VREF/GAIN
0x001
1
VADCm
0x000
0
VADCm - 1/512 VREF/GAIN
0x3FF
-1
:.
:.
:.
VADCm - 511/512 VREF/GAIN
0x201
-511
VADCm - VREF/GAIN
0x200
-512
Example:
ADMUX = 0xED (ADC3 - ADC2, 10x gain, 2.56V reference, left adjusted result)
Voltage on ADC3 is 300 mV, voltage on ADC2 is 500 mV.
ADCR = 512 * 10 * (300 - 500) / 2560 = -400 = 0x270
ADCL will thus read 0x00, and ADCH will read 0x9C. Writing zero to ADLAR right adjusts
the result: ADCL = 0x70, ADCH = 0x02.
22.9
22.9.1
Register Description
ADMUX – ADC Multiplexer Selection Register
Bit
7
6
5
4
3
2
1
0
REFS1
REFS0
ADLAR
MUX4
MUX3
MUX2
MUX1
MUX0
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
ADMUX
• Bit 7:6 – REFS1:0: Reference Selection Bits
These bits select the voltage reference for the ADC, as shown in Table 22-3. 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 22-3.
Voltage Reference Selections for ADC
REFS1
REFS0
Voltage Reference Selection
0
0
AREF, Internal Vref turned off
0
1
AVCC with external capacitor at AREF pin
1
0
Reserved
1
1
Internal 2.56V Voltage Reference with external capacitor at 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 “ADCL and ADCH – The ADC Data Register” on
page 224.
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• Bits 4:0 – MUX4:0: Analog Channel and Gain 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. See Table 22-4 for details. 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 22-4.
MUX4:0
Single Ended
Input
00000
ADC0
00001
ADC1
00010
ADC2
00011
ADC3
00100
ADC4
00101
ADC5
00110
ADC6
00111
ADC7
Positive Differential
Input
Negative Differential Input
Gain
N/A
01000
ADC0
ADC0
10x
01001
ADC1
ADC0
10x
(1)
01010
ADC0
ADC0
200x
01011(1)
ADC1
ADC0
200x
01100
ADC2
ADC2
10x
01101
ADC3
ADC2
10x
(1)
01110
ADC2
ADC2
200x
01111(1)
ADC3
ADC2
200x
10000
ADC0
ADC1
1x
10001
ADC1
ADC1
1x
ADC2
ADC1
1x
10011
ADC3
ADC1
1x
10100
ADC4
ADC1
1x
10101
ADC5
ADC1
1x
10110
ADC6
ADC1
1x
10111
ADC7
ADC1
1x
11000
ADC0
ADC2
1x
11001
ADC1
ADC2
1x
11010
ADC2
ADC2
1x
11011
ADC3
ADC2
1x
11100
ADC4
ADC2
1x
10010
222
Input Channel and Gain Selections
N/A
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Table 22-4.
MUX4:0
Input Channel and Gain Selections (Continued)
Single Ended
Input
11101
11110
1.22 V (VBG)
11111
0 V (GND)
Note:
22.9.2
Positive Differential
Input
Negative Differential Input
Gain
ADC5
ADC2
1x
N/A
1. The differential input channels are not tested for devices in PDIP Package. This feature is only
guaranteed to work for devices in TQFP and QFN/MLF Packages
ADCSRA – ADC Control and Status Register A
Bit
7
6
5
4
3
2
1
0
ADEN
ADSC
ADATE
ADIF
ADIE
ADPS2
ADPS1
ADPS0
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
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.
• 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 SFIOR.
• 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-ModifyWrite 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.
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• 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 22-5.
22.9.3
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
ADCL and ADCH – The ADC Data Register
ADLAR = 0
Bit
Read/Write
Initial Value
15
14
13
12
11
10
9
8
–
–
–
–
–
–
ADC9
ADC8
ADCH
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADC1
ADC0
ADCL
7
6
5
4
3
2
1
0
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
ADLAR = 1
Bit
Read/Write
Initial Value
15
14
13
12
11
10
9
8
ADC9
ADC8
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADCH
ADC1
ADC0
–
–
–
–
–
–
ADCL
7
6
5
4
3
2
1
0
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
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 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.
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• ADC9:0: ADC Conversion Result
These bits represent the result from the conversion, as detailed in “ADC Conversion Result” on
page 219.
22.9.4
SFIOR – Special FunctionIO Register
Bit
7
6
5
4
3
2
1
0
ADTS2
ADTS1
ADTS0
–
ACME
PUD
PSR2
PSR10
Read/Write
R/W
R/W
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
SFIOR
• Bit 7:5 – ADTS2:0: 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 ADTS2:0 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[2:0]=0) will not cause a trigger event, even if the ADC Interrupt Flag is set.
Table 22-6.
ADC Auto Trigger Source Selections
ADTS2
ADTS1
ADTS0
Trigger Source
0
0
0
Free Running mode
0
0
1
Analog Comparator
0
1
0
External Interrupt Request 0
0
1
1
Timer/Counter0 Compare Match
1
0
0
Timer/Counter0 Overflow
1
0
1
Timer/Counter1 Compare Match B
1
1
0
Timer/Counter1 Overflow
1
1
1
Timer/Counter1 Capture Event
• Bit 4 – Res: Reserved Bit
This bit is reserved for future use. To ensure compatibility with future devices, this bit must be
written to zero when SFIOR is written.
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23. JTAG Interface and On-chip Debug System
23.1
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 Breakpoints on Single Address or Address Range
– Data Memory Breakpoints 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®
23.2
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 280 and “IEEE 1149.1 (JTAG) Boundary-scan” on page
232, respectively. The On-chip Debug support is considered being private JTAG instructions,
and distributed within ATMEL and to selected third party vendors only.
Figure 23-1 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 JTAG Serial Programming via the JTAG interface. The
Internal Scan Chain and Break Point Scan Chain are used for On-chip Debugging only.
23.3
TAP – Test Access Port
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:
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• 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.
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 and the JTD bit in MCUCSR is cleared, the TAP
input signals are internally pulled high and the JTAG is enabled for Boundary-scan and programming. In this case, the TAP output pin (TDO) is left floating in states where the JTAG TAP
controller is not shifting data, and must therefore be connected to a pull-up resistor or other
hardware having pull-ups (for instance the TDI-input of the next device in the scan chain). 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 23-1. Block Diagram
I/O PORT 0
DEVICE BOUNDARY
BOUNDARY SCAN CHAIN
TDI
TDO
TCK
TMS
JTAG PROGRAMMING
INTERFACE
TAP
CONTROLLER
AVR CPU
BREAKPOINT
UNIT
BYPASS
REGISTER
INTERNAL
SCAN
CHAIN
PC
Instruction
FLOW CONTROL
UNIT
DIGITAL
PERIPHERAL
UNITS
BREAKPOINT
SCAN CHAIN
ADDRESS
DECODER
JTAG / AVR CORE
COMMUNICATION
INTERFACE
OCD STATUS
AND CONTROL
Analog inputs
M
U
X
Address
Data
Control & Clock lines
ID
REGISTER
FLASH
MEMORY
ANALOG
PERIPHERIAL
UNITS
INSTRUCTION
REGISTER
I/O PORT n
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Figure 23-2. 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
Shift-DR
Shift-IR
0
1
Exit1-DR
0
Pause-DR
0
0
Pause-IR
1
1
0
Exit2-DR
Exit2-IR
1
1
Update-DR
23.4
1
Exit1-IR
0
1
0
1
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 Boundaryscan circuitry, JTAG programming circuitry, or On-chip Debug system. The state transitions
depicted in Figure 23-2 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 TestLogic-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.
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The TMS input must be held low during input of the 3 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.
• 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 Run-Test/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 231.
23.5
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 232.
23.6
Using the On-chip Debug System
As shown in Figure 23-1, 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, 2 Program Memory Break Points, and 2 combined Break Points. Together, the 4 Break Points can be
configured as either:
• 4 single Program Memory Break Points
• 3 Single Program Memory Break Point + 1 single Data Memory Break Point
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• 2 single Program Memory Break Points + 2 single Data Memory Break Points
• 2 single Program Memory Break Points + 1 Program Memory Break Point with mask (“range
Break Point”)
• 2 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 230.
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 any Lock bits are set.
Otherwise, the On-chip Debug system would have provided a back-door into a secured device.
The AVR JTAG ICE from Atmel is a powerful development tool for On-chip Debugging of all
AVR 8-bit RISC Microcontrollers with IEEE 1149.1 compliant JTAG interface. The JTAG ICE
and the AVR Studio user interface give the user complete control of the internal resources of the
microcontroller, helping to reduce development time by making debugging easier. The JTAG
ICE performs real-time emulation of the microcontroller while it is running in a target system.
Please refer to the Support Tools section on the AVR pages on www.atmel.com for a full
description of the AVR JTEG ICE. AVR Studio can be downloaded free from Software section
on the same web site.
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 breakpoints (using the BREAK instruction) and up to two data memory breakpoints, alternatively combined as a mask (range) Break
Point.
23.7
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.
23.7.1
PRIVATE0; $8
Private JTAG instruction for accessing On-chip Debug system.
23.7.2
PRIVATE1; $9
Private JTAG instruction for accessing On-chip Debug system.
23.7.3
PRIVATE2; $A
Private JTAG instruction for accessing On-chip Debug system.
23.7.4
PRIVATE3; $B
Private JTAG instruction for accessing On-chip Debug system.
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23.8
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 MCUSR 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 280.
23.9
23.9.1
Register Description
OCDR – On-chip Debug Register
Bit
7
6
5
4
3
2
1
MSB/IDRD
0
LSB
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
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
7 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.
Refer to the debugger documentation for further information on how to use this register.
23.10 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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24. IEEE 1149.1 (JTAG) Boundary-scan
24.1
Features
•
•
•
•
•
24.2
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
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 IR-Register. 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 MCUCSR 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.
24.3
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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24.3.1
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.
24.3.2
Device Identification Register
Figure 24-1 shows the structure of the Device Identification Register.
Figure 24-1. The Format of the Device Identification Register
MSB
Bit
Device ID
24.3.2.1
31
LSB
28
27
12
11
1
0
Version
Part Number
Manufacturer ID
1
4 bits
16 bits
11 bits
1 bit
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. However, some
revisions deviate from this rule, and the relevant version number is shown in Table 24-1.
Table 24-1.
JTAG Version Numbers
Version
JTAG Version Number (Hex)
ATmega16A revision TBD
24.3.2.2
TBD
Part Number
The part number is a 16-bit code identifying the component. The JTAG Part Number for
ATmega16A is listed in Table 24-2.
Table 24-2.
24.3.2.3
Part Number
JTAG Part Number (Hex)
ATmega16A
0x9403
Manufacturer ID
The Manufacturer ID is a 11 bit code identifying the manufacturer. The JTAG manufacturer ID
for ATMEL is listed in Table 24-3.
Table 24-3.
Manufacturer
ATMEL
24.3.3
AVR JTAG Part Number
Manufacturer ID
JTAG Manufacturer ID (Hex)
0x01F
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 unimplemented 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
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Sources” on page 25) 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 24-2.
Figure 24-2. Reset Register
To
TDO
From other Internal and
External Reset Sources
From
TDI
D
Q
Internal Reset
ClockDR · AVR_RESET
24.3.4
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 235 for a complete description.
24.4
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-impedant 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.
24.4.1
EXTEST; $0
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 IRregister 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.
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24.4.2
IDCODE; $1
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.
24.4.3
SAMPLE_PRELOAD; $2
Mandatory JTAG instruction for pre-loading the output latches and talking 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.
24.4.4
AVR_RESET; $C
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.
24.4.5
BYPASS; $F
Mandatory JTAG instruction selecting the Bypass Register for Data Register.
The active states are:
• Capture-DR: Loads a logic “0” into the Bypass Register.
• Shift-DR: The Bypass Register cell between TDI and TDO is shifted.
24.5
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.
24.5.1
Scanning the Digital Port Pins
Figure 24-3 shows the Boundary-scan Cell for a bi-directional port pin with pull-up function. The
cell consists of a standard Boundary-scan cell for the Pull-up Enable – PUExn – function, and 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.
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The Boundary-scan logic is not included in the figures in the datasheet. Figure 24-4 shows a
simple digital Port Pin as described in the section “I/O Ports” on page 49. The Boundary-scan
details from Figure 24-3 replaces the dashed box in Figure 24-4.
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 24-4 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, and a scan chain is inserted on the interface between the digital logic and the analog circuitry.
Figure 24-3. Boundary-scan Cell for Bidirectional Port Pin with Pull-up Function.
ShiftDR
To Next Cell
EXTEST
Pullup Enable (PUE)
Vcc
0
FF2
LD2
1
0
D
Q
D
Q
1
G
Output Control (OC)
FF1
LD1
0
D
Q
D
Q
0
1
1
G
Output Data (OD)
0
1
FF0
LD0
0
Port Pin (PXn)
0
D
Q
D
Q
1
1
G
Input Data (ID)
From Last Cell
236
ClockDR
UpdateDR
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 24-4. General Port Pin Schematic Diagram(1)
PUExn
PUD
Q
D
DDxn
Q CLR
RESET
OCxn
WDx
Q
Pxn
ODxn
D
PORTxn
Q CLR
WPx
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:
Note:
24.5.2
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:
WPx:
RRx:
RPx:
CLK I/O :
WRITE DDRx
READ DDRx
WRITE PORTx
READ PORTx REGISTER
READ PORTx PIN
I/O CLOCK
1. See Boundary-scan description for details.
Boundary-scan and the Two-wire Interface
The 2 Two-wire Interface pins SCL and SDA have one additional control signal in the scanchain; Two-wire Interface Enable – TWIEN. As shown in Figure 24-5, the TWIEN signal enables
a tri-state buffer with slew-rate control in parallel with the ordinary digital port pins. A general
scan cell as shown in Figure 24-9 is attached to the TWIEN signal.
Notes:
1. A separate scan chain for the 50 ns spike filter on the input is not provided. The ordinary scan
support for digital port pins suffice for connectivity tests. The only reason for having TWIEN in
the scan path, is to be able to disconnect the slew-rate control buffer when doing boundaryscan.
2. Make sure the OC and TWIEN signals are not asserted simultaneously, as this will lead to
drive contention.
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8154B–AVR–07/09
Figure 24-5. Additional Scan Signal for the Two-wire Interface
PUExn
OCxn
ODxn
TWIEN
Pxn
SRC
Slew-rate Limited
IDxn
24.5.3
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 24-6 is
inserted both for the 5V reset signal; RSTT, and the 12V reset signal; RSTHV.
Figure 24-6. Observe-only Cell
To
Next
Cell
ShiftDR
From System Pin
To System Logic
FF1
0
D
Q
1
From
Previous
Cell
24.5.4
ClockDR
Scanning the Clock Pins
The AVR devices have many clock options selectable by fuses. These are: Internal RC Oscillator, External RC, External Clock, (High Frequency) Crystal Oscillator, Low Frequency Crystal
Oscillator, and Ceramic Resonator.
Figure 24-7 shows how each Oscillator with external connection is supported in the scan chain.
The Enable signal is supported with a general boundary-scan cell, while the Oscillator/Clock output is attached to an observe-only cell. In addition to the main clock, the Timer Oscillator is
scanned in the same way. The output from the internal RC Oscillator is not scanned, as this
Oscillator does not have external connections.
238
ATmega16A
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ATmega16A
Figure 24-7. Boundary-scan Cells for Oscillators and Clock Options
To
Next
Cell
From Digital Logic
ShiftDR
XTAL2/TOSC2
Oscillator
EXTEST
To
Next
Cell
ShiftDR
0
ENABLE
OUTPUT
1
FF1
0
D
Q
D
Q
0
1
D
G
From
Previous
Cell
ClockDR
To System Logic
XTAL1/TOSC1
Q
1
UpdateDR
From
Previous
Cell
ClockDR
Table 24-4 summaries the scan registers for the external clock pin XTAL1, Oscillators with
XTAL1/XTAL2 connections as well as 32 kHz Timer Oscillator.
Table 24-4.
Scan Signals for the Oscillators(1)(2)(3)
Scanned Clock Line
when not Used
Enable Signal
Scanned Clock Line
Clock Option
EXTCLKEN
EXTCLK (XTAL1)
External Clock
0
OSCON
OSCCK
External Crystal
External Ceramic Resonator
0
RCOSCEN
RCCK
External RC
1
OSC32EN
OSC32CK
Low Freq. External Crystal
0
TOSKON
TOSCK
32 kHz Timer Oscillator
0
Notes:
1. Do not enable more than one clock source as main clock at a time.
2. Scanning an Oscillator output gives unpredictable results as there is a frequency drift between
the Internal Oscillator and the JTAG TCK clock. If possible, scanning an external clock is
preferred.
3. The clock configuration is programmed by fuses. As a fuse is not changed run-time, the clock
configuration is considered fixed for a given application. The user is advised to scan the same
clock option as to be used in the final system. The enable signals are supported in the scan
chain because the system logic can disable clock options in sleep modes, thereby disconnecting the Oscillator pins from the scan path if not provided. The INTCAP Fuses are not
supported in the scan-chain, so the boundary scan chain can not make a XTAL Oscillator
requiring internal capacitors to run unless the fuse is correctly programmed.
24.5.5
Scanning the Analog Comparator
The relevant Comparator signals regarding Boundary-scan are shown in Figure 24-8. The
Boundary-scan cell from Figure 24-9 is attached to each of these signals. The signals are
described in Table 24-5.
The Comparator need not be used for pure connectivity testing, since all analog inputs are
shared with a digital port pin as well.
239
8154B–AVR–07/09
Figure 24-8. Analog Comparator
BANDGAP
REFERENCE
ACBG
ACO
AC_IDLE
ACME
ADCEN
ADC MULTIPLEXER
OUTPUT
Figure 24-9. General Boundary-scan Cell used for Signals for Comparator and ADC
To
Next
Cell
ShiftDR
EXTEST
From Digital Logic/
From Analog Ciruitry
0
1
To Analog Circuitry/
To Digital Logic
0
D
Q
D
Q
1
G
From
Previous
Cell
240
ClockDR
UpdateDR
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 24-5.
Boundary-scan Signals for the Analog Comparator
Signal
Name
Direction as Seen from
the Comparator
Recommended Input
when Not in Use
AC_IDLE
Input
Turns off Analog
comparator when true
ACO
Output
Analog Comparator
Output
ACME
Input
Uses output signal from
ADC mux when true
0
Depends upon µC code being
executed
ACBG
Input
Bandgap Reference
enable
0
Depends upon µC code being
executed
Description
1
Output Values when
Recommended Inputs are Used
Depends upon µC code being
executed
Will become input to µC
code being executed
0
24.5.6
Scanning the ADC
Figure 24-10 shows a block diagram of the ADC with all relevant control and observe signals. The Boundary-scan cell from
Figure 24-9 is attached to each of these signals. The ADC need not be used for pure connectivity testing, since all analog
inputs are shared with a digital port pin as well.
