Sleep mode (WFI instruction) in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enter sleep mode using the WFI instruction.

ARM Architecture

asm volatile ("[1]" );

Drag options to blanks, or click blank then click option'

AWFI

BWFE

CNOP

DSVC

Attempts:

3 left

💡 Hint

Common Mistakes

Using WFE instead of WFI

Using NOP which does nothing

Using SVC which triggers a supervisor call

The WFI instruction tells the processor to enter sleep mode and wait for an interrupt.

2fill in blank

medium

Complete the code to enable sleep mode by setting the SLEEPDEEP bit before WFI.

ARM Architecture

SCB->SCR |= [1];
asm volatile ("WFI");

Drag options to blanks, or click blank then click option'

ASCB_SCR_SLEEPONEXIT_Msk

BSCB_SCR_SEVONPEND_Msk

CSCB_SCR_SLEEPDEEP_Msk

DSCB_SCR_SLEEP_Msk

Attempts:

3 left

💡 Hint

Common Mistakes

Using SLEEPONEXIT instead of SLEEPDEEP

Using SEVONPEND which controls event behavior

Using a non-existent SLEEP mask

The SLEEPDEEP bit in the System Control Register (SCR) must be set to enter deep sleep mode before executing WFI.

3fill in blank

hard

Fix the error in the code to correctly enter sleep mode with WFI.

ARM Architecture

SCB->SCR &= ~[1];
asm volatile ("WFI");

Drag options to blanks, or click blank then click option'

ASCB_SCR_SLEEP_Msk

BSCB_SCR_SLEEPONEXIT_Msk

CSCB_SCR_SEVONPEND_Msk

DSCB_SCR_SLEEPDEEP_Msk

Attempts:

3 left

💡 Hint

Common Mistakes

Using SLEEPONEXIT instead of SLEEPDEEP

Using SEVONPEND which is unrelated

Using a non-existent SLEEP mask

The SLEEPDEEP bit in the System Control Register (SCR) must be cleared to enter normal sleep mode (not deep sleep) before executing WFI.

4fill in blank

hard

Fill both blanks to set the processor to deep sleep mode and then enter sleep.

ARM Architecture

SCB->SCR [1]= [2];
asm volatile ("WFI");

Drag options to blanks, or click blank then click option'

A|

B&

CSCB_SCR_SLEEPDEEP_Msk

D~SCB_SCR_SLEEPDEEP_Msk

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND (&) instead of OR (|) to set bits

Using the complement (~) mask incorrectly

Clearing bits instead of setting

Use bitwise OR (|) to set the SLEEPDEEP bit before executing WFI to enter deep sleep mode.

5fill in blank

hard

Fill all three blanks to clear the deep sleep bit, set the sleep on exit bit, and then enter sleep mode.

ARM Architecture

SCB->SCR [1]= [2];
SCB->SCR [3]= SCB_SCR_SLEEPONEXIT_Msk;
asm volatile ("WFI");

Drag options to blanks, or click blank then click option'

A&

B~SCB_SCR_SLEEPDEEP_Msk

C|

DSCB_SCR_SLEEPDEEP_Msk

Attempts:

3 left

💡 Hint

Common Mistakes

Using OR to clear bits

Using AND without complement to clear bits

Mixing up bit masks

Clear the SLEEPDEEP bit using AND with its complement, then set SLEEPONEXIT using OR before WFI.

Practice

1. What is the primary purpose of the WFI instruction in ARM processors?

easy

A. To execute a debugging routine

B. To reset the processor immediately

C. To increase the processor clock speed temporarily

D. To put the processor into a low-power sleep mode until an interrupt occurs

Solution

1. Step 1: Understand the function of WFI

   The WFI instruction stands for "Wait For Interrupt" and is designed to reduce power consumption by putting the processor to sleep.

2. Step 2: Identify when the processor wakes up

   The processor wakes up only when an interrupt signal is received, allowing it to save energy during idle periods.

