Goodbye delay(), hello millis()

Getting rid of the delay

Using the delay() function is nice, but the more we want to do, the more it just brings us problems. During a delay() call, you can’t respond to inputs, you can't process any data and you can’t change any outputs. The delay() ties up 100% of the processor. So, if any part of your code uses a delay(), everything else is dead in the water for the duration. So let's see how we can go on without using it:)

Usng millis()

One simple technique for implementing timing is to make a schedule and keep an eye on the clock. Instead of a world-stopping delay, you just check the clock regularly so you know when it is time to act. Meanwhile the processor is still free for other tasks to do their thing. A very simple example of this is the BlinkWithoutDelay example sketch that comes with the IDE.

/*
  Blink without Delay

  Turns on and off a light emitting diode (LED) connected to a digital pin,
  without using the delay() function. This means that other code can run at the
  same time without being interrupted by the LED code.

  The circuit:
  - Use the onboard LED.
  - Note: Most Arduinos have an on-board LED you can control. On the UNO, MEGA
    and ZERO it is attached to digital pin 13, on MKR1000 on pin 6. LED_BUILTIN
    is set to the correct LED pin independent of which board is used.
    If you want to know what pin the on-board LED is connected to on your
    Arduino model, check the Technical Specs of your board at:
    https://www.arduino.cc/en/Main/Products

  created 2005
  by David A. Mellis
  modified 8 Feb 2010
  by Paul Stoffregen
  modified 11 Nov 2013
  by Scott Fitzgerald
  modified 9 Jan 2017
  by Arturo Guadalupi

  This example code is in the public domain.

  http://www.arduino.cc/en/Tutorial/BlinkWithoutDelay
*/

// constants won't change. Used here to set a pin number:
const int ledPin =  3;// the number of the LED pin

// Variables will change:
int ledState = LOW;             // ledState used to set the LED

// Generally, you should use "unsigned long" for variables that hold time
// The value will quickly become too large for an int to store
unsigned long previousMillis = 0;        // will store last time LED was updated

// constants won't change:
const long interval = 1000;           // interval at which to blink (milliseconds)

void setup() {
  // set the digital pin as output:
  pinMode(ledPin, OUTPUT);
}

void loop() {
  // here is where you'd put code that needs to be running all the time.

  // check to see if it's time to blink the LED; that is, if the difference
  // between the current time and last time you blinked the LED is bigger than
  // the interval at which you want to blink the LED.
  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= interval) {
    // save the last time you blinked the LED
    previousMillis = currentMillis;

    // if the LED is off turn it on and vice-versa:
    if (ledState == LOW) {
      ledState = HIGH;
    } else {
      ledState = LOW;
    }

    // set the LED with the ledState of the variable:
    digitalWrite(ledPin, ledState);
  }
}

State + Machine = State Machine
Note that we have variables to keep track of whether the LED is ON or OFF. And variables to keep track of when the last change happened. That is the State part of the State Machine.

We also have code that looks at the state and decides when and how it needs to change. That is the Machine part. Every time through the loop we ‘run the machine’ and the machine takes care of updating the state.

For the advanced group

For the advanced people, write a sketch where using millis() you can change the times of the on and off times separately.

another way

Ok, lets make it a bit more efficient
A variant that has a different on-time and off-time.