Wake-up from sleep with interrupt in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after waking up from sleep?

Consider this Arduino code that puts the microcontroller to sleep and wakes it up using an external interrupt. What will be printed to the serial monitor after the interrupt triggers?

Arduino

volatile bool wokeUp = false;

void wakeUp() {
  wokeUp = true;
}

void setup() {
  Serial.begin(9600);
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), wakeUp, FALLING);
  Serial.println("Going to sleep...");
  delay(1000);
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  sleep_enable();
  sleep_cpu();
  sleep_disable();
  if (wokeUp) {
    Serial.println("Woke up from interrupt!");
  } else {
    Serial.println("Did not wake up.");
  }
}

void loop() {
  // Empty loop
}

A Going to sleep...\nDid not wake up.

B Woke up from interrupt!\nGoing to sleep...

C Going to sleep...\nWoke up from interrupt!

D Did not wake up.\nGoing to sleep...

💡 Hint

Think about whether FALLING mode wakes from SLEEP_MODE_PWR_DOWN.

The program prints "Going to sleep..." first. Then it goes to sleep. FALLING cannot wake from SLEEP_MODE_PWR_DOWN; only LOW can. So wokeUp remains false, printing "Did not wake up."

🧠 Conceptual

intermediate

Which interrupt mode wakes the Arduino from sleep?

When using attachInterrupt() to wake an Arduino from sleep, which interrupt mode will successfully wake the device from SLEEP_MODE_PWR_DOWN?

A FALLING

B CHANGE

C LOW

D RISING

💡 Hint

Only certain interrupt modes can wake the microcontroller from the deepest sleep mode.

In SLEEP_MODE_PWR_DOWN, only interrupts triggered by a LOW level on the interrupt pin can wake the Arduino. Modes like RISING, FALLING, or CHANGE do not wake it from this sleep mode.

🔧 Debug

advanced

Why does this Arduino code not wake up from sleep?

Examine the code below. The Arduino never wakes up after going to sleep. What is the cause?

Arduino

volatile bool wokeUp = false;

void wakeUp() {
  wokeUp = true;
}

void setup() {
  Serial.begin(9600);
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), wakeUp, FALLING);
  Serial.println("Going to sleep...");
  delay(1000);
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  sleep_enable();
  sleep_cpu();
  sleep_disable();
  if (wokeUp) {
    Serial.println("Woke up from interrupt!");
  } else {
    Serial.println("Did not wake up.");
  }
}

void loop() {}

A The interrupt number 2 is incorrect; should use digitalPinToInterrupt(2).

B The sleep_disable() is called before sleep_cpu().

C The variable wokeUp is not declared volatile.

D The interrupt mode FALLING does not wake from SLEEP_MODE_PWR_DOWN.

💡 Hint

Check the interrupt mode used with SLEEP_MODE_PWR_DOWN.

In SLEEP_MODE_PWR_DOWN, only LOW mode wakes the Arduino. FALLING does not.

📝 Syntax

advanced

Identify the syntax error in this sleep code

Which option contains the correct syntax to put the Arduino into power-down sleep mode and wake on an interrupt?

A

set_sleep_mode(SLEEP_MODE_PWR_DOWN);
sleep_enable();
sleep_cpu();
sleep_disable

B

set_sleep_mode(SLEEP_MODE_PWR_DOWN);
sleep_enable();
sleep_cpu();
sleep_disable();

C

set_sleep_mode(SLEEP_MODE_PWR_DOWN);
sleep_enable();
sleep_cpu;
sleep_disable();

D

set_sleep_mode(SLEEP_MODE_PWR_DOWN)
sleep_enable()
sleep_cpu()
sleep_disable()

💡 Hint

Look for missing semicolons or missing parentheses in function calls.

Option B correctly uses semicolons and calls sleep_cpu() as a function. Options A, B, and C have missing semicolons or missing parentheses causing syntax errors.

🚀 Application

expert

How many times does the Arduino wake up in this loop?

Given the code below, how many times will the Arduino print "Woke up!" if the button connected to pin 2 is pressed 3 times?

Arduino

volatile int wakeCount = 0;

void wakeUp() {
  wakeCount++;
}

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

void loop() {
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  sleep_enable();
  sleep_cpu();
  sleep_disable();
  if (wakeCount > 0) {
    Serial.println("Woke up!");
    wakeCount = 0;
  }
  delay(500);
}

A 1

B 0

C 3

D Depends on how long the button is held

💡 Hint

Consider how the LOW interrupt mode behaves and how the wakeCount variable is reset.

The interrupt mode LOW triggers repeatedly as long as the pin is held LOW. However, the code resets wakeCount to 0 after waking once. So if the button is pressed and released 3 times, the Arduino wakes once per loop iteration, but since the loop resets wakeCount, it only prints "Woke up!" once per loop. Because the button presses happen quickly, the Arduino will print "Woke up!" once per loop cycle, not three times separately.

Practice

1. What is the main purpose of using an interrupt to wake an Arduino from sleep mode?

easy

A. To allow the Arduino to respond immediately to an external event while saving power

B. To make the Arduino run faster during normal operation

C. To reset the Arduino automatically after a delay

D. To increase the Arduino's memory capacity

Solution

1. Step 1: Understand sleep mode purpose

   Arduino sleep mode reduces power consumption by stopping most activities.

