millis() for non-blocking timing in Arduino

Introduction

millis() helps you keep track of time without stopping your program. It lets your Arduino do other things while waiting.

You want to blink an LED on and off without stopping other code.

You need to check a sensor every few seconds but still read buttons instantly.

You want to run a timer that counts up while your program runs.

You want to avoid using delay() because it pauses everything.

You want to make your program responsive and multitask easily.

Syntax

Arduino

unsigned long previousMillis = 0;
const unsigned long interval = 1000; // time in milliseconds

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    // do something every interval
  }
}

millis() returns the number of milliseconds since the Arduino started.

Use unsigned long to store millis() values to avoid errors when the number gets big.

Examples

This example toggles an LED every 1 second without stopping the program.

Arduino

unsigned long previousMillis = 0;
const unsigned long interval = 1000;

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    // toggle LED
  }
}

This example reads a sensor every 5 seconds while the rest of the program runs freely.

Arduino

unsigned long previousMillis = 0;
const unsigned long interval = 5000;

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    // read sensor every 5 seconds
  }
}

This shows how other code can run while waiting for the interval to pass.

Arduino

unsigned long previousMillis = 0;
const unsigned long interval = 1000;

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    // do something
  }
  // other code runs here without delay
}

Sample Program

This program blinks the built-in LED on pin 13 every second without stopping the program. It prints the LED state to the Serial Monitor each time it changes.

Arduino

#define LED_PIN 13

unsigned long previousMillis = 0;
const unsigned long interval = 1000; // 1 second
bool ledState = false;

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Starting non-blocking LED blink");
}

void loop() {
  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;

    ledState = !ledState; // toggle LED state
    digitalWrite(LED_PIN, ledState ? HIGH : LOW);

    Serial.print("LED is now ");
    Serial.println(ledState ? "ON" : "OFF");
  }

  // Other code can run here without delay
}

Important Notes

Time complexity: Checking millis() and comparing times is very fast, almost constant time.

Space complexity: Uses only a few variables, so very low memory use.

Common mistake: Using int instead of unsigned long for millis() values causes errors when millis() gets large.

Use millis() timing instead of delay() when you want your Arduino to do many things at once without freezing.

Summary

millis() returns the time since the Arduino started, letting you track time without stopping your program.

Use a variable to remember the last time you did something, then check if enough time passed.

This method helps your Arduino multitask and stay responsive.

Practice

1. What does the millis() function in Arduino return?

easy

A. The current date and time

B. The number of milliseconds since the Arduino board started running the current program

C. The number of microseconds since the last reset

D. The current time in seconds

Solution

1. Step 1: Understand what millis() measures

   millis() returns the time in milliseconds since the Arduino started running the program.

2. Step 2: Compare options with the definition

   Only The number of milliseconds since the Arduino board started running the current program correctly describes this behavior; others mention seconds, microseconds, or date/time which are incorrect.

3. Final Answer:

   The number of milliseconds since the Arduino board started running the current program -> Option B

4. Quick Check:

   millis() = milliseconds since start [OK]

Hint: Remember millis() counts milliseconds since start [OK]

Common Mistakes:

• Confusing millis() with delay()
• Thinking millis() returns seconds
• Assuming millis() gives current date/time

2. Which of the following is the correct way to store the current time using millis() in Arduino?

easy

A. char currentTime = millis();

B. int currentTime = millis();

C. float currentTime = millis();

D. unsigned long currentTime = millis();

Solution

1. Step 1: Identify the correct data type for millis()

   millis() returns an unsigned long value representing milliseconds.

2. Step 2: Match the data type with variable declaration

   Only unsigned long currentTime = millis(); uses unsigned long, which can hold large millisecond values without overflow.

