Arduino Delay Function: Why you need it and How to Use it. This is an Extremely Useful function which you can use to get small delays. However, sometimes it's not the right function to use - there is another!

Arduino delay:

• It is useful for initializing some chips (sometimes you need to wait for them).
  
  
• You do actually need a delay() function to allow human interaction - processors are too fast!
  
  
• Another function with finer resolution is delayMicroseconds().
  
  
• There is an alternative non-blocking delay function: millis() (a bit more complex to use).
  
  
• Find out a gotcha in using delay().

Why you need the Arduino Delay function

The Arduino delay() function allows you to pause the program execution for a specified number of milliseconds, which is useful when you need to synchronize events or actions with real-world timing.

It helps control the speed of programs and operations.

The delay() function is commonly used when initializing peripherals, waiting for readings to settle, or pacing animated sequences. Without delay, code would execute at full speed and it would be difficult to perceive what is happening.

There are two reasons for using the delay function:

1. Chip initialization.
2. Human interaction.

You will want to use delay() to slow down the processor e.g. to see an LED flashing or to slow it down between sending commands to devices

Some chips require specific delays when you initialize them for instance the Hitachi HD44780 needs milliseconds delay at a specific time in its startup process. The delay function is appropriate in that process since the delay is small and the processor does not need to do anything else during startup.

Microcontrollers operate at a very fast rate (16MHz for an Arduino Uno - the ESP8266 runs at 80 or 160MHz!) and this is a good thing since you can do more processing. The problem is that to interact with the real world, you need to slow the processor down since real world actions do not go at MHz rates! This is where the Arduino delay function is useful.

Parameters for delay()

The delay() function requires one unsigned long integer parameter, which specifies the number of milliseconds to pause program execution for. Values must be between 0 and approximately 50 days due to data type limitations (to understand this more there is an analysis of the millis() function, which examines this limit). Common delay times include:

• Short delays between 0-100ms for blinking LEDs, button debouncing etc.
• Medium delays 100-1000ms for user feedback, animation sequences
• Long delays 1-60,000ms (1 minute) for sleep modes, slow sampling rates

It is a function that stays in a loop for the number of milliseconds that you give it as an argument. A millisecond is 1/000^th of a second so there are 1000 milliseconds in one second ( 1000ms = 1s ).

For a 1 second delay you would use the function delay() in your program as follows:

    delay(1000);

Arduino Delay Function LED Example

For quick simple programs the Arduino delay() function provides a convenient slow-down mechanism. Here's an example program for flashing the on-board LED attached to pin 13 of the Arduino uno - the pin set to the relevant LED pin on different boards.

// Simple LED flash

#define LED LED_BUILTIN

//////////////////////////////////////////////
void setup(void) {

  pinMode(LED,OUTPUT);

}
//////////////////////////////////////////////
// Arduino Delay LED flash
void loop(){

  delay(500);
  digitalWrite(LED,HIGH);
  delay(500);
  digitalWrite(LED,LOW);

}

Code Explanation

All it does is initialise the pin as an output in setup(). Then it repeatedly executes the code in loop(). That is, it waits for half a second, then sets the LED on, waits another half a second and sets the LED off. So over the course of 1 second the LED is turned on for half a second and off for half a second i.e. a rate of 1Hz. Then that action is repeated endlessly.

Useful Debug code

When you get into difficulties in using microcontrollers (and you will!). Always go back to basics and use a program such as the one above, since it is one of the simplest, and it will give you confidence that the hardware is working e.g. that the power supply wires or ground wires are connected correctly.

This is more important when you attach a development board such as a solderless breadboard where connections can get unstuck!

Delay details

Delay variants

Arduino Delay milliseconds

You have seen the function delay() which takes one parameter representing milliseconds.

Arduino Delay microseconds

Another function you can use delayMicroseconds() which again takes a parameter but this time representing micro seconds. Each microsecond is a millionth of a second i.e. 1/1e6 seconds.

Arduino Delay nanoseconds

This is too small a delay for a microcontroller. For an Arduino Uno the basic instruction takes 1/16e6 or 62.5ns. So having a function that delays for a number of nano seconds can't be done. You can do it with a faster processor but even a 240MHz processor's instruction is ~4ns so you can't get lower than that.

In practice it is not used as the times involved are small to be useful to external operation.

Nano second delays only become important in circuit design using fast logic chips (not microcontrollers) i.e. for normal HC chips a propagation delay is ~20ns. You can get very high speed (ECL) chips that run at 100MHz so operating with signals of the order of 10ns pulses is needed for these chips.

Example delay values

Here are some common delay values for common time delays.

One Second delay Example

Use delay(1000) to pause for 1 second.

30 seconds delay Example

Use delay(30000) to pause for 30 seconds.

1 minute delay Example

Use delay(60000) to pause for 1 minute (60 seconds).

15 minute delay Example

Use delay(15*60000) to pause for 15 * 1 minute.

1 hour delay Example

Use delay(60*60000) to pause for 1 hour (60 minutes).

1 Day delay Example

Use delay(24 * 60 * 60 * 1000) to pause for 1day.

