Interrupt-driven button handling in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an interrupt in Arduino programming?

An interrupt is a signal that temporarily stops the main program to run a special function called an interrupt service routine (ISR). It helps respond quickly to events like button presses.

beginner

Why use interrupt-driven button handling instead of polling?

Interrupts let the Arduino react immediately to button presses without constantly checking the button state, saving processing time and power.

beginner

What is an ISR (Interrupt Service Routine)?

An ISR is a small function that runs automatically when an interrupt occurs. It should be short and fast to avoid delaying the main program.

intermediate

How do you attach an interrupt to a button pin in Arduino?

Use the attachInterrupt() function with the interrupt number (digitalPinToInterrupt(pin)), the ISR function name, and the trigger mode (e.g., RISING, FALLING).

intermediate

What precautions should you take inside an ISR?

Keep the ISR short, avoid using delay(), Serial.print(), or complex code. Use volatile variables if sharing data with the main program.

What does attachInterrupt() do in Arduino?

A Delays the program execution

B Starts the main program loop

C Reads the button state continuously

D Links a pin interrupt to a function to run when triggered

attachInterrupt() connects a hardware interrupt on a pin to a function (ISR) that runs when the interrupt triggers.

Which trigger mode calls the ISR when the button signal goes from LOW to HIGH?

A RISING

B CHANGE

C LOW

D FALLING

RISING triggers the ISR when the signal changes from LOW to HIGH.

Why should ISRs be short and fast?

A To use more memory

B To avoid blocking other interrupts and the main program

C To print debug messages

D To delay the program

Long ISRs block other interrupts and slow down the main program, causing problems.

What keyword should you use for variables shared between ISR and main code?

A static

B const

C volatile

D register

volatile tells the compiler the variable can change unexpectedly, so it always reads the latest value.

Which Arduino pins support external interrupts on most boards?

A Pins 2 and 3

B Pins 0 and 1

C Pins 4 and 5

D All analog pins

On many Arduino boards, pins 2 and 3 support external interrupts.

Explain how interrupt-driven button handling works on Arduino.

Think about how the Arduino reacts immediately to a button press without waiting.

What are the best practices when writing an ISR for a button press?

Consider what could cause problems if the ISR takes too long or uses certain functions.

Practice

1. What is the main advantage of using interrupts for button handling on an Arduino?

easy

A. The Arduino can react immediately when the button is pressed without waiting.

B. It makes the button press slower but more reliable.

C. It allows the button to be pressed multiple times at once.

D. It disables other parts of the program while the button is pressed.

Solution

1. Step 1: Understand what interrupts do

   Interrupts pause the main program to run a special function immediately when an event happens, like a button press.

2. Step 2: Compare interrupt behavior to normal checking

   Without interrupts, the program must keep checking the button, which can delay response. Interrupts react instantly.

3. Final Answer:

   The Arduino can react immediately when the button is pressed without waiting. -> Option A

4. Quick Check:

   Interrupts = Immediate reaction [OK]

Hint: Interrupts react instantly, no waiting needed [OK]

Common Mistakes:

• Thinking interrupts slow down button response
• Believing interrupts disable other code permanently
• Confusing interrupts with polling multiple presses

2. Which of the following is the correct syntax to attach an interrupt to pin 2 that calls the function buttonPressed on a FALLING edge?

easy

A. attachInterrupt(digitalPinToInterrupt(2), buttonPressed, FALLING);

B. attachInterrupt(2, buttonPressed, RISING);

C. attachInterrupt(buttonPressed, 2, FALLING);

D. attachInterrupt(2, FALLING, buttonPressed);

Solution

1. Step 1: Recall attachInterrupt syntax

   The correct syntax is attachInterrupt(digitalPinToInterrupt(pin), ISR_function, mode).

2. Step 2: Match parameters to the question

   Pin is 2, function is buttonPressed, mode is FALLING edge.

