Multiple timed events with millis() 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 delay() not ideal for multiple timed events?

delay() pauses the entire program, stopping all actions. This makes it hard to run multiple events at once.

intermediate

How can you track multiple timed events using millis()?

By saving the last time each event happened and checking if enough time has passed using millis(), you can run each event independently without stopping the program.

beginner

What is the purpose of storing a variable like previousMillis in timing code?

previousMillis stores the last time an event occurred, so you can compare it with the current millis() to decide if the event should run again.

advanced

What happens when millis() overflows after about 50 days?

millis() resets to zero after about 50 days, but timing code using subtraction still works correctly because of how unsigned numbers behave.

What does millis() return in Arduino?

A Current time of day

B Seconds since the program started

C Milliseconds since the program started

D Microseconds since the program started

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

Why is delay() not good for multiple timed events?

A It speeds up the program

B It pauses the whole program

C It only works once

D It uses too much memory

delay() stops all code from running, so you can't do other things at the same time.

How do you check if enough time has passed for an event using millis()?

A Compare current <code>millis()</code> with stored previous time

B Use <code>delay()</code> for the event duration

C Reset <code>millis()</code> manually

D Use <code>micros()</code> instead

You compare the current millis() value with the last time the event ran to see if the interval passed.

What variable name is commonly used to store the last event time?

A timerCount

B lastDelay

C eventTime

D previousMillis

previousMillis is a common name to hold the last time an event happened.

What happens when millis() overflows after ~50 days?

A Timing still works correctly due to unsigned math

B Program crashes

C Timing stops working

D Arduino resets automatically

The overflow is handled by unsigned subtraction, so timing code still works fine.

Explain how to use millis() to run two different timed events without stopping the program.

Think about saving times and checking intervals separately for each event.

Describe why using delay() is not suitable for multiple timed events and how millis() solves this problem.

Consider what happens when the program is paused.

Practice

1. What is the main advantage of using millis() for timing multiple events in Arduino instead of delay()?

easy

A. It resets the Arduino automatically.

B. It stops the program until the time passes.

C. It makes the program run slower.

D. It allows the program to run other tasks while waiting.

Solution

1. Step 1: Understand delay() behavior

   delay() pauses the whole program, stopping all other actions.

2. Step 2: Understand millis() advantage

   millis() returns elapsed time without stopping the program, so other tasks can run simultaneously.

3. Final Answer:

   It allows the program to run other tasks while waiting. -> Option D

4. Quick Check:

   millis() enables multitasking [OK]

Hint: Remember: delay() stops, millis() doesn't [OK]

Common Mistakes:

• Thinking millis() pauses the program
• Confusing delay() with non-blocking timing
• Believing millis() resets Arduino

2. Which of the following is the correct way to declare a variable to store the last time an event occurred using millis()?

easy

A. unsigned long lastEventTime = 0;

B. int lastEventTime = 0;

C. float lastEventTime = 0.0;

D. char lastEventTime = '0';

Solution

1. Step 1: Identify the correct data type for time

   Since millis() returns an unsigned long value, the variable must be unsigned long.

2. Step 2: Check variable initialization

   Initializing to 0 is correct to mark the start time.

3. Final Answer:

   unsigned long lastEventTime = 0; -> Option A

4. Quick Check:

   Use unsigned long for millis() times [OK]

Hint: Use unsigned long for time variables with millis() [OK]

Common Mistakes:

• Using int which can overflow quickly
• Using float which is not precise for time
• Using char which is for characters, not numbers

3. What will be the output of the following Arduino code snippet if millis() returns 5000 at the moment of checking?

unsigned long previousMillis = 3000;
unsigned long interval = 1500;

if (millis() - previousMillis >= interval) {
  Serial.println("Event triggered");
} else {
  Serial.println("Waiting");
}

medium

A. Waiting

B. No output

C. Event triggered

D. Compilation error

Solution

1. Step 1: Calculate elapsed time

   Elapsed time = 5000 (current millis) - 3000 (previousMillis) = 2000 ms.

2. Step 2: Compare elapsed time with interval

   Interval is 1500 ms. Since 2000 >= 1500, the condition is true, so "Event triggered" should print.

3. Final Answer:

   Event triggered -> Option C

4. Quick Check:

   Elapsed time 2000 >= 1500 triggers event [OK]

Hint: Subtract previousMillis from millis() to check elapsed time [OK]

Common Mistakes:

• Mixing up >= and > operators
• Forgetting to subtract previousMillis
• Assuming output without calculation

4. Identify the error in this code snippet that tries to blink two LEDs at different intervals using millis():

unsigned long previousMillis1 = 0;
unsigned long previousMillis2 = 0;
const long interval1 = 1000;
const long interval2 = 2000;

void loop() {
  if (millis() - previousMillis1 >= interval1) {
    digitalWrite(LED1, !digitalRead(LED1));
    previousMillis1 = millis();
  }
  if (millis() - previousMillis1 >= interval2) {
    digitalWrite(LED2, !digitalRead(LED2));
    previousMillis2 = millis();
  }
}

medium

A. LED pins are not defined.

B. Second if condition uses previousMillis1 instead of previousMillis2.

C. Intervals should be unsigned long, not long.

D. digitalWrite cannot toggle LEDs.

Solution

1. Step 1: Check timing variables in conditions

   The second if condition incorrectly uses previousMillis1 instead of previousMillis2, causing wrong timing for LED2.

2. Step 2: Understand impact of error

   This mistake means LED2 timing depends on LED1 timing, so LED2 won't blink at its own interval.

3. Final Answer:

   Second if condition uses previousMillis1 instead of previousMillis2. -> Option B

4. Quick Check:

   Each event needs its own previousMillis variable [OK]

Hint: Use separate previousMillis for each timed event [OK]

Common Mistakes:

• Reusing the same previousMillis variable for multiple events
• Not updating previousMillis after event
• Confusing interval variables

5. You want to control three LEDs blinking at 500ms, 1000ms, and 1500ms intervals respectively without using delay(). Which approach correctly manages all three timed events using millis()?

hard

A. Use three separate previousMillis variables and check each with its own interval inside loop().

B. Use one previousMillis variable and reset it after each LED toggles.

C. Use delay(500) and toggle all LEDs together.

D. Use millis() only once and toggle LEDs based on dividing millis() by intervals.

Solution

1. Step 1: Understand independent timing needs

   Each LED needs its own timer to blink independently at different intervals.

2. Step 2: Evaluate options

   Use three separate previousMillis variables and check each with its own interval inside loop(), allowing independent timing without blocking.

3. Final Answer:

   Use three separate previousMillis variables and check each with its own interval inside loop(). -> Option A

4. Quick Check:

   Separate timers for each event [OK]

Hint: Assign each event its own timer variable for independent control [OK]

Common Mistakes:

• Using one timer for all events causing sync issues
• Using delay which blocks other events
• Trying to calculate toggles from one millis() value without state