millis() for non-blocking timing in Arduino - Mini Project: Build & Apply

Using millis() for Non-Blocking Timing in Arduino

📖 Scenario: You want to blink an LED on your Arduino board without stopping the rest of your program. Using delay() stops everything, so you will learn how to use millis() to blink the LED without blocking other code.

🎯 Goal: Build a program that blinks the built-in LED on and off every second using millis() for timing, so the program can do other things without waiting.

📋 What You'll Learn

Create a variable to store the LED pin number

Create a variable to store the interval time in milliseconds

Use millis() to check elapsed time without blocking

Toggle the LED state every interval

Print the LED state to the Serial Monitor

💡 Why This Matters

🌍 Real World

Non-blocking timing is important in real devices to keep them responsive while doing tasks like blinking LEDs or reading sensors.

💼 Career

Understanding how to use <code>millis()</code> for timing is a key skill for embedded systems and Arduino programming jobs.

1

Set up LED pin and initial variables

Create a variable called ledPin and set it to LED_BUILTIN. Create a variable called previousMillis and set it to 0. Create a variable called interval and set it to 1000 (milliseconds).

Arduino

// Your code here

Hint

Use int for ledPin, unsigned long for previousMillis, and const long for interval.

2

Initialize the LED pin and Serial in setup()

Write a setup() function. Inside it, use pinMode(ledPin, OUTPUT); to set the LED pin as output. Also, start serial communication with Serial.begin(9600);.

Arduino

int ledPin = LED_BUILTIN;
unsigned long previousMillis = 0;
const long interval = 1000;

void setup() {
  // Your code here
}

Hint

Remember to open the serial monitor at 9600 baud to see the output.

3

Write the non-blocking timing logic in loop()

Write a loop() function. Inside it, create a variable currentMillis and set it to millis(). Use an if statement to check if currentMillis - previousMillis is greater than or equal to interval. If yes, update previousMillis to currentMillis, toggle the LED state using digitalWrite().

Arduino

int ledPin = LED_BUILTIN;
unsigned long previousMillis = 0;
const long interval = 1000;

void setup() {
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  // Your code here
}

Hint

Use a static bool ledState to remember the LED state between loops.

4

Print the LED state to Serial Monitor

Add a Serial.print("LED is "); and Serial.println() inside the if block in loop() to print "ON" when the LED is on and "OFF" when it is off.

Arduino

int ledPin = LED_BUILTIN;
unsigned long previousMillis = 0;
const long interval = 1000;

void setup() {
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  unsigned long currentMillis = millis();
  static bool ledState = LOW;
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    ledState = !ledState;
    digitalWrite(ledPin, ledState);
    // Print LED state here
  }
}

Hint

Use the ternary operator ledState ? "ON" : "OFF" to print the correct state.

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