Figure 24-10. Analog to Digital Converter
VCCREN
AREF
IREFEN
2.56V
ref
To Comparator
PASSEN
MUXEN_7
ADC_7
MUXEN_6
ADC_6
MUXEN_5
ADC_5
MUXEN_4
ADC_4
ADCBGEN
SCTEST
1.22V
ref
EXTCH
MUXEN_3
ADC_3
MUXEN_2
ADC_2
MUXEN_1
ADC_1
MUXEN_0
ADC_0
PRECH
AREF
AREF
DACOUT
10-bit DAC
+
COMP
G10
G20
-
ADCEN
COMP
DAC_9..0
ACTEN
+
10x
NEGSEL_2
-
ADC_2
NEGSEL_1
ADC_0
20x
HOLD
-
GNDEN
ADC_1
NEGSEL_0
+
ST
ACLK
AMPEN
241
8154B–AVR–07/09
The signals are described briefly in Table 24-6.
Table 24-6.
Boundary-scan Signals for the ADC
Recommended
Input when Not
in Use
Output Values when Recommended
Inputs are used, and CPU is not
Using the ADC
Comparator Output
0
0
Input
Clock signal to gain stages
implemented as Switch-cap filters
0
0
ACTEN
Input
Enable path from gain stages to
the comparator
0
0
ADCBGEN
Input
Enable Band-gap reference as
negative input to comparator
0
0
ADCEN
Input
Power-on signal to the ADC
0
0
AMPEN
Input
Power-on signal to the gain stages
0
0
DAC_9
Input
Bit 9 of digital value to DAC
1
1
DAC_8
Input
Bit 8 of digital value to DAC
0
0
DAC_7
Input
Bit 7 of digital value to DAC
0
0
DAC_6
Input
Bit 6 of digital value to DAC
0
0
DAC_5
Input
Bit 5 of digital value to DAC
0
0
DAC_4
Input
Bit 4 of digital value to DAC
0
0
DAC_3
Input
Bit 3 of digital value to DAC
0
0
DAC_2
Input
Bit 2 of digital value to DAC
0
0
DAC_1
Input
Bit 1 of digital value to DAC
0
0
DAC_0
Input
Bit 0 of digital value to DAC
0
0
EXTCH
Input
Connect ADC channels 0 - 3 to bypass path around gain stages
1
1
G10
Input
Enable 10x gain
0
0
G20
Input
Enable 20x gain
0
0
GNDEN
Input
Ground the negative input to
comparator when true
0
0
HOLD
Input
Sample&Hold signal. Sample
analog signal when low. Hold
signal when high. If gain stages
are used, this signal must go
active when ACLK is high.
1
1
IREFEN
Input
Enables Band-gap reference as
AREF signal to DAC
0
0
MUXEN_7
Input
Input Mux bit 7
0
0
MUXEN_6
Input
Input Mux bit 6
0
0
MUXEN_5
Input
Input Mux bit 5
0
0
MUXEN_4
Input
Input Mux bit 4
0
0
Signal
Name
Direction as Seen
from the ADC
Description
COMP
Output
ACLK
242
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 24-6.
Boundary-scan Signals for the ADC (Continued)
Recommended
Input when Not
in Use
Output Values when Recommended
Inputs are used, and CPU is not
Using the ADC
Input Mux bit 3
0
0
Input
Input Mux bit 2
0
0
MUXEN_1
Input
Input Mux bit 1
0
0
MUXEN_0
Input
Input Mux bit 0
1
1
NEGSEL_2
Input
Input Mux for negative input for
differential signal, bit 2
0
0
NEGSEL_1
Input
Input Mux for negative input for
differential signal, bit 1
0
0
NEGSEL_0
Input
Input Mux for negative input for
differential signal, bit 0
0
0
PASSEN
Input
Enable pass-gate of gain stages.
1
1
PRECH
Input
Precharge output latch of
comparator. (Active low)
1
1
SCTEST
Input
Switch-cap TEST enable. Output
from x10 gain stage send out to
Port Pin having ADC_4
0
0
ST
Input
Output of gain stages will settle
faster if this signal is high first two
ACLK periods after AMPEN goes
high.
0
0
VCCREN
Input
Selects Vcc as the ACC reference
voltage.
0
0
Signal
Name
Direction as Seen
from the ADC
Description
MUXEN_3
Input
MUXEN_2
Note:
Incorrect setting of the switches in Figure 24-10 will make signal contention and may damage the part. There are several input
choices to the S&H circuitry on the negative input of the output comparator in Figure 24-10. Make sure only one path is selected
from either one ADC pin, Bandgap reference source, or Ground.
If the ADC is not to be used during scan, the recommended input values from Table 24-6 should
be used. The user is recommended not to use the Differential Gain stages during scan. Switchcap based gain stages require fast operation and accurate timing which is difficult to obtain
when used in a scan chain. Details concerning operations of the differential gain stage is therefore not provided.
The AVR ADC is based on the analog circuitry shown in Figure 24-10 with a successive approximation algorithm implemented in the digital logic. When used in Boundary-scan, the problem is
usually to ensure that an applied analog voltage is measured within some limits. This can easily
be done without running a successive approximation algorithm: apply the lower limit on the digital DAC[9:0] lines, make sure the output from the comparator is low, then apply the upper limit
on the digital DAC[9:0] lines, and verify the output from the comparator to be high.
The ADC need not be used for pure connectivity testing, since all analog inputs are shared with
a digital port pin as well.
When using the ADC, remember the following:
• The Port Pin for the ADC channel in use must be configured to be an input with pull-up
disabled to avoid signal contention.
243
8154B–AVR–07/09
• In Normal mode, a dummy conversion (consisting of 10 comparisons) is performed when
enabling the ADC. The user is advised to wait at least 200 ns after enabling the ADC before
controlling/observing any ADC signal, or perform a dummy conversion before using the first
result.
• The DAC values must be stable at the midpoint value 0x200 when having the HOLD signal
low (Sample mode).
As an example, consider the task of verifying a 1.5V ± 5% input signal at ADC channel 3 when
the power supply is 5.0V and AREF is externally connected to VCC.
The lower limit is:
The upper limit is:
1024 ⋅ 1,5V ⋅ 0,95 ⁄ 5V = 291 = 0x123
1024 ⋅ 1,5V ⋅ 1,05 ⁄ 5V = 323 = 0x143
The recommended values from Table 24-6 are used unless other values are given in the algorithm in Table 24-7. Only the DAC and Port Pin values of the Scan-chain are shown. The column
“Actions” describes what JTAG instruction to be used before filling the Boundary-scan Register
with the succeeding columns. The verification should be done on the data scanned out when
scanning in the data on the same row in the table.
Table 24-7.
Step
Algorithm for Using the ADC
Actions
ADCEN
DAC
MUXEN
HOLD
PRECH
PA3.
Data
PA3.
Control
PA3.
Pullup_
Enable
1
SAMPLE_
PRELOAD
1
0x200
0x08
1
1
0
0
0
2
EXTEST
1
0x200
0x08
0
1
0
0
0
3
1
0x200
0x08
1
1
0
0
0
4
1
0x123
0x08
1
1
0
0
0
5
1
0x123
0x08
1
0
0
0
0
1
0x200
0x08
1
1
0
0
0
7
1
0x200
0x08
0
1
0
0
0
8
1
0x200
0x08
1
1
0
0
0
9
1
0x143
0x08
1
1
0
0
0
10
1
0x143
0x08
1
0
0
0
0
1
0x200
0x08
1
1
0
0
0
6
11
Verify the
COMP bit
scanned
out to be 0
Verify the
COMP bit
scanned
out to be 1
Using this algorithm, the timing constraint on the HOLD signal constrains the TCK clock frequency. As the algorithm keeps HOLD high for five steps, the TCK clock frequency has to be at
least five times the number of scan bits divided by the maximum hold time, thold,max.
244
ATmega16A
8154B–AVR–07/09
ATmega16A
24.6
Boundary-scan Order
Table 24-8 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 pin-out order as far as possible. Therefore, the bits of Port A is scanned in
the opposite bit order of the other ports. 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 24-3, PXn. Data corresponds to FF0, PXn. Control
corresponds to FF1, and PXn. Pullup_enable corresponds to FF2. Bit 2, 3, 4, and 5 of Port C is
not in the scan chain, since these pins constitute the TAP pins when the JTAG is enabled.
Table 24-8.
ATmega16A Boundary-scan Order
Bit Number
Signal Name
Module
140
AC_IDLE
Comparator
139
ACO
138
ACME
137
ACBG
136
COMP
ADC
(1)
135
PRIVATE_SIGNAL1
134
ACLK
133
ACTEN
132
PRIVATE_SIGNAL2(2)
131
ADCBGEN
130
ADCEN
129
AMPEN
128
DAC_9
127
DAC_8
126
DAC_7
125
DAC_6
124
DAC_5
123
DAC_4
122
DAC_3
121
DAC_2
120
DAC_1
119
DAC_0
118
EXTCH
117
G10
116
G20
115
GNDEN
114
HOLD
113
IREFEN
112
MUXEN_7
245
8154B–AVR–07/09
Table 24-8.
246
ATmega16A Boundary-scan Order (Continued)
Bit Number
Signal Name
111
MUXEN_6
110
MUXEN_5
109
MUXEN_4
108
MUXEN_3
107
MUXEN_2
106
MUXEN_1
105
MUXEN_0
104
NEGSEL_2
103
NEGSEL_1
102
NEGSEL_0
101
PASSEN
100
PRECH
99
SCTEST
98
ST
97
VCCREN
96
PB0.Data
95
PB0.Control
94
PB0.Pullup_Enable
93
PB1.Data
92
PB1.Control
91
PB1.Pullup_Enable
90
PB2.Data
89
PB2.Control
88
PB2.Pullup_Enable
87
PB3.Data
86
PB3.Control
85
PB3.Pullup_Enable
84
PB4.Data
83
PB4.Control
82
PB4.Pullup_Enable
81
PB5.Data
80
PB5.Control
79
PB5.Pullup_Enable
78
PB6.Data
77
PB6.Control
76
PB6.Pullup_Enable
Module
Port B
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 24-8.
ATmega16A Boundary-scan Order (Continued)
Bit Number
Signal Name
Module
75
PB7.Data
74
PB7.Control
73
PB7.Pullup_Enable
72
RSTT
71
RSTHV
70
EXTCLKEN
69
OSCON
68
RCOSCEN
67
OSC32EN
66
EXTCLK (XTAL1)
65
OSCCK
64
RCCK
63
OSC32CK
62
TWIEN
TWI
61
PD0.Data
Port D
60
PD0.Control
59
PD0.Pullup_Enable
58
PD1.Data
57
PD1.Control
56
PD1.Pullup_Enable
55
PD2.Data
54
PD2.Control
53
PD2.Pullup_Enable
52
PD3.Data
51
PD3.Control
50
PD3.Pullup_Enable
49
PD4.Data
48
PD4.Control
47
PD4.Pullup_Enable
46
PD5.Data
45
PD5.Control
44
PD5.Pullup_Enable
43
PD6.Data
42
PD6.Control
41
PD6.Pullup_Enable
40
PD7.Data
Reset Logic
(Observe-Only)
Enable signals for main clock/Oscillators
Clock input and Oscillators for the main clock
(Observe-Only)
247
8154B–AVR–07/09
Table 24-8.
248
ATmega16A Boundary-scan Order (Continued)
Bit Number
Signal Name
39
PD7.Control
38
PD7.Pullup_Enable
37
PC0.Data
36
PC0.Control
35
PC0.Pullup_Enable
34
PC1.Data
33
PC1.Control
32
PC1.Pullup_Enable
31
PC6.Data
30
PC6.Control
29
PC6.Pullup_Enable
28
PC7.Data
27
PC7.Control
26
PC7.Pullup_Enable
25
TOSC
24
TOSCON
23
PA7.Data
22
PA7.Control
21
PA7.Pullup_Enable
20
PA6.Data
19
PA6.Control
18
PA6.Pullup_Enable
17
PA5.Data
16
PA5.Control
15
PA5.Pullup_Enable
14
PA4.Data
13
PA4.Control
12
PA4.Pullup_Enable
11
PA3.Data
10
PA3.Control
9
PA3.Pullup_Enable
8
PA2.Data
7
PA2.Control
6
PA2.Pullup_Enable
5
PA1.Data
Module
Port C
32 kHz Timer Oscillator
Port A
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 24-8.
ATmega16A Boundary-scan Order (Continued)
Bit Number
Signal Name
4
PA1.Control
3
PA1.Pullup_Enable
2
PA0.Data
1
PA0.Control
0
PA0.Pullup_Enable
Notes:
Module
1. PRIVATE_SIGNAL1 should always be scanned in as zero.
2. PRIVATE:SIGNAL2 should always be scanned in as zero.
24.7
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. A BSDL file for ATmega16A is
available.
24.8
24.8.1
Register Description
MCUCSR – MCU Control and Status Register
The MCU Control and Status Register contains control bits for general MCU functions, and provides information on which reset source caused an MCU Reset.
Bit
7
6
5
4
3
2
1
0
JTD
ISC2
–
JTRF
WDRF
BORF
EXTRF
PORF
Read/Write
R/W
R/W
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
MCUCSR
See Bit Description
• Bit 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.
If the JTAG interface is left unconnected to other JTAG circuitry, the JTD bit should be set to
one. The reason for this is to avoid static current at the TDO pin in the JTAG interface.
• 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.
249
8154B–AVR–07/09
25. Boot Loader Support – Read-While-Write Self-Programming
25.1
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:
25.2
1. A page is a section in the flash consisting of several bytes (see Table 26-5 on page 267)
used during programming. The page organization does not affect normal operation.
Overview
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.
25.3
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 25-2). The size of the different sections is configured by the
BOOTSZ Fuses as shown in Table 25-6 on page 262 and Figure 25-2. These two sections can
have different level of protection since they have different sets of Lock bits.
25.3.1
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), see Table 25-2 on page 254. The Application section can never store any
Boot Loader code since the SPM instruction is disabled when executed from the Application
section.
25.3.2
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), see Table 25-3 on page 254.
250
ATmega16A
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ATmega16A
25.4
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-WhileWrite (NRWW) section. The limit between the RWW- and NRWW sections is given in Table 257 on page 262 and Figure 25-2 on page 253. 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.
25.4.1
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 ongoing 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 a
call/jmp/lpm 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 Register (SPMCR) 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. See
“SPMCR – Store Program Memory Control Register” on page 255. for details on how to clear
RWWSB.
25.4.2
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 25-1.
Read-While-Write Features
Which Section does the Zpointer Address during the
Programming?
Which Section can be
Read during
Programming?
Is the CPU
Halted?
Read-WhileWrite
Supported?
RWW section
NRWW section
No
Yes
NRWW section
None
Yes
No
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Figure 25-1. Read-While-Write vs. No Read-While-Write
Read-While-Write
(RWW) Section
Z-pointer
Addresses RWW
Section
Z-pointer
Addresses NRWW
Section
No Read-While-Write
(NRWW) Section
CPU is Halted
during the Operation
Code Located in
NRWW Section
Can be Read during
the Operation
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Figure 25-2. Memory Sections(1)
Program Memory
BOOTSZ = '10'
Program Memory
BOOTSZ = '11'
$0000
No Read-While-Write Section
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
Read-While-Write Section
$0000
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'
$0000
No Read-While-Write Section
Note:
25.5
Read-While-Write Section
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
Read-While-Write Section
$0000
Application flash Section
End RWW, End Application
Start NRWW, Start Boot Loader
Boot Loader Flash Section
Flashend
1. The parameters in the figure above are given in Table 25-6 on page 262.
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 Table 25-2 and Table 25-3 for further details. The Boot Lock bits can be set in software and
in Serial or Parallel Programming mode, but they can 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 3) does not
control reading nor writing by LPM/SPM, if it is attempted.
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Table 25-2.
BLB0 Mode
BLB02
BLB01
1
1
1
No restrictions for SPM or 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 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.
1
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
4
Note:
0
0
Protection
1. “1” means unprogrammed, “0” means programmed
Table 25-3.
Boot Lock Bit1 Protection Modes (Boot Loader Section)(1)
BLB1 mode
BLB12
BLB11
1
1
1
No restrictions for SPM or 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 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
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
4
Note:
25.6
Boot Lock Bit0 Protection Modes (Application Section)(1)
0
0
Protection
1. “1” means unprogrammed, “0” means programmed
Entering the Boot Loader Program
Entering the Boot Loader takes place by a jump or call from the application program. This may
be initiated by a trigger such as a command received via USART, or SPI interface. Alternatively,
the Boot Reset Fuse 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.
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Table 25-4.
Boot Reset Fuse(1)
BOOTRST
Note:
25.6.1
Reset Address
1
Reset Vector = Application reset (address $0000)
0
Reset Vector = Boot Loader reset (see Table 25-6 on page 262)
1. “1” means unprogrammed, “0” means programmed
SPMCR – Store Program Memory Control Register
The Store Program Memory Control Register contains the control bits needed to control the Boot
Loader operations.
Bit
7
6
5
4
3
2
1
0
SPMIE
RWWSB
–
RWWSRE
BLBSET
PGWRT
PGERS
SPMEN
Read/Write
R/W
R
R
R/W
R/W
R/W
R/W
R/W
Initial value
0
0
0
0
0
0
0
0
SPMCR
• 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 SPMCR 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 – Res: Reserved Bit
This bit is a reserved bit in the ATmega16A and always read as zero.
• 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 Zpointer 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 LPM instruction within three cycles after BLBSET and SPMEN are set in the SPMCR Register, will read either the Lock bits or the Fuse bits (depending on Z0 in the Z-pointer) into the
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destination register. See “Reading the Fuse and Lock Bits from Software” on page 259 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.
25.7
Addressing the Flash during Self-Programming
The Z-pointer is used to address the SPM commands.
Bit
15
14
13
12
11
10
9
8
ZH (R31)
Z15
Z14
Z13
Z12
Z11
Z10
Z9
Z8
ZL (R30)
Z7
Z6
Z5
Z4
Z3
Z2
Z1
Z0
7
6
5
4
3
2
1
0
Since the Flash is organized in pages (see Table 26-5 on page 267), 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 25-3. 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 only SPM operation that does not use the Z-pointer is Setting the Boot Loader Lock bits.
The content of the Z-pointer is ignored and will have no effect on the operation. The LPM
instruction does also use the Z pointer to store the address. Since this instruction addresses the
Flash byte by byte, also the LSB (bit Z0) of the Z-pointer is used.
