#include <avr/io.h>
#include <avr/interrupt.h>

const int ledPin = 13;

volatile bool ledState = LOW;

void setupTimer() {
  cli();
  
  TCCR1A = 0; 
  TCCR1B = (1 << WGM12) | (1 << CS12); 
  OCR1A = 62499;
  
  TIMSK1 = (1 << OCIE1A);
  
  sei();
}

void setup() {
  pinMode(ledPin, OUTPUT);
  setupTimer();
}

void loop() {

}

ISR(TIMER1_COMPA_vect) {

  ledState = !ledState;
  digitalWrite(ledPin, ledState);
}