How to Half-Duplex Int Driven UART (AVR ASM)

CODE

;**** A P P L I C A T I O N N O T E A V R 3 0 5 ************************
;*
;* Title : Half Duplex Interrupt Driven Software UART
;* Version : 1.1
;* Last updated : 97.08.27
;* Target : AT90Sxxxx (All AVR Device)
;*
;* Support email : avr@atmel.com
;*
;* Code Size : 32 Words
;* Low Register Usage : 0
;* High Register Usage : 4
;* Interrupt Usage : None
;*
;* DESCRIPTION
;* This Application note contains a very code efficient software UART.
;* The example program receives one character and echoes it back.
;***************************************************************************

.include "1200def.inc"

;***** Pin definitions

.equ RxD =0 ;Receive pin is PD0
.equ TxD =1 ;Transmit pin is PD1

;***** Global register variables

.def bitcnt =R16 ;bit counter
.def temp =R17 ;temporary storage register

.def Txbyte =R18 ;Data to be transmitted
.def RXbyte =R19 ;Received data

.cseg
.org 0

;***************************************************************************
;*
;* "putchar"
;*
;* This subroutine transmits the byte stored in the "Txbyte" register
;* The number of stop bits used is set with the sb constant
;*
;* Number of words :14 including return
;* Number of cycles :Depens on bit rate
;* Low registers used :None
;* High registers used :2 (bitcnt,Txbyte)
;* Pointers used :None
;*
;***************************************************************************
.equ sb =1 ;Number of stop bits (1, 2, ...)

putchar: ldi bitcnt,9+sb;1+8+sb (sb is # of stop bits)
com Txbyte ;Inverte everything
sec ;Start bit

putchar0: brcc putchar1;If carry set
cbi PORTD,TxD; send a '0'
rjmp putchar2;else

putchar1: sbi PORTD,TxD; send a '1'
nop

putchar2: rcall UART_delay;One bit delay
rcall UART_delay

lsr Txbyte ;Get next bit
dec bitcnt ;If not all bit sent
brne putchar0; send next
;else
ret ; return

;***************************************************************************
;*
;* "getchar"
;*
;* This subroutine receives one byte and returns it in the "Rxbyte" register
;*
;* Number of words :14 including return
;* Number of cycles :Depens on when data arrives
;* Low registers used :None
;* High registers used :2 (bitcnt,Rxbyte)
;* Pointers used :None
;*
;***************************************************************************

getchar: ldi bitcnt,9;8 data bit + 1 stop bit

getchar1: sbic PIND,RxD;Wait for start bit
rjmp getchar1

rcall UART_delay;0.5 bit delay

getchar2: rcall UART_delay;1 bit delay
rcall UART_delay

clc ;clear carry
sbic PIND,RxD;if RX pin high
sec ;

dec bitcnt ;If bit is stop bit
breq getchar3; return
;else
ror Rxbyte ; shift bit into Rxbyte
rjmp getchar2; go get next

getchar3: ret

;***************************************************************************
;*
;* "UART_delay"
;*
;* This delay subroutine generates the required delay between the bits when
;* transmitting and receiving bytes. The total execution time is set by the
;* constant "b":
;*
;* 3搓 + 7 cycles (including rcall and ret)
;*
;* Number of words :4 including return
;* Low registers used :None
;* High registers used :1 (temp)
;* Pointers used :None
;*
;***************************************************************************
; Some b values: (See also table in Appnote documentation)
;
; 1 MHz crystal:
; 9600 bps - b=14
; 19200 bps - b=5
; 28800 bps - b=2
;
; 2 MHz crystal:
; 19200 bps - b=14
; 28800 bps - b=8
; 57600 bps - b=2

; 4 MHz crystal:
; 19200 bps - b=31
; 28800 bps - b=19
; 57600 bps - b=8
; 115200 bps - b=2

.equ b =31;19200 bps @ 4 MHz crystal

UART_delay: ldi temp,b
UART_delay1: dec temp
brne UART_delay1

ret

;***** Program Execution Starts Here

;***** Test program

reset: sbi PORTD,TxD;Init port pins
sbi DDRD,TxD

ldi Txbyte,12;Clear terminal
rcall putchar

forever: rcall getchar
mov Txbyte,Rxbyte
rcall putchar ;Echo received char
rjmp forever