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Figure 25-3. Addressing the Flash during SPM(1)
BIT
15
ZPCMSB
ZPAGEMSB
Z - REGISTER
1 0
0
PCMSB
PROGRAM
COUNTER
PAGEMSB
PCPAGE
PCWORD
PAGE ADDRESS
WITHIN THE FLASH
WORD ADDRESS
WITHIN A PAGE
PROGRAM MEMORY
PAGE
PAGE
INSTRUCTION WORD
PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
Notes:
25.8
1. The different variables used in Figure 25-3 are listed in Table 25-8 on page 263.
2. PCPAGE and PCWORD are listed in Table 26-5 on page 267.
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
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page. See “Simple Assembly Code Example for a Boot Loader” on page 260 for an assembly
code example.
25.8.1
Performing Page Erase by SPM
To execute Page Erase, set up the address in the Z-pointer, write “X0000011” to SPMCR and
execute SPM within four clock cycles after writing SPMCR. 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 must
be written zero 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.
25.8.2
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 SPMCR and execute SPM within four clock cycles after writing SPMCR. The content of PCWORD in the Z-register 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
SPMCR. 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.
Note:
25.8.3
If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be
lost.
Performing a Page Write
To execute Page Write, set up the address in the Z-pointer, write “X0000101” to SPMCR and
execute SPM within four clock cycles after writing SPMCR. 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 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.
25.8.4
Using the SPM Interrupt
If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt when the
SPMEN bit in SPMCR is cleared. This means that the interrupt can be used instead of polling
the SPMCR 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 44.
25.8.5
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.
25.8.6
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 SPMCR will be set as long as the RWW
section is busy. During self-programming the Interrupt Vector table should be moved to the BLS
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ATmega16A
as described in “Interrupts” on page 44, 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 260 for an example.
25.8.7
Setting the Boot Loader Lock Bits by SPM
To set the Boot Loader Lock bits, write the desired data to R0, write “X0001001” to SPMCR and
execute SPM within four clock cycles after writing SPMCR. 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
7
6
5
4
3
2
1
0
R0
1
1
BLB12
BLB11
BLB02
BLB01
1
1
See Table 25-2 and Table 25-3 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 SPMCR.
The Z-pointer is don’t care during this operation, but for future compatibility it is recommended to
load the Z-pointer with $0001 (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.
25.8.8
EEPROM Write Prevents Writing to SPMCR
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 (EEWE) in the EECR Register and verifies
that the bit is cleared before writing to the SPMCR Register.
25.8.9
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 $0001 and set the BLBSET and SPMEN bits in SPMCR. When an LPM instruction
is executed within three CPU cycles after the BLBSET and SPMEN bits are set in SPMCR, 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 LPM instruction is executed
within three CPU cycles or no SPM instruction is executed within four CPU cycles. When BLBSET and SPMEN are cleared, LPM will work as described in the Instruction set Manual.
Bit
7
6
5
4
3
2
1
0
Rd
–
–
BLB12
BLB11
BLB02
BLB01
LB2
LB1
The algorithm for reading the Fuse Low bits is similar to the one described above for reading the
Lock bits. To read the Fuse Low bits, load the Z-pointer with $0000 and set the BLBSET and
SPMEN bits in SPMCR. When an LPM instruction is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCR, the value of the Fuse Low bits (FLB) will be loaded
in the destination register as shown below. Refer to Table 26-4 on page 266 for a detailed
description and mapping of the Fuse Low bits.
Bit
7
6
5
4
3
2
1
0
Rd
FLB7
FLB6
FLB5
FLB4
FLB3
FLB2
FLB1
FLB0
Similarly, when reading the Fuse High bits, load $0003 in the Z-pointer. When an LPM instruction is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCR,
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the value of the Fuse High bits (FHB) will be loaded in the destination register as shown below.
Refer to Table 26-3 on page 265 for detailed description and mapping of the Fuse High bits.
Bit
7
6
5
4
3
2
1
0
Rd
FHB7
FHB6
FHB5
FHB4
FHB3
FHB2
FHB1
FHB0
Fuse and Lock bits that are programmed, will be read as zero. Fuse and Lock bits that are
unprogrammed, will be read as one.
25.8.10
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 SPMCR Register and thus the Flash from unintentional writes.
25.8.11
Programming Time for Flash when using SPM
The Calibrated RC Oscillator is used to time Flash accesses. Table 25-5 shows the typical programming time for Flash accesses from the CPU.
Table 25-5.
SPM Programming Time.
Symbol
Flash write (Page Erase, Page Write,
and write Lock bits by SPM)
25.8.12
Min Programming Time
Max Programming Time
3.7 ms
4.5 ms
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),
; 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
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ATmega16A
; 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)
;init loop variable
ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256
subi YL, low(PAGESIZEB)
;restore pointer
sbci YH, high(PAGESIZEB)
Rdloop:
lpm r0, Z+
ld
r1, Y+
cpse r0, r1
jmp Error
sbiw loophi:looplo, 1
;use subi for PAGESIZEB<=256
brne Rdloop
; return to RWW section
; verify that RWW section is safe to read
Return:
in
temp1, SPMCR
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
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Do_spm:
; check for previous SPM complete
Wait_spm:
in
temp1, SPMCR
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, EEWE
rjmp Wait_ee
; SPM timed sequence
out SPMCR, spmcrval
spm
; restore SREG (to enable interrupts if originally enabled)
out SREG, temp2
ret
25.8.13
ATmega16A Boot Loader Parameters
In Table 25-6 through Table 25-8, the parameters used in the description of the self programming are given.
Table 25-6.
Boot Size Configuration(1)
Boot
Size
End
Application
section
Boot Reset
Address (start
Boot Loader
Section)
BOOTSZ0
1
1
128
words
2
$0000 $1F7F
$1F80 $1FFF
$1F7F
$1F80
1
0
256
words
4
$0000 $1EFF
$1F00 $1FFF
$1EFF
$1F00
0
1
512
words
8
$0000 $1DFF
$1E00 $1FFF
$1DFF
$1E00
0
0
1024
words
16
$0000 $1BFF
$1C00 $1FFF
$1BFF
$1C00
1. The different BOOTSZ Fuse configurations are shown in Figure 25-2
Table 25-7.
Read-While-Write Limit(1)
Section
Pages
Address
Read-While-Write section (RWW)
112
$0000 - $1BFF
No Read-While-Write section (NRWW)
16
$1C00 - $1FFF
Note:
262
Boot
Loader
Flash
Section
BOOTSZ1
Note:
Pages
Application
Flash
Section
1. For details about these two section, see “NRWW – No Read-While-Write Section” on page
251 and “RWW – Read-While-Write Section” on page 251
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 25-8.
Explanation of Different Variables used in Figure 25-3 and the Mapping to the Zpointer
Corresponding
Z-value(1)
Variable
PCMSB
12
Most significant bit in the Program Counter. (The
Program Counter is 13 bits PC[12:0])
5
Most significant bit which is used to address the
words within one page (64 words in a page requires
6 bits PC [5:0]).
PAGEMSB
Z13
Bit in Z-register that is mapped to PCMSB. Because
Z0 is not used, the ZPCMSB equals PCMSB + 1.
Z6
Bit in Z-register that is mapped to PAGEMSB.
Because Z0 is not used, the ZPAGEMSB equals
PAGEMSB + 1.
PC[12:6]
Z13:Z7
Program Counter page address: Page select, for
Page Erase and Page Write
PC[5:0]
Z6:Z1
Program Counter word address: Word select, for
filling temporary buffer (must be zero during page
write operation)
ZPCMSB
ZPAGEMSB
PCPAGE
PCWORD
Note:
Description
1. Z15:Z14: always ignored
Z0: should be zero for all SPM commands, byte select for the LPM instruction.
See “Addressing the Flash during Self-Programming” on page 256 for details about the use of
Z-pointer during Self-Programming.
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26. Memory Programming
26.1
Program And Data Memory Lock Bits
The ATmega16A provides six Lock bits which can be left unprogrammed (“1”) or can be programmed (“0”) to obtain the additional features listed in Table 26-2. The Lock bits can only be
erased to “1” with the Chip Erase command.
Table 26-1.
Lock Bit Byte(1)
Lock Bit Byte
Description
Default Value
7
–
1 (unprogrammed)
6
–
1 (unprogrammed)
BLB12
5
Boot Lock bit
1 (unprogrammed)
BLB11
4
Boot Lock bit
1 (unprogrammed)
BLB02
3
Boot Lock bit
1 (unprogrammed)
BLB01
2
Boot Lock bit
1 (unprogrammed)
LB2
1
Lock bit
1 (unprogrammed)
LB1
0
Lock bit
1 (unprogrammed)
Note:
Bit No.
1. “1” means unprogrammed, “0” means programmed
Table 26-2.
Lock Bit Protection Modes
Memory Lock Bits(2)
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 SPI/JTAG 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 SPI/JTAG Serial Programming mode.
The 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 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 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
264
Protection Type
0
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 26-2.
Lock Bit Protection Modes (Continued)
Memory Lock Bits(2)
Protection Type
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 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 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
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:
0
1. Program the Fuse bits before programming the Lock bits.
2. “1” means unprogrammed, “0” means programmed
26.2
Fuse Bits
The ATmega16A has two fuse bytes. Table 26-3 and Table 26-4 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 26-3.
Fuse High Byte
Fuse High
Byte
Bit No.
Description
Default Value
OCDEN(4)
7
Enable OCD
1 (unprogrammed, OCD disabled)
(5)
6
Enable JTAG
0 (programmed, JTAG enabled)
SPIEN(1)
5
Enable SPI Serial Program and
Data Downloading
0 (programmed, SPI prog. enabled)
CKOPT(2)
4
Oscillator options
1 (unprogrammed)
EESAVE
3
EEPROM memory is preserved
through the Chip Erase
1 (unprogrammed, EEPROM not
preserved)
BOOTSZ1
2
Select Boot Size (see Table 25-6
for details)
0 (programmed)(3)
BOOTSZ0
1
Select Boot Size (see Table 25-6
for details)
0 (programmed)(3)
BOOTRST
0
Select reset vector
1 (unprogrammed)
JTAGEN
Notes:
1. The SPIEN Fuse is not accessible in SPI Serial Programming mode.
2. The CKOPT Fuse functionality depends on the setting of the CKSEL bits. See See “Clock
Sources” on page 25. for details.
3. The default value of BOOTSZ1:0 results in maximum Boot Size. See Table 25-6 on page 262.
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4. Never ship a product with the OCDEN Fuse programmed regardless of the setting of Lock bits
and the 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.
5. If the JTAG interface is left unconnected, the JTAGEN fuse should if possible be disabled. This
to avoid static current at the TDO pin in the JTAG interface.
Table 26-4.
Fuse Low
Byte
Fuse Low Byte
Bit No.
Description
Default Value
BODLEVEL
7
Brown-out Detector trigger level
1 (unprogrammed)
BODEN
6
Brown-out Detector enable
1 (unprogrammed, BOD disabled)
SUT1
5
Select start-up time
1 (unprogrammed)(1)
SUT0
4
Select start-up time
0 (programmed)(1)
CKSEL3
3
Select Clock source
0 (programmed)(2)
CKSEL2
2
Select Clock source
0 (programmed)(2)
CKSEL1
1
Select Clock source
0 (programmed)(2)
CKSEL0
0
Select Clock source
1 (unprogrammed)(2)
Notes:
1. The default value of SUT1:0 results in maximum start-up time. SeeTable 8-9 on page 29 for
details.
2. The default setting of CKSEL3:0 results in internal RC Oscillator @ 1MHz. See Table 8-1 on
page 25 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.
26.2.1
26.3
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.
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.
For the ATmega16A the signature bytes are:
1. $000: $1E (indicates manufactured by Atmel)
2. $001: $94 (indicates 16KB Flash memory)
3. $002: $03 (indicates ATmega16A device when $001 is $94)
26.4
Calibration Byte
The ATmega16A stores four different calibration values for the internal RC Oscillator. These
bytes resides in the signature row High Byte of the addresses 0x0000, 0x0001, 0x0002, and
0x0003 for 1, 2, 4, and 8 Mhz respectively. During Reset, the 1 MHz value is automatically
loaded into the OSCCAL Register. If other frequencies are used, the calibration value has to be
loaded manually, see “OSCCAL – Oscillator Calibration Register” on page 31 for details.
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ATmega16A
26.5
Page Size
Table 26-5.
No. of Words in a Page and no. of Pages in the Flash
Flash Size
8K words (16K bytes)
Table 26-6.
26.6
Page Size
PCWORD
No. of Pages
PCPAGE
PCMSB
64 words
PC[5:0]
128
PC[12:6]
12
No. of Words in a Page and no. of Pages in the EEPROM
EEPROM Size
Page Size
PCWORD
No. of Pages
PCPAGE
EEAMSB
512 bytes
4 bytes
EEA[1:0]
128
EEA[8:2]
8
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 ATmega16A. Pulses are assumed to be at
least 250 ns unless otherwise noted.
26.6.1
Signal Names
In this section, some pins of the ATmega16A are referenced by signal names describing their
functionality during parallel programming, see Figure 26-1 and Table 26-7. 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 26-9.
When pulsing WR or OE, the command loaded determines the action executed. The different
Commands are shown in Table 26-10.
Figure 26-1. Parallel Programming
+5V
RDY/BSY
PD1
OE
PD2
WR
PD3
BS1
PD4
XA0
PD5
XA1
PD6
PAGEL
PD7
+12 V
BS2
VCC
+5V
AVCC
PB7 - PB0
DATA
RESET
PA0
XTAL1
GND
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Table 26-7.
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 (“0” selects Low byte, “1” selects High
byte)
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
PA0
I
Byte Select 2 (“0” selects Low byte, “1” selects 2’nd
High byte)
DATA
PB7-0
I/O
Table 26-8.
Bidirectional Data bus (Output when OE is low)
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
Table 26-9.
XA1 and XA0 Coding
XA1
XA0
Action when XTAL1 is Pulsed
0
0
Load Flash or EEPROM Address (High or low address byte determined by 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 26-10. Command Byte Bit Coding
Command Byte
268
Command Executed
1000 0000
Chip Erase
0100 0000
Write Fuse Bits
0010 0000
Write Lock Bits
0001 0000
Write Flash
0001 0001
Write EEPROM
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 26-10. Command Byte Bit Coding
Command Byte
26.7
26.7.1
Command Executed
0000 1000
Read Signature Bytes and Calibration byte
0000 0100
Read Fuse and Lock bits
0000 0010
Read Flash
0000 0011
Read EEPROM
Parallel Programming
Enter Programming Mode
The following algorithm puts the device in Parallel Programming mode:
1. Apply 4.5 - 5.5V between VCC and GND, and wait at least 100 µs.
2. Set RESET to “0” and toggle XTAL1 at least 6 times
3. Set the Prog_enable pins listed in Table 26-8 on page 268 to “0000” and wait at least
100 ns.
4. Apply 11.5 - 12.5V to RESET. Any activity on Prog_enable pins within 100 ns after
+12V has been applied to RESET, will cause the device to fail entering Programming
mode.
Note, if External Crystal or External RC configuration is selected, it may not be possible to apply
qualified XTAL1 pulses. In such cases, the following algorithm should be followed:
1. Set Prog_enable pins listed in Table 26-8 on page 268 to “0000”.
2. Apply 4.5 - 5.5V between VCC and GND simultaneously as 11.5 - 12.5V is applied to
RESET.
3. Wait 100 µs.
4. Re-program the fuses to ensure that External Clock is selected as clock source
(CKSEL3:0 = 0b0000) If Lock bits are programmed, a Chip Erase command must be
executed before changing the fuses.
5. Exit Programming mode by power the device down or by bringing RESET pin to 0b0.
6. Entering Programming mode with the original algorithm, as described above.
26.7.2
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 $FF, 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.
26.7.3
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
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changed. A Chip Erase must be performed before the Flash and/or the EEPROM are
reprogrammed.
Note:
1. The EEPRPOM memory is preserved during chip erase if the EESAVE Fuse is programmed.
1. Load Command “Chip Erase”
2. Set XA1, XA0 to “10”. This enables command loading.
3. Set BS1 to “0”.
4. Set DATA to “1000 0000”. This is the command for Chip Erase.
5. Give XTAL1 a positive pulse. This loads the command.
6. Give WR a negative pulse. This starts the Chip Erase. RDY/BSY goes low.
7. Wait until RDY/BSY goes high before loading a new command.
26.7.4
Programming the Flash
The Flash is organized in pages, see Table 26-5 on page 267. 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
1. Set XA1, XA0 to “00”. This enables address loading.
2. Set BS1 to “0”. This selects low address.
3. Set DATA = Address Low byte ($00 - $FF).
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 ($00 - $FF).
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 ($00 - $FF).
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 26-3 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 26-2 on page 271. Note that if less than
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ATmega16A
8 bits are required to address words in the page (pagesize < 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
1. Set XA1, XA0 to “00”. This enables address loading.
2. Set BS1 to “1”. This selects high address.
3. Set DATA = Address High byte ($00 - $FF).
4. Give XTAL1 a positive pulse. This loads the address High byte.
H. Program Page
1. Set BS1 = “0”
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 26-3 for signal waveforms)
I. Repeat B through H until the entire Flash is programmed or until all data has been
programmed.
J. 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 26-2. Addressing the Flash which is Organized in Pages
PCMSB
PROGRAM
COUNTER
PAGEMSB
PCPAGE
PAGE ADDRESS
WITHIN THE FLASH
PROGRAM MEMORY
PAGE
PCWORD
WORD ADDRESS
WITHIN A PAGE
PAGE
INSTRUCTION WORD
PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
Note:
1. PCPAGE and PCWORD are listed in Table 26-5 on page 267.
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8154B–AVR–07/09
Figure 26-3. Programming the Flash Waveforms(1)
F
A
DATA
$10
B
C
ADDR. LOW DATA LOW
D
DATA HIGH
E
B
C
D
E
G
XX
ADDR. LOW
DATA LOW
DATA HIGH
XX
ADDR. HIGH
H
XX
XA1
XA0
BS1
XTAL1
WR
RDY/BSY
RESET +12V
OE
PAGEL
BS2
Note:
Programming the
EEPROM
1. “XX” is don’t care. The letters refer to the programming description above.
The EEPROM is organized in pages, see Table 26-6 on page 267. 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 270 for details on Command, Address and
Data loading):
1. A: Load Command “0001 0001”.
2. G: Load Address High Byte ($00 - $FF)
3. B: Load Address Low Byte ($00 - $FF)
4. C: Load Data ($00 - $FF)
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 BS1 to “0”.
