A N 115
S O FT W A R E UART E X A M P L E S
Relevant Devices
Implementation Options
This application note applies to the following The essential trade-off to consider when impledevices:
menting a software UART (SW UART) is between
hardware usage and speed/efficiency. Designs that
C8051F000, C8051F001, C8051F002,
utilize more hardware are likely to consume less
C8051F005, C8051F006, C8051F010,
CPU bandwidth and allow higher bit rates. This
C8051F011, C8051F012, C8051F012,
trade-off is discussed below.
C8051F015, C8051F016, C8051F017,
C8051F220, C8051F221, C8051F226,
Baud Rate Sources
C8051F230, C8051F231, C8051F236.
An interrupt must be generated for each bit that is
transferred; at a full-duplex 115.2 kbps, that’s an
Introduction
interrupt every 4.3 µs. The method of generating
these interrupts (baud rate source) determines to a
This application note presents a discussion about
large extent how much overhead the implementasoftware UART implementation on C8051Fxxx
tion consumes. Available options include: 8-bit
devices. Two complete examples are given: a C
timers, 16-bit timers, and the Programmable
program using the PCA as the baud rate source, and
Counter Array (PCA). Note that for full-duplex
an assembly program using Timer 0 as the baud
operation, two baud rate sources are required (one
rate source.
each for transmit and receive).
Key Features
The two software examples were designed to
closely mimic the hardware UART while still preserving hardware resources and CPU bandwidth.
The following is a list of key features found in both
examples:
•
•
•
•
•
An interface similar to the hardware UART,
with user-level transmit and receive interrupts.
Interrupt or polled mode access support.
Full-duplex communication up to 57.6 kbps
using an 18.432 MHz clock source.
State-based, interrupt-driven implementation,
requiring minimal CPU overhead.
Minimal hardware usage:
- ‘C’ example uses two PCA modules.
- Assembly example uses Timer 0 in Mode 3.
Rev. 1.1 12/03
The use of 8-bit timers allows one of the 16-bit
hardware timers to be used for both transmit and
receive baud rate generation. Timer 0 offers this
capability in Mode 3. Note that when Timer 0 is in
this mode, Timer 1 functionality is reduced; however, Timer 1 may still provide baud rate generation for the hardware UART (HW UART). Using
8-bit timers preserves hardware resources, but does
introduce some overhead and latency issues. These
issues are discussed in Example 2.
An alternative to the above solution is the use of
16-bit auto-reload timers. In this case two of the 16
bit hardware timers are occupied by the SW UART-one for transmit and one for receive. Any of the
available timers will suffice, but the auto-reload
feature on Timer 2 and Timer 3 reduces overhead,
and eliminates any interrupt latency issues. Additionally, 16-bit timers support a wider range of
baud rates.
Copyright © 2003 by Silicon Laboratories
AN115-DS11
AN11 5
The Programmable Counter Array (PCA) also provides an excellent solution for the SW UART, as
demonstrated in the provided ‘C’ example. The
PCA consists of a dedicated 16-bit counter/timer
and five 16-bit capture/compare modules. Each of
these modules may be configured to trigger an
interrupt when the PCA counter matches the associated compare module’s contents. Since the PCA
counter runs uninterrupted, this solution avoids the
problem of accumulated interrupt latency. The
PCA implementation is not available on
C8051F2xx devices.
Additional Considerations
Each of the above timer sources may be clocked by
SYSCLK or an external signal. In the provided
examples, baud rate sources are clocked by
SYSCLK, which is derived from an external
18.432 MHz crystal. Any baud rate/crystal frequency combination is allowed, though software
overhead limits the maximum baud rate-toSYSCLK ratio.
Program Structure
In software timer mode, the PCA can generate an
interrupt when the PCA counter matches a value in
of one of the compare modules. Since the PCA
counter runs uninterrupted, the modules can be
updated each bit time to accurately produce the
next bit time. In addition, the PCA offers a capture
function that is useful in START bit detection.
The PCA modules may be routed via the crossbar
to external signals. These signals (called CEXn for
module n) can be used to trigger PCA counter captures. This feature is exploited in the SW UART
receiver. START bit recognition is accomplished
with module 0 configured to capture the PCA
counter contents upon a falling edge on the RX pin.
This function offers two benefits: (1) START bit
detection is easily accomplished; and (2) since the
capture is performed immediately as the edge is
detected, the bit sample timing is immune to interrupt latency.
Implementation
START bit detection is also a concern for the SW
UART receiver. C8051F00x and C8051F01x The transmit and receive operations for Example 1
devices offer many external interrupt sources, sev- are implemented as two independent state
eral of which can be configured to detect falling
edges. Both example programs utilize external
interrupts for START detection.
Example 1: Programmable
Counter Array
Implementation
Example 1 uses two PCA modules to generate the
receive and transmit baud rates (modules 0 and 1,
respectively). The modules are configured in software timer mode to generate baud rate interrupts.
An introduction to the PCA can be found in
AN007.
2
Rev. 1.1
AN115
Figure 1. Transmit and Receive State Machines
Receive State Machine
Transmit State Machine
State 0: Start bit detected.
- Load Baud Rate source for 3/2 bit time.
- Increment state variable.
State 0: Transmit START bit.
- Drop TX pin as START condition.
- Update Baud Rate source for next bit time.
- Increment state variable.
State 1-8: Bit Received.
- Shift value of RX pin into RX shift register.
- Update Baud Rate source for next bit time.
- Increment state variable.
State 1-9:Transmit Bit.
- Shift LSB of Transmit Data onto TX pin.
- Shift STOP bit into MSB of Transmit Data.
- Update Baud Rate source for next bit time.
- Increment state variable.
State 9: Capture STOP bit.
- Indicate Receive complete.
- Trigger user-level interrupt if enabled.
- Reset Receiver to Idle state.
State 10: STOP bit transmitted.
- Indicate Transmit complete.
- Trigger user-level interrupt if enabled.
- Reset Transmitter to Idle state.
Interrupt Sources
1) START detection
2) Bit Time Generator
Interrupt Sources
1) User (begin TX)
2) Bit Time Generator
machines in the PCA ISR. The state machines are mode after the START bit is detected, and 3/2 bitillustrated in Figure 1.
time is added to the module 0 capture register. The
extra 1/2 bit-time is used only after the start bit is
Receive State Machine
detected, so that sampling will occur during the
When the SW UART is initialized, the PCA middle of the next bit period (see Figure 2). When
module 0 is configured for negative-edge capture the PCA counter reaches the value held in the
mode. Its input, CEX0, is routed via the crossbar to module 0 capture registers, the first bit-sampling
a GPIO pin (P0.2, SW_RX). With the state interrupt (LSB in this case) occurs.
machine in State 0, an interrupt is generated when a
falling edge is detected on SW_RX. Since the mod- States 1-8 execute on module match interrupts. In
ule is in capture mode, the contents of the PCA each state, bits are sampled from SW_RX and
counter are loaded into the module 0 capture regis- shifted into the RXSHIFT variable. The PCA
ters. Note that this value is independent of interrupt module 0 contents are updated in each state to prolatency. Module 0 is switched to software timer vide the next bit-time interrupt (1 bit time is added
Figure 2. SW UART Bit Timing
MARK
SPACE
START
BIT
D0
D1
D2
D3
D4
D5
D6
D7
STOP
BIT
BIT TIMES
BIT SAMPLING
3/2 Bit-Time
1 Bit-Time
Rev. 1.1
3
AN11 5
to the compare registers). The state variable is also The initialization and polled mode programming
procedure for Example 1 is shown in Figure 3.
incremented.
State 9 captures the STOP bit, posts SRI, and Figure 3. Example 1 User-Level Polled
returns the receiver to Idle state.
Mode Interface
Transmit State Machine
A user initiates a transmit by forcing a PCA
module 1 interrupt (setting CCF1=1). In State 0,
the TX pin is forced low to generate the START
condition. The PCA counter is read, and this value
plus one bit-time is loaded into to the module 1
capture registers. Note that a few SYSCLKs will
pass between the time the START bit is generated
and when the PCA counter is read. This is the only
instance in Example 1 where interrupt latency
affects the bit time. The effect is negligible (worst
case ~ 1/16 bit-time for 57.6 kbps and an
18.432 MHz SYSCLK).
Initialization
1) Define SYSCLK and desired
BAUD_RATE.
2) Call SW_UART_INIT.
3) Set SES if user-level interrupt
support is desired.
4) Set SREN to enable the SW UART
receiver.
5) Call SW_UART_ENABLE.
To Receive
To Transmit
STXBSY
1
States 1-9 are executed on module match interrupts. In each state, a bit is shifted out of the LSB
of TDR, and a ‘1’ shifted in the MSB of TDR to
represent the STOP bit. One bit time is added to the
PCA module 1 capture registers to generate the
next bit time. After 9 shifts, the data byte + STOP
bit have been transmitted. The Transmit Complete
indicator (STI) is set, the Transmit Busy indicator
(STXBSY) is cleared, and the TX state variable is
reset.
