Wake-up from sleep with interrupt in Arduino

Introduction

Sometimes, your Arduino needs to save power by sleeping. It can wake up when something important happens, like pressing a button. This is done using interrupts.

You want your Arduino to sleep to save battery and wake up when a button is pressed.

You want to pause your program until a sensor detects movement.

You want to reduce power use but still respond quickly to an event.

You want to wake your Arduino from sleep when a signal arrives on a pin.

Syntax

Arduino

attachInterrupt(digitalPinToInterrupt(pin), ISR, mode);
// pin: the pin number to watch
// ISR: the function to run when interrupt happens
// mode: when to trigger (LOW, CHANGE, RISING, FALLING)

The ISR function must be very short and fast.

Use digitalPinToInterrupt(pin) to get the correct interrupt number for the pin.

Examples

Wake up when pin 2 goes from LOW to HIGH.

Arduino

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

Wake up when pin 3 goes from HIGH to LOW.

Arduino

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

Sample Program

This program puts the Arduino to sleep to save power. When you press the button on pin 2, it triggers an interrupt that wakes the Arduino. The program then prints "Woke up!" and goes back to sleep.

Arduino

#include <avr/sleep.h>

volatile bool wokeUp = false;

void wakeUp() {
  wokeUp = true; // This runs when interrupt happens
}

void setup() {
  pinMode(2, INPUT_PULLUP); // Button on pin 2
  Serial.begin(9600);
  attachInterrupt(digitalPinToInterrupt(2), wakeUp, FALLING);
  Serial.println("Going to sleep. Press button on pin 2 to wake up.");
}

void loop() {
  if (!wokeUp) {
    set_sleep_mode(SLEEP_MODE_PWR_DOWN);
    sleep_enable();
    sleep_cpu(); // Arduino sleeps here
    sleep_disable();
  }
  if (wokeUp) {
    Serial.println("Woke up!");
    wokeUp = false; // Reset flag
    delay(1000); // Wait a bit before sleeping again
  }
}

Important Notes

Interrupts can wake the Arduino from deep sleep modes.

Keep the interrupt service routine (ISR) very short to avoid problems.

Use INPUT_PULLUP for buttons to avoid floating pins.

Summary

Use interrupts to wake Arduino from sleep when an event happens.

Attach interrupt to a pin with attachInterrupt() and define a short ISR.

Put Arduino to sleep with sleep_mode() and it wakes on interrupt.

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()