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 26-4
for signal waveforms)
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ATmega16A
Figure 26-4. Programming the EEPROM Waveforms
K
A
DATA
0x11
G
B
ADDR. HIGH ADDR. LOW
C
E
B
C
E
DATA
XX
ADDR. LOW
DATA
XX
L
XA1
XA0
BS1
XTAL1
WR
RDY/BSY
RESET +12V
OE
PAGEL
BS2
26.7.5
Reading the Flash
The algorithm for reading the Flash memory is as follows (refer to “Programming the Flash” on
page 270 for details on Command and Address loading):
1. A: Load Command “0000 0010”.
2. G: Load Address High Byte ($00 - $FF)
3. B: Load Address Low Byte ($00 - $FF)
4. Set OE to “0”, and BS1 to “0”. The Flash word Low byte can now be read at DATA.
5. Set BS1 to “1”. The Flash word High byte can now be read at DATA.
6. Set OE to “1”.
26.7.6
Reading the EEPROM
The algorithm for reading the EEPROM memory is as follows (refer to “Programming the Flash”
on page 270 for details on Command and Address loading):
1. A: Load Command “0000 0011”.
2. G: Load Address High Byte ($00 - $FF)
3. B: Load Address Low Byte ($00 - $FF)
4. Set OE to “0”, and BS1 to “0”. The EEPROM Data byte can now be read at DATA.
5. Set OE to “1”.
26.7.7
Programming the Fuse Low Bits
The algorithm for programming the Fuse Low bits is as follows (refer to “Programming the Flash”
on page 270 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.
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3. Set BS1 to “0” and BS2 to “0”.
4. Give WR a negative pulse and wait for RDY/BSY to go high.
26.7.8
Programming the Fuse High Bits
The algorithm for programming the Fuse high bits is as follows (refer to “Programming the Flash”
on page 270 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 BS1 to “1” and BS2 to “0”. This selects high data byte.
4. Give WR a negative pulse and wait for RDY/BSY to go high.
5. Set BS1 to “0”. This selects low data byte.
Figure 26-5. Programming the Fuses
Write Fuse Low byte
DATA
A
C
$40
DATA
XX
Write Fuse high byte
A
C
$40
DATA
XX
XA1
XA0
BS1
BS2
XTAL1
WR
RDY/BSY
RESET +12V
OE
PAGEL
26.7.9
Programming the Lock Bits
The algorithm for programming the Lock bits is as follows (refer to “Programming the Flash” on
page 270 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.
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.
26.7.10
274
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 270 for details on Command loading):
ATmega16A
8154B–AVR–07/09
ATmega16A
1. A: Load Command “0000 0100”.
2. Set OE to “0”, BS2 to “0” and BS1 to “0”. The status of the Fuse Low bits can now be
read at DATA (“0” means programmed).
3. Set OE to “0”, BS2 to “1” and BS1 to “1”. The status of the Fuse High bits can now be
read at DATA (“0” means programmed).
4. Set OE to “0”, BS2 to “0” and BS1 to “1”. The status of the Lock bits can now be read at
DATA (“0” means programmed).
5. Set OE to “1”.
Figure 26-6. Mapping between BS1, BS2 and the Fuse- and Lock Bits during Read
0
Fuse Low Byte
DATA
Lock Bits
0
1
BS1
Fuse High Byte
1
BS2
26.7.11
Reading the Signature Bytes
The algorithm for reading the Signature bytes is as follows (refer to “Programming the Flash” on
page 270 for details on Command and Address loading):
1. A: Load Command “0000 1000”.
2. B: Load Address Low Byte ($00 - $02).
3. Set OE to “0”, and BS1 to “0”. The selected Signature byte can now be read at DATA.
4. Set OE to “1”.
26.7.12
Reading the Calibration Byte
The algorithm for reading the Calibration byte is as follows (refer to “Programming the Flash” on
page 270 for details on Command and Address loading):
1. A: Load Command “0000 1000”.
2. B: Load Address Low Byte, $00.
3. Set OE to “0”, and BS1 to “1”. The Calibration byte can now be read at DATA.
4. Set OE to “1”.
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26.8
Serial Downloading
Both the Flash and EEPROM memory arrays can be programmed using the serial SPI bus while
RESET is pulled to GND. The serial interface consists of pins SCK, MOSI (input), and MISO
(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 26-11 on page 276, the pin
mapping for SPI programming is listed. Not all parts use the SPI pins dedicated for the internal
SPI interface.
26.8.1
SPI Serial Programming Pin Mapping
Table 26-11. Pin Mapping SPI Serial Programming
Symbol
Pins
I/O
Description
MOSI
PB5
I
Serial Data in
MISO
PB6
O
Serial Data out
SCK
PB7
I
Serial Clock
Figure 26-7. SPI Serial Programming and Verify(1)
+2.7 - 5.5V
VCC
+2.7 - 5.5V(2)
MOSI
PB5
MISO
PB6
SCK
PB7
AVCC
XTAL1
RESET
GND
Notes:
1. If the device is clocked by the Internal Oscillator, it is no need to connect a clock source to the
XTAL1 pin.
2. VCC -0.3V < AVCC < VCC +0.3V, however, AVCC should always be within 2.7 - 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 $FF.
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: > 2 CPU clock cycles for fck < 12 MHz, 3 CPU clock cycles for fck ≥ 12 MHz
High: > 2 CPU clock cycles for fck < 12 MHz, 3 CPU clock cycles for fck ≥ 12 MHz
276
ATmega16A
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ATmega16A
26.8.2
SPI Serial Programming Algorithm
When writing serial data to the ATmega16A, data is clocked on the rising edge of SCK.
When reading data from the ATmega16A, data is clocked on the falling edge of SCK. See Figure
27-5 for timing details.
To program and verify the ATmega16A in the SPI Serial Programming mode, the following
sequence is recommended (See four byte instruction formats in Figure 26-13 on page 278):
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 20 ms and enable SPI Serial Programming by sending the Programming Enable serial instruction to pin MOSI.
3. The SPI Serial Programming instructions will not work if the communication is out of
synchronization. When in sync. the second byte ($53), will echo back when issuing the
third byte of the Programming Enable instruction. Whether the echo is correct or not, all
four bytes of the instruction must be transmitted. If the $53 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 page size is found in Table 26-5 on page
267. The memory page is loaded one byte at a time by supplying the 6 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 7 MSB of the address. If polling is not used,
the user must wait at least tWD_FLASH before issuing the next page. (See Table 26-12).
Accessing the SPI 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 26-12). In a chip
erased device, no $FFs 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 MISO.
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.
26.8.3
Data Polling Flash
When a page is being programmed into the Flash, reading an address location within the page
being programmed will give the value $FF. At the time the device is ready for a new page, the
programmed value will read correctly. This is used to determine when the next page can be written. Note that the entire page is written simultaneously and any address within the page can be
used for polling. Data polling of the Flash will not work for the value $FF, so when programming
this value, the user will have to wait for at least tWD_FLASH before programming the next page. As
a chip erased device contains $FF in all locations, programming of addresses that are meant to
contain $FF, can be skipped. See Table 26-12 for tWD_FLASH value
277
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26.8.4
Data Polling EEPROM
When a new byte has been written and is being programmed into EEPROM, reading the
address location being programmed will give the value $FF. At the time the device is ready for a
new byte, the programmed value will read correctly. This is used to determine when the next
byte can be written. This will not work for the value $FF, but the user should have the following in
mind: As a chip erased device contains $FF in all locations, programming of addresses that are
meant to contain $FF, can be skipped. This does not apply if the EEPROM is re-programmed
without chip erasing the device. In this case, data polling cannot be used for the value $FF, and
the user will have to wait at least tWD_EEPROM before programming the next byte. See Table 26-12
for tWD_EEPROM value.
Table 26-12. Minimum Wait Delay before Writing the Next Flash or EEPROM Location
26.8.5
Symbol
Minimum Wait Delay
tWD_FUSE
4.5 ms
tWD_FLASH
4.5 ms
tWD_EEPROM
9.0 ms
tWD_ERASE
9.0 ms
Serial Programming Instruction set
Table 26-13 on page 278 and Figure 26-8 on page 280 describes the Instruction set.
Table 26-13. Serial Programming Instruction Set (Hexadecimal values)
Instruction Format
(1)
Instruction /Operation
Byte 1
Byte 2
Byte 3
Byte4
Programming Enable
$AC
$53
$00
$00
Chip Erase (Program Memory/EEPROM)
$AC
$80
$00
$00
Poll RDY/BSY
$F0
$00
$00
data byte out
Load Extended Address byte(1)
$4D
$00
Extended adr
$00
Load Program Memory Page, High byte
$48
adr MSB
adr LSB
high data byte in
$40
adr MSB
adr LSB
low data byte in
$C1
$00
adr LSB
data byte in
Read Program Memory, High byte
$28
adr MSB
adr LSB
high data byte out
Read Program Memory, Low byte
$20
adr MSB
adr LSB
low data byte out
Read EEPROM Memory
$A0
adr MSB
adr LSB
data byte out
Read Lock bits
$58
$00
$00
data byte out
Read Signature Byte
$30
$00
0000 000aa
data byte out
Read Fuse bits
$50
$00
$00
data byte out
Read Fuse High bits
$58
$08
$00
data byte out
Read Extended Fuse Bits
$50
$08
$00
data byte out
Load Instructions
Load Program Memory Page, Low byte
Load EEPROM Memory Page (page access)
(1)
Read Instructions
278
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ATmega16A
Table 26-13. Serial Programming Instruction Set (Hexadecimal values) (Continued)
Instruction Format
Instruction(1)/Operation
Byte 1
Byte 2
Byte 3
Byte4
$38
$00
$0b00 000bb
data byte out
$4C
000a aaaa
aa00 0000
$00
$C0
adr MSB
adr LSB
data byte in
$C2
adr MSB
adr LSB
$00
Write Lock bits
$AC
$E0
$00
data byte in
Write Fuse bits
$AC
$A0
$00
data byte in
Write Fuse High bits
$AC
$A8
$00
data byte in
Write Extended Fuse Bits
$AC
$A4
$00
data byte in
Read Calibration Byte
Write Instructions
Write Program Memory Page
Write EEPROM Memory
Write EEPROM Memory Page (page access)
Notes:
(1)
1. Not all instructions are applicable for all parts.
2. a = address
3. Bits are programmed ‘0’, unprogrammed ‘1’.
4. To ensure future compatibility, unused Fuses and Lock bits should be unprogrammed (‘1’) .
5. Refer to the correspondig section for Fuse and Lock bits, Calibration and Signature bytes and Page size.
6. See htt://www.atmel.com/avr for Application Notes regarding programming and programmers.
If the LSB in RDY/BSY data byte out is ‘1’, a programming operation is still pending. Wait until
this bit returns ‘0’ before the next instruction is carried out.
Within the same page, the low data byte must be loaded prior to the high data byte.
After data is loaded to the page buffer, program the EEPROM page, see Figure 26-8 on page
280.
279
8154B–AVR–07/09
Figure 26-8. Serial Programming Instruction example
Serial Programming Instruction
Load Program Memory Page (High/Low Byte)/
Load EEPROM Memory Page (page access)
Byte 1
Byte 2
Byte 3
Adr MSB
A
Bit 15 B
Write Program Memory Page/
Write EEPROM Memory Page
Byte 1
Byte 4
Byte 2
Adr LSB
Adr MSB
Bit 15 B
0
Byte 3
Byte 4
Adrr LSB
B
0
Page Buffer
Page Offset
Page 0
Page 1
Page 2
Page Number
Page N-1
Program Memory/
EEPROM Memory
26.9
SPI Serial Programming Characteristics
For characteristics of the SPI module, see “SPI Timing Characteristics” on page 298.
26.10 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.
As a definition in this datasheet, the LSB is shifted in and out first of all Shift Registers.
26.10.1
280
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.
ATmega16A
8154B–AVR–07/09
ATmega16A
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 26-9.
Figure 26-9. 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
0
1
1
Capture-DR
Capture-IR
0
0
0
Shift-DR
1
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
Shift-IR
1
0
1
Update-IR
0
1
0
281
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26.10.2
AVR_RESET ($C)
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.
26.10.3
PROG_ENABLE ($4)
The AVR specific public JTAG instruction for enabling programming via the JTAG port. The 16bit 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.
26.10.4
PROG_COMMANDS ($5)
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:
• 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 (not always
required, see Table 26-14 below).
26.10.5
PROG_PAGELOAD ($6)
The AVR specific public JTAG instruction to directly load the Flash data page via the JTAG port.
The 1024 bit Virtual Flash Page Load Register is selected as Data Register. This is a virtual
scan chain with length equal to the number of bits in one Flash page. Internally the Shift Register
is 8-bit. Unlike most JTAG instructions, the Update-DR state is not used to transfer data from the
Shift Register. The data are automatically transferred to the Flash page buffer byte by byte in the
Shift-DR state by an internal state machine. This is the only active state:
• Shift-DR: Flash page data are shifted in from TDI by the TCK input, and automatically loaded
into the Flash page one byte at a time.
Note:
26.10.6
282
The JTAG instruction PROG_PAGELOAD can only be used if the AVR device is the first device in
JTAG scan chain. If the AVR cannot be the first device in the scan chain, the byte-wise programming algorithm must be used.
PROG_PAGEREAD ($7)
The AVR specific public JTAG instruction to read one full Flash data page via the JTAG port.
The 1032 bit Virtual Flash Page Read Register is selected as Data Register. This is a virtual
scan chain with length equal to the number of bits in one Flash page plus 8. Internally the Shift
Register is 8-bit. Unlike most JTAG instructions, the Capture-DR state is not used to transfer
data to the Shift Register. The data are automatically transferred from the Flash page buffer byte
by byte in the Shift-DR state by an internal state machine. This is the only active state:
ATmega16A
8154B–AVR–07/09
ATmega16A
• Shift-DR: Flash data are automatically read one byte at a time and shifted out on TDO by the
TCK input. The TDI input is ignored.
Note:
26.10.7
The JTAG instruction PROG_PAGEREAD can only be used if the AVR device is the first device in
JTAG scan chain. If the AVR cannot be the first device in the scan chain, the byte-wise programming algorithm must be used.
Data Registers
The Data Registers are selected by the JTAG Instruction Registers described in section “Programming Specific JTAG Instructions” on page 280. The Data Registers relevant for
programming operations are:
• Reset Register
• Programming Enable Register
• Programming Command Register
• Virtual Flash Page Load Register
• Virtual Flash Page Read Register
26.10.8
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 25) 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 24-2 on page 234.
26.10.9
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 1010_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.
283
8154B–AVR–07/09
Figure 26-10. Programming Enable Register
TDI
D
A
T
A
$A370
=
D
Q
Programming Enable
ClockDR & PROG_ENABLE
TDO
26.10.10 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 Table 26-14. The state sequence when shifting
in the programming commands is illustrated in Figure 26-12.
Figure 26-11. 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
284
ATmega16A
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ATmega16A
Table 26-14. JTAG Programming Instruction Set
a = address high bits, b = address low bits, H = 0 – Low byte, 1 – High Byte, o = data out, i = data in, x = don’t care
Instruction
TDI sequence
TDO sequence
Notes
1a. Chip erase
0100011_10000000
0110001_10000000
0110011_10000000
0110011_10000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
1b. Poll for chip erase complete
0110011_10000000
xxxxxox_xxxxxxxx
2a. Enter Flash Write
0100011_00010000
xxxxxxx_xxxxxxxx
2b. Load Address High Byte
0000111_aaaaaaaa
xxxxxxx_xxxxxxxx
2c. Load Address Low Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
2d. Load Data Low Byte
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
2e. Load Data High Byte
0010111_iiiiiiii
xxxxxxx_xxxxxxxx
2f. Latch Data
0110111_00000000
1110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
2g. Write Flash Page
0110111_00000000
0110101_00000000
0110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
2h. Poll for Page Write complete
0110111_00000000
xxxxxox_xxxxxxxx
(2)
3a. Enter Flash Read
0100011_00000010
xxxxxxx_xxxxxxxx
3b. Load Address High Byte
0000111_aaaaaaaa
xxxxxxx_xxxxxxxx
3c. Load Address Low Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
3d. Read Data Low and High Byte
0110010_00000000
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
xxxxxxx_oooooooo
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
4e. Latch Data
0110111_00000000
1110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
4f. Write EEPROM Page
0110011_00000000
0110001_00000000
0110011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
4g. Poll for Page Write complete
0110011_00000000
xxxxxox_xxxxxxxx
(2)
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
(2)
(9)
(9)
Low byte
High byte
(9)
(9)
285
8154B–AVR–07/09
Table 26-14. JTAG Programming Instruction Set (Continued)
a = address high bits, b = address low bits, H = 0 – Low byte, 1 – High Byte, o = data out, i = data in, x = don’t care
Instruction
TDI sequence
TDO sequence
5d. Read Data Byte
0110011_bbbbbbbb
0110010_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
0100011_01000000
xxxxxxx_xxxxxxxx
6b. Load Data Low Byte
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
(3)
6c. Write Fuse High byte
0110111_00000000
0110101_00000000
0110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
6d. Poll for Fuse Write complete
0110111_00000000
xxxxxox_xxxxxxxx
(2)
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
(3)
6f. Write Fuse Low byte
0110011_00000000
0110001_00000000
0110011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
6g. Poll for Fuse Write complete
0110011_00000000
xxxxxox_xxxxxxxx
(2)
7a. Enter Lock Bit Write
0100011_00100000
xxxxxxx_xxxxxxxx
7b. Load Data Byte
0010011_11iiiiii
xxxxxxx_xxxxxxxx
(4)
7c. Write Lock Bits
0110011_00000000
0110001_00000000
0110011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
7d. Poll for Lock Bit Write complete
0110011_00000000
xxxxxox_xxxxxxxx
(2)
8a. Enter Fuse/Lock Bit Read
0100011_00000100
xxxxxxx_xxxxxxxx
8b. Read Fuse High Byte
0111110_00000000
0111111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
8c. Read Fuse Low Byte(7)
0110010_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
8d. Read Lock Bits(8)
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxoooooo
(5)
8e. Read Fuses and Lock Bits
0111110_00000000
0110010_00000000
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
xxxxxxx_oooooooo
xxxxxxx_oooooooo
(5)
Fuse High Byte
Fuse Low Byte
Lock bits
9a. Enter Signature Byte Read
0100011_00001000
xxxxxxx_xxxxxxxx
9b. Load Address Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
9c. Read Signature Byte
0110010_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
10a. Enter Calibration Byte Read
0100011_00001000
xxxxxxx_xxxxxxxx
6a. Enter Fuse Write
(6)
6e. Load Data Low Byte
(7)
(8)
(6)
286
Notes
ATmega16A
8154B–AVR–07/09
ATmega16A
Table 26-14. JTAG Programming Instruction Set (Continued)
a = address high bits, b = address low bits, H = 0 – Low byte, 1 – High Byte, o = data out, i = data in, x = don’t care
Instruction
TDI sequence
TDO sequence
10b. Load Address Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
10c. Read Calibration Byte
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
11a. Load No Operation Command
0100011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
Notes:
Notes
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 High byte is listed in Table 26-3 on page 265
7. The bit mapping for Fuses Low byte is listed in Table 26-4 on page 266
8. The bit mapping for Lock bits byte is listed in Table 26-1 on page 264
9. Address bits exceeding PCMSB and EEAMSB (Table 26-5 and Table 26-6) are don’t care
287
8154B–AVR–07/09
Figure 26-12. 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
0
1
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
26.10.11 Virtual Flash Page Load Register
The Virtual Flash Page Load Register is a virtual scan chain with length equal to the number of
bits in one Flash page. Internally the Shift Register is 8-bit, and the data are automatically transferred to the Flash page buffer byte by byte. Shift in all instruction words in the page, starting
with the LSB of the first instruction in the page and ending with the MSB of the last instruction in
the page. This provides an efficient way to load the entire Flash page buffer before executing
Page Write.
