Limitations of delay() & How to Do Timers Correctly

Arduino IDE

Arduino Uno Rev3

<h3 >Delay(): uses and limitations</h3>First ExamplePush one button and turn a LED ON for 15 secs (15000 milliseconds)(Note: This is pseudo code, ignoring the syntax of &#34;{&#34; and so on,  concentrating on the flow of the code. Also we are assuming a perfect button without contact bounce. As it happens, most examples here will work with a &#34;noisy&#34; button, as the LED timing hides the bounce.)<pre ><code >if digitalread==Down then digitalWrite ON delay 15sec digitalWrite OFF </code></pre>Results: It works, yes, but looking closely: The button can change up or down as much as you like in the 15 seconds - it simply is not looked at. The delay starts counting when button is pushed. If the button is still down when 15 seconds expire, the LED will blink Off for a very short time (the time it takes loop to start again). This may or may not be what you intended.Second ExampleThe same with two buttons and two LEDs. The intention is each button controls one LED, each staying lit for 15 seconds.<pre ><code >if digitalreadA==Down then digitalWriteA ON delay 15 sec digitalWriteA OFF if digitalreadB==Down then digitalWriteB ON delay 15 sec digitalWriteB OFF </code></pre>Result: This does NOT WORK the way it usually is intended - whilst A or B is on, we are totally ignoring the other button. This code can not turn on both LEDs no matter how you push the buttons.The code is not the problem either. As long as we are using <code >delay() </code> then no combination of <code >if then </code> and <code >digitalRead</code> will allow the choice of one or both LEDs by button pushes. This is why <code >delay()</code> is fatal and very wrong if you need to do more than one single thing at a time.<h3 >Using a millis() timer instead</h3>How to get similar delay. Timer is not a feature or function, it is simply comparing <code >millis()</code> value regularly to determine when the desired time has elapsed.Example 1: Implementing a simple timer<pre ><code >//Psudocode unsigned long Timer // ALWAYS use unsigned long for timers, not int (variable declaration outside setup and loop, of course) if digitalRead == Down then &#34;set timer&#34; Timer = millis digitalWrite ON if &#34;timer expired&#34; millis - Timer &gt;= 15000UL then digitalWrite Off (the &#34;UL&#34; after the number is a syntax detail that is important when dealing with large numbers in millis and micros, therefore it is shown although this is pseudo code) </code></pre>Result: It works just as well as the delay version, but slightly differently: The timer is constantly reset as long as the button is pushed. Thus only when releasing does it start to &#34;count&#34; the 15 seconds. If you push shortly inside the 15 seconds period, it starts counting from anew. Notice in particular that <code >digitalRead</code> is looking at the input as fast as <code >loop()</code> runs.The code is doing a <code >digitalWrite</code> OFF, even when the LED is not lit for every <code >loop()</code> pass, and likewise <code >digitalWrite</code> ON as long as the button is pushed. Fortunately this has no ill effects (apart from taking a handful of microseconds time).Example 2: Two buttons and LEDsThis is rather simple, we just &#34;repeat&#34; the above code.<pre ><code >//Psudocode unsigned long TimerA &#34;ALWAYS use unsigned long for timers...&#34; unsigned long TimerB if digitalReadA==Down then TimerA= millis() , digitalWriteA ON if millis()-TimerA &gt;= 15000UL then digitalWriteA Off if digitalReadB==Down then TimerB= millis() , digitalWriteB ON if millis()-TimerB &gt;= 15000UL then digitalWriteB Off </code></pre>Result: This works! Two separate timers start and stop independently.As shown in the single button/LED case, we are done with handling the A case in just a few tens of microseconds, whether we are counting seconds or not, and thus free to handle the B case. We thus examine and react to either buttons hundreds of times each millisecond.Interesting side issue: The 4 if statements can be freely rearranged - the end effect is the same. There are some subtle differences only active for the few microseconds until loop() goes round again.<h3 >More complex millis() timers</h3>The effect that the timer is to start on button push or button release, or if a button should be ignored once the timer has started may be a requirement. Timer code can be made to handle all variations by storing the desired state and skipping <code >digitalRead</code>. This will work fine with two buttons, too.Example 3: Timer with State change<pre ><code >//Psudocode boolean LEDon = false unsigned long Timer &#34;ALWAYS use unsigned long&#34; if not LEDon and digitalRead==Down then &#34;set timer&#34; Timer= millis digitalWrite ON LEDon = true if LEDon and millis-Timer &gt;= 15000UL then digitalWrite Off LEDon = false </code></pre>Result: This will not do a <code >digitalRead</code> or reset the timer, once the LED has turned on, ie. we are starting the timer on button push (like attempt 1 earlier). Likewise it only tests the timer if the LED is on, and only turns it off once (which make very little difference, so I would normally omit the LED on test in the 2nd part).And this will work fine with two or more buttons and LEDs, as the timers are independent and the code will not block.Example 4: One button, one LED - each with their own timer.A more useful example of doing two timing things &#34;at once&#34;:<pre ><code >//Psudocode boolean LEDon = false boolean buttondown = false unsigned long LEDtimer = 0 &#34;ALWAYS use unsigned long...&#34; unsigned long buttontimer = 0 if millis - buttontimer &gt;= 5UL &#34;debounce 5 millisec&#34; and not buttondown and digitalReadButton==Down then buttondown = true buttontimer = millis if millis - buttontimer &gt;= 5UL and buttondown and digitalReadButton==Up then buttondown = false buttontimer = millis if not LEDon and buttondown //can use &#34;not buttondown&#34; then LEDtimer = millis digitalWrite ON ledon = true if millis - LEDtimer &gt;= 15000UL then digitalWrite Off LEDon = false </code></pre>Result: The button timer will purposefully skip <code >digitalRead</code> of the button for 5 milliseconds after it has changed. This filters away the &#34;noise&#34; in any mechanical button. The Led on/off code can be set to trigger on button down or up.<h3 >A Challenge</h3>If you use a <code >previousbuttonup</code> indicator instead of <code >LEDon</code>, you can ensure that the button has to be released before a new timer cycle can start - but that is left as an exercise, and is more about state transition than about timers.So, this was the last time you used delay(), right? There will be exceptions, of course.

Limitations of delay() & How to Do Timers Correctly

The Arduino delay() function has a (usually unintended) side effect, this lesson tells you what it is and how to avoid it if needed.