millis() for non-blocking timing in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the millis() function return in Arduino?

millis() returns the number of milliseconds since the Arduino board began running the current program.

beginner

Why is using millis() better than delay() for timing in Arduino?

millis() allows the program to keep running other code without stopping, while delay() pauses everything, blocking the program.

beginner

How do you check if a certain time has passed using millis()?

Save the start time with unsigned long startTime = millis(); then check if millis() - startTime >= interval to see if the interval passed.

beginner

What data type should you use to store the value returned by millis()?

You should use unsigned long because millis() returns a large number that can grow beyond the range of smaller types.

beginner

Explain what 'non-blocking timing' means in Arduino programming.

Non-blocking timing means the program can do other tasks while waiting for a time interval to pass, instead of stopping everything like delay() does.

What does millis() return?

A Seconds since the program started

B Microseconds since the program started

C Milliseconds since the program started

D Current time of day

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

Which data type is best to store millis() value?

A unsigned long

B int

C float

D char

unsigned long can hold large positive numbers needed for millis().

Why avoid using delay() for timing in Arduino?

A It uses too much memory

B It blocks the program from running other code

C It is slower than <code>millis()</code>

D It causes the Arduino to reset

delay() stops the program from doing anything else during the wait.

How do you check if a time interval has passed using millis()?

A Check if <code>millis() - startTime >= interval</code>

B Use <code>millis() + interval</code>

C Compare <code>millis()</code> directly to the interval

D Use <code>delay(interval)</code>

Subtracting the start time from current millis() tells how much time passed.

What is the main benefit of non-blocking timing?

A It uses less power

B It reduces memory usage

C It makes the Arduino run faster

D It allows the program to run other tasks while waiting

Non-blocking timing lets your program do other things instead of stopping.

Describe how to use millis() to perform an action every 2 seconds without stopping the rest of the program.

Think about saving the time when you last did the action and checking how much time passed.

Explain why delay() can cause problems in Arduino programs and how millis() helps solve this.

Consider what happens when you want to do many things at once.

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