blink.c

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

#define LED_ON _BV(PORTB5)
#define LED_OFF ~_BV(PORTB5);

int main () {
  DDRB |= _BV(DDB5);

  while(1) {
    PORTB |= LED_ON;
    _delay_ms(300);

    PORTB &= LED_OFF;
    _delay_ms(300);
  }
}