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.