int ledState;
int ledPin = 4;
void setup() {
  // put your setup code here, to run once:
  pinMode(ledPin, OUTPUT);        
  cli();                    
  TCCR1A = 0;                 
  TCCR1B = 0;                  
  TCCR1B |= B00000100;        
  TIMSK1 |= B00000010;         
  OCR1A = 31250;               
  sei();                     
  }
void loop() { 
  // put your main code here, to run repeatedly:
}
ISR(TIMER1_COMPA_vect){
  TCNT1  = 0;                  
  ledState = !ledState;      
  digitalWrite(ledPin,ledState);  
}