#include <avr/io.h>
#include <avr/interrupt.h>

#ifndef _BV
#define _BV(x) (1 << x)
#endif

#define F_CPU 16000000

bool led;
ISR(TIMER1_COMPA_vect){
  TCNT1  = 0;
  led = !led;
}

int main(void){
  DDRD |= _BV(6);

  cli();                    
  TCCR1A = 0;                 
  TCCR1B = 0; 
  TCCR1B |= 4;
  TIMSK1 |= 2;        
  OCR1A = 62500;             
  sei();      
  
  while(1){
    if(led){
      PORTD |= _BV(6);
    } else {
      PORTD &= ~_BV(6);
    }
  }
  return 0;
}