SRI
0
0
1
1) Read RDR.
2) Clear SRI
1) Write data to TDR.
2) Set CCF1 to
initiate transmit.
3) Set STXBSY
STI
0
Done?
Program Interface
No
The SW UART supports both polled and interruptdriven interfacing. Polled support is configured by
disabling user-level interrupts (SES=0). The transmit and receive indicators (STI and SRI, respectively) can then be polled for transfer completions.
1
Yes
1) Clear STI
End RX
End TX
Done?
Yes
4
Rev. 1.1
No
AN115
The initialization routine, SW_UART_INIT, configures the PCA, interrupts, and state variables for
use in the SW UART. The SW_UART_ENABLE
routine enables the SW UART. The SREN bit must
be set to enable the receiver. Note that the
TIME_COUNT constant is calculated by the software from the BAUD_RATE and SYSCLK constants.
Figure 4. Example 1 User-Level
Interrupt Mode Interface
IE7 Interrupt
Clear IE7 Interrupt
Flag
If user-level interrupt support is enabled (SES=1),
an IE7 interrupt is generated each time a transmit
or receive is completed. As with the hardware
UART, user software must check the transmit/
receive complete flags to determine the source of
the interrupt. In the event that a transmit and
receive are completed simultaneously, the user
software will receive only one interrupt. The IE7
ISR must be capable of handling this situation. Two
options are available: (1) service both transmit and
receive in the same ISR execution, or (2) service
one (STI or SRI) and force an interrupt so that the
ISR is called again to service the other. The second
option is recommended to minimize ISR execution
time.
Test code is provided to interface the SW UART
with the HW UART. Connect jumper wires as
shown in Figure 5.
SRI
Figure 5. Example 1 Test
Configuration
0
1
STI
0
1) Read RDR.
2) Clear SRI
P0.0 (HW_TX)
1
P0.3 (SW_RX)
1) Clear STI
2) Write new data to
TDR
3) Set CCF1 to
initate transmit.
4) Set STXBSY
P0.1 (HW_RX)
P0.2 (SW_TX)
STI || SRI
1) Re-trigger IE7
1
The test code routines configure and enable the
HW UART in Mode 1 using Timer 1 as the baud
rate source. Timer 1 is also configured. Different
baud rates and crystals may be tested by changing
Exit ISR
the BAUD_RATE and SYSCLK constants. Both
HW and SW UART baud rate counts are calculated
To use the software in interrupt mode, set SES=1. by the software from these constants. The testing
The programming procedure for interrupt mode is routines transmit 15 characters in both directions.
shown in Figure 4.
0
Rev. 1.1
5
AN11 5
To test the SW UART in polled mode, comment the With Timer 0 in Mode 3, Timer 1 may not set the
TF1 flag, generate an interrupt, or be clocked by
line
external signals. However, Timer 1 may still oper; INTERRUPT_TEST();
ate as a baud rate generator for the HW UART if
configured in Mode 2 (8-bit timer w/auto-reload).
and uncomment the line
While Timer 0 is in Mode 3, Timer 1 may be
enabled/disabled through its mode settings. Timer
POLLED_TEST();
1 is disabled in Mode 3, and enabled in all other
modes.
Reverse the above steps to test the SW UART in
interrupt mode. Uncomment the line
With Timer 1 as the HW UART baud rate source,
this solution is perhaps the most efficient use of
INTERRUPT_TEST();
hardware resources. The downside is increased
software overhead (relative to the 16-bit timer soluAnd comment the line
tion). Timer 0 Mode 3 does not offer auto-reload
capabilities; the manual timer reload requires a 16; POLLED_TEST();
bit move in each interrupt service routine (ISR)
The longest states in Example 2 require 113 iteration. In addition, interrupt latency will affect
SYSCLKs (TX States 1-9). For an 18.432 MHz the bit-time accuracy. A correction constant can be
crystal, a SW UART transmit or receive operation factored into the timer preload values to compenwill require a worst case 6 µs per bit transferred sate for typical interrupt latency, but variations in
(113*TSYSCLK). At 57.6 kbps, that’s ~35% of CPU interrupt latency are unaccounted for.
bandwidth for a transmit or receive (70% for fullduplex). For the Example 1 software compiled with Slower baud rates may require more than 8-bits of
the Keil compiler, the full-duplex overhead may be timer counts for each bit time. With SYSCLK at
18.432 MHz and Timer 0 in SYSCLK/1 mode,
approximated by the following equation:
baud rates below 72 kbps require more than 256
timer counts. Available options include:
FD Overhead(%) ~ = BAUD_RATE/81,000
Per the above equation, baud rates above 80 kbps
are not supported for full duplex operation. The
overhead penalty is only incurred while the SW
UART is performing a transfer. The code listing
begins on page 10.
1) Use Timer 0 in SYSCLK/12 mode. Slower baud
rates may be achieved with 8 bits, but standard
baud rate/SYSCLK combinations are more difficult
to obtain.
2) Use Timer 0 in SYSCLK/1 mode, and keep an
upper timer byte manually in the timer ISR. Note
Example 2: 8-Bit Timer
that this method will generate an interrupt every
256 SYSCLKs for each transmit and receive,
Implementation
regardless of the baud rate (an interrupt each time
In Example 1 the SW UART uses Timer 0 in the lower 8-bits overflow). The Example 2 softMode 3. In this mode, Timer 0 is split into two 8- ware demonstrates option #2.
bit timers: one is used for transmitting and one for
receiving. TL0 is used as the receive timer; TH0 is
Program Structure
used as the transmit timer.
The transmit and receive operations for Example 2
are implemented as two independent state
6
Rev. 1.1
AN115
machines in the Timer 0 and Timer 1 ISRs (see
Figure 1 on page 3). The Timer 0 ISR is used to
manage the receive state machine; the Timer 1 ISR
manages the transmit state machine. The /INT0
ISR starts the receive state machine, but is disabled
when the receive state is non-zero.
mov
TH0, #-LOW(TX_BT);
Notice that BCTHI is loaded with the unsigned bittime high byte, but TH0 is loaded with the negative
of the bit-time low byte. This is because Timer 0
(as all hardware timers) is an up-counter, while
BCTHI counts down. TH0 will overflow and genThe SW UART receiver captures the START bit erate an interrupt as it overflows from 0xFF to
using an external interrupt source, /INT0, config- 0x00; BCTHI is decremented upon each interrupt,
ured for active-low edge-sensitive input. The /INT0 and indicates a bit time when it equals zero.
interrupt is enabled when waiting for a START bit,
and disabled during transfers. /INT0 is routed to the For States 1-9, one state is executed each time
GPIO pins via the crossbar. Details on crossbar BCTHI reaches zero. In each State, the LSB of the
Transmit data register (TDR) is shifted onto the TX
configuration can be found in AN001.
pin. The TX timer is loaded with 1 bit time, and a
Since all timer loading is performed manually in ‘1’ is shifted into the MSB of TDR to represent the
the ISR, interrupt latency must be compensated for. STOP bit in State 9 (TDR should hold 0xFF after
A ‘slop constant’ is subtracted from each timer pre- the transfer is complete).
load value to adjust for this interrupt latency and
code executed between the timer overflow and the State 10 sets the Transmit Complete indicator
operation of reloading the new timer values. These (STI), clears the Transmit Busy indicator (STXconstants are independent of the SYSCLK fre- BSY), and triggers an IE7 interrupt if user-level
quency or baud rate; however, they do not account interrupt support is enabled.
for variations in interrupt latency.
Receive State Machine
In State 0, /INT0 is used as the RX input (configImplementation
ured falling-edge active, HIGH priority). An /INT0
NOTE: For this discussion, assume the baud rate is interrupt means a START condition has been
slow enough that the 8-bit timers are not sufficient. detected. The /INT0 ISR loads the RX timer (TL0
The direct RAM bytes labeled BCRHI and BCTHI + BCRHI) with 3/2 bit-time (see Figure 2). BCRHI
are used to manually keep the upper bytes for the is decremented each time TL0 overflows.
receive and transmit timers, respectively.
States 1-8 execute when BCRHI reaches zero. In
Transmit State Machine
each state, the SW_RX pin is sampled and shifted
When the SW UART is initialized and enabled, the into the LSB of the RXSHIFT variable. The RX
TX interrupt is set pending but still disabled. The timer is also reloaded to generate the next sampling
user initiates the transfer by enabling the transmit time. State 9 captures the STOP bit, but framing
interrupt (Note that TH0, the upper byte of error detection is not provided (the STOP bit polarTimer 0, generates the TX interrupts).
ity is not checked). If user-level interrupts are
enabled, this state enables and triggers the IE7
In State 0, the TX pin is asserted low to produce the interrupt.
