// demo-Code non-blocking timing

unsigned long MyBlinkTimer = 0;                   // Timer-variables MUST be of type unsigned long
const byte    OnBoard_LED = 13;

byte ledState = LOW;


void setup() {
  Serial.begin(115200);
  Serial.println("Setup-Start");
  pinMode(OnBoard_LED,OUTPUT);
}


void loop() {

  // check if 500 milliseconds have passed by since last time 
  // function TimePeriodIsOver resulted in "true"
  if ( TimePeriodIsOver(MyBlinkTimer,500) ) {
    // when REALLY 500 milliseconds have passed by
    if (ledState == LOW) {
      ledState = HIGH; // change to HIGH
    }
    else {            // ledState is HIGH
      ledState = LOW; // change to LOW      
    }

    digitalWrite(OnBoard_LED,ledState);
  }  

}

// easy to use helper-function for non-blocking timing
boolean TimePeriodIsOver (unsigned long &startOfPeriod, unsigned long TimePeriod) {
  unsigned long currentMillis  = millis();
  if ( currentMillis - startOfPeriod >= TimePeriod ) {
    // more time than TimePeriod has elapsed since last time if-condition was true
    startOfPeriod = currentMillis; // a new period starts right here so set new starttime
    return true;
  }
  else return false;            // actual TimePeriod is NOT yet over
}