288
ATmega16A
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ATmega16A
Figure 26-13. Virtual Flash Page Load Register
STROBES
TDI
State
Machine
ADDRESS
Flash
EEPROM
Fuses
Lock Bits
D
A
T
A
TDO
26.10.12 Virtual Flash Page Read Register
The Virtual Flash Page Read Register is a virtual scan chain with length equal to the number of
bits in one Flash page plus 8. Internally the Shift Register is 8-bit, and the data are automatically
transferred from the Flash data page byte by byte. The first 8 cycles are used to transfer the first
byte to the internal Shift Register, and the bits that are shifted out during these 8 cycles should
be ignored. Following this initialization, data are shifted out starting with the LSB of the first
instruction in the page and ending with the MSB of the last instruction in the page. This provides
an efficient way to read one full Flash page to verify programming.
Figure 26-14. Virtual Flash Page Read Register
STROBES
TDI
State
Machine
ADDRESS
Flash
EEPROM
Fuses
Lock Bits
D
A
T
A
TDO
289
8154B–AVR–07/09
26.10.13 Programming Algorithm
All references below of type “1a”, “1b”, and so on, refer to Table 26-14.
26.10.14 Entering Programming Mode
1. Enter JTAG instruction AVR_RESET and shift 1 in the Reset Register.
2. Enter instruction PROG_ENABLE and shift 1010_0011_0111_0000 in the Programming Enable Register.
26.10.15 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 0000_0000_0000_0000 in the programming Enable Register.
4. Enter JTAG instruction AVR_RESET and shift 0 in the Reset Register.
26.10.16 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 27-7 on page 303).
26.10.17 Programming the Flash
Before programming the Flash a Chip Erase must be performed. See “Performing Chip Erase”
on page 290.
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash write using programming instruction 2a.
3. Load address High byte using programming instruction 2b.
4. Load address Low byte using programming instruction 2c.
5. Load data using programming instructions 2d, 2e and 2f.
6. Repeat steps 4 and 5 for all instruction words in the page.
7. Write the page using programming instruction 2g.
8. Poll for Flash write complete using programming instruction 2h, or wait for tWLRH (refer
to Table 27-7 on page 303).
9. Repeat steps 3 to 7 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 and 2c. PCWORD (refer to
Table 26-5 on page 267) 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, starting with the
LSB of the first instruction in the page and ending with the MSB of the last instruction in
the page.
6. Enter JTAG instruction PROG_COMMANDS.
7. Write the page using programming instruction 2g.
290
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ATmega16A
8. Poll for Flash write complete using programming instruction 2h, or wait for tWLRH (refer
to Table 27-7 on page 303).
9. Repeat steps 3 to 8 until all data have been programmed.
26.10.18 Reading the Flash
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash read using programming instruction 3a.
3. Load address using programming instructions 3b and 3c.
4. Read data using programming instruction 3d.
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.
3. Load the page address using programming instructions 3b and 3c. PCWORD (refer to
Table 26-5 on page 267) is used to address within one page and must be written as 0.
4. Enter JTAG instruction PROG_PAGEREAD.
5. Read the entire page by shifting out all instruction words in the page, starting with the
LSB of the first instruction in the page and ending with the MSB of the last instruction in
the page. Remember that the first 8 bits shifted out should be ignored.
6. Enter JTAG instruction PROG_COMMANDS.
7. Repeat steps 3 to 6 until all data have been read.
26.10.19 Programming the EEPROM
Before programming the EEPROM a Chip Erase must be performed. See “Performing Chip
Erase” on page 290.
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 27-7 on page 303).
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
26.10.20 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
291
8154B–AVR–07/09
26.10.21 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 27-7 on page 303).
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 27-7 on page 303).
26.10.22 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 27-7 on page 303).
26.10.23 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.
26.10.24 Reading the Signature Bytes
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Signature byte read using programming instruction 9a.
3. Load address $00 using programming instruction 9b.
4. Read first signature byte using programming instruction 9c.
5. Repeat steps 3 and 4 with address $01 and address $02 to read the second and third
signature bytes, respectively.
26.10.25 Reading the Calibration Byte
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Calibration byte read using programming instruction 10a.
3. Load address $00 using programming instruction 10b.
4. Read the calibration byte using programming instruction 10c.
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ATmega16A
27. Electrical Characteristics
27.1
Absolute Maximum Ratings*
Operating Temperature.................................. -55°C to +125°C
*NOTICE:
Storage Temperature ..................................... -65°C to +150°C
Voltage on any Pin except RESET
with respect to Ground ................................-0.5V to VCC+0.5V
Voltage on RESET with respect to Ground......-0.5V to +13.0V
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.
Maximum Operating Voltage ............................................ 6.0V
DC Current per I/O Pin ............................................... 40.0 mA
DC Current VCC and GND Pins................. 200.0mA PDIP and
400.0mA TQFP/MLF
27.2
DC Characteristics
TA = -40°C to 85°C, VCC = 2.7V to 5.5V (Unless Otherwise Noted)
Symbol
Parameter
Condition
Min
VIL
Input Low Voltage except
XTAL1 and RESET pins
VCC=2.7 - 5.5
VIH
Input High Voltage except
XTAL1 and RESET pins
VIH1
Typ
Max
Units
-0.5
0.2 VCC(1)
V
VCC=2.7 - 5.5
0.6 VCC(2)
VCC +0.5
V
Input High Voltage
XTAL1 pin
VCC=2.7 - 5.5
0.7 VCC(2)
VCC +0.5
V
VIL1
Input Low Voltage
XTAL1 pin
VCC=2.7 - 5.5
-0.5
0.1 VCC(1)
V
VIH2
Input High Voltage
RESET pin
VCC=2.7 - 5.5
0.9 VCC(2)
VCC +0.5
V
VIL2
Input Low Voltage
RESET pin
VCC=2.7 - 5.5
-0.5
0.2 VCC
V
VOL
Output Low Voltage(3)
(Ports A,B,C,D)
IOL = 20 mA, VCC = 5V
IOL = 10 mA, VCC = 3V
0.7
0.5
V
V
VOH
Output High Voltage(4)
(Ports A,B,C,D)
IOH = -20 mA, VCC = 5V
IOH = -10 mA, VCC = 3V
IIL
Input Leakage
Current I/O Pin
Vcc = 5.5V, pin low
(absolute value)
1
µA
IIH
Input Leakage
Current I/O Pin
Vcc = 5.5V, pin high
(absolute value)
1
µA
RRST
Reset Pull-up Resistor
30
85
kΩ
Rpu
I/O Pin Pull-up Resistor
20
50
kΩ
4.2
2.2
V
V
60
293
8154B–AVR–07/09
TA = -40°C to 85°C, VCC = 2.7V to 5.5V (Unless Otherwise Noted) (Continued)
Symbol
Parameter
Power Supply Current
ICC
Power-down Mode(5)
Condition
Typ
Max
Units
Active 1 MHz, VCC = 3V
0.6
Active 4 MHz, VCC = 3V
1.9
5
mA
Active 8 MHz, VCC = 5V
7
15
mA
mA
Idle 1 MHz, VCC = 3V
0.2
Idle 4 MHz, VCC = 3V
0.6
2
mA
Idle 8 MHz, VCC = 5V
2.7
7
mA
WDT enabled, VCC = 3V
<8
15
µA
WDT disabled, VCC = 3V
<1
4
µA
40
mV
50
nA
VACIO
Analog Comparator
Input Offset Voltage
VCC = 5V
Vin = VCC/2
IACLK
Analog Comparator
Input Leakage Current
VCC = 5V
Vin = VCC/2
tACPD
Analog Comparator
Propagation Delay
VCC = 2.7V
VCC = 4.0V
Notes:
Min
-50
750
500
mA
ns
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 (20 mA at Vcc = 5V, 10 mA at Vcc = 3V) under steady state
conditions (non-transient), the following must be observed:
PDIP Package:
1] The sum of all IOL, for all ports, should not exceed 200 mA.
2] The sum of all IOL, for port A0 - A7, should not exceed 100 mA.
3] The sum of all IOL, for ports B0 - B7,C0 - C7, D0 - D7 and XTAL2, should not exceed 100 mA.
TQFP and QFN/MLF Package:
1] The sum of all IOL, for all ports, should not exceed 400 mA.
2] The sum of all IOL, for ports A0 - A7, should not exceed 100 mA.
3] The sum of all IOL, for ports B0 - B4, should not exceed 100 mA.
4] The sum of all IOL, for ports B3 - B7, XTAL2, D0 - D2, should not exceed 100 mA.
5] The sum of all IOL, for ports D3 - D7, should not exceed 100 mA.
6] The sum of all IOL, for ports C0 - C7, should not exceed 100 mA.
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 (20 mA at Vcc = 5V, 10 mA at Vcc = 3V) under steady state
conditions (non-transient), the following must be observed:
PDIP Package:
1] The sum of all IOH, for all ports, should not exceed 200 mA.
2] The sum of all IOH, for port A0 - A7, should not exceed 100 mA.
3] The sum of all IOH, for ports B0 - B7,C0 - C7, D0 - D7 and XTAL2, should not exceed 100 mA.
TQFP and QFN/MLF Package:
1] The sum of all IOH, for all ports, should not exceed 400 mA.
2] The sum of all IOH, for ports A0 - A7, should not exceed 100 mA.
3] The sum of all IOH, for ports B0 - B4, should not exceed 100 mA.
4] The sum of all IOH, for ports B3 - B7, XTAL2, D0 - D2, should not exceed 100 mA.
5] The sum of all IOH, for ports D3 - D7, should not exceed 100 mA.
6] The sum of all IOH, for ports C0 - C7, should not exceed 100 mA.If IOH exceeds the test condition, VOH may exceed the
related specification. Pins are not guaranteed to source current greater than the listed test condition.
5. Minimum VCC for Power-down is 2.5V.
294
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ATmega16A
27.3
Speed Grades
Figure 27-1. Maximum Frequency vs VCC.
16 MHz
8 MHz
Safe Operating Area
2.7V
27.4
27.4.1
4.5V
5.5V
Clock Characteristics
External Clock Drive Waveforms
Figure 27-2. External Clock Drive Waveforms
V IH1
V IL1
27.4.2
External Clock Drive
Figure 27-3. External Clock Drive(1)
VCC = 2.7V to 5.5V
VCC = 4.5V to 5.5V
Min
Max
Min
Max
Units
0
8
0
16
MHz
Symbol
Parameter
1/tCLCL
Oscillator Frequency
tCLCL
Clock Period
125
62.5
ns
tCHCX
High Time
50
25
ns
tCLCX
Low Time
50
25
ns
295
8154B–AVR–07/09
Figure 27-3. External Clock Drive(1) (Continued)
VCC = 2.7V to 5.5V
Symbol
Parameter
tCLCH
Rise Time
tCHCL
Fall Time
ΔtCLCL
Change in period from
one clock cycle to the
next
Note:
27.5
Min
Max
Units
1.6
0.5
μs
1.6
0.5
μs
2
2
%
External RC Oscillator, Typical Frequencies (VCC = 5)
R [kΩ](1)
C [pF]
f(2)
33
22
650 kHz
10
22
1. R should be in the range 3 kΩ - 100 kΩ, and C should be at least 20 pF.
2. The frequency will vary with package type and board layout.
2.0 MHz
System and Reset Characteristics
Table 27-2.
Symbol
VPOT
Reset, Brown-out and Internal Voltage ReferenceCharacteristics
Parameter
Condition
Min
Typ
Max
Units
Power-on Reset Threshold
Voltage (rising)
1.4
2.3
V
Power-on Reset Threshold
Voltage (falling)(1)
1.3
2.3
V
0.9VCC
V
1.5
µs
VRST
RESET Pin Threshold
Voltage
tRST
Minimum pulse width on
RESET Pin
Brown-out Reset Threshold
Voltage(2)
BODLEVEL = 1
2.5
2.7
3.2
VBOT
BODLEVEL = 0
3.6
4.0
4.5
Minimum low voltage period
for Brown-out Detection
BODLEVEL = 1
2
µs
tBOD
BODLEVEL = 0
2
µs
VHYST
Brown-out Detector
hysteresis
50
mV
VBG
Bandgap reference voltage
tBG
IBG
Notes:
296
Max
1. Refer to “External Clock” on page 30 for details.
Table 27-1.
Notes:
Min
VCC = 4.5V to 5.5V
0.1 VCC
V
1.15
1.23
1.4
V
Bandgap reference start-up
time
40
70
µs
Bandgap reference current
consumption
10
µA
1. The Power-on Reset will not work unless the supply voltage has been below VPOT (falling).
ATmega16A
8154B–AVR–07/09
ATmega16A
2. VBOT may be below nominal minimum operating voltage for some devices. For devices where
this is the case, the device is tested down to VCC = VBOT during the production test. This guarantees that a Brown-out Reset will occur before VCC drops to a voltage where correct
operation of the microcontroller is no longer guaranteed. The test is performed using
BODLEVEL = 1 and BODLEVEL = 0 for ATmega16A.
27.6
External Interrupts Characteristics
Table 27-3.
Symbol
tINT
27.7
Asynchronous External Interrupt Characteristics
Parameter
Condition
Min
Minimum pulse width for asynchronous
external interrupt
Typ
Max
50
Units
ns
Two-wire Serial Interface Characteristics
Table 27-4 describes the requirements for devices connected to the Two-wire Serial Bus. The ATmega16A Two-wire Serial
Interface meets or exceeds these requirements under the noted conditions.
Timing symbols refer to Figure 27-4.
Table 27-4.
Two-wire Serial Bus Requirements
Symbol
Parameter
VIL
Min
Max
Units
Input Low-voltage
-0.5
0.3 VCC
V
VIH
Input High-voltage
0.7 VCC
VCC + 0.5
V
Vhys(1)
Hysteresis of Schmitt Trigger Inputs
0.05 VCC(2)
–
V
VOL(1)
Output Low-voltage
0
tr(1)
Rise Time for both SDA and SCL
tof(1)
Output Fall Time from VIHmin to VILmax
tSP(1)
Spikes Suppressed by Input Filter
Ii
Input Current each I/O Pin
Ci(1)
Capacitance for each I/O Pin
fSCL
SCL Clock Frequency
Rp
tHD;STA
tLOW
tHIGH
Condition
3 mA sink current
Low Period of the SCL Clock
High period of the SCL clock
tSU;STA
Set-up time for a repeated START condition
V
300
ns
20 + 0.1Cb(3)(2)
250
ns
(2)
ns
20 + 0.1Cb
10 pF < Cb < 400 pF(3)
0
0.1VCC < Vi < 0.9VCC
50
-10
10
µA
–
10
pF
fCK(4) > max(16fSCL, 250kHz)(5)
0
400
kHz
fSCL ≤ 100 kHz
V CC – 0,4V
---------------------------3mA
1000ns
------------------Cb
Ω
fSCL > 100 kHz
V CC – 0,4V
---------------------------3mA
300ns
---------------Cb
Ω
fSCL ≤ 100 kHz
4.0
–
µs
fSCL > 100 kHz
Value of Pull-up resistor
Hold Time (repeated) START Condition
0.4
(3)(2)
0.6
–
µs
(6)
fSCL ≤ 100 kHz
4.7
–
µs
fSCL > 100 kHz(7)
1.3
–
µs
fSCL ≤ 100 kHz
4.0
–
µs
fSCL > 100 kHz
0.6
–
µs
fSCL ≤ 100 kHz
4.7
–
µs
fSCL > 100 kHz
0.6
–
µs
297
8154B–AVR–07/09
Table 27-4.
Two-wire Serial Bus Requirements (Continued)
Symbol
Parameter
tHD;DAT
Data hold time
tSU;DAT
tSU;STO
tBUF
Notes:
Data setup time
Setup time for STOP condition
Bus free time between a STOP and START
condition
Condition
Min
Max
Units
fSCL ≤ 100 kHz
0
3.45
µs
fSCL > 100 kHz
0
0.9
µs
fSCL ≤ 100 kHz
250
–
ns
fSCL > 100 kHz
100
–
ns
fSCL ≤ 100 kHz
4.0
–
µs
fSCL > 100 kHz
0.6
–
µs
fSCL ≤ 100 kHz
4.7
–
µs
fSCL > 100 kHz
1.3
–
µs
1. In ATmega16A, this parameter is characterized and not 100% tested.
2. Required only for fSCL > 100 kHz.
3. Cb = capacitance of one bus line in pF.
4. fCK = CPU clock frequency
5. This requirement applies to all ATmega16A Two-wire Serial Interface operation. Other devices connected to the Two-wire
Serial Bus need only obey the general fSCL requirement.
6. The actual low period generated by the ATmega16A Two-wire Serial Interface is (1/fSCL - 2/fCK), thus fCK must be greater than
6 MHz for the low time requirement to be strictly met at fSCL = 100 kHz.
7. The actual low period generated by the ATmega16A Two-wire Serial Interface is (1/fSCL - 2/fCK), thus the low time requirement will not be strictly met for fSCL > 308 kHz when fCK = 8 MHz. Still, ATmega16A devices connected to the bus may
communicate at full speed (400 kHz) with other ATmega16A devices, as well as any other device with a proper tLOW acceptance margin.
Figure 27-4. Two-wire Serial Bus Timing
tof
tHIGH
tLOW
tr
tLOW
SCL
tSU;STA
tHD;STA
tHD;DAT
SDA
tSU;DAT
tSU;STO
tBUF
27.8
SPI Timing Characteristics
Figure 27-5. SPI Serial Programming Waveforms
SERIAL DATA INPUT
(MOSI)
MSB
LSB
SERIAL DATA OUTPUT
(MISO)
MSB
LSB
SERIAL CLOCK INPUT
(SCK)
SAMPLE
298
ATmega16A
8154B–AVR–07/09
ATmega16A
.