START condition, and the timer is loaded with 1 bit
time to produce the next interrupt.
Program Interface
; Load high byte into BCTHI
mov
BCTHI, #HIGH(TX_BT);
; Load low byte into TH0
Example 2 supports both polled and interrupt
driven interfacing. The initialization ritual and programming procedure for polled mode is shown in
Rev. 1.1
7
AN11 5
Figure 6. The TIME_COUNT constant must be
explicitly defined in this example.
Figure 6. Example 2 User-Level Polled
Mode Interface
Initialization
1) Define TIME_COUNT according to
desired baud rate and SYSCLK.
2) Call SW_UART_INIT.
3) Set SES if user-level interrupt
support is desired.
4) Set SREN to enable the SW UART
receiver.
5) Call SW_UART_ENABLE.
To Receive
To Transmit
STXBSY
1
0
SRI
0
1
1) Read RDR.
2) Clear SRI
1) Write data to TDR.
2) Set ETI to initiate
transmit.
3) Set STXBSY
STI
0
Done?
No
1
Yes
1) Clear STI
End RX
End TX
Done?
Yes
8
No
Rev. 1.1
AN115
Figure 7 shows the IE7 ISR programming sequence one function and re-triggers itself to service the
for interrupt mode. Note that the receive operation other.
is serviced first, since it is the most sensitive to
latency.
Test code is provided. To test the polled mode code,
in the MAIN routine uncomment the line
Figure 7. Example 2 User-Level
ajmp PolledRX_PolledTX
Interrupt Interface
and comment the line
IE7 Interrupt
; ajmp InterruptRX_InterruptTX
To run the interrupt mode test code, reverse the
above steps. Comment the line
Clear IE7 Interrupt Flag
; ajmp PolledRX_PolledTX
and uncomment the line
SRI
0
ajmp InterruptRX_InterruptTX
1
STI
Adding a jumper wire between SW_GPIO_TX and
SW_GPIO_RX provides quick and easy evaluation
of the SW UART. Note that this evaluation method
is only useful with the interrupt mode test code.
0
1) Read RDR.
2) Clear SRI
1
1) Clear STI
2) Write new data to
TDR
3) Set ETI to initate
transmit.
4) Set STXBSY
STI
0
With a SYSCLK of 18.432 MHz, the software
given in Example 2 operates in full-duplex at a
maximum of 57.6 kbps. The code listing begins on
page 21.
1
1) Re-trigger IE7
SRI
0
1
1) Re-trigger IE7
Exit ISR
To handle the situation of a transmit and receive
completing simultaneously, this example services
Rev. 1.1
9
AN11 5
//-----------------------------------------------------------------------------------//
// Copyright 2003 Cygnal, Inc.
//
// FILE NAME: AN015_1.c
// TARGET DEVICE: C8051F00x, C8051F01x
// CREATED ON: 03/10/01
// CREATED BY: JS
//
// Software UART program, using PCA as baud rate source.
// PCA module 0 is used as receive baud rate source and START detector. For START
// detection, module 0 is configured in negative-edge capture mode. For all other
// SW_UART operations, module 0 is configured as a software timer. Module match
// interrupts are used to generate the baud rate. Module 1 generates the transmit
// baud rate in software timer mode.
// Code assumes an external crystal is attached between the XTAL1 and XTAL2 pins.
// The frequency of the external crystal should be defined in the SYSCLK constant.
//
// INITIALIZATION PROCEDURE:
// 1) Define SYSCLK according to external crystal frequency.
// 2) Define desired BAUD_RATE.
// 3) Call SW_UART_INIT().
// 4) Set SREN to enable SW_UART receiver.
// 5) Set SES only if user-level interrupt support is desired.
// 6) Call SW_UART_ENABLE().
//
// TO TRANSMIT:
// 1) Poll STXBSY for zero.
// 2) Write data to TDR.
// 3) Set CCF1 to initiate transmit.
// 4) STI will be set upon transmit completion. An IE7 interrupt is generated if
//
user-level interrupts are enabled.
//
// TO RECEIVE:
// 1) If in polled mode, poll SRI. If in interrupt mode, check SRI in IE7 Interrupt
//
Service Routine.
// 2) Read data from RDR.
//
// Test code is included, for both polled and interrupt mode. Test code assumes
// the HW_UART pins and SW_UART pins are connected externally:
// P0.0 (HW_UART TX) -> P0.3 (SW_UART RX)
// P0.1 (HW_UART RX) -> P0.2 (SW_UART TX)
//
// To use the test code in polled mode, comment out the call to the INTERRUPT_TEST()
// at the bottom of the main routine, and uncomment the call to POLLED_TEST(). To
// test the interrupt mode, comment out the POLLED_TEST() call and uncomment the
// INTERRUPT_TEST() call.
//
// The test routines configure the HW_UART to operate with Timer 1 as the baud rate
// source. The Timer 1 preload values are auto-calculated from the SYSCLK and BAUD_RATE
// constants.
//
//----------------------------------------------------------------------------------// Includes
//----------------------------------------------------------------------------------#include <c8051f000.h>
// SFR declarations
//----------------------------------------------------------------------------------// Global CONSTANTS
10
Rev. 1.1
AN115
//----------------------------------------------------------------------------------#define
#define
BAUD_RATE
SYSCLK
57600
18432000
// User-definable SW_UART baud rate
// System clock derived from
// 18.432MHz XTL
#define
TIME_COUNT
SYSCLK/BAUD_RATE/4
// Number of PCA counts for one
// bit-time. (PCA configured to count
// SYSCLK/4)
#define
TH_TIME_COUNT
TIME_COUNT*3/2
//
//
//
//
//
#define
HW_TIME_COUNT
SYSCLK/BAUD_RATE/16 //
//
//
//
3/2 bit-time, for use after receiving
a START bit. RX should be LOW for one
bit-time after the edge of the START,
and the first bit sample starts in
the middle of the next bit-time.
Time count for HW_UART baud rate
generation. Auto-calculated from the
SYSCLK and BAUD_RATE constants
defined above.
//----------------------------------------------------------------------------------//Global VARIABLES
//----------------------------------------------------------------------------------bit
bit
bit
bit
bit
SRI;
STI;
STXBSY;
SREN;
SES;
//
//
//
//
//
//
SW_UART
SW_UART
SW_UART
SW_UART
SW_UART
Support
Receive Complete Indicator
Transmit Complete Indicator
TX Busy flag
RX Enable
User-level Interrupt
Enable
sbit SW_RX = P0^2;
sbit SW_TX = P0^3;
// SW_UART Receive pin
// SW_UART Transmit pin
char TDR;
char RDR;
// SW_UART TX Data Register
// SW_UART RX Data Register (latch)
// Test Variables
char k, m;
char idata SW_BUF[20];
// Test indices.
// SW_UART test receive buffer.
bit HW_DONE;
bit SW_DONE;
//
//
//
//
HW transfer complete flag
(15 characters transmitted.)
SW transfer complete flag
(15 characters transmitted.)
//-----------------------------------------------------------------------------------// Function PROTOTYPES
//-----------------------------------------------------------------------------------void
void
void
void
void
void
SW_UART_INIT();
SW_UART_ENABLE();
PCA_ISR();
INTERRUPT_TEST(void);
POLLED_TEST(void);
USER_ISR(void);
void HW_UART_INIT(void);
//
//
//
//
//
//
//
//
SW_UART
SW_UART
SW_UART
SW_UART
SW_UART
SW_UART
routine
HW_UART
Rev. 1.1
initialization routine
enable routine
interrupt service routine
test routine (interrupt mode)
test routine (polled mode)
test interrupt service
initialization and setup
11
AN11 5
void HW_UART_ISR(void);
// routine
// HW_UART interrupt service routine
//-----------------------------------------------------------------------------------// MAIN Routine
//-----------------------------------------------------------------------------------// - Disables Watchdog Timer
// - Configures external crystal; switches SYSCLK to external crystal when stable.
// - Configures crossbar and ports.
// - Initializes and enables SW_UART.
// - Calls Test Routines.
//
void MAIN (void){
int delay;
// Delay counter.
OSCXCN = 0x66;
WDTCN = 0xDE;
WDTCN = 0xAD;
// Port Setup
XBR0 = 0x0C;
XBR2 = 0x40;
// Enable external crystal
// disable watchdog timer
// HW_UART routed to pins P0.0 and P0.1;
// CEX0 routed to pin P0.2.
// Enable crossbar, pull-ups enabled.
PRT0CF = 0x09;
// P0.0 (HW TX), and P0.3 (SW TX)
// configured for push-pull output.
delay=256;
while(delay--);
// Delay >1 ms
while (!(OSCXCN & 0x80));
//
//
//
//
//
Wait until external crystal has
started.
Switch to external oscillator
Disable internal oscillator; enable
missing clock detector.
//
//
//
//
Call Polled mode SW_UART test
routine.
