millis() Tutorial: Arduino Multitasking

After working through these exercises, check out this article on how to avoid rollover or reset mills().

After learning how to flash a single LED on your Arduino, you are probably looking for a way to make cool patterns, but feel limited by the use of delay(). If you ask in the forums, you get told to look at the “Blink Without Delay” example. This example introduces the idea of replacing delay() with a state machine. If you’re confused how to use it, this tutorial is setup to take you from blinking two LEDs with delay, to using an alternate method, right down to how you can use millis().

The millis() function is one of the most powerful functions of the Arduino library. This function returns the number of milliseconds the current sketch has been running since the last reset. At first, you might be thinking, well that’s not every useful! But consider how you tell time during the day. Effectively, you look at how many minutes have elapsed since midnight. That’s the idea behind millis()!

Instead of “waiting a certain amount of time” like you do with delay(), you can use millis() to ask “how much time has passed”? Let’s start by looking at a couple of ways you can use delay() to flash LEDs.

Example #1: Basic Delay

You are probably already familiar with this first code example The Arduino IDE includes this example as “Blink.”

void setup() {
   pinMode(13, OUTPUT);
}

void loop() {
   digitalWrite(13, HIGH);   // set the LED on
   delay(1000);              // wait for a second
   digitalWrite(13, LOW);    // set the LED off
   delay(1000);              // wait for a second
}

Reading each line of loop() in sequence the sketch:

  1. Turns on Pin 13’s LED,
  2. Waits 1 second (or 1000milliseconds),
  3. Turns off Pin 13’s LED and,
  4. Waits 1 second.
  5. Then the entire sequence repeats.

The potential issue is that while you are sitting at the delay(), your code can’t be doing anything else. So let’s look at an example where you aren’t “blocking” for that entire 1000 milliseconds.

Example #2: Basic Delay with for() loops

For our 2nd example, we are only going to delay for 1ms, but do so inside of a for() loop.

void setup() {
   pinMode(13, OUTPUT);
}

void loop() {
   digitalWrite(13, HIGH);   // set the LED on
   for (int x=0; x < 1000; x++) {     // Wait for 1 second
      delay(1);
   }
   digitalWrite(13, LOW);   // set the LED on
   for (int x=0; x < 1000; x++) {     // Wait for 1 second
      delay(1);
   }
}

This new sketch will accomplish the same sequence as Example #1. The difference is that the Arduino is only “delayed” for one millisecond at a time. A clever trick would be to call other functions inside of that for() loop. However, your timing will be off because those instructions will add additional delay.

Example #3: for() loops with 2 LEDs

In this example, we’ve added a second LED on Pin 12 (with a current limiting resistor!). Now let’s see how we could write the code from Example #2 to flash the 2nd LED.

void setup() {
   pinMode(13, OUTPUT);
   pinMode(12, OUTPUT);
}

void loop() {
   digitalWrite(13, HIGH);   // set the LED on
   for (int x=0; x < 1000; x++) {             // wait for a secoond
      delay(1);
      if (x==500) {
         digitalWrite(12, HIGH);
      }
   }
   digitalWrite(13, LOW);    // set the LED off
   for (int x=0; x < 1000; x++) {             // wait for a secoond
      delay(1);
      if (x==500) {
            digitalWrite(12, LOW);
      }
   }
}

Starting with the first for() loop, while Pin 13 is high, Pin 12 will turn on after  500 times through the for() loop.  It would appear that Pin 13 and Pin 12 were flashing in Sequence.  Pin 12 would turn on 1/2 second after Pin 13 turns on.  1/2 second later Pin 13 turns off, followed by Pin 12 another 1/2 second.

This code is pretty complicated, isn’t it?

If you wanted to add other LEDs or change the sequence, you have to start getting ingenious with all of the if-statements. Just like example #2, the timing of these LEDs is going to be off. The if() statement and digitalWrite() function all take time, adding to the “delay()”.

Now let’s look at how millis() gets around this problem.

The millis() Function

Going back to the definition of the millis() function: it counts the number of milliseconds the sketch has been running.

Step back and think about that for a second. In Example #3, we are trying to flash LEDs based on a certain amount of time. Every 500ms we want one of the LEDs to do something different. So what if we write the code to see how much time as passed, instead of, waiting for time to pass?

Example #4: Blink with millis()

This code is the same “Blink” example from #1 re-written to make use of millis(). A slightly more complicated design, because you have to include a couple of more variables. One to know how long to wait, and one to know the state of LED on Pin 13 needs to be.

unsigned long interval=1000; // the time we need to wait
unsigned long previousMillis=0; // millis() returns an unsigned long.

bool ledState = false; // state variable for the LED

void setup() {
 pinMode(13, OUTPUT);
 digitalWrite(13, ledState);
}

void loop() {
 unsigned long currentMillis = millis(); // grab current time

 // check if "interval" time has passed (1000 milliseconds)
 if ((unsigned long)(currentMillis - previousMillis) >= interval) {
 
   ledState = !ledState; // "toggles" the state
   digitalWrite(13, ledState); // sets the LED based on ledState
   // save the "current" time
   previousMillis = millis();
 }
}

Let’s look at the code from the very beginning.

unsigned long interval=1000;     // the time we need to wait
unsigned long previousMillis=0;  // millis() returns an unsigned long.

millis() returns an unsigned long.

