;**** 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		: rev. 1.2 (24-04-2002), reset vector added
;*			: rev. 1.1 (27.08.1997)
;* Last updated		: 24-04-2002
;* 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.
;***************************************************************************


;***** Pin definitions

;RxD	= 0x01			;Receive pin is PD0
.equ TxD	= 0x07			;Transmit pin is PA0

;***** 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
;	rjmp	reset
;***************************************************************************
;*
;* "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	= 0x01		;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	PORTB,TxD	;    send a '0'
		rjmp	putchar2	;else	

putchar1:	sbi	PORTB,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





;***************************************************************************
;*
;* "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·b + 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	=66	;  66 = 9600 bps @ 4 MHz crystal


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

		ret

;***** Program Execution Starts Here

;***** Test program