void setup() {
  TCCR1A = 0;
  TCCR1B = 0;
  bitSet(TCCR1B, CS12);  // 256 prescaler
  bitSet(TIMSK1, TOIE1); // timer overflow interrupt
  pinMode(LED_BUILTIN, OUTPUT);
}

ISR(TIMER1_OVF_vect) {
  digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
}

void loop() {
}

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