3. Final Answer:

   attachInterrupt(digitalPinToInterrupt(2), buttonPressed, FALLING); -> Option A

4. Quick Check:

   attachInterrupt(pin, function, mode) = attachInterrupt(digitalPinToInterrupt(2), buttonPressed, FALLING); [OK]

Hint: Use digitalPinToInterrupt(pin) for correct pin number [OK]

Common Mistakes:

• Swapping function and pin arguments
• Using pin number directly without digitalPinToInterrupt
• Mixing up trigger modes like RISING and FALLING

3. What will be the output on the serial monitor after pressing the button once in this code?

volatile bool pressed = false;

void setup() {
  Serial.begin(9600);
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), buttonISR, FALLING);
}

void loop() {
  if (pressed) {
    Serial.println("Button pressed!");
    pressed = false;
  }
}

void buttonISR() {
  pressed = true;
}

medium

A. Compilation error due to volatile variable

B. Button pressed! (printed repeatedly forever)

C. No output at all

D. Button pressed! (printed once)

Solution

1. Step 1: Understand ISR and flag usage

   The ISR sets the volatile flag 'pressed' to true when the button is pressed.

2. Step 2: Check loop behavior

   The loop prints "Button pressed!" once when 'pressed' is true, then resets it to false, so it prints only once per press.

3. Final Answer:

   Button pressed! (printed once) -> Option D

4. Quick Check:

   ISR sets flag, loop prints once [OK]

Hint: ISR sets flag; loop prints once per press [OK]

Common Mistakes:

• Thinking the message prints repeatedly without reset
• Assuming volatile causes errors
• Ignoring that flag resets after printing

4. Identify the error in this interrupt-driven button code:

volatile int count = 0;

void setup() {
  Serial.begin(9600);
  pinMode(3, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(3), countPresses, FALLING);
}

void loop() {
  Serial.println(count);
  delay(1000);
}

void countPresses() {
  count++;
}

medium

A. Using delay() in loop causes missed interrupts

B. Modifying a multi-byte variable without disabling interrupts

C. Incrementing a non-volatile variable in ISR

D. attachInterrupt() called with wrong pin number

Solution

1. Step 1: Check variable type and ISR safety

   'count' is volatile int (multi-byte). Incrementing it in ISR and reading in loop can cause inconsistent values.

2. Step 2: Understand atomicity issue

   Incrementing multi-byte variables is not atomic; main code reading 'count' may see partial updates causing errors.

3. Final Answer:

   Modifying a multi-byte variable without disabling interrupts -> Option B

4. Quick Check:

   Multi-byte variable access must be atomic [OK]

Hint: Use atomic operations or disable interrupts for multi-byte vars [OK]

Common Mistakes:

• Ignoring atomicity of multi-byte variables
• Thinking delay() blocks interrupts
• Assuming attachInterrupt pin is wrong without checking

5. You want to count how many times a button connected to pin 4 is pressed, but only if the button is pressed and held for at least 2 seconds. Which approach best uses interrupts to achieve this?

hard

A. Poll the button state in loop without interrupts and count presses.

B. Use attachInterrupt on RISING edge to increment count immediately on press.

C. Use attachInterrupt on FALLING edge to start a timer, then check duration in loop before incrementing count.

D. Use attachInterrupt on CHANGE edge and increment count inside ISR.

Solution

1. Step 1: Understand requirement for press duration

   Counting only if pressed for 2 seconds means timing the press length, not just detecting press.

2. Step 2: Use interrupt to detect press start and timer in loop

   Interrupt on FALLING edge detects press start; loop checks if button held 2 seconds before counting.

3. Final Answer:

   Use attachInterrupt on FALLING edge to start a timer, then check duration in loop before incrementing count. -> Option C

4. Quick Check:

   Interrupt detects press start; loop checks hold time [OK]

Hint: Interrupt starts timer; loop checks hold duration [OK]

Common Mistakes:

• Incrementing count immediately without timing
• Using CHANGE edge causing multiple triggers
• Polling without interrupts misses instant detection