When using variables associated with millis() or micros(), ALWAYS declare them as an unsigned long

The variable interval is the amount of time we are going to wait. The variable previousMillis is used so we can see how long it has been since something happened.

bool ledState = false; // state variable for the LED

This code uses a variable for the state of the LED. Instead of directly writing a “HIGH” or “LOW” with digitalWrite(), we will write the value of this variable.

The setup() function is pretty standard, so let’s skip directly to the loop():

void loop() {
   unsigned long currentMillis = millis(); // grab current time

Each time loop() repeats, the first thing we do is grab the current value of millis(). Instead of repeated calls to millis(), we will use this time like a timestamp.

void loop() {
   if ((unsigned long)(currentMillis - previousMillis) >= interval) {

Holy cow does that look complicated! It really isn’t, so don’t be afraid to just “copy and paste” this one. First, it is an if()-statement. Second the (unsigned long)  isn’t necessary (but I’m not going to sit around for 49 days to make sure). Third “(currentMillis – previousMillis) >= interval)” is the magic.

What it boils down to, this code will only become TRUE after millis() is at least “interval” larger than the previously stored value of millis, in previousMillis. In other words, nothing in the if()-statement will execute until millis() gets 1000 milliseconds larger than previousMillis.

The reason we use this subtraction is that it will handle the roll-over of millis. You don’t need to do anything else.

  ledState = !ledState;

This line of code will set the value of ledState to the corresponding value. If it is true, it becomes false. If it is false, it becomes true.

digitalWrite(13, ledState); // sets the LED based on ledState

Instead of writing a HIGH or LOW directly, we are using a state variable for the LED. You’ll understand why in the next example.

previousMillis = millis();

The last thing you do inside of the if-statement is to set previousMillis to the current time stamp. This value allows the if-statement to track when at least interval (or 1000ms) has passed.

Example #5: Adding a 2nd LED with millis()

Adding a second flashing LED is straightforward with the millis() code. Duplicate the if-statement with a second waitUntil and LEDstate variable.

// each "event" (LED) gets their own tracking variable
unsigned long previousMillisLED12=0;
unsigned long previousMillisLED13=0;

// different intervals for each LED
int intervalLED12 = 500;
int intervalLED13 = 1000;

// each LED gets a state varaible
boolean LED13state = false;     // the LED will turn ON in the first iteration of loop()
boolean LED12state = false;     // need to seed the light to be OFF

void setup() {
   pinMode(13, OUTPUT);
   pinMode(12, OUTPUT);
}
void loop() {
   // get current time stamp
   // only need one for both if-statements
   unsigned long currentMillis = millis();

   // time to toggle LED on Pin 12?
   if ((unsigned long)(currentMillis - previousMillisLED12) >= intervalLED12) {
      LED12state = !LED12state;
      digitalWrite(12, LED12state);
      // save current time to pin 12's previousMillis
      previousMillisLED12 = currentMillis;
   }

// time to toggle LED on Pin 13?
  if ((unsigned long)(currentMillis - previousMillisLED13) >= intervalLED13) {
      LED13state = !LED13state;
      digitalWrite(13, LED13state);
      // save current time to pin 13's previousMillis
      previousMillisLED13 = currentMillis;
  }
}

This code has some small changes from the previous example. There are separate previousMillis and state variables for both pins 12 and 13.

To get an idea of what’s going on with this code change the interval values for each. Both LEDs will blink independently of each other.

Other Examples

Check out this list of millis() examples: https://www.baldengineer.com/millis-cookbook.html
There are examples to create a Police Light Strobe Effect, Time your Code, Change the ON-time and OFF-time for an LED, and others.

Conclusion

At first it seems that using the millis() function will make a sketch more complicated than using delay(). In some ways, this is true. However, the trade-off is that the sketch becomes significantly more flexible. Animations become much simpler. Talking to LCDs while reading buttons becomes no problem. Virtually, you can make your Arduino multitask.

After working through these exercises, check out this article on how to avoid rollover or reset mills().

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121 thoughts on “millis() Tutorial: Arduino Multitasking

  1. I’m trying to use a plurality of leds as a six string strobe tuner for guitar… Aparently this works with micros too, and I’m sure I’ll require the micro resolution… Rounding to the nearest 4th micro has me kinda spooked. Is there no way around this?

  2. I’m trying to get the code for 2 butons controlling 1 led(when they are pressed simultaneously the led turns on) end a buton, when is presed 2 times , turns a led on for a couple of seconds.My problem is that i ned them to work simultaneos, de delay() function esent ideal, the mili() function , i can not get my head around it.

