Timer interrupts with TimerOne library in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this TimerOne interrupt example?

Consider this Arduino sketch using the TimerOne library to toggle an LED every 1 second. What will be printed on the Serial Monitor?

Arduino

volatile bool ledState = false;

void setup() {
  Serial.begin(9600);
  pinMode(13, OUTPUT);
  Timer1.initialize(1000000); // 1 second
  Timer1.attachInterrupt(toggleLED);
}

void loop() {
  // main loop does nothing
}

void toggleLED() {
  ledState = !ledState;
  digitalWrite(13, ledState);
  Serial.println(ledState ? "LED ON" : "LED OFF");
}

A Only LED ON is printed repeatedly every second

B No output is printed because Serial.println cannot be used in interrupts

C Only LED OFF is printed repeatedly every second

D LED ON and LED OFF alternate every second on the Serial Monitor

💡 Hint

Think about what the toggleLED function does each time the interrupt fires.

The interrupt toggles the ledState boolean every second. It prints "LED ON" when ledState is true and "LED OFF" when false, so the output alternates every second.

🧠 Conceptual

intermediate

Which statement about TimerOne interrupts is true?

Choose the correct statement about using TimerOne interrupts in Arduino sketches.

A TimerOne interrupts can be used to execute code at precise intervals without blocking the main loop

B TimerOne interrupts automatically disable all other interrupts when running

C TimerOne interrupts require the use of delay() inside the interrupt function

D TimerOne interrupts can only be used with Arduino Uno and not other boards

💡 Hint

Think about how interrupts help with timing and multitasking.

TimerOne interrupts allow code to run at precise intervals without blocking the main loop. They do not disable all other interrupts, and delay() should not be used inside interrupts. TimerOne works on many AVR-based boards, not only Uno.

🔧 Debug

advanced

Why does this TimerOne interrupt code cause the LED to flicker irregularly?

Examine the code below. The LED connected to pin 13 flickers irregularly instead of toggling every 0.5 seconds. What is the cause?

Arduino

volatile int ledState = LOW;

void setup() {
  pinMode(13, OUTPUT);
  Timer1.initialize(500000); // 0.5 seconds
  Timer1.attachInterrupt(toggleLED);
}

void loop() {
  // empty
}

void toggleLED() {
  ledState = !ledState;
  digitalWrite(13, ledState);
}

A digitalWrite cannot be called inside an interrupt function

B Timer1.initialize is set to 500000 microseconds which is too fast for the LED

C ledState is declared as int and toggled with ! operator, causing unexpected values

D pinMode(13, OUTPUT) must be called inside the interrupt function

💡 Hint

Consider the data type and how the ! operator works on integers.

The ! operator on an int toggles between 0 and 1, but ledState was initialized to LOW (0). The toggle works but ledState becomes 1 or 0, which is fine for digitalWrite. However, using int instead of bool can cause confusion. The flicker is likely due to the improper use of ! on int instead of bool.

📝 Syntax

advanced

Which option fixes the syntax error in this TimerOne setup?

Identify the option that corrects the syntax error in the TimerOne initialization code below:

Timer1.initialize(1000000)
Timer1.attachInterrupt(toggleLED);

A Add a semicolon after Timer1.initialize(1000000);

B Change Timer1.attachInterrupt(toggleLED) to Timer1.attachInterrupt(toggleLED);

C Remove the parentheses from Timer1.initialize

D Replace Timer1.initialize(1000000) with Timer1.init(1000000);

💡 Hint

Check the end of each statement for proper punctuation.

In Arduino (C++), each statement must end with a semicolon. The first line is missing a semicolon, causing a syntax error.

🚀 Application

expert

How many times will the LED toggle in 10 seconds with this TimerOne code?

Given this code snippet, how many times will the LED connected to pin 13 toggle in 10 seconds?

volatile bool ledState = false;

void setup() {
  pinMode(13, OUTPUT);
  Timer1.initialize(2500000); // 2.5 seconds
  Timer1.attachInterrupt(toggleLED);
}

void loop() {}

void toggleLED() {
  ledState = !ledState;
  digitalWrite(13, ledState);
}

A 5 times

B 4 times

C 10 times

D 20 times

💡 Hint

Calculate how many 2.5 second intervals fit into 10 seconds.

The interrupt fires every 2.5 seconds. In 10 seconds, 10 / 2.5 = 4 interrupts occur, so the LED toggles 4 times.

Practice

1. What is the main purpose of using the TimerOne library in Arduino programming?

easy

A. To run code at precise time intervals using interrupts

B. To control the speed of the Arduino board

C. To manage power consumption automatically

D. To connect Arduino to the internet

Solution

1. Step 1: Understand TimerOne library role

   The TimerOne library is designed to help run code at exact time intervals by using hardware timer interrupts.

