byte state = 0;
void setup() {
cli();
TCCR1A = 0;
TCCR1B = 0;
TCCR1B |= B00000001;
TIMSK1 |= B00000010;
OCR1A = 15624;
sei();
}

void loop(){
  digitalWrite(LED_BUILTIN,state);
}

ISR(TIMER1_COMPA_vect){
TCNT1 = 0;
state = !state;
}