Call Interrupt mode SW_UART test
routine.
OSCICN = 0x0C;
OSCICN = 0x88;
POLLED_TEST();
// INTERRUPT_TEST();
while(1);
before polling XTLVLD.
// Spin forever
}
//-----------------------------------------------------------------------------------// Functions
//-----------------------------------------------------------------------------------//-----------------------------------------------------------------------------------// INTERRUPT_TEST: SW_UART Interrupt Mode Test
// Test code to transmit and receive 15 characters to/from the HW_UART (in interrupt
// mode), with SW_UART in interrupt mode.
// - Initializes and enables the SW_UART & HW_UART
// - Clears all test variables & counters
// - Transfers 15 characters from HW to SW_UART, and 15 characters from SW to HW_UART,
//
simultaneously.
//
void INTERRUPT_TEST(void){
12
Rev. 1.1
AN115
SW_UART_INIT();
SW_UART_ENABLE();
SREN = 1;
SES = 1;
//
//
//
//
HW_UART_INIT();
k=m=0;
// Configure HW_UART for testing
// routine.
// Clear user ISR counters.
HW_DONE=0;
SW_DONE=0;
// Clear transfer complete indicators
//
IE |= 0x10;
STI = 1;
// Enable HW_UART interrupts
// Indicate transmit complete to
// initiate first transfer.
EIE2 |= 0x20;
PRT1IF |= 0x80;
// Start SW_TX by enabling
// and forcing an IE7 interrupt
TI = 1;
// Initiate a HW_UART transmit
// by forcing TX interrupt.
// Wait for transfers to finish.
while(!(HW_DONE&SW_DONE));
Initialize SW_UART
Enable SW_UART
Enable SW_UART Receiver
User-level interrupt support enabled.
}
//-----------------------------------------------------------------------------------// POLLED_TEST: SW_UART Polled Mode Test
// Test code to transmit and receive 15 characters to/from the HW_UART, with SW_UART
// in polled mode.
// - Initializes and enables the SW_UART & HW_UART
// - Clears all test variables & counters
// - Sends 15 characters from the HW_UART to be received by SW_UART.
// - Sends 15 characters from the SW_UART to be received by the HW_UART.
//
void POLLED_TEST(void){
SW_UART_INIT();
SW_UART_ENABLE();
SREN = 1;
SES = 0;
//
//
//
//
Initialize SW_UART
Enable SW_UART
Enable SW_UART Receiver
Disable user-level interrupt support.
HW_UART_INIT();
//
//
//
//
//
//
Configure HW_UART for testing
routine.
Clear test counter variables.
Clear transfer complete indicators
k=m=0;
HW_DONE = 0;
SW_DONE = 0;
IE |= 0x10;
TI = 1;
Enable HW_UART interrupts.
// Initiate a HW_UART transmit
// by forcing TX interrupt.
// Receive 15 characters with SW_UART; transmit with HW_UART.
while(SREN){
// Run while SW_UART Receiver
// is enabled.
if (SRI){
// If Receive Complete:
SR = 0;
// Clear receive flag.
SW_BUF[k++] = RDR;
// Read receive buffer.
if (k==15)
// If 15 characters have been received:
SREN = 0;
// Disable SW_UART Receiver.
Rev. 1.1
13
AN11 5
}
// Indicate 15 characters received
}
// Transmit 15 characters with SW_UART; receive with HW_UART.
while(STXBSY);
// Poll Busy flag.
STXBSY = 1;
// Claim SW_UART Transmitter
TDR=m++;
// Load TX data.
CCF1=1;
// Initiate first SW_UART TX
// by forcing a PCA module 1 interrupt.
while(!SW_DONE){
if (STI){
STI = 0;
if (m<16){
STXBSY = 1;
TDR = m++;
CCF1 = 1;
}
else
SW_DONE=1;
}
// SW_UART transmitting here
// HW_UART receiving.
//
//
//
//
//
//
//
//
//
If Transmit Complete:
Clear transmit flag.
Transmit 15 characters.
Claim SW_UART Transmitter
Transmit, increment variable.
Force module 1 interrupt to
initiate TX.
If this is 15th character,
Indicate last character transmitted.
}
}
//-----------------------------------------------------------------------------------// HW_UART_INIT: HW_UART Initialization Routine
// Sets up HW_UART for use in SW_UART testing.
// - HW_UART in Mode 1
// - Timer 1 used as baud rate source, clocked by SYSCLK.
//
void HW_UART_INIT(void) {
PCON |= 0x80;
// SMOD=1 (HW_UART uses Timer 1 overflow
// with no divide down).
TMOD = 0x20;
CKCON |= 0x10;
TH1 = -HW_TIME_COUNT;
TL1 = -HW_TIME_COUNT;
//
//
//
//
TR1 = 1;
// Start Timer 1
RI=0;
TI=0;
// Clear HW_UART receive and transmit
// complete indicators.
SCON = 0x50;
// Configure HW_UART for Mode 1,
// receiver enabled.
Configure Timer 1 for use by HW_UART
Timer 1 derived from SYSCLK
Timer 1 initial value
Timer 1 reload value
}
//-----------------------------------------------------------------------------------// SW_UART_INIT: SW_UART initialization routine
// Prepares SW_UART for use.
// - Configures PCA: Module 0 in negative capture mode; module 1 in software
//
timer mode; PCA time base = SYSCLK/4; PCA interrupt disabled; PCA counter
//
disabled.
// - Clears pending PCA module 0 and 1 interrupts
// - Resets TX and RX state variables
//
14
Rev. 1.1
AN115
void SW_UART_INIT(void){
PCA0CPM0 = 0x10;
// Module 0 in negative capture mode;
// module 0 interrupt disabled.
PCA0CPM1 = 0x48;
// Module 1 in software timer mode;
// module 1 interrupt disabled.
PCA0CN = 0;
PCA0MD = 0x02;
// Leave PCA disabled
// PCA timebase = SYSCLK/4; PCA counter
// interrupt disabled.
CCF0 = 0;
CCF1 = 0;
// Clear pending PCA module 0 and
// module 1 capture/compare interrupts.
SRI = 0;
STI = 0;
// Clear Receive complete flag.
// Clear Transmit complete flag.
SW_TX = 1;
STXBSY = 0;
// TX line initially high.
// Clear SW_UART Busy flag
}
//-----------------------------------------------------------------------------------// SW_UART_ENABLE: SW_UART Enable Routine
// Enables SW_UART for use.
// - Enables PCA module 0 interrupts
// - Enables PCA module 1 interrupts
// - Starts PCA counter.
//
void SW_UART_ENABLE(void){
PCA0CPM0 |= 0x01;
PCA0CPM1 |= 0x01;
// Enable module 0 (receive) interrupts.
// Enable module 1 (transmit)
// interrupts.
CR = 1;
EIE1 |= 0x08;
EA = 1;
// Start PCA counter.
// Enable PCA interrupts
// Globally enable interrupts
}
//-----------------------------------------------------------------------------------// Interrupt Service Routines
//-----------------------------------------------------------------------------------//
// PCA_ISR: PCA Interrupt Service Routine.
// This ISR is triggered by both transmit and receive functions, for each bit that
// is transmitted or received.
// - Checks module 0 interrupt flag (CCF0); if set, services receive state.
// - Checks module 1 interrupt flag (CCF1); if set, services transmit state.
//
void PCA_ISR(void) interrupt 9 {
static char SUTXST = 0;
static char SURXST = 0;
static unsigned char RXSHIFT;
// SW_UART TX state variable
// SW_UART RX state variable
// SW_UART RX Shift Register
unsigned int PCA_TEMP;
// Temporary storage variable for
Rev. 1.1
15
AN11 5
// manipulating PCA module
// high & low bytes.
// Check receive interrupt flag first; service if CCF0 is set.
if (CCF0){
CCF0 = 0;
// Clear interrupt flag.
switch (SURXST){
// State 0: START bit received.
// In this state, a negative edge on SW_TX has caused the interrupt,
// meaning a START has been detected and the PCA0CP0 registers have
// captured the value of PCA0.
// - Check for receive enable and good START bit
// - Switch PCA module 0 to software timer mode
// - Add 3/2 bit time to module 0 capture registers to sample LSB.
// - Increment RX state variable.
case 0:
if (SREN & ~SW_RX){
// Check for receive enable and a good
// START bit.
PCA_TEMP = (PCA0CPH0 << 8);
PCA_TEMP |= PCA0CPL0;
// Read module 0 contents into
// PCA_TEMP.
PCA_TEMP += TH_TIME_COUNT;
// Add 3/2 bit times to PCA_TEMP
PCA0CPL0 = PCA_TEMP;
PCA0CPH0 = (PCA_TEMP >> 8);
// Restore PCA0CPL0 and PCA0CPH0
// with the updated value
PCA0CPM0 = 0x49;
// Change module 0 to software
// timer mode, interrupts enabled.
