Arduino sleep modes

Introduction

Arduino sleep modes help save battery by putting the board into low power states when it is not doing anything important.

When your Arduino project runs on batteries and you want it to last longer.

When your device waits for a button press or sensor event and can sleep in the meantime.

When you want to reduce power use in a remote sensor or data logger.

When your Arduino controls something only occasionally and can rest between tasks.

Syntax

Arduino

1. Include the sleep library:
#include <avr/sleep.h>

2. Set the sleep mode:
set_sleep_mode(SLEEP_MODE_IDLE); // or other modes like SLEEP_MODE_PWR_DOWN

3. Put Arduino to sleep:
sleep_enable();
sleep_cpu();
sleep_disable();

You must include <avr/sleep.h> to use sleep functions.

Choose the sleep mode based on how much power you want to save and what should stay active.

Examples

This example puts Arduino into the IDLE sleep mode once in setup, then wakes up immediately (for demo).

Arduino

#include <avr/sleep.h>

void setup() {
  Serial.begin(9600);
  Serial.println("Going to sleep now");
  set_sleep_mode(SLEEP_MODE_IDLE);
  sleep_enable();
  sleep_cpu();
  sleep_disable();
  Serial.println("Woke up!");
}

void loop() {
  // Nothing here
}

This example uses the deepest sleep mode (POWER DOWN) to save the most power. Wakes after about 8 seconds using the Watchdog timer.

Arduino

#include <avr/sleep.h>
#include <avr/wdt.h>

void goToSleep() {
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  wdt_enable(WDTO_8S);
  sleep_enable();
  sleep_cpu();
  sleep_disable();
  wdt_disable();
}

void setup() {
  Serial.begin(9600);
  Serial.println("Sleeping deeply...");
  goToSleep();
  Serial.println("Awake now!");
}

void loop() {}

Sample Program

This program puts Arduino into a power-save sleep mode 5 times, woken each time by the 1 second watchdog timer (total ~5 seconds), then prints a message.

Arduino

#include <avr/sleep.h>
#include <avr/wdt.h>

void setup() {
  Serial.begin(9600);
  delay(1000); // Wait for serial to start
  Serial.println("Arduino will sleep for 5 seconds...");

  set_sleep_mode(SLEEP_MODE_PWR_SAVE);
  sleep_enable();

  // Sleep 5 times, ~1s each using watchdog timer
  for (int i = 0; i < 5; i++) {
    wdt_enable(WDTO_1S);
    sleep_cpu();
    wdt_disable();
  }

  sleep_disable();
  Serial.println("Arduino woke up after sleep!");
}

void loop() {
  // Nothing here
}

Important Notes

Not all Arduino boards support all sleep modes; check your board's datasheet.

To wake from deep sleep, you usually need an interrupt like a button press or timer.

Use sleep_disable() after waking to return to normal operation.

Summary

Arduino sleep modes help save power by stopping or slowing parts of the chip.

Use set_sleep_mode() and sleep_cpu() to enter sleep.

Wake up using interrupts or timers to continue your program.

Practice

1. Which Arduino function is used to set the type of sleep mode before entering sleep?

easy

A. set_sleep_mode()

B. sleep_cpu()

C. attachInterrupt()

D. delay()

Solution

1. Step 1: Understand sleep mode setup

   The function set_sleep_mode() is used to choose which sleep mode the Arduino will enter.

2. Step 2: Differentiate from sleep execution

   The function sleep_cpu() actually puts the CPU to sleep, but does not set the mode.

3. Final Answer:

   set_sleep_mode() -> Option A

4. Quick Check:

   Set mode = set_sleep_mode() [OK]

Hint: Set mode first with set_sleep_mode() before sleeping [OK]

Common Mistakes:

• Confusing set_sleep_mode() with sleep_cpu()
• Thinking attachInterrupt() sets sleep mode
• Using delay() to enter sleep

2. Which of the following is the correct syntax to put the Arduino into sleep mode after setting it?

easy

A. sleep_mode();

B. sleep_cpu();

C. sleep();

D. enter_sleep();

Solution

1. Step 1: Identify the sleep function

   The function sleep_cpu() is the correct call to put the CPU into sleep mode after setting it.

