void setup() {
  cli(); // Disable global interrupts
  
  // Set up the LED pin as output
  DDRB |= (1 << DDB5); // Set pin 13 as output
  
  // Configure Timer1 for Normal mode
  TCCR1A = 0; // Set entire TCCR1A register to 0, ensure WGM10 and WGM11 are 0 for Normal mode
  TCCR1B = 0; // Ensure WGM12 and WGM13 are 0 for Normal mode
  
  // In Normal mode, the timer overflows from 65535 back to 0. Let's use a prescaler to adjust the timing.
  // Set CS12 bit for 256 prescaler, the closest we can easily configure for a 1-second overflow without external calculations.
  TCCR1B |= (1 << CS12);
  
  // Enable timer overflow interrupt for Timer1
  TIMSK1 |= (1 << TOIE1);
  
  sei(); // Enable global interrupts
}

ISR(TIMER1_OVF_vect) {
  // ISR called upon timer overflow
  // Toggle the LED connected to pin 13
  PORTB ^= (1 << PORTB5);
}

void loop() {
  // Nothing needed here - the ISR handles the LED blinking
}