Sleep mode (WFI instruction) in ARM Architecture - Step-by-Step Execution

Concept Flow - Sleep mode (WFI instruction)

CPU executes WFI

↓

CPU checks for interrupts

↓

Enter sleep

↓

Wait for interrupt

↓

Interrupt occurs

↓

CPU wakes up and resumes execution

The CPU executes the WFI instruction, checks for interrupts, and if none are pending, it enters sleep mode until an interrupt wakes it up.

Execution Sample

ARM Architecture

WFI  ; Wait For Interrupt
// CPU sleeps until an interrupt occurs
// Then resumes execution after waking up

This code puts the CPU into sleep mode until an interrupt wakes it up.

Analysis Table

Step	Action	Interrupt Pending?	CPU State	Result
1	Execute WFI instruction	No	Check interrupts	CPU checks if any interrupt is pending
2	Interrupt check	No	Enter sleep mode	CPU goes to sleep waiting for interrupt
3	Wait	No	Sleeping	CPU remains in low power state
4	Interrupt occurs	Yes	Wake up	CPU wakes up from sleep
5	Resume execution	Yes	Running	CPU continues normal operation after interrupt
6	Execute next instruction	Depends	Running	CPU processes interrupt or next code
7	End	N/A	Running	WFI cycle complete

💡 CPU wakes up when an interrupt occurs, ending sleep mode and resuming execution

State Tracker

Variable	Start	After Step 2	After Step 4	Final
CPU State	Running	Sleeping	Waking up	Running
Interrupt Pending	No	No	Yes	Depends

Key Insights - 3 Insights

Why does the CPU not sleep if an interrupt is already pending when WFI is executed?

Because at Step 2 in the execution_table, the CPU checks for pending interrupts before sleeping. If an interrupt is pending, it skips sleep and immediately wakes up to handle it.

What happens if no interrupt occurs after WFI is executed?

As shown in Step 3, the CPU stays in sleep mode indefinitely, waiting for an interrupt to wake it up.

Does WFI clear the interrupt or just wait for it?

WFI only waits for an interrupt; it does not clear it. The interrupt is handled after the CPU wakes up, as seen in Step 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the CPU state immediately after executing the WFI instruction if no interrupt is pending?

A Sleeping

B Running

C Waking up

D Halted

💡 Hint

Refer to Step 2 and Step 3 in the execution_table where CPU enters sleep mode when no interrupt is pending.

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

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Check the 'CPU State' and 'Result' columns in the execution_table for when the CPU changes from sleeping to running.

If an interrupt is pending before WFI executes, what will the CPU do?

A Enter sleep mode anyway

B Ignore the interrupt and continue running

C Wake up immediately without sleeping

D Reset the interrupt

💡 Hint

Look at Step 2 in the execution_table where the CPU checks for interrupts before sleeping.

Concept Snapshot

WFI (Wait For Interrupt) puts the CPU into low power sleep mode.
CPU checks for pending interrupts before sleeping.
If interrupt pending, CPU wakes immediately.
CPU stays asleep until an interrupt occurs.
After waking, CPU resumes normal execution.

Full Transcript

The WFI instruction causes the CPU to enter sleep mode to save power. When the CPU executes WFI, it first checks if any interrupt is pending. If yes, it wakes immediately without sleeping. If no interrupt is pending, the CPU enters sleep mode and waits. The CPU stays asleep until an interrupt occurs. When an interrupt happens, the CPU wakes up and resumes normal execution. This process helps reduce power consumption during idle times.