Arduino sleep modes - Step-by-Step Execution

Concept Flow - Arduino sleep modes

Start Program

↓

Setup Sleep Mode

↓

Enable Sleep

↓

Enter Sleep

↓

Sleep Mode Active

↓

Wake Up Interrupt

↓

Resume Execution

↓

Repeat or End

The Arduino sets up a sleep mode, enables it, then enters sleep. It stays asleep until an interrupt wakes it, then resumes execution.

Execution Sample

Arduino

#include <avr/sleep.h>
void setup() {
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  sleep_enable();
  sleep_cpu();
}
void loop() {}

This code puts the Arduino into the deepest sleep mode once during setup.

Execution Table

Step	Action	Sleep Mode Set	Sleep Enabled	CPU State	Output
1	Start setup()	None	Disabled	Awake	None
2	Call set_sleep_mode(SLEEP_MODE_PWR_DOWN)	Power Down	Disabled	Awake	None
3	Call sleep_enable()	Power Down	Enabled	Awake	None
4	Call sleep_cpu()	Power Down	Enabled	Sleeping	None
5	CPU sleeps until interrupt	Power Down	Enabled	Sleeping	None
6	Interrupt wakes CPU	Power Down	Disabled	Awake	None
7	Resume normal execution	Power Down	Disabled	Awake	None
8	End of setup(), loop() runs empty	Power Down	Disabled	Awake	None

💡 CPU wakes up on interrupt, sleep_cpu() ends, program continues

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 6	Final
sleep_mode	None	Power Down	Power Down	Power Down	Power Down	Power Down
sleep_enabled	Disabled	Disabled	Enabled	Enabled	Disabled	Disabled
cpu_state	Awake	Awake	Awake	Sleeping	Awake	Awake

Key Moments - 3 Insights

Why does the CPU state change from 'Sleeping' back to 'Awake' after an interrupt?

When an interrupt occurs, it wakes the CPU from sleep mode, so the CPU state changes from 'Sleeping' to 'Awake' as shown between steps 5 and 6 in the execution table.

Does the Arduino keep sleeping forever after sleep_cpu() is called?

No, the Arduino stays asleep only until an interrupt wakes it. After waking, the program continues normally, as seen in step 7.

Is sleep_enable() enough to put the Arduino to sleep?

No, sleep_enable() just prepares the sleep mode. The actual sleep starts when sleep_cpu() is called, as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the CPU state immediately after calling sleep_cpu()?

A Sleeping

B Awake

C Disabled

D Power Down

💡 Hint

Check the 'CPU State' column at step 4 in the execution table.

At which step does the CPU wake up from sleep mode?

A Step 3

B Step 5

C Step 6

D Step 8

💡 Hint

Look for the step where 'CPU State' changes from 'Sleeping' to 'Awake' in the execution table.

If sleep_enable() was not called, what would be the CPU state after sleep_cpu()?

A Sleeping

B Awake

C Power Down

D Undefined

💡 Hint

Refer to the variable_tracker for 'sleep_enabled' and 'cpu_state' changes.

Concept Snapshot

Arduino Sleep Modes Quick Guide:
- Use set_sleep_mode() to choose sleep type.
- Call sleep_enable() to prepare sleep.
- Call sleep_cpu() to enter sleep.
- CPU sleeps until an interrupt wakes it.
- After waking, program continues normally.

Full Transcript

This visual trace shows how Arduino sleep modes work step-by-step. First, the program starts awake. Then it sets the sleep mode to Power Down. Next, sleep is enabled. When sleep_cpu() is called, the CPU goes to sleep. It stays asleep until an interrupt wakes it up. After waking, the CPU state returns to awake and the program continues. Variables like sleep_mode and sleep_enabled track the setup, while cpu_state shows if the CPU is awake or sleeping. Key moments include understanding that sleep_cpu() actually puts the CPU to sleep, and interrupts wake it back up. The quizzes test understanding of CPU state changes and the role of sleep_enable().