Table 27-5.
SPI Timing Parameters
Description
Mode
Min
Typ
Max
1
SCK period
Master
See Table 18-5
2
SCK high/low
Master
50% duty cycle
3
Rise/Fall time
Master
3.6
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 • tSCK
11
SCK high/low
Slave
2 • tSCK
12
Rise/Fall time
Slave
13
Setup
Slave
10
14
Hold
Slave
10
15
SCK to out
Slave
16
SCK to SS high
Slave
17
SS high to tri-state
Slave
18
SS low to SCK
Slave
ns
1.6
15
µs
ns
20
10
2 • tSCK
Figure 27-6. 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
7
MOSI
(Data Output)
MSB
8
...
LSB
299
8154B–AVR–07/09
Figure 27-7. SPI Interface Timing Requirements (Slave Mode)
18
SS
10
9
16
SCK
(CPOL = 0)
11
11
SCK
(CPOL = 1)
13
MOSI
(Data Input)
14
12
MSB
...
LSB
15
MISO
(Data Output)
300
MSB
17
...
LSB
X
ATmega16A
8154B–AVR–07/09
ATmega16A
27.9
ADC Characteristics
Table 27-6.
Symbol
ADC Characteristics
Parameter
Resolution
Condition
Min(1)
Typ(1)
Max(1)
Units
Single Ended Conversion
10
Bits
Differential Conversion
Gain = 1x or 10x
8
Bits
Differential Conversion
Gain = 200x
7
Bits
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200 kHz
1.5
2.5
LSB
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 1 MHz
3
4
LSB
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200 kHz
Noise Reduction mode
1.5
LSB
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 1 MHz
Noise Reduction mode
3
LSB
Integral Non-linearity (INL)
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200 kHz
1
LSB
Differential Non-linearity (DNL)
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200 kHz
0.5
LSB
Gain Error
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200 kHz
1
LSB
Offset Error
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200 kHz
Conversion Time
Free Running Conversion
Absolute Accuracy (Including INL, DNL,
Quantization Error, Gain, and Offset Error).
Clock Frequency
AVCC
Analog Supply Voltage
VREF
Reference Voltage
LSB
13
260
50
1000
kHz
(3)
(2)
VCC - 0.3
VCC + 0.3
V
Single Ended Conversion
2.0
AVCC
V
Differential Conversion
2.0
AVCC - 0.2
V
Single ended channels
GND
VREF
V
0
VREF
V
Input voltage
Differential channels
VIN
µs
Single ended channels
38.5
kHz
4
kHz
Input bandwidth
Differential channels
301
8154B–AVR–07/09
Table 27-6.
Symbol
ADC Characteristics (Continued)
Parameter
Condition
Min(1)
Typ(1)
Max(1)
Units
2.3
2.6
2.9
V
VINT
Internal Voltage Reference
RREF
Reference Input Resistance
32
kΩ
RAIN
Analog Input Resistance
100
MΩ
Notes:
1. Values are guidelines only.
2. Minimum for AVCC is 2.7V.
3. Maximum for AVCC is 5.5V.
27.10 Parallel Programming Characteristics
Figure 27-8. Parallel Programming Timing, Including some General Timing Requirements
t XLWL
t XHXL
XTAL1
t DVXH
t XLDX
Data & Contol
(DATA, XA0/1, BS1, BS2)
t PLBX
t BVPH
PAGEL
t BVWL
t WLBX
t PHPL
t WL
WR
WH
t PLWL
WLRL
RDY/BSY
t WLRH
Figure 27-9. Parallel Programming Timing, Loading Sequence with Timing Requirements(1)
LOAD ADDRESS
(LOW BYTE)
LOAD DATA LOAD DATA
(HIGH BYTE)
LOAD DATA
(LOW BYTE)
t XLXH
tXLPH
LOAD ADDRESS
(LOW BYTE)
tPLXH
XTAL1
BS1
PAGEL
DATA
ADDR0 (Low Byte)
DATA (Low Byte)
DATA (High Byte)
ADDR1 (Low Byte)
XA0
XA1
Note:
302
1. The timing requirements shown in Figure 27-8 (i.e., tDVXH, tXHXL, and tXLDX) also apply to loading operation.
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 27-10. 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. The timing requirements shown in Figure 27-8 (i.e., tDVXH, tXHXL, and tXLDX) also apply to reading operation.
Table 27-7.
Parallel Programming Characteristics, VCC = 5 V ± 10%
Symbol
Parameter
Min
VPP
Programming Enable Voltage
11.5
IPP
Programming Enable Current
tDVXH
Data and Control Valid before XTAL1 High
67
ns
tXLXH
XTAL1 Low to XTAL1 High
200
ns
tXHXL
XTAL1 Pulse Width High
150
ns
tXLDX
Data and Control Hold after XTAL1 Low
67
ns
tXLWL
XTAL1 Low to WR Low
0
ns
tXLPH
XTAL1 Low to PAGEL high
0
ns
tPLXH
PAGEL low to XTAL1 high
150
ns
tBVPH
BS1 Valid before PAGEL High
67
ns
tPHPL
PAGEL Pulse Width High
150
ns
tPLBX
BS1 Hold after PAGEL Low
67
ns
tWLBX
BS2/1 Hold after WR Low
67
ns
tPLWL
PAGEL Low to WR Low
67
ns
tBVWL
BS1 Valid to WR Low
67
ns
tWLWH
WR Pulse Width Low
150
ns
tWLRL
WR Low to RDY/BSY Low
tWLRH
(1)
WR Low to RDY/BSY High
(2)
tWLRH_CE
WR Low to RDY/BSY High for Chip Erase
tXLOL
XTAL1 Low to OE Low
Typ
Max
Units
12.5
V
250
μA
0
1
μs
3.7
4.5
ms
7.5
9
ms
0
ns
303
8154B–AVR–07/09
Table 27-7.
Symbol
Parameter
tBVDV
BS1 Valid to DATA valid
tOLDV
tOHDZ
Notes:
304
Parallel Programming Characteristics, VCC = 5 V ± 10% (Continued)
Min
Max
Units
250
ns
OE Low to DATA Valid
250
ns
OE High to DATA Tri-stated
250
ns
0
Typ
1.
tWLRH is valid for the Write Flash, Write EEPROM, Write Fuse bits and Write Lock bits
commands.
2.
tWLRH_CE is valid for the Chip Erase command.
ATmega16A
8154B–AVR–07/09
ATmega16A
28. 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.
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 Power-down mode with Watchdog Timer disabled represents the differential current drawn by the Watchdog Timer.
28.0.1
Active Supply Current
Figure 28-1. Active Supply Current vs. Low Frequency (0.1 - 1.0 MHz
ACTIVE SUPPLY CURRENT vs. LOW FREQUENCY
0.1 - 1.0 MHz
1.4
5.5 V
1.2
5.0 V
ICC (mA)
1
4.5 V
4.0 V
3.6 V
3.3 V
0.8
0.6
2.7 V
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)
305
8154B–AVR–07/09
Figure 28-2. Active Supply Current vs. Frequency (1 - 16 MHz)
ACTIVE SUPPLY CURRENT vs. FREQUENCY
1 - 16 MHz
16
5.5 V
14
5.0 V
12
4.5 V
ICC (mA)
10
4.0 V
8
3.6 V
6
3.3 V
4
2.7 V
2
0
0
2
4
6
8
10
12
14
16
Frequency (MHz)
Figure 28-3. Active Supply Current vs. VCC (Internal RC Oscillator, 8 MHz)
ACTIVE SUPPLY CURRENT vs. VCC
INTERNAL RC OSCILLATOR, 8 MHz
10
85 °C
25 °C
9
8
-40 °C
ICC (mA)
7
6
5
4
3
2
1
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
306
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 28-4. Active Supply Current vs. VCC (Internal RC Oscillator, 4 MHz)
ACTIVE SUPPLY CURRENT vs. VCC
INTERNAL RC OSCILLATOR, 4 MHz
6
85 °C
25 °C
5
-40 °C
ICC (mA)
4
3
2
1
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 28-5. Active Supply Current vs. VCC (Internal RC Oscillator, 2 MHz)
ACTIVE SUPPLY CURRENT vs. VCC
INTERNAL RC OSCILLATOR, 2 MHz
3
85 °C
25 °C
-40 °C
2.5
ICC (mA)
2
1.5
1
0.5
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
307
8154B–AVR–07/09
Figure 28-6. Active Supply Current vs. VCC (Internal RC Oscillator, 1 MHz)
ACTIVE SUPPLY CURRENT vs. VCC
INTERNAL RC OSCILLATOR, 1 MHz
1.6
85 °C
25 °C
1.4
-40 °C
1.2
ICC (mA)
1
0.8
0.6
0.4
0.2
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 28-7. Active Supply Current vs. VCC (32 kHz External Oscillator)
ACTIVE SUPPLY CURRENT vs. VCC
EXTERNAL OSCILLATOR, 32 kHz
140
25 °C
120
ICC (uA)
100
80
60
40
20
0
2,5
3
3,5
4
4,5
5
5,5
VCC (V)
308
ATmega16A
8154B–AVR–07/09
ATmega16A
28.0.2
Idle Supply Current
Figure 28-8. Idle Supply Current vs. Frequency (0.1 - 1.0 MHz)
IDLE SUPPLY CURRENT vs. LOW FREQUENCY
0.1 - 1.0 MHz
0.5
5.5 V
ICC (mA)
0.45
0.4
5.0 V
0.35
4.5 V
0.3
4.0 V
0.25
3.6 V
3.3 V
0.2
2.7 V
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 28-9. Idle Supply Current vs. Frequency (1 - 16 MHz)
IDLE SUPPLY CURRENT vs. FREQUENCY
1 - 16 MHz
8
7
5.5 V
6
5.0 V
I CC (mA)
5
4.5 V
4
4.0 V
3
3.6 V
2
3.3 V
1
2.7 V
0
0
2
4
6
8
10
12
14
16
Frequency (MHz)
309
8154B–AVR–07/09
Figure 28-10. Idle Supply Current vs. VCC (Internal RC Oscillator, 8 MHz)
IDLE SUPPLY CURRENT vs. VCC
INTERNAL RC OSCILLATOR, 8 MHz
5
-40 °C
4.5
25 °C
85 °C
4
ICC (mA)
3.5
3
2.5
2
1.5
1
0.5
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 28-11. Idle Supply Current vs. VCC (Internal RC Oscillator, 4 MHz)
IDLE SUPPLY CURRENT vs. VCC
INTERNAL RC OSCILLATOR, 4 MHz
2.5
85 °C
25 °C
-40 °C
ICC (mA)
2
1.5
1
0.5
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
310
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 28-12. Idle Supply Current vs. VCC (Internal RC Oscillator, 2 MHz)
IDLE SUPPLY CURRENT vs. VCC
INTERNAL RC OSCILLATOR, 2 MHz
1.2
85 °C
25 °C
-40 °C
1
ICC (mA)
0.8
0.6
0.4
0.2
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 28-13. Idle Supply Current vs. VCC (Internal RC Oscillator, 1 MHz)
IDLE SUPPLY CURRENT vs. VCC
INTERNAL RC OSCILLATOR, 1 MHz
0.7
85 °C
25 °C
-40 °C
0.6
ICC (mA)
0.5
0.4
0.3
0.2
0.1
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
311
8154B–AVR–07/09
Figure 28-14. Idle Supply Current vs. VCC (32 kHz External Oscillator)
IDLE SUPPLY CURRENT vs. V CC
EXTERNAL OSCILLATOR, 32 kHz
30
25
25 °C
ICC (uA)
20
15
10
5
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
28.0.3
Power-Down Supply Current
Figure 28-15. Power-Down Supply Current vs. VCC (Watchdog Timer Disabled)
POWER-DOWN SUPPLY CURRENT vs. VCC
WATCHDOG TIMER DISABLED
2
1.8
1.6
85 °C
-40 °C
1.4
ICC (uA)
1.2
25 °C
1
0.8
0.6
0.4
0.2
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
312
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 28-16. Power-Down Supply Current vs. VCC (Watchdog Timer Enabled)
POWER-DOWN SUPPLY CURRENT vs. VCC
WATCHDOG TIMER ENABLED
20
85 °C
-40 °C
25 °C
18
16
14
ICC (uA)
12
10
8
6
4
2
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
28.0.4
Power-Save Supply Current
Figure 28-17. Power-Save Supply Current vs. VCC (Watchdog Timer Disabled)
POWER-SAVE SUPPLY CURRENT vs. VCC
WATCHDOG TIMER DISABLED
14
25 °C
12
ICC (uA)
10
8
6
4
2
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
313
8154B–AVR–07/09
28.0.5
Standby Supply Current
Figure 28-18. Standby Supply Current vs. VCC (455 kHz Resonator, Watchdog Timer Disabled)
STANDBY SUPPLY CURRENT vs. VCC
WATCHDOG TIMER DISABLED
0.16
0.14
6MHz_xtal
6MHz_res
0.12
ICC (mA)
0.1
4MHz_res
4MHz_xtal
0.08
0.06
2MHz_res
2MHz_xtal
450kHz_res
0.04
1MHz_res
0.02
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
28.0.6
Pin Pullup
Figure 28-19. I/O Pin Pull-Up Resistor Current vs. Input Voltage (VCC = 5V)
I/O PIN PULL-UP RESISTOR CURRENT vs. INPUT VOLTAGE
VCC = 5V
140
120
IOP (uA)
100
80
60
40
25 °C
-40 °C
85 °C
20
0
0
1
2
3
4
5
6
VOP (V)
314
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 28-20. I/O Pin Pull-Up Resistor Current vs. Input Voltage (VCC = 2.7V)
I/O PIN PULL-UP RESISTOR CURRENT vs. INPUT VOLTAGE
VCC = 2.7V
80
70
60
IOP (uA)
50
40
30
20
25 °C
-40 °C
85 °C
10
0
0
0.5
1
1.5
2
2.5
3
VOP (V)
Figure 28-21. Reset Pull-Up Resistor Current vs. Reset Pin Voltage (VCC = 5V)
RESET PULL-UP RESISTOR CURRENT vs. RESET PIN VOLTAGE
VCC = 5V
120
100
25 °C
-40 °C
IRESET (uA)
80
85 °C
60
40
20
0
0
1
2
3
4
5
6
VRESET(V)
315
8154B–AVR–07/09
Figure 28-22. Reset Pull-Up Resistor Current vs. Reset Pin Voltage (VCC = 2.7V)
RESET PULL-UP RESISTOR CURRENT vs. RESET PIN VOLTAGE
V CC = 2.7V
60
25 °C
-40 °C
50
IRESET (uA)
40
85 °C
30
20
10
0
0
0,5
1
1,5
2
2,5
3
VRESET(V)
28.0.7
Pin Driver Strength
Figure 28-23. I/O Pin Source Current vs. Output Voltage (VCC = 5V)
I/O PIN SOURCE CURRENT vs. OUTPUT VOLTAGE
VCC = 5V
80
-40 °C
25 °C
IOH (mA)
60
85 °C
40
20
0
0
1
2
3
4
5
6
VOH (V)
316
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 28-24. I/O Pin Source Current vs. Output Voltage (VCC = 2.7V)
I/O PIN SOURCE CURRENT vs. OUTPUT VOLTAGE
VCC = 2.7V
30
-40 °C
25 °C
25
85 °C
IOH (mA)
20
15
10
5
0
0
0.5
1
1.5
2
2.5
3
VOH (V)
Figure 28-25. I/O Pin Sink Current vs. Output Voltage (VCC = 5V)
I/O PIN SINK CURRENT vs. OUTPUT VOLTAGE
VCC = 5V
100
-40 °C
80
25 °C
85 °C
IOL (mA)
60
40
20
0
0
0,5
1
1,5
2
2,5
VOL (V)
317
8154B–AVR–07/09
Figure 28-26. I/O Pin Sink Current vs. Output Voltage (VCC = 2.7V)
I/O PIN SINK CURRENT vs. OUTPUT VOLTAGE
VCC = 2.7V
40
35
-40 °C
30
25 °C
IOL (mA)
25
85 °C
20
15
10
5
0
0
0.5
1
1.5
2
2.5
VOL (V)
28.0.8
Pin Thresholds And Hysteresis
Figure 28-27. I/O Pin Input Threshold Voltage vs. VCC (VIH, I/O Pin Read As '1')
I/O PIN INPUT THRESHOLD VOLTAGE vs. VCC
VIH, IO PIN READ AS '1'
3
85 °C
25 °C
-40 °C
2.5
Threshold (V)
2
1.5
1
0.5
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
318
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 28-28. I/O Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin Read As '0')
I/O PIN INPUT THRESHOLD VOLTAGE vs. VCC
VIL, IO PIN READ AS '0'
2.5
25 °C
85 °C
2
Threshold (V)
-40 °C
1.5
1
0.5
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 28-29. I/O Pin Input Hysteresis vs. VCC
I/O PIN INPUT HYSTERESIS vs. VCC
0.6
0.55
85 °C
25 °C
Input Hysteresis (mV)
0.5
-40 °C
0.45
0.4
0.35
0.3
0.25
0.2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
319
8154B–AVR–07/09
Figure 28-30. Reset Input Threshold Voltage vs. VCC (VIH, Reset Pin Read As '1')
RESET INPUT THRESHOLD VOLTAGE vs. VCC
VIH, IO PIN READ AS '1'
2.5
2
Threshold (V)
-40 °C
1.5
25 °C
85 °C
1
0.5