SURXST++;
}
break;
// Update RX state variable.
// States 1-8: Bit Received
// - Sample SW_RX pin
// - Shift new bit into RXSHIFT
// - Add 1 bit time to module 0 capture registers
// - Increment RX state variable
case 1:
case 2:
case 3:
case 4:
case 5:
case 6:
case 7:
case 8:
16
RXSHIFT = RXSHIFT >> 1;
if (SW_RX)
RXSHIFT |= 0x80;
// Shift right 1 bit
// If SW_RX=1,
// shift '1' into RXSHIFT MSB
PCA_TEMP = (PCA0CPH0 << 8);
PCA_TEMP |= PCA0CPL0;
// Read module 0 contents into
// PCA_TEMP.
PCA_TEMP += TIME_COUNT;
// Add 1 bit time to PCA_TEMP
PCA0CPL0 = PCA_TEMP;
// Restore PCA0CPL0 and PCA0CPH0
Rev. 1.1
AN115
PCA0CPH0 = (PCA_TEMP >> 8);
// with the updated value
SURXST++;
break;
// Update RX state variable.
// State 9: 8-bits received, Capture STOP bit.
// - Move RXSHIFT into RDR.
// - Set SRI (indicate receive complete).
// - Prepare module 0 for next transfer.
// - Reset RX state variable.
// - Trigger IE7 if user-level interrupt support is enabled.
case 9:
RDR = RXSHIFT;
// Move received data to receive
// register.
// Set receive complete indicator.
SRI = 1;
PCA0CPM0 = 0x11;
// Switch module 0 to negative capture
// mode; interrupt enabled for START
// detection.
SURXST = 0;
// Reset RX state variable.
if (SES){
//
//
//
//
EIE2 |= 0x20;
PRT1IF |= 0x80;
If user-level interrupt support
enabled
Enable IE7.
Trigger IE7.
}
break;
}
}
// Check Transmit interrupt; service if CCF1 is set.
else if (CCF1){
CCF1 = 0;
// Clear interrupt flag
switch (SUTXST){
// State 0: Transmit Initiated.
// Here, the user has loaded a byte to transmit into TDR, and set the
// module 1 interrupt to initiate the transfer.
// - Transmit START bit (drop SW_TX)
// - Read PCA0, add one bit time, & store in module 1 capture registers
//
for first bit.
// - Increment TX state variable.
case 0:
SW_TX = 0;
// Drop TX pin as START bit.
PCA_TEMP = PCA0L;
PCA_TEMP |= (PCA0H << 8);
// Read PCA counter value into
// PCA_TEMP.
PCA_TEMP += TIME_COUNT;
// Add 1 bit time.
PCA0CPL1 = PCA_TEMP;
PCA0CPH1 = (PCA_TEMP >> 8);
// Store updated match value into
// module 1 capture/compare registers.
PCA0CPM1 |= 0x48;
// Enable module 1 software timer.
Rev. 1.1
17
AN11 5
SUTXST++;
break;
// Update TX state variable.
// States 1-9: Transmit Bit.
// - Output LSB of TDR onto TX
// - Shift TDR 1 bit right.
// - Shift a '1' into MSB of TDR for STOP bit in State 9.
// - Add 1 bit time to module 1 capture register
case 1:
case 2:
case 3:
case 4:
case 5:
case 6:
case 7:
case 8:
case 9:
SW_TX = (TDR & 0x01);
TDR >>= 1;
TDR |= 0x80;
//
//
//
//
Output LSB of TDR onto SW_TX pin.
Shift TDR right 1 bit.
Shift '1' into MSB of TDR for
STOP bit in State 9.
PCA_TEMP = (PCA0CPH1 << 8);
PCA_TEMP |= PCA0CPL1;
// Read module 1 contents into
// PCA_TEMP.
PCA_TEMP += TIME_COUNT;
// Add 1 bit time to PCA_TEMP
PCA0CPL1 = PCA_TEMP;
PCA0CPH1 = (PCA_TEMP >> 8);
// Restore PCA0CPL1 and PCA0CPH1
// with the updated value
SUTXST++;
break;
// Update TX state variable.
// State 10: Last bit has been transmitted. Transmit STOP bit
// and end transfer.
// - Transmit STOP bit
// - Set TX Complete indicator, clear Busy flag
// - Reset TX state
// - Prepare module 1 for next transfer.
// - Trigger IE7 interrupt if user-level interrupts enabled.
case 10:
STI = 1;
SUTXST = 0;
SW_TX = 1;
// Indicate TX complete.
// Reset TX state.
// SW_TX should remain high.
PCA0CPM1 = 0x01;
// Disable module 1 software timer;
// leave interrupt enabled for next
// transmit.
if (SES){
//
//
//
//
EIE2 |= 0x20;
PRT1IF |= 0x80;
}
STXBSY = 0;
break;
If user-level interrupt support
enabled:
Enable IE7.
Trigger IE7.
// SW_UART TX free.
}
18
Rev. 1.1
AN115
}
}
//-----------------------------------------------------------------------------------// USER_ISR: User SW_UART Interrupt Service Routine (IE7 ISR)
// If interrupt-mode test code is enabled, this ISR
// transmits 15 characters and receives 15 characters. This routine is triggered each
// time a SW_UART transmit or receive is completed.
// - Checks receive complete indicator, and services.
// - Checks transmit complete indicator, and services.
// - Checks for transmits or receives that completed during the ISR; if so, triggers the
//
interrupt again.
//
void USER_ISR(void) interrupt 19 {
// IE7 Interrupt Service Routine
PRT1IF &= ~(0x80);
// Clear IE7 interrupt flag
if (SRI){
SRI = 0;
SW_BUF[k++] = RDR;
if (k==15){
SREN=0;
}
}
//
//
//
//
//
//
else if (STI){
STI = 0;
// If Transmit Complete:
// Clear transmit flag.
if (m<15){
STXBSY = 1;
TDR = m++;
CCF1 = 1;
}
else
SW_DONE=1;
//
//
//
//
//
If Receive Complete:
Clear receive flag.
Read receive buffer.
If 15 characters have been received:
Disable SW_UART Receiver.
Indicate 15 characters received.
If less than 15 characters:
Claim SW_UART Transmitter.
Increment variable, transmit.
Force module 1 interrupt to initiate
TX
// Indicate last character transmitted.
}
if (STI|SRI)
PRT1IF |= 0x80;
// If SRI or STI is set, re-trigger
// interrupt to service.
}
//-----------------------------------------------------------------------------------// HW_UART_ISR: Hardware UART Interrupt Service Routine
// Transmits characters from 1 to 15, and receives 15 characters.
// - Checks receive interrupt, and services.
// - Checks transmit interrupt, and services.
//
void HW_UART_ISR(void) interrupt 4 {
static char i=0;
static char j=0;
static idata char HW_BUF[20];
// Transmit data variable.
// Receive data index.
// Receive data buffer.
if (RI){
// If Receive Complete:
RI=0;
HW_BUF[j++] = SBUF;
if (j==15)
REN=0;
//
//
//
//
Clear receive flag
Read receive buffer
If 15 characters received:
Disable HW_UART receiver.
Rev. 1.1
19
AN11 5
}
else if (TI){
TI = 0;
if (i<15)
SBUF=i++;
else
HW_DONE=1;
// If Transmit Complete:
//
//
//
//
//
Clear transmit flag
If characters left to transmit:
Increment variable, transmit.
If 15 characters transmitted,
Indicate HW TX finished.
}
}
// End of Example 1, Software UART with PCA
20
Rev. 1.1
AN115
;----------------------------------------------------------------------------; Cygnal, INC.
;
;
; FILE NAME
: AN015_2.ASM
; TARGET MCU : C8051F0xx
; DESCRIPTION : Example source code for a software UART
;
; IMPLEMENTATION NOTES:
; - uses Timer0 in Mode3 (two 8-bit timers)
; - Timer0 run/overflow is used for RX state machine
; - Timer1 overflow is used for TX state machine
; - 8N1, no Framing error detection
; - uses IE7 as user-level interrupt
; - uses single-byte PC offset for state table implementation
; - uses /INT0 falling as START bit detect (primitive START bit verification)
;
;----------------------------------------------------------------------------;----------------------------------------------------------------------------; EQUATES
;----------------------------------------------------------------------------$MOD8F000
; SW UART constants
SW_TX_GPIO
EQU
P0.4
SW_RX_GPIO
EQU
P0.2
TIME_COUNT
EQU
; SW UART TX GPIO pin (can be any GPIO pin)
; SW UART RX GPIO pin (must be /INT0)
320
; Note: 320 is the limit for reliable
; FD operation...
