Wake-up from sleep with interrupt in Arduino - Step-by-Step Execution

Concept Flow - Wake-up from sleep with interrupt

Start program

↓

Setup interrupt

↓

Enter sleep mode

↓

Interrupt occurs?

No→Stay asleep

Yes↓

Wake up

↓

Run interrupt code

↓

Return to main loop or sleep

The program sets up an interrupt, then goes to sleep. When the interrupt triggers, it wakes up and runs the interrupt code.

Execution Sample

Arduino

#include <avr/sleep.h>

void wakeUp() {
  sleep_disable();
  // Interrupt code
}

void setup() {
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), wakeUp, FALLING);
}

void loop() {
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  sleep_enable();
  sleep_mode();
}

This code puts the Arduino to sleep and wakes it up when pin 2 detects a falling edge interrupt.

Execution Table

Step	Action	Interrupt Pin State	Sleep State	Interrupt Triggered	Output/Effect
1	Setup pin 2 as input with pull-up	HIGH	Awake	No	Pin 2 ready for interrupt
2	Attach interrupt on pin 2 falling edge	HIGH	Awake	No	Interrupt configured
3	Enter sleep mode	HIGH	Sleeping	No	Arduino sleeps, waiting
4	Pin 2 goes LOW (button pressed)	LOW	Sleeping	Yes	Interrupt triggers wakeUp()
5	WakeUp function runs	LOW	Awake	Yes	Arduino wakes and runs code
6	Return to loop or sleep again	HIGH	Awake or Sleeping	No	Cycle repeats
7	Pin 2 stays HIGH, no interrupt	HIGH	Sleeping	No	Arduino remains asleep

💡 Execution stops sleeping only when interrupt triggers (pin 2 falling edge detected)

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
Interrupt Pin State	HIGH	HIGH	LOW	LOW	HIGH
Sleep State	Awake	Sleeping	Sleeping	Awake	Awake or Sleeping
Interrupt Triggered	No	No	Yes	Yes	No

Key Moments - 3 Insights

Why does the Arduino stay asleep even if the pin state changes?

The Arduino only wakes up when the interrupt condition (falling edge) occurs, not just any pin change. See execution_table row 7 where pin is HIGH and no interrupt triggers.

What happens inside the wakeUp() function?

The wakeUp() function runs immediately when the interrupt triggers, waking the Arduino from sleep. This is shown in execution_table row 5.

Why do we use INPUT_PULLUP for the pin mode?

INPUT_PULLUP keeps the pin HIGH by default, so pressing a button pulls it LOW, creating a falling edge to trigger the interrupt. This setup is in execution_table row 1.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Sleep State at Step 4 when the interrupt triggers?

A Awake

B Unknown

C Sleeping

D Reset

💡 Hint

Check the Sleep State column at Step 4 in the execution_table.

At which step does the Arduino wake up from sleep?

A Step 5

B Step 3

C Step 4

D Step 7

💡 Hint

Look for the step where the wakeUp() function runs and Sleep State changes to Awake.

If the pin never goes LOW, what happens to the Arduino?

A It stays awake

B It stays asleep indefinitely

C It wakes up periodically

D It resets

💡 Hint

See execution_table row 7 where pin stays HIGH and no interrupt triggers.

Concept Snapshot

Setup interrupt on a pin with attachInterrupt(pin, ISR, mode)
Use INPUT_PULLUP to keep pin HIGH by default
Call sleep_mode() to enter sleep
Arduino wakes when interrupt triggers (e.g., falling edge)
ISR runs immediately on wake-up
After ISR, program continues or sleeps again

Full Transcript

This example shows how an Arduino can go to sleep and wake up using an interrupt. First, the program sets pin 2 as input with a pull-up resistor. Then it attaches an interrupt to pin 2 that triggers on a falling edge (when the pin goes from HIGH to LOW). The Arduino then enters sleep mode. When the pin 2 detects a falling edge, the interrupt triggers the wakeUp() function, waking the Arduino. After running the interrupt code, the Arduino can continue running or go back to sleep. The key is that the Arduino stays asleep until the interrupt triggers, saving power.

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()