2. Step 2: Check other options

   sleep_mode() is not a valid function, nor are sleep() or enter_sleep().

3. Final Answer:

   sleep_cpu(); -> Option B

4. Quick Check:

   Sleep call = sleep_cpu() [OK]

Hint: Use sleep_cpu() to enter sleep after setting mode [OK]

Common Mistakes:

• Using sleep_mode() instead of sleep_cpu()
• Trying to call sleep() which doesn't exist
• Confusing function names

3. What will the following Arduino code do?

set_sleep_mode(SLEEP_MODE_PWR_DOWN);
sleep_enable();
sleep_cpu();
// CPU sleeps here
sleep_disable();

medium

A. The CPU enters power-down sleep mode and stays asleep until an interrupt wakes it

B. The CPU enters power-down sleep mode and wakes immediately

C. The CPU does not sleep because sleep_enable() is missing

D. The CPU enters idle mode instead of power-down

Solution

1. Step 1: Analyze sleep mode setting

   The code sets the sleep mode to power-down with set_sleep_mode(SLEEP_MODE_PWR_DOWN);.

2. Step 2: Understand sleep flow

   Calling sleep_enable() then sleep_cpu() puts the CPU to sleep in power-down mode. It stays asleep until an interrupt or reset wakes it. After waking, sleep_disable() disables sleep.

3. Final Answer:

   The CPU enters power-down sleep mode and stays asleep until an interrupt wakes it -> Option A

4. Quick Check:

   Power-down sleep = stays asleep until interrupt [OK]

Hint: Power-down sleep waits for interrupt to wake CPU [OK]

Common Mistakes:

• Assuming CPU wakes immediately without interrupt
• Forgetting sleep_enable() disables sleep
• Confusing idle mode with power-down

4. Identify the error in this Arduino sleep code snippet:

set_sleep_mode(SLEEP_MODE_IDLE);
sleep_cpu();
sleep_enable();

medium

A. set_sleep_mode() is missing a parameter

B. No error, code is correct

C. sleep_cpu() should be replaced with sleep_mode()

D. sleep_enable() must be called before sleep_cpu()

Solution

1. Step 1: Check function order

   The function sleep_enable() must be called before sleep_cpu() to enable sleep mode properly.

2. Step 2: Verify other parts

   set_sleep_mode() is correctly called with a valid mode. sleep_cpu() is the correct function to enter sleep.

3. Final Answer:

   sleep_enable() must be called before sleep_cpu() -> Option D

4. Quick Check:

   Enable sleep before sleeping [OK]

Hint: Call sleep_enable() before sleep_cpu() to avoid errors [OK]

Common Mistakes:

• Calling sleep_cpu() before sleep_enable()
• Confusing sleep_cpu() with sleep_mode()
• Missing parameters in set_sleep_mode()

5. You want to save maximum power on your Arduino but still wake it up using a button press on pin 2. Which sleep mode and wake method should you use?

hard

A. Use SLEEP_MODE_IDLE and poll the button in loop()

B. Use SLEEP_MODE_STANDBY and wake with delay()

C. Use SLEEP_MODE_PWR_DOWN and attach an external interrupt on pin 2

D. Use SLEEP_MODE_ADC and check button state with analogRead()

Solution

1. Step 1: Choose lowest power sleep mode

   SLEEP_MODE_PWR_DOWN uses the least power by shutting down most parts of the chip.

2. Step 2: Select wake method

   To wake from power-down mode, an external interrupt on pin 2 (using attachInterrupt) is needed because polling or delay() won't wake the CPU.

3. Final Answer:

   Use SLEEP_MODE_PWR_DOWN and attach an external interrupt on pin 2 -> Option C

4. Quick Check:

   Lowest power + external interrupt wake = Use SLEEP_MODE_PWR_DOWN and attach an external interrupt on pin 2 [OK]

Hint: Lowest power sleep needs external interrupt wake [OK]

Common Mistakes:

• Using idle mode which uses more power
• Trying to wake with delay() which doesn't work
• Polling button instead of using interrupt