uart.c

/**
 * Copyright (c) 2017, Łukasz Marcin Podkalicki <lpodkalicki@gmail.com>
 * The ASM code is based on Ralph Doncaster's project (https://github.com/nerdralph/nerdralph/tree/master/avr/libs/bbuart)
 * Software UART for ATtiny13
 */

#include <avr/interrupt.h>
#include "uart.h"

char
uart_getc(void)
{
#ifdef	UART_RX_ENABLED
	char c;
	uint8_t sreg;

	sreg = SREG;
	cli();
	PORTB &= ~(1 << UART_RX);
	DDRB &= ~(1 << UART_RX);
	__asm volatile(
		" ldi r18, %[rxdelay2] \n\t" // 1.5 bit delay
		" ldi %0, 0x80 \n\t" // bit shift counter
		"WaitStart: \n\t"
		" sbic %[uart_port]-2, %[uart_pin] \n\t" // wait for start edge
		" rjmp WaitStart \n\t"
		"RxBit: \n\t"
		// 6 cycle loop + delay - total = 5 + 3*r22
		// delay (3 cycle * r18) -1 and clear carry with subi
		" subi r18, 1 \n\t"
		" brne RxBit \n\t"
		" ldi r18, %[rxdelay] \n\t"
		" sbic %[uart_port]-2, %[uart_pin] \n\t" // check UART PIN
		" sec \n\t"
		" ror %0 \n\t"
		" brcc RxBit \n\t"
		"StopBit: \n\t"
		" dec r18 \n\t"
		" brne StopBit \n\t"
		: "=r" (c)
		: [uart_port] "I" (_SFR_IO_ADDR(PORTB)),
		[uart_pin] "I" (UART_RX),
		[rxdelay] "I" (RXDELAY),
		[rxdelay2] "I" (RXDELAY2)
		: "r0","r18","r19"
	);
	SREG = sreg;
	return c;
#else
	return (-1);
#endif /* !UART_RX_ENABLED */
}

void
uart_putc(char c)
{
#ifdef	UART_TX_ENABLED
	uint8_t sreg;

	sreg = SREG;
	cli();
	PORTB |= 1 << UART_TX;
	DDRB |= 1 << UART_TX;
	__asm volatile(
		" cbi %[uart_port], %[uart_pin] \n\t" // start bit
		" in r0, %[uart_port] \n\t"
		" ldi r30, 3 \n\t" // stop bit + idle state
		" ldi r28, %[txdelay] \n\t"
		"TxLoop: \n\t"
		// 8 cycle loop + delay - total = 7 + 3*r22
		" mov r29, r28 \n\t"
		"TxDelay: \n\t"
		// delay (3 cycle * delayCount) - 1
		" dec r29 \n\t"
		" brne TxDelay \n\t"
		" bst %[ch], 0 \n\t"
		" bld r0, %[uart_pin] \n\t"
		" lsr r30 \n\t"
		" ror %[ch] \n\t"
		" out %[uart_port], r0 \n\t"
		" brne TxLoop \n\t"
		:
		: [uart_port] "I" (_SFR_IO_ADDR(PORTB)),
		[uart_pin] "I" (UART_TX),
		[txdelay] "I" (TXDELAY),
		[ch] "r" (c)
		: "r0","r28","r29","r30"
	);
	SREG = sreg;
#endif /* !UART_TX_ENABLED */
}

void uart_putu(uint16_t x)
{
        char buff[8] = {0};
        char *p = buff+6;
        do { *(p--) = (x % 10) + '0'; x /= 10; } while(x);
        uart_puts((const char *)(p+1));
}

void
uart_puts(const char *s)
{
     	while (*s) uart_putc(*(s++));
}