2. Step 2: Compare options with library purpose

   Only To run code at precise time intervals using interrupts correctly describes this purpose. Other options describe unrelated features.

3. Final Answer:

   To run code at precise time intervals using interrupts -> Option A

4. Quick Check:

   TimerOne = precise timed interrupts [OK]

Hint: TimerOne = precise timing with interrupts [OK]

Common Mistakes:

• Thinking TimerOne controls Arduino speed
• Confusing interrupts with internet connection
• Assuming it manages power automatically

2. Which of the following is the correct way to start TimerOne with a 1-second interval?

easy

A. Timer1.initialize(1000000);

B. Timer1.start(1000);

C. Timer1.setInterval(1);

D. Timer1.begin(1000);

Solution

1. Step 1: Recall TimerOne initialization syntax

   The TimerOne library uses initialize() with microseconds as argument to set the timer interval.

2. Step 2: Match 1 second to microseconds

   1 second = 1,000,000 microseconds, so initialize(1000000) is correct.

3. Final Answer:

   Timer1.initialize(1000000); -> Option A

4. Quick Check:

   initialize uses microseconds = 1000000 [OK]

Hint: Use initialize() with microseconds for timing [OK]

Common Mistakes:

• Using milliseconds instead of microseconds
• Using non-existent start() or begin() methods
• Confusing setInterval() with initialize()

3. What will be the output on the Serial Monitor if this code runs?

void blink() {
  Serial.println("Tick");
}

void setup() {
  Serial.begin(9600);
  Timer1.initialize(500000);
  Timer1.attachInterrupt(blink);
}

void loop() {
  // empty
}

medium

A. No output because loop is empty

B. Prints "Tick" once and stops

C. Prints "Tick" every 0.5 seconds repeatedly

D. Prints "Tick" continuously without delay

Solution

1. Step 1: Understand Timer1 setup

   Timer1 is initialized to 500000 microseconds (0.5 seconds) and blink() is attached as interrupt.

2. Step 2: Analyze interrupt behavior

   Every 0.5 seconds, blink() runs and prints "Tick" to Serial Monitor repeatedly.

3. Final Answer:

   Prints "Tick" every 0.5 seconds repeatedly -> Option C

4. Quick Check:

   attachInterrupt triggers blink() repeatedly [OK]

Hint: attachInterrupt runs function repeatedly at interval [OK]

Common Mistakes:

• Thinking loop() must print for output
• Assuming interrupt runs only once
• Confusing timing units causing wrong intervals

4. Identify the error in this TimerOne usage:

void blink() {
  digitalWrite(13, !digitalRead(13));
}

void setup() {
  pinMode(13, OUTPUT);
  Timer1.initialize(1000000);
  Timer1.attachInterrupt(blink());
}

void loop() {}

medium

A. Timer1.initialize() argument should be in milliseconds

B. Missing Serial.begin() in setup()

C. Pin 13 cannot be used with TimerOne interrupts

D. Incorrect call: attachInterrupt(blink()) should be attachInterrupt(blink)

Solution

1. Step 1: Check attachInterrupt usage

   attachInterrupt expects a function pointer, so it should be attachInterrupt(blink), not attachInterrupt(blink()).

2. Step 2: Verify other parts

   Serial.begin() is not needed here, pin 13 can be toggled, and initialize() uses microseconds correctly.

3. Final Answer:

   Incorrect call: attachInterrupt(blink()) should be attachInterrupt(blink) -> Option D

4. Quick Check:

   attachInterrupt needs function name, no parentheses [OK]

Hint: Pass function name without () to attachInterrupt [OK]

Common Mistakes:

• Calling function with () inside attachInterrupt
• Confusing microseconds with milliseconds
• Thinking Serial.begin() is always required

5. You want to toggle an LED every 200ms using TimerOne. Which code snippet correctly sets this up?

hard

A. Timer1.initialize(200);
Timer1.attachInterrupt(blink);

B. Timer1.initialize(200000);
Timer1.attachInterrupt([](){ digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN)); });

C. Timer1.start(200000);
Timer1.attachInterrupt(blink());

D. Timer1.setInterval(200000);
Timer1.attachInterrupt(blink);

Solution

1. Step 1: Convert 200ms to microseconds

   200 milliseconds = 200,000 microseconds, so initialize(200000) is correct.

2. Step 2: Check attachInterrupt syntax and function

   Using a lambda function without parentheses is valid and toggles LED correctly. Other options use wrong methods or call functions incorrectly.

3. Final Answer:

   Timer1.initialize(200000); Timer1.attachInterrupt([](){ digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN)); }); -> Option B

4. Quick Check:

   initialize with microseconds + correct attachInterrupt [OK]

Hint: Use initialize(microseconds) and attachInterrupt with function [OK]

Common Mistakes:

• Using milliseconds instead of microseconds
• Calling function with () inside attachInterrupt
• Using non-existent start() or setInterval() methods