Interrupt-driven button handling in Arduino

Introduction

Interrupt-driven button handling lets your Arduino react immediately when a button is pressed, without waiting or checking all the time.

You want your Arduino to respond quickly to a button press.

You want to save power by not constantly checking the button.

You have other tasks running and don't want to miss button presses.

You want to avoid delays caused by checking buttons in the main loop.

Syntax

Arduino

attachInterrupt(digitalPinToInterrupt(pin), ISR_function, mode);

pin is the Arduino pin number connected to the button.

ISR_function is the name of the function to run when the button is pressed.

mode defines when the interrupt triggers: RISING, FALLING, or CHANGE.

Examples

Run buttonPressed function when pin 2 goes from HIGH to LOW (button press).

Arduino

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

Run buttonPressed function when pin 3 goes from LOW to HIGH (button release).

Arduino

attachInterrupt(digitalPinToInterrupt(3), buttonPressed, RISING);

Sample Program

This program uses an interrupt to detect when the button connected to pin 2 is pressed (goes from HIGH to LOW). The interrupt sets a flag. The main loop checks the flag and prints a message when the button is pressed.

Arduino

#define BUTTON_PIN 2
volatile bool buttonPressedFlag = false;

void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP); // Button connected to pin 2 with internal pull-up
  Serial.begin(9600);
  attachInterrupt(digitalPinToInterrupt(BUTTON_PIN), handleButtonPress, FALLING);
}

void loop() {
  if (buttonPressedFlag) {
    Serial.println("Button was pressed!");
    buttonPressedFlag = false; // Reset flag
  }
}

void handleButtonPress() {
  buttonPressedFlag = true; // Set flag to indicate button press
}

Important Notes

Use volatile for variables shared between interrupt and main code to avoid errors.

Keep interrupt service routines (ISR) very short and fast.

Use INPUT_PULLUP mode for buttons to avoid needing extra resistors.

Summary

Interrupts let your Arduino react instantly to button presses.

Use attachInterrupt() with a pin, function, and trigger mode.

Keep ISR code short and use flags to communicate with the main program.

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