; FOR SYSCLK = 18.432 MHz:
; 115200 = 160
; 57600 = 320
; 38400 = 480
; 19200 = 960
; 9600 = 1920
; 4800 = 3840
; 2400 = 7680
TX_CORR
EQU
41
; (41) Timer preload correction value in
; cycles for TX
; (47) Timer preload correction value in
; cycles for RX
; (113)Timer preload correction value for
; 3/2 RX
RX_CORR
EQU
47
THALF_CORR
EQU
113
TX_BT
EQU
RX_BT
EQU
THALF_BT
EQU
TIME_COUNT - TX_CORR; actual 16-bit bit counter cycle value
; TX
TIME_COUNT - RX_CORR; actual 16-bit bit counter cycle value
; RX
TIME_COUNT*3/2 - THALF_CORR ; actual 16-bit 1.5 bit cycle value
; RX
RX_BUFSIZE
EQU
16
; size of RX buffer in chars
;----------------------------------------------------------------------------; VARIABLES
;-----------------------------------------------------------------------------
Rev. 1.1
21
AN11 5
BSEG
org 0h
SRI:
STI:
STXBSY:
SREN:
SES:
DBIT
DBIT
DBIT
DBIT
DBIT
1
1
1
1
1
;
;
;
;
;
SW
SW
SW
SW
SW
UART
UART
UART
UART
UART
Receive complete flag
Transmit complete flag
TX BSY flag
RX Enable
user-Interrupt Support Enable
DS
DS
DS
DS
DS
DS
DS
1
1
1
1
1
1
1
;
;
;
;
;
;
;
SW UART TX data register
SW UART RX data register
SW UART RX shift register
SW UART RX state variable
SW UART TX state variable
MSB of 16-bit bit timer for SW UART RX
MSB of 16-bit bit timer for SW UART TX
DSEG at 30h
TDR:
RDR:
RXSHIFT:
SURXST:
SUTXST:
BCRHI:
BCTHI:
;test variables
RX_TAIL:
DS
1
TX_VAL:
DS
1
;------------------; Indirect address space variables
; write pointer to RX message buffer
; value to transmit
ISEG at 80h
RX_BUF:
DS
RX_BUFSIZE
; RX message buffer
1
; placeholder in symbol table for
; beginning of hardware stack
;------------------; STACK
STACK_TOP:
DS
;----------------------------------------------------------------------------; MACRO DEFINITIONS
;----------------------------------------------------------------------------;----------------------------------------------------------------------------; RESET AND INTERRUPT VECTOR TABLE
;----------------------------------------------------------------------------CSEG
22
org
ljmp
00h
Reset
; RESET initialization vector
org
ljmp
03h
INT0_ISR
; Software UART RX START bit detect
org
ljmp
0bh
Timer0_ISR
; Software UART RX state machine interrupt
org
ljmp
1bh
Timer1_ISR
; Software UART TX state machine interrupt
org
ljmp
9bh
IE7_ISR
; user-level Software UART interrupt
Rev. 1.1
AN115
;----------------------------------------------------------------------------; MAIN PROGRAM CODE
;----------------------------------------------------------------------------org
0B3h
ajmp
PolledRX_PolledTX
ajmp
InterruptRX_InterruptTX; leave one of these lines uncommented
sjmp
$
Main:
;
; leave one of these lines uncommented
; spin forever...
;----------------------------------------------------------------------------; MAIN SUBROUTINES
;----------------------------------------------------------------------------;----------------------------------------------------------------------------; PolledRX_PolledTX
;----------------------------------------------------------------------------; This routine demonstrates polled access to the SW UART.
;
; The transmitter transmits a sequence from $00 to $ff
;
; The receiver receives characters and stores them in a circular buffer.
;
PolledRX_PolledTX:
acall SW_UART_Init
; initialize SW UART (leave in a
; disabled state)
setb
clr
SREN
SES
acall
SW_UART_Enable
; enable SW UART receiver
; disable user-level interrupt
; support
; enable SW UART
; transmit message -- polled mode
jb
STXBSY, $
; wait for SW TX available
TX_LOOP:
; transmit chars $00 to $ff
clr
a
setb
STXBSY
;
mov
TDR, a
;
setb
ET1
;
inc
a
;
jnb
STI, $
;
clr
STI
;
jnz
TX_LOOP
claim SW UART Transmitter
write char to transmit data reg
initiate SW TX operation
set next value to write
wait for TX complete
clear TX complete indicator
TX_LOOP_END:
mov
RX_TAIL, #RX_BUF
; initialize TAIL pointer
; receive message -- polled mode
RX_LOOP:
mov
r0, RX_TAIL
; indirect address to write character to
jnb
SRI, $
; wait for RX character
clr
SRI
; clear RX complete indicator
mov
@r0, RDR
; store it
inc
RX_TAIL
; advance TAIL pointer
mov
a, RX_TAIL
; handle TAIL wrapping
add
a, #-(RX_BUF + RX_BUFSIZE)
Rev. 1.1
23
AN11 5
jnc
mov
RX_LOOP
RX_TAIL, #RX_BUF
; wrap TAIL
sjmp
RX_LOOP
; repeat forever...
;----------------------------------------------------------------------------; InterruptRX_InterruptTX
;----------------------------------------------------------------------------; This routine demonstrates interrupt access to the SW UART.
;
; The receiver receives characters and stores them in a circular buffer.
; Both the transmit and receive routines are located in the IE7_ISR handler.
;
InterruptRX_InterruptTX:
acall
SW_UART_Init
setb
setb
SES
SREN
;
;
;
;
initialize SW UART (leave in a
disabled state)
Enable user-level interrupt support
Enable SW UART receiver
mov
RX_TAIL, #RX_BUF
; initialize TAIL pointer
acall
SW_UART_Enable
; enable SW UART
setb
orl
orl
STI
EIE2, #00100000b
PRT1IF, #10000000b
; kick-start SW UART transmitter
; by enabling IE7
; and activating IE7
sjmp
$
;----------------------------------------------------------------------------; INTERRUPT VECTORS
;----------------------------------------------------------------------------;----------------------------------------------------------------------------; Reset Interrupt Vector
;
; This routine initializes the device and all peripherals and variables.
;
- External oscillator started (SYSCLK will be switched to external osc.
;
once XTLVLD goes high)
;
- Watchdog timer is disabled
;
- Crossbar and GPIO output modes are defined
;
- H/W stack pointer is initialized
;
- interrupt priorities and enables are initialized here
;
- /INT0
;
- Timer0
;
- Timer1
Reset:
24
mov
OSCXCN, #01100110b
; Enable Crystal osc., divide by 1 mode
; XFCN = '110' for 18.432 MHz crystal
; External oscillator will be selected
; below after XTLVLD has gone high
; indicating that the external osc
; has started and settled (several
; hundred microseconds from now).
mov
mov
WDTCN, #0deh
WDTCN, #0adh
; disable watchdog timer
Rev. 1.1
AN115
; set up Crossbar and Port
mov
XBR0, #00000100b
mov
XBR1, #10000100b
mov
XBR2, #01000000b
orl
PRT0CF, #00011101b
orl
PRT1CF, #01000000b
mov
SP, #STACK_TOP
I/O
; enable HW UART on P0.0 (TX), P0.1 (RX)
; enable /INT0 on P0.2; /SYSCLK on P0.3
; enable crossbar w/ pull-ups enabled
; enable P0.0, 0.2, 0.3, and 0.4 as push-pull
; P0.4 is SW UART TX pin
; P0.2 is SW UART RX pin
; enable P1.6 (LED on target board) as
; push-pull
; init stack pointer to end of allocated RAM
; Wait >1 ms before checking external crystal for stability
clr
a
mov
r0, a
; Clear r0
djnz
djnz
djnz
r0, $
r0, $
r0, $
; Delay ~380 µs
; Delay ~380 µs
; Delay ~380 µs
mov
jnb
a, OSCXCN
acc.7, OSC_WAIT
; spin here until crystal osc is stable
orl
OSCICN, #00001000b
OSC_WAIT:
anl
orl
; Select external osc. as
; system clock source
OSCICN, #NOT(00000100b); Disable internal oscillator
OSCICN, #10000000b ; Enable missing clock detector
; this must be done AFTER
; selecting external osc as system
; clock source
setb
EA
ljmp
Main
; enable GLOBAL interrupts
;----------------------------------------------------------------------------; Timer0_ISR / INT0_ISR
;
; These interrupts start and drive the SW UART receive state machine
;
SWRX_STATE_TABLE:
; each table entry is 1 byte
DB
SWRX_S0 - SWRX_STATE_TABLE ; IDLE / START detect
DB
SWRX_S1 - SWRX_STATE_TABLE ; b0
DB
SWRX_S2 - SWRX_STATE_TABLE ; b1
DB
SWRX_S3 - SWRX_STATE_TABLE ; b2
DB
SWRX_S4 - SWRX_STATE_TABLE ; b3
DB
SWRX_S5 - SWRX_STATE_TABLE ; b4
DB
SWRX_S6 - SWRX_STATE_TABLE ; b5
DB
SWRX_S7 - SWRX_STATE_TABLE ; b6
DB
SWRX_S8 - SWRX_STATE_TABLE ; b7
DB
SWRX_S9 - SWRX_STATE_TABLE ; STOP bit capture
INT0_ISR:
Timer0_ISR:
push
push
PSW
acc
; resource preservation
Rev. 1.1
25
AN11 5
mov
a, BCRHI
jz
dec
ajmp
SWRX_PROCESS_STATE
BCRHI
Timer0_ISR_EXIT
SWRX_PROCESS_STATE:
push
DPH
push
DPL
mov
mov
movc
jmp
; resource preservation
a, SURXST
; read state offset from table
DPTR, #SWRX_STATE_TABLE
a, @A+DPTR
; 'a' now contains state offset (PC)
@A+DPTR
; execute state
Timer0_ISR_END:
pop
pop
Timer0_ISR_EXIT:
pop
pop
reti
; if BCRHI is non-zero, we need to roll
; through the timer again...