The Problem with Arduino Delay

So far, everything looks great; Delays work fine and LEDs flash on and off, but delay() has a skeleton in the cupboard.

delay() is a lazy function - it is a DO NOTHING function or a blocking function.

When you use delay(), even though it is sometimes useful, you are throwing away processing power straight out of the window never to be recovered!

When you start to write more substantial programs you will find that using delay interferes with operations.

For example if you wanted to read a input button and react to that button quickly e.g. within 100ms. If you had used a 200ms on/200ms off delay to flash an LED on and off, you could not meet that requirement; 400ms would be lost waiting in the delay routines. This is time that could be better spent in doing useful processing.

So the question now becomes how do you actually create a delay but at the same time use a delay that uses up minimal processor time.

There are actually two ways to do this:

• Use the millis() function
• Use Interrupts to detect the button press.

The first option is easier and is explained more here.

The second is more advanced and involves using low level interrupt routines and more hardware (to allow several keys to operate into one interrupt pin).

Arduino Delay Gotchas

This Arduino delay gotcha is fairly subtle and you may have already come across it. It is all to do with scoping ( C/C++ rules of when a variable is visible to other functions) and how the Arduino environment (although convenient) does hide what is going on behind the scenes - and this can catch you out.

The following code is trying to control the delay time by passing a variable to the delay function which is changed when a button is pressed.

#define LED LED_BUILTIN
#define INPUT 5

//////////////////////////////////////////////
void loop(){
uint16_t dly=200;

  delay(dly);
  digitalWrite(LED,HIGH);
  delay(dly);
  digitalWrite(LED,LOW);

  if (digitalRead(INPUT)) dly=100; else dly=10;

}

When you look at the code it looks perfectly reasonable; Use a variable to hold the delay value and change the value dependent of the state of an input button press, however it does not work! All that happens is that the Arduino delay value is only ever set to 200.

The reason is that the functions setup() and loop() are artificial constructs that make programming easier (sort of).

The setup() and loop() functions are inherited from the "processing" environment which is a graphics coding system based on openGL. These two functions make it easier to explain how a program is written i.e. you first initialise variables and ports in setup() and then execute code repeatedly in loop().

In fact the underlying compiler only starts a program from main() and ends at the end of main() - there is no loop! You roll your own loops as required! The actual code that lies above the familiar Arduino words 'setup' and 'loop' is something like this:

// main construct

void main(void) {

    setup();         // Call the user defined code for setup

    while(1) loop(); // Continuously call the user defined code for loop.

}

As shown above when 'main()' is called, first of all the code within function setup() is executed and then the function loop() is called repeatedly (in a so-called endless loop). This is standard programming practice and all that the Arduino code has done is removed the outer layer so it sort-of simplifies the proceedings so that you only need to write those functions ( setup() and loop() ).

However you can't actually see that loop() is called (hidden in the Arduino code similar to the code above) since you are never aware of the main() function.

The key point is that scoping rules force all local variables to be initialised at the start of any function and lost at exit of that function. Therefore the value of dly is lost at the end of loop() and so the code that sets the new value has no effect. All that happens is that each time the function is started the dly variable is initialised to 200.

To make a function remember values between calls you need to preceded the variable with the word static - meaning that the value of that variable is going no where and is retained between calls to the function.

#define LED LED_BUILTIN
#define INPUT 5

//////////////////////////////////////////////
// Arduino Delay LED flash that works
void loop(){
static uint16_t dly=200;

  delay(dly);
  digitalWrite(LED,HIGH);
  delay(dly);
  digitalWrite(LED,LOW);

  if (digitalRead(INPUT)) dly=100; else dly=10;

}

More Usual Compiler Operation

Or how "normal" compilers work - not the Arduino compiler!

The real reason that there is a problem is that in a "standard compiler" program you could have written the whole thing as follows using the main() function.

// main construct Full program non-Arduino compiler code.

// Arduino pins
#define INPUT 5
#define LED 13

void main(void) {
uint16_t dly=200;

    pinMode(LED,OUTPUT);       // This was the code within setup()

    while(1) {}                // Endless loop code

      delay(dly);              // This was the code within loop()
      digitalWrite(LED,HIGH);
      delay(dly);
      digitalWrite(LED,LOW);

      if (digitalRead(INPUT)) dly=100; else dly=10;
    }
}

In this case the definition of dly does not need to be static since the variable's scope is the whole of the main() function. In this case anything within main() can see and use the variable dly.

This is the sort of code that you will have to write for most microcontrollers since compiler makers do not mess about giving you the easy route that the Arduino IDE/compiler does!

Conclusions

The Arduino delay function is a useful function allowing you to interact with your design. It slows the processor down enough so that the high speed of the processor is reduced to allow a person to see what is going on.

The problem is that it is a blocking function, which means that it wastes processor time, and you should really not use the Arduinino delay function, instead use the millis() function.

The millis() function allows non blocking delays so that the processor can get on with other operations even while the delay is being calculated.

You can find more on the millis() function in this link. It is slightly more complicated to use but will ley your programs work better.

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