void setup() {
  DDRB |= B00100000;  // Set pin 13 (PB5) to output using direct port manipulation
  // Equivalent to pinMode(13, OUTPUT), but faster and more efficient.

  cli();              // Disable global interrupts during timer setup

  TCCR1A = 0;         // Clear Timer1 control register A
  TCCR1B = 0;         // Clear Timer1 control register B

  OCR1A = 15624;      // Set Compare Match value for 1-second intervals
  // Calculation:
  // Target time = 1 second
  // Prescaler = 1024
  // Timer frequency = 16 MHz / 1024 = 15625 Hz
  // OCR1A = (Target time × Timer frequency) - 1 = (1 × 15625) - 1 = 15624

  TCCR1B |= (1 << WGM12);  // Configure Timer1 in CTC (Clear Timer on Compare Match) mode

  TCCR1B |= (1 << CS10);   // Set prescaler bits CS10 and CS12 for 1024 prescaler
  TCCR1B |= (1 << CS12);

  TIMSK1 |= (1 << OCIE1A); // Enable Timer1 Compare Match A interrupt

  sei();              // Re-enable global interrupts
}

void loop() {
  // Main program code can run here, unaffected by the LED blinking
}

ISR(TIMER1_COMPA_vect) {
  PORTB ^= B00100000; // Toggle pin 13 using XOR
}