2. Step 2: Role of interrupts in sleep mode

   Interrupts allow the Arduino to wake up immediately when an external event occurs, saving power while staying responsive.

3. Final Answer:

   To allow the Arduino to respond immediately to an external event while saving power -> Option A

4. Quick Check:

   Interrupt wakes Arduino from sleep to save power [OK]

Hint: Interrupts wake Arduino instantly during sleep to save power [OK]

Common Mistakes:

• Thinking interrupts speed up normal running
• Confusing interrupts with reset functions
• Believing interrupts increase memory

2. Which of the following is the correct way to attach an interrupt to pin 2 that triggers on a RISING signal?

easy

A. attachInterrupt(2, ISR, FALLING);

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

C. attachInterrupt(digitalPinToInterrupt(3), ISR, CHANGE);

D. attachInterrupt(2, ISR, LOW);

Solution

1. Step 1: Use digitalPinToInterrupt() for pin mapping

   Pin 2 must be converted using digitalPinToInterrupt(2) for portability.

2. Step 2: Set correct trigger mode

   RISING triggers interrupt when signal goes from LOW to HIGH.

3. Final Answer:

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

4. Quick Check:

   Correct pin and RISING trigger = attachInterrupt(digitalPinToInterrupt(2), ISR, RISING); [OK]

Hint: Always use digitalPinToInterrupt(pin) with correct trigger [OK]

Common Mistakes:

• Using pin number directly without digitalPinToInterrupt()
• Choosing wrong trigger mode like FALLING or LOW
• Attaching interrupt to wrong pin number

3. What will be the output on the serial monitor after running this code if a button connected to pin 2 is pressed once?

void ISR() {
  Serial.println("Woke up!");
}

void setup() {
  Serial.begin(9600);
  attachInterrupt(digitalPinToInterrupt(2), ISR, RISING);
  sleep_enable();
  sleep_mode();
  Serial.println("Sleeping...");
}

void loop() {
  // Empty loop
}

medium

A. No output at all

B. Woke up!\nSleeping...

C. Sleeping... (no further output)

D. Sleeping...\nWoke up!

Solution

1. Step 1: Analyze print locations

   "Sleeping..." prints after sleep_mode(), so it executes after waking up.

2. Step 2: ISR execution order

   ISR prints "Woke up!" immediately upon interrupt, before resuming code after sleep_mode().

3. Final Answer:

   Sleeping...\nWoke up! -> Option D

4. Quick Check:

   Code after sleep_mode() runs after ISR, so output order is Sleeping... then Woke up! [OK]

Hint: ISR runs immediately on interrupt, but code after sleep_mode() runs after ISR [OK]

Common Mistakes:

• Thinking "Sleeping..." prints before sleep
• Believing no output after wake
• Expecting Sleeping... before ISR

4. Identify the error in this code snippet that tries to wake Arduino from sleep on pin 3 interrupt:

void ISR() {
  // Empty ISR
}

void setup() {
  attachInterrupt(3, ISR, FALLING);
  sleep_enable();
  sleep_mode();
}

void loop() {
  // Empty
}

medium

A. attachInterrupt() uses pin number directly instead of digitalPinToInterrupt()

B. ISR function should not be empty

C. sleep_mode() should be called inside loop()

D. sleep_enable() must be called after sleep_mode()

Solution

1. Step 1: Check attachInterrupt() usage

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

2. Step 2: Validate other parts

   Empty ISR is allowed; sleep_mode() can be in setup; sleep_enable() must be before sleep_mode().

3. Final Answer:

   attachInterrupt() uses pin number directly instead of digitalPinToInterrupt() -> Option A

4. Quick Check:

   Pin mapping requires digitalPinToInterrupt() [OK]

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

Common Mistakes:

• Not using digitalPinToInterrupt() for pin mapping
• Thinking ISR must have code
• Misplacing sleep_enable() call

5. You want to put your Arduino to sleep and wake it up only when a button connected to pin 4 is pressed. Which sequence correctly implements this behavior?

hard

A. attachInterrupt(digitalPinToInterrupt(4), wakeISR, LOW); sleep_enable(); sleep_mode();

B. attachInterrupt(4, wakeISR, RISING); sleep_disable(); sleep_mode();

C. attachInterrupt(digitalPinToInterrupt(4), wakeISR, FALLING); sleep_enable(); sleep_mode();

D. attachInterrupt(digitalPinToInterrupt(4), wakeISR, CHANGE); sleep_enable(); sleep_disable();

Solution

1. Step 1: Attach interrupt correctly

   Use digitalPinToInterrupt(4) and FALLING trigger for button press (assuming active LOW button).

2. Step 2: Enable sleep and enter sleep mode

   Call sleep_enable() before sleep_mode() to allow waking on interrupt.

3. Final Answer:

   attachInterrupt(digitalPinToInterrupt(4), wakeISR, FALLING); sleep_enable(); sleep_mode(); -> Option C

4. Quick Check:

   Correct pin, trigger, and sleep calls = attachInterrupt(digitalPinToInterrupt(4), wakeISR, FALLING); sleep_enable(); sleep_mode(); [OK]

Hint: Use digitalPinToInterrupt() with sleep_enable() before sleep_mode() [OK]

Common Mistakes:

• Using sleep_disable() before sleep_mode()
• Wrong trigger mode like LOW or CHANGE for button press
• Not enabling sleep before entering sleep_mode()