USART test. Atmega8. WITH interrupts.

#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>

#define XTAL 8000000L
#define BAUDRATE 9600L
#define BAUDDIVIDER (XTAL/(16*BAUDRATE)-1)

#define BUFFER_SIZE 16

void init(void);
void initTransmition(void);

char buffer[BUFFER_SIZE] = "0123456789ABCDEF";
unsigned char bufferIndex = 0;

int main(void) {
    init();
    sei();

    while(1) {
        if(PINB & (1 << PINB0)) {
            initTransmition();
        }
        PORTB = 0b00000000;
        _delay_ms(1000);
        PORTB = 0b10000000;
        _delay_ms(1000);
    }

    return 0;

}

void init(void) {
    DDRB = 0b11111110;
    PORTB = 0x00;
    UBRRH = BAUDDIVIDER >> 8;
    UBRRL = BAUDDIVIDER & 0xFF;

    UCSRA = 0;
    UCSRB = (1 << RXEN) | (1 << TXEN) | (0 << RXCIE) | (0 << TXCIE) | (0 << UDRIE);
    UCSRC = (1 << URSEL) | (1 << UCSZ0) | (1 << UCSZ1);
}

void initTransmition(void) {
    bufferIndex = 0;
    UDR = buffer[0];
    UCSRB |= (1 << UDRIE);
}

ISR(USART_UDRE_vect) {
    bufferIndex++;
    if(bufferIndex == BUFFER_SIZE) {
        UCSRB &= ~(1 << UDRIE);
    } else {
        UDR = buffer[bufferIndex];
    }
}