3. Final Answer:

   unsigned long currentTime = millis(); -> Option D

4. Quick Check:

   Use unsigned long for millis() values [OK]

Hint: Use unsigned long to store millis() time [OK]

Common Mistakes:

• Using int which can overflow quickly
• Using float or char which are incorrect types
• Not declaring variable before assignment

3. What will the following Arduino code print to the Serial Monitor?

unsigned long previousMillis = 0;
const long interval = 1000;

void setup() {
  Serial.begin(9600);
}

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    Serial.println("Tick");
  }
}

medium

A. Prints "Tick" once and stops

B. Prints "Tick" continuously without delay

C. Prints "Tick" every 1000 milliseconds without stopping the program

D. Causes a compile error due to variable scope

Solution

1. Step 1: Understand the timing logic

   The code checks if 1000 milliseconds have passed since last print using millis() and updates previousMillis accordingly.

2. Step 2: Analyze the output behavior

   When 1000 ms pass, it prints "Tick" and continues looping without blocking, so it prints every second repeatedly.

3. Final Answer:

   Prints "Tick" every 1000 milliseconds without stopping the program -> Option C

4. Quick Check:

   Non-blocking timing prints "Tick" every second [OK]

Hint: Check millis() difference to print repeatedly [OK]

Common Mistakes:

• Thinking it prints only once
• Confusing with delay() causing blocking
• Assuming compile error due to variable scope

4. Identify the error in this Arduino code using millis() for timing:

unsigned long previousMillis;
const long interval = 2000;

void setup() {
  Serial.begin(9600);
}

void loop() {
  if (millis() - previousMillis >= interval) {
    Serial.println("Hello");
  }
}

medium

A. previousMillis is never updated, so "Hello" prints continuously

B. interval should be unsigned long, not long

C. Serial.begin() is missing in setup()

D. millis() cannot be used in loop()

Solution

1. Step 1: Check how previousMillis is used

   The code checks the time difference but never updates previousMillis after printing.

2. Step 2: Understand the effect of missing update

   Without updating, the condition stays true, so "Hello" prints repeatedly without delay.

3. Final Answer:

   previousMillis is never updated, so "Hello" prints continuously -> Option A

4. Quick Check:

   Update previousMillis to avoid continuous printing [OK]

Hint: Always update previousMillis after action [OK]

Common Mistakes:

• Forgetting to update previousMillis
• Thinking interval type causes error
• Assuming Serial.begin() is missing

5. You want to blink an LED every 500 milliseconds without stopping other code from running. Which code snippet correctly uses millis() for this non-blocking timing? A)

unsigned long previousMillis = 0;
const long interval = 500;
void loop() {
  if (millis() - previousMillis >= interval) {
    previousMillis = millis();
    digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
  }
  // other code runs here
}

B)

void loop() {
  delay(500);
  digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
}

C)

unsigned long previousMillis = 0;
const long interval = 500;
void loop() {
  if (millis() >= previousMillis + interval) {
    digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
    previousMillis = millis();
  }
}

D)

unsigned long previousMillis = 0;
const long interval = 500;
void loop() {
  if (millis() - previousMillis > interval) {
    digitalWrite(LED_BUILTIN, HIGH);
    delay(500);
    digitalWrite(LED_BUILTIN, LOW);
  }
}

hard

A. Correct non-blocking blink using millis() and toggling LED

B. Uses delay(), which blocks other code from running

C. Correct logic but may cause overflow issues with addition

D. Uses delay() inside if, causing blocking and incorrect blink

Solution

1. Step 1: Identify non-blocking timing usage

   Correct non-blocking blink using millis() and toggling LED uses millis() difference and updates previousMillis correctly, toggling LED without delay.

2. Step 2: Compare other options for blocking or logic issues

   The other snippets either use delay(), which blocks other code from running, or use addition in the condition, which can cause overflow issues with large millisecond values.

3. Final Answer:

   Correct non-blocking blink using millis() and toggling LED -> Option A

4. Quick Check:

   Use millis() difference and update previousMillis [OK]

Hint: Toggle LED using millis() difference, avoid delay() [OK]

Common Mistakes:

• Using delay() causing blocking
• Not updating previousMillis properly
• Using addition risking overflow bugs