; ALL RX states return here
DPL
DPH
; resource restoration
acc
PSW
; resource restoration
;SWRX_S0: RX IDLE state
; At this point, a falling edge has been detected on /INT0.
; We first check to see if the SW UART receiver is enabled. If it is, we check
; once to see if the RX pin is still low (START bit valid). If it is, we set up
; Timer0 to count for 3/2 bit time in order to capture the LSB. Here, we also
; disable /INT0 interrupts.
; - Check for SREN = '1': IF '1':
;
- Load TL0 with 3/2 bit time value
;
- Start Timer
;
- Enable TF0 interrupt
;
- Disable /INT0 interrupt
;
- INC state variable to S1
; - IF SREN = '0' (SW UART RX disabled)
;
- exit gracefully, next state is S0
;
SWRX_S0:
jnb
SREN, SWRX_S0_END
; Check to see if SW UART RX is enabled
; if not, exit and remain at IDLE state
26
jb
SW_RX_GPIO, SWRX_S0_END; check to see if START bit is good
clr
EX0
; disable /INT0
clr
clr
TR0
TF0
; Stop Timer0 (low)
; Clear any pending interrupts
mov
mov
BCRHI, #HIGH(THALF_BT); set Timer0 (low) + BCRHI for 1.5 bit
TL0, #-LOW(THALF_BT); times from now (we assume the start
; bit is good)
setb
setb
ET0
TR0
; enable Timer0 interrupts
; Start Timer0L
inc
SURXST
; next state is SWRX_S1 (we assume START bit
; is good)
Rev. 1.1
AN115
SWRX_S0_END:
ajmp
Timer0_ISR_END
;SWRX_S1 thru SWRX_S8: Capture b0..b7
; At this point, we've determined that the START bit is valid, and we're going to
; query RX_GPIO at bit intervals, shifting the results into RXSHIFT.
; - If BCRHI is non-zero, then we need to spin through the timer again
;
- DEC BCRHI
;
- let timer roll over on its own
;
- leave state as is
; - If BCRHI is zero:
;
- stop timer
;
- Move RX_GPIO state into Carry
;
- Right shift Carry into RXSHIFT
;
- set up timer to capture the next bit
;
- enable timer
;
- advance state variable
;
SWRX_S1:
SWRX_S2:
SWRX_S3:
SWRX_S4:
SWRX_S5:
SWRX_S6:
SWRX_S7:
SWRX_S8:
clr
TR0
; Stop Timer0 (low)
clr
TF0
; Clear any pending interrupts
mov
mov
BCRHI, #HIGH(RX_BT) ; load bit time value into 16-bit virtual
; counter
TL0, #-LOW(RX_BT)
setb
TR0
; START RX bit timer
mov
C, SW_RX_GPIO
; Move RX state into Carry prior to rshift
mov
rrc
mov
a, RXSHIFT
a
RXSHIFT, a
; right shift Carry into shift register
; re-store
inc
SURXST
; advance state variable
ajmp
Timer0_ISR_END
SWRX_S2_END:
;SWRX_S9: Capture STOP bit
; At this point, we've shifted all the data bits into RXSHIFT, and we're ready to
; sample the STOP bit. Here, we indicate that we've received a character, and reset
; the state machine back to IDLE. In this implementation, we don't actually capture
; the STOP bit; we assume it's good. Here's where we would add support for Framing
; Error detection.
; - If BCRHI is non-zero, then we need to spin through the timer again
;
- DEC BCRHI
;
- let timer roll over on its own
;
- leave state as is
; - If BCRHI is zero:
;
- stop timer
Rev. 1.1
27
AN11 5
;
;
;
;
;
;
;
;
;
SWRX_S9:
-
Move RXSHIFT into RDR
Set SRI
Disable timer interrupt
Enable /INT0 interrupt
Reset state variable to IDLE
Check to see if User-level interrupt support is enabled (EIS): If so:
- Enable IE7
- Toggle P1.7 to activate IE7
clr
mov
TR0
RDR, RXSHIFT
; Stop Timer0L
; move data from shift reg to data reg
setb
clr
clr
setb
mov
SRI
ET0
IE0
EX0
SURXST, #00
;
;
;
;
;
jnb
SES, SWRX_S9_END
orl
orl
EIE2, #00100000b
PRT1IF, #10000000b
; check to see if user-level interrupt
; support is enabled
; enable IE7; leave priority alone
; activate IE7
ajmp
Timer0_ISR_END
set SW UART SRI bit to indicate RX complete
Disable Timer0L interrupt
Disable pending /INT0 interrupts
Enable /INT0 interrupt
reset RX state to IDLE
SWRX_S9_END:
;----------------------------------------------------------------------------; Timer1_ISR (note that this is actually called by the upper-half of Timer0
; which is operating in Mode 3)
;
; This interrupt drives the SW UART transmit state machine
;
SWTX_STATE_TABLE:
DB
DB
DB
DB
DB
DB
DB
DB
DB
DB
DB
SWTX_S0
SWTX_S1
SWTX_S2
SWTX_S3
SWTX_S4
SWTX_S5
SWTX_S6
SWTX_S7
SWTX_S8
SWTX_S9
SWTX_S10
-
;
;
SWTX_STATE_TABLE;
SWTX_STATE_TABLE;
SWTX_STATE_TABLE;
SWTX_STATE_TABLE;
SWTX_STATE_TABLE;
SWTX_STATE_TABLE;
SWTX_STATE_TABLE;
SWTX_STATE_TABLE;
SWTX_STATE_TABLE;
SWTX_STATE_TABLE;
SWTX_STATE_TABLE;
each table entry is 1 byte; 11 entries
total
START bit
b0
b1
b2
b3
b4
b5
b6
b7
STOP bit onset edge
STOP bit terminus
Timer1_ISR:
push
push
PSW
acc
; resource preservation
mov
a, BCTHI
; if BCTHI is non-zero, we need to roll
; through the timer again...
jz
dec
ajmp
SWTX_PROCESS_STATE
BCTHI
Timer1_ISR_EXIT
SWTX_PROCESS_STATE:
push
DPH
28
; resource preservation
Rev. 1.1
AN115
push
DPL
mov
mov
movc
jmp
a, SUTXST
; read state offset from table
DPTR, #SWTX_STATE_TABLE
a, @A+DPTR
; acc now contains state offset
@A+DPTR
; execute State x
Timer1_ISR_END:
pop
pop
Timer1_ISR_EXIT:
pop
pop
; ALL TX states return here
DPL
DPH
; resource restoration
acc
PSW
; resource restoration
reti
;SWTX_S0: TX START bit state
; At this point, user code has placed the char to be transmitted in TDR and has
; called the Timer1 interrupt handler explicitly by setting TF1.
; - Clear STI
; - Drop TX_GPIO (START bit onset edge)
; - Configure TH0, BCTHI for next bit time, which will be the LSB
; - Enable TH0
; - Set next state to SWTX_S1
;
SWTX_S0:
mov
BCTHI, #HIGH(TX_BT) ; load bit time value into 16-bit virtual
; counter
mov
TH0, #-LOW(TX_BT)
clr
SW_TX_GPIO
; START bit onset edge
clr
TF1
; clear any pending interrupts
inc
SUTXST
ajmp
Timer1_ISR_END
; next state is SWTX_S1
SWTX_S0_END:
;SWTX_S1 thru SWTX_S9: TX b0..b7 and STOP bit
; At this point, we start shifting the character in TDR out the TX_GPIO pin, bit
; by bit, one bit per state transition. We shift in an extra '1' at the MSB which
; becomes the STOP bit.
