[ARDUINO] hello world nie działa

#define F_CPU 16000000
#define BAUD 9600
#define PRESCALE (((F_CPU/16)/BAUD) -1)

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

int main (void)
{
    uint8_t data;
    char helloworld[] = "Hello World!";
    int i = 0;

    UCSR0C = 9;
    UCSR0B = 24;

    UBRR0 = PRESCALE;

    for (;;){

        while ( !(UCSR0A & (1<<RXC0)) );
        data = UDR0; //odczytanie
        while ( !( UCSR0A & (1<<UDRE0)) );
        UDR0 = (int)helloworld[i]; //wysyłanie spowrotem

        i++; if (i > strlen(helloworld)) { i = 0; }
    }
    return 0;
}