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

int main(void)
{
    /* PB4 output */
    DDRD  |=  _BV(PB4);
    PORTB &= ~_BV(PB4);

    for (;;)
    {
        _delay_ms(1000);
        PORTB ^= _BV(PB4);
    }

    return 0;
}