Rising, falling, and change triggers in Arduino - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to attach an interrupt that triggers on a rising edge.

Arduino

attachInterrupt(digitalPinToInterrupt(2), handleInterrupt, [1]);

Drag options to blanks, or click blank then click option'

ARISING

BCHANGE

CFALLING

DLOW

Attempts:

3 left

💡 Hint

Common Mistakes

Using FALLING instead of RISING will trigger on the opposite edge.

Using CHANGE triggers on both edges, not just rising.

The RISING mode triggers the interrupt when the signal goes from LOW to HIGH.

2fill in blank

medium

Complete the code to attach an interrupt that triggers on a falling edge.

Arduino

attachInterrupt(digitalPinToInterrupt(3), interruptHandler, [1]);

Drag options to blanks, or click blank then click option'

AFALLING

BRISING

CHIGH

DCHANGE

Attempts:

3 left

💡 Hint

Common Mistakes

Using RISING instead of FALLING triggers on the opposite edge.

Using HIGH is not a valid interrupt mode.

The FALLING mode triggers the interrupt when the signal goes from HIGH to LOW.

3fill in blank

hard

Fix the error in the interrupt mode to trigger on any change.

Arduino

attachInterrupt(digitalPinToInterrupt(4), onChange, [1]);

Drag options to blanks, or click blank then click option'

AFALLING

BCHANGE

CRISING

DLOW

Attempts:

3 left

💡 Hint

Common Mistakes

Using RISING or FALLING triggers only one edge, not both.

Using LOW triggers level-sensitive (while low), not on edges.

The CHANGE mode triggers the interrupt on both rising and falling edges.

4fill in blank

hard

Fill both blanks to create arrays that map pins to interrupt modes for rising and falling triggers.

Arduino

const int pins[] = {2, 3};
const int modes[] = { [1], [2] };

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ARISING

BFALLING

CCHANGE

DLOW

Attempts:

3 left

💡 Hint

Common Mistakes

Using CHANGE for both pins triggers on any change, not specific edges.

Using LOW is invalid for edge-triggered interrupts in this context.

Pin 2 uses RISING and pin 3 uses FALLING interrupt modes.

5fill in blank

hard

Fill all three blanks to create a map of pins to interrupt modes and attach interrupts accordingly.

Arduino

const int pins[] = {5, 6, 7};
const int modes[] = { [1], [2], [3] };

for (int i = 0; i < 3; i++) {
  attachInterrupt(digitalPinToInterrupt(pins[i]), interruptHandlers[i], modes[i]);
}

Drag options to blanks, or click blank then click option'

ACHANGE

BRISING

CFALLING

DLOW

Attempts:

3 left

💡 Hint

Common Mistakes

Using CHANGE mode for all pins triggers on any change, not specific modes.

Forgetting that all listed modes (LOW, CHANGE, RISING, FALLING) are valid.

Pin 5 uses LOW (though usually not recommended), pin 6 uses RISING, and pin 7 uses FALLING modes.

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