; - If BCTHI is non-zero, then we need to spin through the timer again
;
- DEC BCTHI
;
- let timer roll over on its own
;
- leave state as is
; - If BCTHI is zero:
;
- stop timer
;
- set up timer for next bit
;
- right-shift TDR
;
- enable timer
;
- output bit
;
- advance state variable
;
SWTX_S1:
SWTX_S2:
SWTX_S3:
SWTX_S4:
SWTX_S5:
SWTX_S6:
Rev. 1.1
29
AN11 5
SWTX_S7:
SWTX_S8:
SWTX_S9:
mov
mov
BCTHI, #HIGH(TX_BT) ; load bit time value into 16-bit virtual
; counter
TH0, #-LOW(TX_BT)
mov
setb
rrc
mov
mov
clr
a, TDR
C
a
TDR, a
SW_TX_GPIO, C
TF1
; re-store value
; output bit on GPIO pin
; clear any pending interrupts
inc
SUTXST
; advance to next state
ajmp
Timer1_ISR_END
; right shift next bit to transmit into Carry
; shift STOP bit into MSB
SWTX_S1_END:
;SWTX_S10 STOP bit complete / reset to IDLE
; At this point, we've shifted the STOP bit out, and we're ready to reset the state
; machine and indicate transmit complete, including initiating a user-level interrupt
; if it's enabled.
; - If BCTHI is non-zero, then we need to spin through the timer again
;
- DEC BCTHI
;
- let timer roll over on its own
;
- leave state as is
; - If BCTHI is zero:
;
- stop timer
;
- set STI
;
- clear STXBSY
;
- check for IE7 support, and activate if enabled
;
- set state variable to S0
;
SWTX_S10:
clr
ET1
; Disable Timer1 interrupts
setb
TF1
; Force a pending Timer1 interrupt. This
; allows the Enable Timer1 interrupt
; operation to immediately trigger a
; transmit operation
mov
setb
clr
SUTXST, #00h
STI
STXBSY
jnb
SES, SWTX_S10_END
orl
orl
EIE2, #00100000b
PRT1IF, #10000000b
ajmp
Timer1_ISR_END
;
;
;
;
;
;
;
;
reset state variable to IDLE state
Set STI to indicate transmit complete
Clear TXBSY to indicate transmitter
available
activate user-level interrupt IE7 if
enabled
enable IE7; leave priority alone
activate IE7
SWTX_S10_END:
;----------------------------------------------------------------------------; IE7_ISR
;
; This is the user-level interrupt handler for the SW UART. Note: this code
; MUST check both SRI and TRI, and if both are set, it must handle one case, and
; re-trigger IE7 for the other case (or handle it in the same call) if that case
30
Rev. 1.1
AN115
; is interrupt handled. This is not required, for example, if the RX case is
; handled in the interrupt and the TX case is polled.
;
; Note, if the TX case is polled, STI should not be cleared here.
;
; In this example, if SRI is set, indicating that a character was received by
; the SW UART, that received character is stored in a circular buffer (RX_BUF).
; If STI is set, indicating transmit complete, the character stored in TX_VAL
; is transmitted (and post incremented).
;
;
IE7_ISR:
push
PSW
push
acc
anl
jbc
jbc
PRT1IF, #NOT(10000000b); clear IE7
SRI, SW_RX_HANDLE
; handle receive first, since
; it's the most sensitive to
; latency
STI, SW_TX_HANDLE
; handle TX case
IE7_ISR_END:
pop
pop
reti
acc
PSW
push
ar0
mov
mov
inc
mov
add
jnc
mov
r0, RX_TAIL
; point r0 to location to store
@r0, RDR
; read value into buffer
RX_TAIL
; update the TAIL pointer
a, RX_TAIL
; wrap pointer if necessary
a, #-(RX_BUF+RX_BUFSIZE)
SW_RX_HANDLE_END
RX_TAIL, #RX_BUF
; wrap the pointer
; all IE7_ISR routines return here...
SW_RX_HANDLE:
SW_RX_HANDLE_END:
jnb
orl
STI, NO_TX_PENDING
PRT1IF, #10000000b
NO_TX_PENDING:
pop
ajmp
ar0
IE7_ISR_END
; resource preservation
; if TX interrupt is pending,
; activate it (IE7)
SW_TX_HANDLE:
setb
mov
setb
inc
SW_TX_HANDLE_END:
jnb
orl
NO_RX_PENDING:
ajmp
STXBSY
TDR, TX_VAL
ET1
TX_VAL
;
;
;
;
claim SW UART Transmitter
load byte to transmit into TDR
start SW UART transmitter
next byte to store
SRI, NO_RX_PENDING
PRT1IF, #10000000b
; if RX interrupt is pending,
; activate it (IE7)
IE7_ISR_END
; exit
;----------------------------------------------------------------------------; SUBROUTINES
;-----------------------------------------------------------------------------
Rev. 1.1
31
AN11 5
;----------------------------------------------------------------------------; SW UART SUBROUTINES (non-user code)
;----------------------------------------------------------------------------;----------------------------------------------------------------------------; SW_UART_Init
;
; Init:
; - /INT0 is falling-edge triggered
; - Timer0 in Mode 3, (2) 8-bit timers, interrupt handlers for TL0, TH0 (TF0, TF1)
;
timers initially disabled...
; - RX/TX State machines and state variables
; - SW UART TX state machine and RX state machine operate at HIGH priority
SW_UART_Init:
; Init /INT0
clr
EX0
; disable /INT0 interrupts
setb
IT0
; /INT0 is falling-edge triggered
clr
IE0
; forcibly clear /INT0 interrupt flag
setb
PX0
; /INT0 is HIGH priority interrupt
; Init
clr
clr
clr
clr
clr
clr
orl
anl
orl
setb
setb
Timer0
ET0
; disable Timer0 interrupts
ET1
; disable Timer1 interrupts
TR0
; Timer0 off
TR1
; Timer1 off
TF0
; forcibly clear interrupt flags
TF1
TMOD, #00000011b
; Timer0 in Mode 3 (2) 8-bit timers
TMOD, #NOT(00001100b); GATE0=0; C/T0 = 0
CKCON, #00001000b
; Timer0 uses system clock as time base
PT0
; Timer0 interrupt is HIGH priority
PT1
; Timer1 interrupt is HIGH priority
; User-level interrupt (IE7) is initialized explicitly by the state
; machines
; Init
clr
mov
mov
mov
mov
clr
clr
clr
clr
clr
State Machines and Variables
a
; Init state machines
SURXST, a
; RX state variable
SUTXST, a
; TX state variable
BCRHI, a
; RX bit timer MSB
BCTHI, a
; TX bit timer MSB
SES
; Disable user-level interrupt support
SREN
; Disable SW UART receiver
TXBSY
; clear TXBSY indicator
SRI
; clear RX complete indicator
STI
; clear TX complete indicator
ret
;----------------------------------------------------------------------------; SW_UART_Enable
;
; The SW_UART is enabled by enabling the interrupt handlers that move the transmit
; and receive state machines from their IDLE states to their corresponding next
; states. /INT0 transitions the RX state machine from IDLE to START. Timer1,
; which is called explicitly by the user code (setb TF1), transitions the
; transmit state machine from IDLE/START to TX_LSB.
32
Rev. 1.1
AN115
;
; The user-level interrupt (IE7) is enabled in the state machines themselves
; after polling EIS (external interrupt support).
;
SW_UART_Enable:
clr
IE0
; clear pending /INT0 interrupts
setb
TF1
; Force a pending Timer1 interrupt
setb
EX0
; enable /INT0 interrupts
clr
ET1
; keep Timer1 interrupts disabled
setb
TR1
; Enable Timer1
ret
;----------------------------------------------------------------------------; SW_UART_Disable
;
; The SW UART is disabled by disabling all of its state machine interrupts,
; including the user-level interrupt (IE7), if the status register indicates that
; it's enabled.
SW_UART_Disable:
clr
EX0
; disable /INT0 interrupts
clr
ET0
; disable Timer0 interrupts
clr
ET1
; disable Timer1 interrupts
jnb
SES, SW_UART_Dis_End; check to see if IE7 use is enabled
anl
EIE2, #NOT(00100000b); disable IE7 interrupts
SW_UART_Dis_End:
ret
;----------------------------------------------------------------------------; End of file.
; End of Example 2, Software UART with Timer 0.
END
Rev. 1.1
33
AN11 5
Contact Information
Silicon Laboratories Inc.
4635 Boston Lane
Austin, TX 78735
Tel: 1+(512) 416-8500
Fax: 1+(512) 416-9669
Toll Free: 1+(877) 444-3032
Email: [email protected]
Internet: www.silabs.com
The information in this document is believed to be accurate in all respects at the time of publication but is subject to change without notice.
Silicon Laboratories assumes no responsibility for errors and omissions, and disclaims responsibility for any consequences resulting from
the use of information included herein. Additionally, Silicon Laboratories assumes no responsibility for the functioning of undescribed features
or parameters. Silicon Laboratories reserves the right to make changes without further notice. Silicon Laboratories makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Silicon Laboratories assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation consequential or incidental damages. Silicon Laboratories products are not designed, intended, or authorized for use in applications intended to
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34
Rev. 1.1