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 28-31. Reset Input Threshold Voltage vs. VCC (VIL, Reset Pin Read As '0')
RESET INPUT THRESHOLD VOLTAGE vs. V CC
VIL, IO PIN READ AS '0'
2.5
Threshold (V)
2
1.5
85 °C
25 °C
-40 °C
1
0.5
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
320
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 28-32. Reset Input Pin Hysteresis vs. VCC
RESET PIN INPUT HYSTERESIS vs. VCC
0.5
Input Hysteresis (mV)
0.4
-40 °C
0.3
0.2
25 °C
0.1
85 °C
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
28.0.9
Bod Thresholds
Figure 28-33. Bod Thresholds vs. Temperature (Bodlevel is 4.0V)
BOD THRESHOLDS vs. TEMPERATURE
BOD LEVEL IS 4.0 V
4.2
Threshold (V)
4.1
Rising VCC
4
3.9
Falling VCC
3.8
3.7
-60
-40
-20
0
20
40
60
80
100
Temperature (C)
321
8154B–AVR–07/09
Figure 28-34. Bod Thresholds vs. Temperature (Bodlevel is 2.7V)
BOD THRESHOLDS vs. TEMPERATURE
BOD LEVEL IS 2.7 V
3
Threshold (V)
2.9
Rising VCC
2.8
2.7
Falling VCC
2.6
2.5
-60
-40
-20
0
20
40
60
80
100
Temperature (C)
Figure 28-35. Bandgap Voltage vs. VCC
BANDGAP VOLTAGE vs. VCC
1.255
85 °C
Bandgap Voltage (V)
1.25
25 °C
1.245
1.24
1.235
-40 °C
1.23
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
322
ATmega16A
8154B–AVR–07/09
ATmega16A
28.0.10
Internal Oscillator Speed
Figure 28-36. Watchdog Oscillator Frequency vs. VCC
WATCHDOG OSCILLATOR FREQUENCY vs. V CC
1340
1320
-40 °C
1300
25 °C
1280
F RC (kHz)
1260
85 °C
1240
1220
1200
1180
1160
1140
1120
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 28-37. Calibrated 8 MHz RC Oscillator Frequency vs. Temperature
CALIBRATED 8 MHz RC OSCILLATOR FREQUENCY vs. TEMPERATURE
9
8
F RC (MHz)
5.5 V
4.0 V
7
2.7 V
6
-60
-40
-20
0
20
40
60
80
100
Temperature
323
8154B–AVR–07/09
Figure 28-38. Calibrated 8 MHz RC Oscillator Frequency vs. VCC
CALIBRATED 8 MHz RC OSCILLATOR FREQUENCY vs. V CC
9
-40 °C
25 °C
8
F RC (MHz)
85 °C
7
6
2.5
3
3.5
4
4.5
5
5.5
V CC (V)
Figure 28-39. Calibrated 8 MHz RC Oscillator Frequency vs. Osccal Value
CALIBRATED 8 MHz RC OSCILLATOR FREQUENCY vs. OSCCAL VALUE
16
14
25 °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)
324
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 28-40. Calibrated 4 MHz RC Oscillator Frequency vs. Temperature
CALIBRATED 4 MHz RC OSCILLATOR FREQUENCY vs. TEMPERATURE
4.2
4.1
F RC (MHz)
4
5.5 V
3.9
4.0 V
3.8
3.7
2.7 V
3.6
-60
-40
-20
0
20
40
60
80
100
Temperature
Figure 28-41. Calibrated 4 MHz RC Oscillator Frequency vs. VCC
CALIBRATED 4 MHz RC OSCILLATOR FREQUENCY vs. VCC
4.2
-40 °C
4.1
25 °C
F RC (MHz)
4
85 °C
3.9
3.8
3.7
3.6
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
325
8154B–AVR–07/09
Figure 28-42. Calibrated 4 MHz RC Oscillator Frequency vs. Osccal Value
CALIBRATED 4 MHz RC OSCILLATOR FREQUENCY vs. OSCCAL VALUE
8
25 °C
7
6
F RC (MHz)
5
4
3
2
1
0
0
16
32
48
64
80
96
112 128 144 160 176 192 208 224 240 256
OSCCAL (X1)
Figure 28-43. Calibrated 2 MHz RC Oscillator Frequency vs. Temperature
CALIBRATED 2 MHz RC OSCILLATOR FREQUENCY vs. TEMPERATURE
2.1
2
F RC (MHz)
5.5 V
4.0 V
1.9
2.7 V
1.8
-60
-40
-20
0
20
40
60
80
100
Temperature
326
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 28-44. Calibrated 2 MHz RC Oscillator Frequency vs. VCC
CALIBRATED 2 MHz RC OSCILLATOR FREQUENCY vs. VCC
2.1
-40 °C
25 °C
2
F RC (MHz)
85 °C
1.9
1.8
1.7
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 28-45. Calibrated 2 MHz RC Oscillator Frequency vs. Osccal Value
CALIBRATED 2 MHz RC OSCILLATOR FREQUENCY vs. OSCCAL VALUE
4
3,5
25 °C
3
F RC (MHz)
2,5
2
1,5
1
0,5
0
0
16
32
48
64
80
96
112 128 144 160 176 192 208 224 240 256
OSCCAL (X1)
327
8154B–AVR–07/09
Figure 28-46. Calibrated 1 MHz RC Oscillator Frequency vs. Temperature
CALIBRATED 1 MHz RC OSCILLATOR FREQUENCY vs. TEMPERATURE
1.05
1
F RC (MHz)
5.5 V
4.0 V
0.95
2.7 V
0.9
-60
-40
-20
0
20
40
60
80
100
Temperature
Figure 28-47. Calibrated 1 MHz RC Oscillator Frequency vs. VCC
CALIBRATED 1 MHz RC OSCILLATOR FREQUENCY vs. V CC
1.05
-40 °C
25 °C
1
F RC (MHz)
85 °C
0.95
0.9
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
328
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 28-48. Calibrated 1 MHz RC Oscillator Frequency vs. Osccal Value
CALIBRATED 1 MHz RC OSCILLATOR FREQUENCY vs. OSCCAL VALUE
1.8
25 °C
1.6
F RC (MHz)
1.4
1.2
1
0.8
0.6
0.4
0
16
32
48
64
80
96
112 128 144 160 176 192 208 224 240 256
OSCCAL (X1)
28.0.11
Current Consumption Of Peripheral Units
Figure 28-49. Brownout Detector Current vs. VCC
BROWNOUT DETECTOR CURRENT vs. VCC
25
ICC (uA)
20
-40 °C
25 °C
85 °C
15
10
5
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
329
8154B–AVR–07/09
Figure 28-50. ADC Current vs. VCC(AREF = AVCC)
ADC CURRENT vs. VCC
AREF = AVCC
350
85 °C
-40 °C
25 °C
300
ICC (uA)
250
200
150
100
50
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 28-51. Aref External Reference Current vs. VCC
AREF EXTERNAL REFERENCE CURRENT vs. VCC
200
85 °C
25 °C
-40 °C
180
160
140
ICC (uA)
120
100
80
60
40
20
0
2,5
3
3,5
4
4,5
5
5,5
VCC (V)
330
ATmega16A
8154B–AVR–07/09
ATmega16A
Figure 28-52. Analog Comparator Current vs. VCC
ANALOG COMPARATOR CURRENT vs. VCC
ICC (uA)
90
80
85 °C
70
25 °C
60
-40 °C
50
40
30
20
10
0
2,5
3
3,5
4
4,5
5
5,5
VCC (V)
Figure 28-53. 32 kHz Tosc Current vs. VCC (Watchdog Timer Disabled)
331
8154B–AVR–07/09
Figure 28-54. Watchdog Timer Current vs. VCC
WATCHDOG TIMER CURRENT vs. V CC
20
-40 °C
25 °C
85 °C
16
ICC (uA)
12
8
4
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 28-55. Programming Current vs. VCC
PROGRAMMING CURRENT vs. VCC
7
-40 °C
6
25 °C
ICC (mA)
5
85 °C
4
3
2
1
0
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
332
ATmega16A
8154B–AVR–07/09
ATmega16A
28.0.12
Reset Supply Current
Figure 28-56. Reset Supply Current vs. Low Frequency (0.1 - 1.0 MHz
RESET SUPPLY CURRENT vs. LOW FREQUENCY
0.1 - 1.0 MHz
2.5
5.5 V
2
ICC (mA)
5.0 V
4.5 V
1.5
4.0 V
3.6 V
3.3 V
1
2.7 V
0.5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Frequency (MHz)
Figure 28-57. Reset Supply Current vs. Frequency (1 - 16 MHz)
RESET SUPPLY CURRENT vs. FREQUENCY
1 - 16 MHz
ICC (mA)
14
12
5.5 V
10
5.0 V
4.5 V
8
4.0 V
6
3.6 V
4
3.3 V
2.7 V
2
0
0
2
4
6
8
10
12
14
16
Frequency (MHz)
333
8154B–AVR–07/09
29. Register Summary
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
$3F ($5F)
SREG
I
T
H
S
V
N
Z
C
9
$3E ($5E)
SPH
–
–
–
–
–
SP10
SP9
SP8
12
SP4
SP3
SP2
SP1
SP0
12
–
–
–
IVSEL
IVCE
47, 69
$3D ($5D)
SPL
$3C ($5C)
OCR0
$3B ($5B)
GICR
SP7
SP6
SP5
Timer/Counter0 Output Compare Register
INT1
INT0
Page
85
INT2
$3A ($5A)
GIFR
INTF1
INTF0
INTF2
–
–
–
–
–
69
$39 ($59)
TIMSK
OCIE2
TOIE2
TICIE1
OCIE1A
OCIE1B
TOIE1
OCIE0
TOIE0
85, 115, 134
85, 115, 134
$38 ($58)
TIFR
OCF2
TOV2
ICF1
OCF1A
OCF1B
TOV1
OCF0
TOV0
$37 ($57)
SPMCR
SPMIE
RWWSB
–
RWWSRE
BLBSET
PGWRT
PGERS
SPMEN
255
$36 ($56)
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
200
$35 ($55)
MCUCR
SM2
SE
SM1
SM0
ISC11
ISC10
ISC01
ISC00
36, 67
$34 ($54)
MCUCSR
JTD
ISC2
–
JTRF
WDRF
BORF
EXTRF
PORF
41, 68, 249
$33 ($53)
TCCR0
FOC0
WGM00
COM01
COM00
WGM01
CS02
CS01
CS00
82
$32 ($52)
TCNT0
$31(1) ($51)(1)
OSCCAL
OCDR
Timer/Counter0 (8 Bits)
84
Oscillator Calibration Register
31
On-Chip Debug Register
231
$30 ($50)
SFIOR
ADTS2
ADTS1
ADTS0
–
ACME
PUD
PSR2
PSR10
65,87,134,205,225
$2F ($4F)
TCCR1A
COM1A1
COM1A0
COM1B1
COM1B0
FOC1A
FOC1B
WGM11
WGM10
109
$2E ($4E)
TCCR1B
ICNC1
ICES1
–
WGM13
WGM12
CS12
CS11
CS10
112
$2D ($4D)
TCNT1H
Timer/Counter1 – Counter Register High Byte
113
$2C ($4C)
TCNT1L
113
$2B ($4B)
OCR1AH
Timer/Counter1 – Counter Register Low Byte
Timer/Counter1 – Output Compare Register A High Byte
$2A ($4A)
OCR1AL
Timer/Counter1 – Output Compare Register A Low Byte
114
$29 ($49)
OCR1BH
Timer/Counter1 – Output Compare Register B High Byte
114
114
$28 ($48)
OCR1BL
Timer/Counter1 – Output Compare Register B Low Byte
114
$27 ($47)
ICR1H
Timer/Counter1 – Input Capture Register High Byte
114
$26 ($46)
ICR1L
Timer/Counter1 – Input Capture Register Low Byte
$25 ($45)
TCCR2
$24 ($44)
TCNT2
Timer/Counter2 (8 Bits)
$23 ($43)
OCR2
Timer/Counter2 Output Compare Register
$22 ($42)
ASSR
$21 ($41)
$20(2) ($40)(2)
FOC2
WGM20
COM21
114
COM20
WGM21
CS22
CS21
CS20
130
133
133
–
–
–
–
AS2
TCN2UB
OCR2UB
TCR2UB
WDTCR
–
–
–
WDTOE
WDE
WDP2
WDP1
WDP0
UBRRH
URSEL
–
–
–
UBRR[11:8]
133
42
170
UCSRC
URSEL
UMSEL
UPM1
UPM0
USBS
UCSZ1
UCSZ0
UCPOL
169
$1F ($3F)
EEARH
–
–
–
–
–
–
–
EEAR8
19
$1E ($3E)
EEARL
EEPROM Address Register Low Byte
$1D ($3D)
EEDR
EEPROM Data Register
$1C ($3C)
EECR
–
–
–
–
EERIE
EEMWE
EEWE
EERE
$1B ($3B)
PORTA
PORTA7
PORTA6
PORTA5
PORTA4
PORTA3
PORTA2
PORTA1
PORTA0
65
$1A ($3A)
DDRA
DDA7
DDA6
DDA5
DDA4
DDA3
DDA2
DDA1
DDA0
65
65
19
20
20
$19 ($39)
PINA
PINA7
PINA6
PINA5
PINA4
PINA3
PINA2
PINA1
PINA0
$18 ($38)
PORTB
PORTB7
PORTB6
PORTB5
PORTB4
PORTB3
PORTB2
PORTB1
PORTB0
65
$17 ($37)
DDRB
DDB7
DDB6
DDB5
DDB4
DDB3
DDB2
DDB1
DDB0
65
$16 ($36)
PINB
PINB7
PINB6
PINB5
PINB4
PINB3
PINB2
PINB1
PINB0
66
$15 ($35)
PORTC
PORTC7
PORTC6
PORTC5
PORTC4
PORTC3
PORTC2
PORTC1
PORTC0
66
$14 ($34)
DDRC
DDC7
DDC6
DDC5
DDC4
DDC3
DDC2
DDC1
DDC0
66
$13 ($33)
PINC
PINC7
PINC6
PINC5
PINC4
PINC3
PINC2
PINC1
PINC0
66
$12 ($32)
PORTD
PORTD7
PORTD6
PORTD5
PORTD4
PORTD3
PORTD2
PORTD1
PORTD0
66
$11 ($31)
DDRD
DDD7
DDD6
DDD5
DDD4
DDD3
DDD2
DDD1
DDD0
66
PIND7
PIND6
PIND5
PIND4
PIND3
PIND2
PIND1
PIND0
$10 ($30)
PIND
$0F ($2F)
SPDR
SPI Data Register
66
145
$0E ($2E)
SPSR
SPIF
WCOL
–
–
–
–
–
SPI2X
144
$0D ($2D)
SPCR
SPIE
SPE
DORD
MSTR
CPOL
CPHA
SPR1
SPR0
143
$0C ($2C)
UDR
$0B ($2B)
UCSRA
$0A ($2A)
UCSRB
$09 ($29)
UBRRL
$08 ($28)
ACSR
ACD
ACBG
$07 ($27)
ADMUX
REFS1
$06 ($26)
ADCSRA
ADEN
$05 ($25)
ADCH
ADC Data Register High Byte
224
$04 ($24)
ADCL
ADC Data Register Low Byte
224
$03 ($23)
TWDR
Two-wire Serial Interface Data Register
$02 ($22)
TWAR
334
USART I/O Data Register
166
RXC
TXC
UDRE
FE
DOR
PE
U2X
MPCM
167
RXCIE
TXCIE
UDRIE
RXEN
TXEN
UCSZ2
RXB8
TXB8
168
ACO
ACI
ACIE
ACIC
ACIS1
ACIS0
205
REFS0
ADLAR
MUX4
MUX3
MUX2
MUX1
MUX0
221
ADSC
ADATE
ADIF
ADIE
ADPS2
ADPS1
ADPS0
223
USART Baud Rate Register Low Byte
TWA6
TWA5
TWA4
170
202
TWA3
TWA2
TWA1
TWA0
TWGCE
203
ATmega16A
8154B–AVR–07/09
ATmega16A
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Page
$01 ($21)
TWSR
TWS7
TWS6
TWS5
TWS4
TWS3
–
TWPS1
TWPS0
202
$00 ($20)
TWBR
Notes:
Two-wire Serial Interface Bit Rate Register
200
1. When the OCDEN Fuse is unprogrammed, the OSCCAL Register is always accessed on this address. Refer to the debugger specific documentation for details on how to use the OCDR Register.
2. Refer to the USART description for details on how to access UBRRH and UCSRC.
3. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory addresses
should never be written.
4. 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 $00 to $1F only.
335
8154B–AVR–07/09
30. Instruction Set Summary
Mnemonics
Operands
Description
Operation
Flags
#Clocks
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
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
1
AND
Rd, Rr
Logical AND Registers
Rd ← Rd • Rr
Z,N,V
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 ← $FF − Rd
Z,C,N,V
1
NEG
Rd
Two’s Complement
Rd ← $00 − Rd
Z,C,N,V,H
1
SBR
Rd,K
Set Bit(s) in Register
Rd ← Rd v K
Z,N,V
1
CBR
Rd,K
Clear Bit(s) in Register
Rd ← Rd • ($FF - 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 ← $FF
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) <<
Z,C
2
FMULS
Rd, Rr
Fractional Multiply Signed
Z,C
2
FMULSU
Rd, Rr
Fractional Multiply Signed with Unsigned
1
R1:R0 ← (Rd x Rr) << 1
R1:R0 ← (Rd x Rr) << 1
Z,C
2
Relative Jump
PC ← PC + k + 1
None
2
Indirect Jump to (Z)
PC ← Z
None
2
3
BRANCH INSTRUCTIONS
RJMP
k
IJMP
JMP
k
Direct Jump
PC ← k
None
RCALL
k
Relative Subroutine Call
PC ← PC + k + 1
None
3
Indirect Call to (Z)
PC ← Z
None
3
ICALL
Direct Subroutine Call
PC ← k
None
4
RET
Subroutine Return
PC ← STACK
None
4
RETI
Interrupt Return
PC ← STACK
I
if (Rd = Rr) PC ← PC + 2 or 3
None
CALL
k
4
CPSE
Rd,Rr
Compare, Skip if Equal
1/2/3
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
SBRC
Rr, b
Skip if Bit in Register Cleared
if (Rr(b)=0) PC ← PC + 2 or 3
None
1
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
336
ATmega16A
8154B–AVR–07/09
ATmega16A
Mnemonics
Operands
Description
Operation
Flags
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
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
None
1
None
1
#Clocks
DATA TRANSFER INSTRUCTIONS
MOV
Rd, Rr
Move Between Registers
MOVW
Rd, Rr
Copy Register Word
Rd ← Rr
Rd+1:Rd ← Rr+1:Rr
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
2
LDS
Rd, k
Load Direct from SRAM
Rd ← (k)
None
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
ST
Y+, Rr
Store Indirect and Post-Inc.
