#ifndef F_CPU
#define F_CPU 1000000UL // or whatever may be your frequency
#endif

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

int main(void)
{
    DDRD = 0b00000100;                       // initialize port D
    while(1)
    {
        // LED on
        PORTD = 0b00000100;            // PD2 = High = Vcc
        _delay_ms(500);                // wait 500 milliseconds

        //LED off
        PORTD = 0b00000000;            // PD2 = Low = 0v
        _delay_ms(500);                // wait 500 milliseconds
    }
}