avrinput/usart.c

#include <avr/io.h>
#include <stdlib.h>
#include "usart.h"
#include "conf.h"
#include "ringbuffer.h"
#include <avr/interrupt.h>

volatile ringbuffer_t usart_tx_buffer;


void
usart_init(void)
{
	usart_tx_buffer = ringbuffer_new(USART_BUFFER_SIZE);
	UBRR0H = (unsigned char)(UBRR_VALUE >> 8);
	UBRR0L = (unsigned char)UBRR_VALUE;
#ifdef USART_U2X_EN
	UCSR0A |= _BV(U2X0);
#endif
	UCSR0B = _BV(TXEN0) | _BV(TXCIE0);
	UCSR0C = _BV(UCSZ00) | _BV(UCSZ01);
	sei();
}

int
usart_putc(char c)
{
	// Basically this is really ugly, 
	// but there is no other way to do it.
	// We let the main thread wait until the
	// sender thread has cleared one byte. 
	while(ringbuffer_put_char(usart_tx_buffer, c));

	// If the data register is already emtpy there is no
	// need to wait for the interrupt.
	// Also it is likely that the interrupt will never happen.
	// (For instance because this is the first byte sent)
	if(UCSR0A & _BV(UDRE0))
	{
		UDR0 = ringbuffer_get_char(usart_tx_buffer);
	}
	return 0;
}


ISR(USART_TX_vect)
{
	if(ringbuffer_nonempty(usart_tx_buffer))
	{
		UDR0 = ringbuffer_get_char(usart_tx_buffer);
	}
}