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.