(Y) ← Rr, Y ← Y + 1
None
2
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
ST
Z, Rr
Store Indirect
(Z) ← Rr
None
2
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
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
Store Program Memory
(Z) ← R1:R0
None
-
In Port
Rd ← P
None
1
SPM
IN
Rd, P
OUT
P, Rr
Out Port
P ← Rr
None
1
PUSH
Rr
Push Register on Stack
STACK ← Rr
None
2
POP
Rd
Pop Register from Stack
Rd ← STACK
None
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
1
SES
Set Signed Test Flag
S←1
S
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
337
8154B–AVR–07/09
Mnemonics
Operands
SET
CLT
SEH
CLH
MCU CONTROL INSTRUCTIONS
NOP
SLEEP
WDR
BREAK
338
Flags
Description
Operation
Set T in SREG
Clear T in SREG
Set Half Carry Flag in SREG
Clear Half Carry Flag in SREG
T←1
T←0
H←1
H←0
T
T
H
H
No Operation
Sleep
Watchdog Reset
Break
(see specific descr. for Sleep function)
(see specific descr. for WDR/timer)
For On-Chip Debug Only
None
None
None
None
#Clocks
1
1
1
1
1
1
1
N/A
ATmega16A
8154B–AVR–07/09
ATmega16A
31. Ordering Information
Speed (MHz)
Power Supply
Ordering Code
(1)
16
Note:
2.7 - 5.5V
ATmega16A-AU
ATmega16A-PU(1)
ATmega16A-MU(1)
Package
Operation Range
44A
40P6
44M1
Industrial
(-40oC to 85oC)
1. Pb-free packaging complies to the European Directive for Restriction of Hazardous Substances (RoHS directive). Also
Halide free and fully Green.
Package Type
44A
44-lead, Thin (1.0 mm) Plastic Gull Wing Quad Flat Package (TQFP)
40P6
40-pin, 0.600” Wide, Plastic Dual Inline Package (PDIP)
44M1
44-pad, 7 x 7 x 1.0 mm body, lead pitch 0.50 mm, Quad Flat No-Lead/Micro Lead Frame Package (QFN/MLF)
339
8154B–AVR–07/09
32. Packaging Information
44A
PIN 1
B
PIN 1 IDENTIFIER
E1
e
E
D1
D
C
0˚~7˚
A1
A2
A
L
COMMON DIMENSIONS
(Unit of Measure = mm)
Notes:
1. This package conforms to JEDEC reference MS-026, Variation ACB.
2. Dimensions D1 and E1 do not include mold protrusion. Allowable
protrusion is 0.25 mm per side. Dimensions D1 and E1 are maximum
plastic body size dimensions including mold mismatch.
3. Lead coplanarity is 0.10 mm maximum.
SYMBOL
MIN
NOM
MAX
A
–
–
1.20
A1
0.05
–
0.15
A2
0.95
1.00
1.05
D
11.75
12.00
12.25
D1
9.90
10.00
10.10
E
11.75
12.00
12.25
E1
9.90
10.00
10.10
B
0.30
–
0.45
C
0.09
–
0.20
L
0.45
–
0.75
e
NOTE
Note 2
Note 2
0.80 TYP
10/5/2001
R
340
2325 Orchard Parkway
San Jose, CA 95131
TITLE
44A, 44-lead, 10 x 10 mm Body Size, 1.0 mm Body Thickness,
0.8 mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP)
DRAWING NO.
REV.
44A
B
ATmega16A
8154B–AVR–07/09
ATmega16A
40P6
D
PIN
1
E1
A
SEATING PLANE
A1
L
B
B1
e
E
0º ~ 15º
C
eB
Notes:
COMMON DIMENSIONS
(Unit of Measure = mm)
REF
1. This package conforms to JEDEC reference MS-011, Variation AC.
2. Dimensions D and E1 do not include mold Flash or Protrusion.
Mold Flash or Protrusion shall not exceed 0.25 mm (0.010").
SYMBOL
MIN
NOM
MAX
A
–
–
4.826
A1
0.381
–
–
D
52.070
–
52.578
E
15.240
–
15.875
E1
13.462
–
13.970
B
0.356
–
0.559
B1
1.041
–
1.651
L
3.048
–
3.556
C
0.203
–
0.381
eB
15.494
–
17.526
e
NOTE
Note 2
Note 2
2.540 TYP
09/28/01
R
2325 Orchard Parkway
San Jose, CA 95131
TITLE
40P6, 40-lead (0.600"/15.24 mm Wide) Plastic Dual
Inline Package (PDIP)
DRAWING NO.
40P6
REV.
B
341
8154B–AVR–07/09
44M1
D
Marked Pin# 1 ID
E
SEATING PLANE
A1
TOP VIEW
A3
A
K
L
Pin #1 Corner
D2
1
2
3
Option A
SIDE VIEW
Pin #1
Triangle
E2
Option B
K
Option C
b
e
Pin #1
Chamfer
(C 0.30)
Pin #1
Notch
(0.20 R)
BOTTOM VIEW
COMMON DIMENSIONS
(Unit of Measure = mm)
SYMBOL
MIN
NOM
MAX
A
0.80
0.90
1.00
A1
–
0.02
0.05
A3
0.20 REF
b
0.18
0.23
0.30
D
6.90
7.00
7.10
D2
5.00
5.20
5.40
E
6.90
7.00
7.10
E2
5.00
5.20
5.40
e
Note: JEDEC Standard MO-220, Fig. 1 (SAW Singulation) VKKD-3.
NOTE
0.50 BSC
L
0.59
0.64
0.69
K
0.20
0.26
0.41
9/26/08
Package Drawing Contact:
[email protected]
342
TITLE
44M1, 44-pad, 7 x 7 x 1.0 mm Body, Lead
Pitch 0.50 mm, 5.20 mm Exposed Pad, Thermally
Enhanced Plastic Very Thin Quad Flat No
Lead Package (VQFN)
GPC
ZWS
DRAWING NO.
REV.
44M1
H
ATmega16A
8154B–AVR–07/09
ATmega16A
33. Errata
The revision letter in this section refers to the revision of the ATmega16A device.
33.1
ATmega16A rev. N to rev. Q
•
•
•
•
First Analog Comparator conversion may be delayed
Interrupts may be lost when writing the timer registers in the asynchronous timer
IDCODE masks data from TDI input
Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request
1. First Analog Comparator conversion may be delayed
If the device is powered by a slow rising VCC, the first Analog Comparator conversion will
take longer than expected on some devices.
Problem Fix/Workaround
When the device has been powered or reset, disable then enable theAnalog Comparator
before the first conversion.
2. Interrupts may be lost when writing the timer registers in the asynchronous timer
The interrupt will be lost if a timer register that is synchronous timer clock is written when the
asynchronous Timer/Counter register (TCNTx) is 0x00.
Problem Fix/Workaround
Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor
0x00 before writing to the asynchronous Timer Control Register (TCCRx), asynchronous
Timer Counter Register (TCNTx), or asynchronous Output Compare Register (OCRx).
3. IDCODE masks data from TDI input
The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are
replaced by all-ones during Update-DR.
Problem Fix / Workaround
–
If ATmega16A is the only device in the scan chain, the problem is not visible.
–
Select the Device ID Register of the ATmega16A by issuing the IDCODE instruction
or by entering the Test-Logic-Reset state of the TAP controller to read out the
contents of its Device ID Register and possibly data from succeeding devices of the
scan chain. Issue the BYPASS instruction to the ATmega16A while reading the
Device ID Registers of preceding devices of the boundary scan chain.
–
If the Device IDs of all devices in the boundary scan chain must be captured
simultaneously, the ATmega16A must be the fist device in the chain.
4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt
request.
Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR register triggers an unexpected EEPROM interrupt request.
Problem Fix / Workaround
Always use OUT or SBI to set EERE in EECR.
343
8154B–AVR–07/09
34. Datasheet Revision History
Please 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.
Rev. 8154B – 07/09
1.
Updated “Errata” on page 343.
2.
Updated the last page with Atmel’s new addresses.
1.
Initial revision (Based on the ATmega16/L datasheet revision 2466R-AVR-05/08)
Rev. 8154A – 06/08
Changes done comparted ATmega16/L datasheet revision 2466R-AVR-05/08:
- Updated description in “Stack Pointer” on page 12.
- All Electrical characteristics is moved to “Electrical Characteristics” on page 293.
- Register descriptions are moved to sub sections at the end of each chapter.
- Added “Speed Grades” on page 295.
- New graphs in “Typical Characteristics” on page 305.
- New “Ordering Information” on page 339.
344
ATmega16A
8154B–AVR–07/09
ATmega16A
Table of Contents
Features ..................................................................................................... 1
1
Pin Configurations ................................................................................... 2
2
Overview ................................................................................................... 3
2.1
Block Diagram ...................................................................................................4
2.2
Pin Descriptions .................................................................................................6
3
Resources ................................................................................................. 7
4
Data Retention .......................................................................................... 7
5
About Code Examples ............................................................................. 7
6
AVR CPU ................................................................................................... 8
7
8
6.1
Overview ............................................................................................................8
6.2
ALU – Arithmetic Logic Unit ...............................................................................9
6.3
Status Register ..................................................................................................9
6.4
General Purpose Register File ........................................................................11
6.5
Stack Pointer ...................................................................................................12
6.6
Instruction Execution Timing ...........................................................................13
6.7
Reset and Interrupt Handling ...........................................................................14
AVR Memories ........................................................................................ 16
7.1
Overview ..........................................................................................................16
7.2
In-System Reprogrammable Flash Program Memory .....................................16
7.3
SRAM Data Memory ........................................................................................17
7.4
EEPROM Data Memory ..................................................................................18
7.5
I/O Memory ......................................................................................................19
7.6
Register Description ........................................................................................19
System Clock and Clock Options ......................................................... 24
8.1
Clock Systems and their Distribution ...............................................................24
8.2
Clock Sources .................................................................................................25
8.3
Default Clock Source .......................................................................................25
8.4
Crystal Oscillator .............................................................................................26
8.5
Low-frequency Crystal Oscillator .....................................................................27
8.6
External RC Oscillator .....................................................................................28
8.7
Calibrated Internal RC Oscillator .....................................................................29
8.8
External Clock .................................................................................................30
i
8154B–AVR–07/09
9
8.9
Timer/Counter Oscillator ..................................................................................30
8.10
Register Description ........................................................................................31
Power Management and Sleep Modes ................................................. 32
9.1
Overview ..........................................................................................................32
9.2
Sleep Modes ....................................................................................................32
9.3
Idle Mode .........................................................................................................33
9.4
ADC Noise Reduction Mode ............................................................................33
9.5
Power-down Mode ...........................................................................................33
9.6
Power-save Mode ............................................................................................33
9.7
Standby Mode .................................................................................................34
9.8
Extended Standby Mode .................................................................................34
9.9
Minimizing Power Consumption ......................................................................34
9.10
Register Description ........................................................................................36
10 System Control and Reset .................................................................... 37
10.1
Resetting the AVR ...........................................................................................37
10.2
Internal Voltage Reference ..............................................................................40
10.3
Watchdog Timer ..............................................................................................41
10.4
Register Description ........................................................................................41
11 Interrupts ................................................................................................ 44
11.1
Overview ..........................................................................................................44
11.2
Interrupt Vectors ..............................................................................................44
12 I/O Ports .................................................................................................. 49
12.1
Overview ..........................................................................................................49
12.2
Ports as General Digital I/O .............................................................................50
12.3
Alternate Port Functions ..................................................................................54
12.4
Register Description ........................................................................................65
13 External Interrupts ................................................................................. 67
13.1
Register Description ........................................................................................67
14 8-bit Timer/Counter0 with PWM ............................................................ 71
ii
14.1
Features ..........................................................................................................71
14.2
Overview ..........................................................................................................71
14.3
Timer/Counter Clock Sources .........................................................................72
14.4
Counter Unit ....................................................................................................72
14.5
Output Compare Unit .......................................................................................73
ATmega16A
8154B–AVR–07/09
ATmega16A
14.6
Compare Match Output Unit ............................................................................75
14.7
Modes of Operation .........................................................................................76
14.8
Timer/Counter Timing Diagrams .....................................................................80
14.9
Register Description ........................................................................................82
15 Timer/Counter0 and Timer/Counter1 Prescalers ................................ 86
15.1
Overview ..........................................................................................................86
15.2
Internal Clock Source ......................................................................................86
15.3
Prescaler Reset ...............................................................................................86
15.4
External Clock Source .....................................................................................86
15.5
Register Description ........................................................................................87
16 16-bit Timer/Counter1 ............................................................................ 88
16.1
Features ..........................................................................................................88
16.2
Overview ..........................................................................................................88
16.3
Accessing 16-bit Registers ..............................................................................91
16.4
Timer/Counter Clock Sources .........................................................................93
16.5
Counter Unit ....................................................................................................93
16.6
Input Capture Unit ...........................................................................................95
16.7
Output Compare Units .....................................................................................97
16.8
Compare Match Output Unit ............................................................................98
16.9
Modes of Operation .........................................................................................99
16.10
Timer/Counter Timing Diagrams ...................................................................107
16.11
Register Description ......................................................................................109
17 8-bit Timer/Counter2 with PWM and Asynchronous Operation ...... 117
17.1
Features ........................................................................................................117
17.2
Overview ........................................................................................................117
17.3
Timer/Counter Clock Sources .......................................................................118
17.4
Counter Unit ..................................................................................................118
17.5
Output Compare Unit .....................................................................................119
17.6
Compare Match Output Unit ..........................................................................121
17.7
Modes of Operation .......................................................................................122
17.8
Timer/Counter Timing Diagrams ...................................................................126
17.9
Asynchronous Operation of the Timer/Counter2 ...........................................128
17.10
Timer/Counter Prescaler ...............................................................................130
17.11
Register Description ......................................................................................130
18 SPI – Serial Peripheral Interface ......................................................... 136
iii
8154B–AVR–07/09
18.1
Features ........................................................................................................136
18.2
Overview ........................................................................................................136
18.3
SS Pin Functionality ......................................................................................141
18.4
Data Modes ...................................................................................................141
18.5
Register Description ......................................................................................143
19 USART ................................................................................................... 146
19.1
Features ........................................................................................................146
19.2
Overview ........................................................................................................146
19.3
Clock Generation ...........................................................................................148
19.4
Frame Formats ..............................................................................................151
19.5
USART Initialization .......................................................................................152
19.6
Data Transmission – The USART Transmitter ..............................................153
19.7
Data Reception – The USART Receiver .......................................................156
19.8
Asynchronous Data Reception ......................................................................160
19.9
Multi-processor Communication Mode ..........................................................163
19.10
Accessing UBRRH/ UCSRC Registers .........................................................164
19.11
Examples of Baud Rate Setting .....................................................................171
20 Two-wire Serial Interface ..................................................................... 175
20.1
Features ........................................................................................................175
20.2
Two-wire Serial Interface Bus Definition ........................................................175
20.3
Data Transfer and Frame Format ..................................................................176
20.4
Multi-master Bus Systems, Arbitration and Synchronization .........................179
20.5
Overview of the TWI Module .........................................................................181
20.6
Using the TWI ................................................................................................183
20.7
Transmission Modes .....................................................................................186
20.8
Multi-master Systems and Arbitration ............................................................199
20.9
Register Description ......................................................................................200
21 Analog Comparator .............................................................................. 204
21.1
Analog Comparator Multiplexed Input ...........................................................204
21.2
Register Description ......................................................................................205
22 Analog to Digital Converter ................................................................. 207
iv
22.1
Features ........................................................................................................207
22.2
Overview ........................................................................................................207
22.3
Operation .......................................................................................................208
22.4
Starting a Conversion ....................................................................................209
ATmega16A
8154B–AVR–07/09
ATmega16A
22.5
Prescaling and Conversion Timing ................................................................210
22.6
Changing Channel or Reference Selection ...................................................213
22.7
ADC Noise Canceler .....................................................................................215
22.8
ADC Conversion Result .................................................................................219
22.9
Register Description ......................................................................................221
22.10
ADLAR = 0 .....................................................................................................224
22.11
ADLAR = 1 .....................................................................................................224
23 JTAG Interface and On-chip Debug System ...................................... 226
23.1
Features ........................................................................................................226
23.2
Overview ........................................................................................................226
23.3
TAP – Test Access Port ................................................................................226
23.4
TAP Controller ...............................................................................................228
23.5
Using the Boundary-scan Chain ....................................................................229
23.6
Using the On-chip Debug System .................................................................229
23.7
On-chip Debug Specific JTAG Instructions ...................................................230
23.8
Using the JTAG Programming Capabilities ...................................................231
23.9
Register Description ......................................................................................231
23.10
Bibliography ...................................................................................................231
24 IEEE 1149.1 (JTAG) Boundary-scan ................................................... 232
24.1
Features ........................................................................................................232
24.2
Overview ........................................................................................................232
24.3
Data Registers ...............................................................................................232
24.4
Boundary-scan Specific JTAG Instructions ...................................................234
24.5
Boundary-scan Chain ....................................................................................235
24.6
Boundary-scan Order ....................................................................................245
24.7
Boundary-scan Description Language Files ..................................................249
24.8
Register Description ......................................................................................249
25 Boot Loader Support – Read-While-Write Self-Programming ......... 250
25.1
Features ........................................................................................................250
25.2
Overview ........................................................................................................250
25.3
Application and Boot Loader Flash Sections .................................................250
25.4
Read-While-Write and no Read-While-Write Flash Sections ........................251
25.5
Boot Loader Lock Bits ...................................................................................253
25.6
Entering the Boot Loader Program ................................................................254
25.7
Addressing the Flash during Self-Programming ............................................256
v
8154B–AVR–07/09
25.8
Self-Programming the Flash ..........................................................................257
26 Memory Programming ......................................................................... 264
26.1
Program And Data Memory Lock Bits ...........................................................264
26.2
Fuse Bits ........................................................................................................265
26.3
Signature Bytes .............................................................................................266
26.4
Calibration Byte .............................................................................................266
26.5
Page Size ......................................................................................................267
26.6
Parallel Programming Parameters, Pin Mapping, and Commands ...............267
26.7
Parallel Programming ....................................................................................269
26.8
Serial Downloading ........................................................................................276
26.9
SPI Serial Programming Characteristics .......................................................280
26.10
Programming via the JTAG Interface ............................................................280
27 Electrical Characteristics .................................................................... 293
27.1
Absolute Maximum Ratings* .........................................................................293
27.2
DC Characteristics .........................................................................................293
27.3
Speed Grades ...............................................................................................295
27.4
Clock Characteristics .....................................................................................295
27.5
System and Reset Characteristics ................................................................296
27.6
External Interrupts Characteristics ................................................................297
27.7
Two-wire Serial Interface Characteristics ......................................................297
27.8
SPI Timing Characteristics ............................................................................298
27.9
ADC Characteristics .....................................................................................301
27.10
Parallel Programming Characteristics ...........................................................302
28 Typical Characteristics ........................................................................ 305
29 Register Summary ............................................................................... 334
30 Instruction Set Summary ..................................................................... 336
31 Ordering Information ........................................................................... 339
32 Packaging Information ........................................................................ 340
33 Errata ..................................................................................................... 343
33.1
ATmega16A rev. N to rev. Q .........................................................................343
34 Datasheet Revision History ................................................................. 344
Table of Contents....................................................................................... i
vi
ATmega16A
8154B–AVR–07/09
ATmega16A
vii
8154B–AVR–07/09
Headquarters
International
Atmel Corporation
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USA
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