    Here is my code:

     const int Pin5 = 5;    
     const int Pin6 = 6; 
     const int Pin3 = 3;
     
     const int ledPin = 9;       
     const int ledPin1 = 10;
     const int ledPin2 = 11;
     
     int buttonPushCounter = 0;   // counter for the number of button presses
     int buttonState = 0;         // current state of the button
     int lastButtonState = 0;     // previous state of the button
     
     
    void setup() {
       
       pinMode(ledPin, OUTPUT);
       pinMode(ledPin1, OUTPUT);
       pinMode(ledPin2, OUTPUT);
      
      pinMode(Pin5, INPUT);
       pinMode(Pin6, INPUT);
       pinMode(Pin3, INPUT);
       
      
     }
     
    void loop() {
       
       int Value = digitalRead(Pin5);
       int Value1 = digitalRead(Pin6);
       int Value2 = digitalRead(Pin3);
      buttonState = digitalRead(Pin3);
    
       if (Value == HIGH &amp; Value1 == HIGH)
       
       {
         digitalWrite(ledPin, HIGH);
         delay(2000);
       } 
       
      else 
      
      {
         digitalWrite(ledPin,LOW); 
      }
      
     if (Value2 != lastButtonState)
     
     {
         
         
         if (buttonState == HIGH)
        
         {
           
           buttonPushCounter++;
          
         } 
       
       lastButtonState = buttonState;
     
       if (buttonPushCounter % 2 == 0) 
       
       {
         digitalWrite(ledPin1, HIGH);
         delay(3000);
         digitalWrite(ledPin1, LOW);
         
       } 
      
       
    }
       
    }
    
    • You are really trying to combine two different things. Using the Button library might help with managing when or how the buttons are pressed. You should avoid variable names like “Value” and certainly don’t want to use conventions like “Value” and “Value1”. That makes it difficult to follow code later on because they don’t describe what you are trying to store. Variables like “LeftButtonValue” or “RedLED” are far more descriptive.

      Using millis() is all about creating a state machine. You need to constantly be checking what is happening with the buttons and reacting when they change. Then you need to have some state variables for your LEDs and constantly calling digitalWrite() using those state variables.

  3. I was wondering if you could help me? I would like to use use millis() with two different delays say light is on for 2 sec and off for 1 sec. On pin 13. Is this possible with millis? I’m sure it is im just not sure how to do it. Could you do an example like that? I would be very grateful.

    Thanks,

    Matt

  4. This is a great tutorial! I’ve got one quick suggestion for an addition though –

    The way it’s presently coded it always makes sure that at least one second passes between times the code executes, but that means that the actual cycle time is the time the code takes to execute plus one second, and if you’re doing something intensive elsewhere in the program this lost time can add up. If you change

    waitUntil = millis() + 1000;
    to
    waitUntil = waitUntil + 1000;

    then the program always tries to execute your code ‘on time.’ One iteration may start late if the program was doing something else, but that doesn’t delay every other time through the code.

  5. Hi James. Thanks for your tutorials. I am just starting with Arduino. I looked at the above sketch and added another LED which I got to flash at it’s own rate. But where I am stumped is I want it to flash twice quickly and then another led to flash twice quickly then back to the last LED. What I am going for is a police car type flasher where the headlights wig wag on their own and the blue and reds flash on their own and strobes flash on their own, normally all at different rates. I don’t expect you to write code for me but can you point me in the right direction?? I can’t seem to get a handle on this. But it’s a great learning tool and I look forward to cracking it finally! LOL…
    Thank you.

    • Hi Andrew, I think the key is going to be creating a State variable for each LED and tracking how many times the LED has strobed. Once you’ve strobed enough times, set a wait state for the amount of time the other LED will take to strobe. Does that make sense?

  6. No, the increment isn’t random. The same instructions are executed between the two calls every time.

    There is an accruacy issue with the code abive. For completeness the code should either: a) assign millis() a variable, check that variable, and increment waitUntilX based on that variable. Or b) adjust flashDelay to account for the time between instructions.

    In practice, if timing isn’t absolutely critical then the above method is okay.

  7. Hi,
    I am curious is the time changes between the fist call to millis() to the second call to millis() within the if statements. In this case there will be a random increment in the duration. Isn’t this true.
    Thanks,
    Sudarshan

  8. Could I send you a sketch using the mills() function? I have been working on the problem for days with no success. It involves the Leah Buechley turn signal sketch, which works fine unmodified. I’m trying to add “chasing light” to the turn signals.I think other delays are interfering with the mills() function.

    Thanks, Don

  9. Looks like the last example sketch comments do not agree with the values of the variables sequenceDelay (100 is referred to as 1000) and flashDelay (250 is referred to as 500).

    Or did I misunderstand?

    Thank you for the excellent tutorial.

    Chad

    • Chad, you are correct. The comments did not agree, the listing has been updated. In debugging the original code I started changing the Delays to make sure the lights (and the code) acted as I expected. I forgot to change them back before posting.

      It is an interesting exercise to change each on independently, to see how the code reacts differently. It is even more fun when you have a few more LEDs.