#ifndef F_CPU
#define F_CPU 16000000UL // or whatever may be your frequency
#endif

#include <avr/io.h>
//#include <avr/uart.h>
#include <util/delay.h> // for _delay_ms()

int main(void) {
  DDRD = 0xff;
  while (1) {
    PORTD = 0xff;
    _delay_ms(100);

    PORTD = 0x00;
    _delay_ms(100);
  }
}