void setup() {
  DDRD = DDRD | (1 << 2);
  TCCR1A = 0;
  TCCR1B = 0;
  TCCR1B = 0b00000011;
  OCR1A = 25000;
  TIMSK1 |= B00000010;
}

ISR(TIMER1_COMPA_vect) {
  OCR1A += 25000;
  PORTD ^= 1 << 2;
}

void loop() {
  // do nothing.
}