avr_demo/hello_uart/uart.c

#include <avr/io.h>
#include <stdio.h>

#ifndef BAUD
#define BAUD 9600
#endif
#include <util/setbaud.h>

/* http://www.cs.mun.ca/~rod/Winter2007/4723/notes/serial/serial.html */

void uart_init(void) {
    UBRR0H = UBRRH_VALUE;
    UBRR0L = UBRRL_VALUE;

    UCSR0C = _BV(UCSZ01) | _BV(UCSZ00); /* 8-bit data */ 
    UCSR0B = _BV(RXEN0) | _BV(TXEN0);   /* Enable RX and TX */    
}

int uart_putchar(char c, FILE *stream) {
    if (c == '\n') {
        uart_putchar('\r', stream);
    }
    loop_until_bit_is_set(UCSR0A, UDRE0);
    UDR0 = c;
    return 0;
}

int uart_getchar(FILE *stream) {
    loop_until_bit_is_set(UCSR0A, RXC0);
    return UDR0;
}