Rising, falling, and change triggers in Arduino

Introduction

These triggers help your Arduino notice when a button or sensor changes from off to on, on to off, or just changes state. This way, your program can react right away.

When you want to detect when a button is pressed down (going from off to on).

When you want to detect when a button is released (going from on to off).

When you want to detect any change in a sensor or switch, whether it turns on or off.

When you want to save power by only reacting when something changes, not all the time.

When you want to trigger an action exactly once when a signal changes.

Syntax

Arduino

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

// pin: the pin number
// ISR: the function to run when triggered
// mode: RISING, FALLING, or CHANGE

RISING triggers when the signal goes from LOW to HIGH.

FALLING triggers when the signal goes from HIGH to LOW.

CHANGE triggers on any change, either LOW to HIGH or HIGH to LOW.

Examples

This runs the buttonPressed function when pin 2 changes from LOW to HIGH.

Arduino

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

This runs the buttonReleased function when pin 3 changes from HIGH to LOW.

Arduino

attachInterrupt(digitalPinToInterrupt(3), buttonReleased, FALLING);

This runs the sensorChanged function whenever pin 4 changes state.

Arduino

attachInterrupt(digitalPinToInterrupt(4), sensorChanged, CHANGE);

Sample Program

This program counts how many times a button connected to pin 2 is pressed. It uses a FALLING trigger because the button connects the pin to ground when pressed (going from HIGH to LOW). The count increases each time the button is pressed, and the number is printed every second.

Arduino

#define buttonPin 2
volatile int count = 0;

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

void loop() {
  // Just print the count every second
  Serial.print("Button pressed: ");
  Serial.print(count);
  Serial.println(" times");
  delay(1000);
}

void countPresses() {
  count++;
}

Important Notes

Interrupt functions (ISR) should be short and fast. Avoid using delay() or Serial.print() inside them.

Use volatile for variables shared between the ISR and main code to keep values correct.

Not all pins support interrupts on every Arduino board. Check your board's documentation.

Summary

RISING triggers when a signal goes from LOW to HIGH.

FALLING triggers when a signal goes from HIGH to LOW.

CHANGE triggers on any change in the signal.

Practice

1. Which interrupt trigger type activates when a signal changes from LOW to HIGH on an Arduino pin?

easy

A. RISING

B. FALLING

C. CHANGE

D. LOW

Solution

1. Step 1: Understand the meaning of RISING trigger

   The RISING trigger activates when the signal goes from LOW (0) to HIGH (1).

2. Step 2: Compare with other triggers

   FALLING triggers on HIGH to LOW, CHANGE triggers on any change, LOW triggers when signal is LOW.

3. Final Answer:

   RISING -> Option A

4. Quick Check:

   LOW to HIGH = RISING [OK]

Hint: RISING means signal goes up from LOW to HIGH [OK]

Common Mistakes:

• Confusing RISING with FALLING
• Thinking CHANGE triggers only on RISING
• Mixing LOW trigger with RISING

2. Which of the following is the correct syntax to attach an interrupt on pin 2 for a FALLING trigger in Arduino?

easy

A. attachInterrupt(2, ISR, RISING);

B. attachInterrupt(digitalPinToInterrupt(2), ISR, FALLING);

C. attachInterrupt(2, ISR, CHANGE);

D. attachInterrupt(digitalPinToInterrupt(2), ISR, LOW);

Solution

1. Step 1: Identify correct function usage

   attachInterrupt requires pin number converted by digitalPinToInterrupt for portability.

2. Step 2: Match trigger type FALLING

   attachInterrupt(digitalPinToInterrupt(2), ISR, FALLING); uses digitalPinToInterrupt(2) and FALLING correctly.

3. Final Answer:

   attachInterrupt(digitalPinToInterrupt(2), ISR, FALLING); -> Option B

4. Quick Check:

   Use digitalPinToInterrupt with FALLING [OK]

Hint: Use digitalPinToInterrupt(pin) with trigger type [OK]

Common Mistakes:

• Using pin number directly without digitalPinToInterrupt
• Mixing trigger types with wrong syntax
• Using LOW trigger incorrectly

3. What will be the output on the serial monitor if the following Arduino code is run and the input pin changes from LOW to HIGH, then HIGH to LOW?

volatile int count = 0;
void ISR() { count++; }
void setup() {
  Serial.begin(9600);
  pinMode(2, INPUT);
  attachInterrupt(digitalPinToInterrupt(2), ISR, CHANGE);
}
void loop() {
  Serial.println(count);
  delay(1000);
}

medium

A. 0 0 0 ... (no change)

B. 1 1 1 ... (count increments once)

C. 2 2 2 ... (count increments twice)

D. Error: ISR cannot increment variable

Solution

1. Step 1: Understand CHANGE trigger behavior

   CHANGE triggers on any signal change: LOW to HIGH and HIGH to LOW, so ISR runs twice.

2. Step 2: Count increments in ISR

   Each ISR call increments count by 1, so after two changes count = 2.

3. Final Answer:

   2 2 2 ... (count increments twice) -> Option C

4. Quick Check:

   CHANGE triggers twice, count = 2 [OK]

Hint: CHANGE triggers on both edges, count increments twice [OK]

Common Mistakes:

• Assuming CHANGE triggers only once
• Forgetting volatile keyword importance
• Thinking ISR cannot modify variables

4. Identify the error in this Arduino code snippet that tries to attach an interrupt on pin 3 for a RISING trigger:

void ISR() {
  // interrupt code
}
void setup() {
  attachInterrupt(3, ISR, RISING);
}

medium

A. ISR function must have parameters

B. attachInterrupt must be called in loop(), not setup()

C. RISING is not a valid trigger type

D. Pin number 3 should be converted using digitalPinToInterrupt()

Solution

1. Step 1: Check attachInterrupt pin argument

   Pin number must be converted with digitalPinToInterrupt(pin) for correct interrupt mapping.

2. Step 2: Validate other parts

   ISR has correct signature (no parameters), RISING is valid, attachInterrupt is correctly called in setup.

3. Final Answer:

   Pin number 3 should be converted using digitalPinToInterrupt() -> Option D

4. Quick Check:

   Use digitalPinToInterrupt for pin numbers [OK]

Hint: Always convert pin with digitalPinToInterrupt() [OK]

Common Mistakes:

• Passing raw pin number directly
• Thinking ISR needs parameters
• Calling attachInterrupt in loop()

5. You want to count how many times a button connected to pin 4 changes state (pressed or released). Which interrupt trigger should you use and why?

hard

A. CHANGE, because it counts both presses and releases

B. FALLING, because it counts only releases

C. RISING, because it counts only presses

D. LOW, because it triggers when button is held down

Solution

1. Step 1: Understand button state changes

   A button press changes signal from HIGH to LOW or LOW to HIGH depending on wiring.

2. Step 2: Choose trigger to count all changes

   CHANGE triggers on any signal change, so it counts both presses and releases.

3. Final Answer:

   CHANGE, because it counts both presses and releases -> Option A

4. Quick Check:

   Count all changes = use CHANGE trigger [OK]

Hint: Use CHANGE to count all button state changes [OK]

Common Mistakes:

• Using RISING or FALLING and missing half the events
• Confusing LOW trigger with change detection
• Not considering both press and release