const byte LEDred = 9;
const byte LEDyel = 10;
const byte LEDgrn = 11;
boolean muxit = false;
volatile byte LEDState = HIGH;

void setup() {
  cli();
  //set timer1 interrupt at 1 Hz
  TCCR1A = 0;// set entire TCCR1A register to 0
  TCCR1B = 0;// same for TCCR1B
  TCNT1  = 0;//initialize counter value to 0
  // set compare match register for 1hz increments
  OCR1A = 15624;// = (16*10^6) / (1*1024) - 1 (must be <65536)
  // turn on CTC mode
  TCCR1B |= (1 << WGM12);
  // Set CS10 and CS12 bits for 1024 prescaler
  TCCR1B |= (1 << CS12) | (1 << CS10);  
  // enable timer compare interrupt
  TIMSK1 |= (1 << OCIE1A);
  sei();
  pinMode(LEDred, OUTPUT);
  pinMode(LEDyel, OUTPUT);
  pinMode(LEDgrn, OUTPUT);
}

void loop() {
if (muxit == true) {
  muxit = false;
  digitalWrite(LEDred, HIGH);
  delay(333);
  digitalWrite(LEDred, LOW);
  digitalWrite(LEDyel, HIGH);
  delay(333);
  digitalWrite(LEDyel, LOW);
  digitalWrite(LEDgrn, HIGH);
  delay(333);
  digitalWrite(LEDgrn, LOW);
    }
}

ISR(TIMER1_COMPA_vect) {//timer1 interrupt 1Hz toggles pin 13 (LED) //generates pulse wave of frequency 1Hz/2 = 0.5kHz (takes two cycles for full wave- toggle high then toggle low)
muxit = true;
// Serial.println(muxit);

}