#define F_CPU 16000000UL // 16 MHz

#include <avr/io.h> // header file file for input output pins
#include <util/delay.h> // header file for delay.

int main (void)
{
  DDRD |= _BV(DDD3);          //set pin D3 of PORTD as output
                              //DDRx; x is the port 
                              //DDxx; xx is the pin#
  while (1) {
    PORTD |= _BV(PORTD3);    //set pin D3 high to turn led on
    _delay_ms(1000);         //delay 1 second

    PORTD &= ~_BV(PORTD3);  // set pin D3 low to turn led off
    _delay_ms(1000);         //delay 1 second.
  }
}