3. Final Answer:

   To put the processor into a low-power sleep mode until an interrupt occurs -> Option D

4. Quick Check:

   WFI = low-power sleep until interrupt [OK]

Hint: WFI means wait until an interrupt wakes the CPU [OK]

Common Mistakes:

• Confusing WFI with reset or clock speed instructions
• Thinking WFI wakes up on any signal, not just interrupts
• Assuming WFI increases performance instead of saving power

2. Which of the following is the correct syntax to execute the WFI instruction in ARM assembly?

easy

A. WFI()

B. WFI

C. wfi

D. wait_for_interrupt

Solution

1. Step 1: Recall ARM assembly instruction format

   ARM assembly instructions are usually uppercase and written without parentheses.

2. Step 2: Identify the correct WFI syntax

   The correct instruction is simply WFI in uppercase letters without parentheses or underscores.

3. Final Answer:

   WFI -> Option B

4. Quick Check:

   ARM instructions are uppercase and simple: WFI [OK]

Hint: ARM instructions are uppercase without parentheses [OK]

Common Mistakes:

• Adding parentheses like a function call
• Using lowercase which is not standard assembly syntax
• Using descriptive names instead of official instruction

3. Consider this ARM assembly snippet:

    MOV R0, #0
    WFI
    MOV R1, #1

What happens to the execution flow when the WFI instruction is reached?

medium

A. The processor enters sleep mode and resumes at the next instruction after an interrupt

B. The processor halts and never executes the next instruction

C. The processor immediately executes the next instruction without delay

D. The processor resets and starts from the beginning

Solution

1. Step 1: Understand WFI behavior in code flow

   When the processor executes WFI, it enters a low-power sleep state and pauses execution.

2. Step 2: Identify when execution resumes

   The processor wakes up only when an interrupt occurs, then continues with the next instruction MOV R1, #1.

3. Final Answer:

   The processor enters sleep mode and resumes at the next instruction after an interrupt -> Option A

4. Quick Check:

   WFI pauses until interrupt, then continues [OK]

Hint: WFI pauses execution until interrupt, then continues [OK]

Common Mistakes:

• Assuming WFI skips next instructions
• Thinking WFI causes a reset
• Believing WFI does nothing and continues immediately

4. You wrote this ARM assembly code:

    WFI()
    MOV R0, #5

But the assembler gives an error. What is the problem?

medium

A. WFI must be lowercase

B. MOV instruction cannot follow WFI

C. Parentheses are not allowed in the WFI instruction

D. R0 register cannot be used after WFI

Solution

1. Step 1: Check WFI syntax rules

   The WFI instruction is a standalone ARM assembly command and does not use parentheses.

2. Step 2: Identify syntax error cause

   Using WFI() causes a syntax error because parentheses are invalid in ARM assembly instructions.

3. Final Answer:

   Parentheses are not allowed in the WFI instruction -> Option C

4. Quick Check:

   WFI must be written without parentheses [OK]

Hint: No parentheses in ARM instructions like WFI [OK]

Common Mistakes:

• Adding parentheses like a function call
• Changing case incorrectly
• Assuming register usage causes error here

5. In an embedded system, you want to save power during idle times but still respond quickly to external events. How does using the WFI instruction help achieve this?

hard

A. It puts the processor to sleep until an interrupt wakes it, reducing power use without missing events

B. It speeds up the processor clock to handle events faster

C. It disables all interrupts to save power

D. It resets the processor to clear idle state

Solution

1. Step 1: Understand power saving with WFI

   The WFI instruction puts the processor into a low-power sleep mode during idle times.

2. Step 2: Recognize how interrupts affect wake-up

   The processor wakes immediately when an interrupt occurs, allowing quick response to external events.

3. Step 3: Connect power saving and responsiveness

   This combination reduces energy use without missing important signals, ideal for embedded systems.

4. Final Answer:

   It puts the processor to sleep until an interrupt wakes it, reducing power use without missing events -> Option A

5. Quick Check:

   WFI = sleep + wake on interrupt = power save + responsiveness [OK]

Hint: WFI sleeps but wakes instantly on interrupts [OK]

Common Mistakes:

• Thinking WFI disables interrupts
• Assuming WFI speeds up processing
• Believing WFI resets the processor