serial.S

/* Optimized AVR305 half-duplex serial uart implementation
 * timing for 81N, 115.2kbps @8Mhz = 69.4 cycles/bit
 * and @16Mhz = 138.9 cycles/bit
 * @author: Ralph Doncaster
 * @version: $Id$
 *
 * http://forum.arduino.cc/index.php?topic=207467.0
 */

#include <avr/io.h>
; correct for avr/io.h 0x20 port offset for io instructions
#define UART_Port (PORTB-0x20)
#define UART_Tx PB4

#define bitcnt r18
#define delayArg 19

#if F_CPU == 1200000
#define TXDELAY 38 ; 9600 Baud
#elif F_CPU == 9600000L
#define TXDELAY 25 ; 115200 Baud
#else
#error
#endif


.global TxByte
; transmit byte in r24 - 15 instructions
; calling code must set Tx line to idle state (high) or 1st byte may be lost
; i.e. PORTB |= (1<<UART_Tx)
TxByte:
 cli
        sbi UART_Port-1, UART_Tx               ; set Tx line to output
        ldi bitcnt, 10                              ; 1 start + 8 bit + 1 stop
        com r24                                     ; invert and set carry
TxLoop:
        ; 10 cycle loop + delay
        brcc tx1
        cbi UART_Port, UART_Tx                  ; transmit a 0
tx1:
        brcs TxDone
        sbi UART_Port, UART_Tx                  ; transmit a 1
TxDone:
        ldi delayArg, TXDELAY
TxDelay:
; delay (3 cycle * delayArg) -1
        dec delayArg
        brne TxDelay
        lsr r24
        dec bitcnt
        brne TxLoop
 reti    ; return and enable interrupts