Deep sleep mode in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enter deep sleep mode on an ARM processor.

ARM Architecture

SCB->SCR |= [1];

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 the wrong SCR bit like SLEEPONEXIT which controls sleep on ISR exit.

Confusing deep sleep with normal sleep bits.

The SCB_SCR_SLEEPDEEP_Msk bit in the System Control Block's SCR register enables deep sleep mode on ARM processors.

2fill in blank

medium

Complete the code to execute the WFI (Wait For Interrupt) instruction to enter sleep.

ARM Architecture

__[1]();

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

AWFI

BWFE

CSEV

DNOP

Attempts:

3 left

💡 Hint

Common Mistakes

Using WFE (Wait For Event) instead of WFI.

Using NOP which does nothing.

The WFI instruction tells the processor to wait for an interrupt, entering sleep mode until an interrupt occurs.

3fill in blank

hard

Fix the error in the code that attempts to clear the deep sleep bit.

ARM Architecture

SCB->SCR &= ~[1];

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

ASCB_SCR_SLEEPONEXIT_Msk

BSCB_SCR_SLEEPDEEP_Msk

CSCB_SCR_SEVONPEND_Msk

DSCB_SCR_SLEEP_Msk

Attempts:

3 left

💡 Hint

Common Mistakes

Clearing the wrong bit mask.

Forgetting to use the bitwise NOT operator.

To clear the deep sleep bit, you must use the SCB_SCR_SLEEPDEEP_Msk mask with bitwise NOT and AND operations.

4fill in blank

hard

Fill both blanks to create a dictionary that maps power modes to their descriptions.

ARM Architecture

power_modes = {"sleep": [1], "deep_sleep": [2]

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

A"Low power mode"

B"Processor halts until interrupt"

C"Processor halts until reset"

D"Lowest power consumption state"

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing descriptions of sleep and deep sleep modes.

Using incorrect descriptions that don't match the mode.

"sleep" mode means the processor halts until an interrupt, while "deep_sleep" is the lowest power consumption state.

5fill in blank

hard

Fill all three blanks to complete the code that sets the processor to deep sleep, executes WFI, and then clears the deep sleep bit.

ARM Architecture

SCB->SCR |= [1];
__[2]();
SCB->SCR &= ~[3];

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

ASCB_SCR_SLEEPDEEP_Msk

BWFI

DWFE

Attempts:

3 left

💡 Hint

Common Mistakes

Using WFE instead of WFI.

Not clearing the deep sleep bit after waking.

First, set the deep sleep bit with SCB_SCR_SLEEPDEEP_Msk, then execute the WFI instruction to enter sleep, and finally clear the deep sleep bit.

Practice

1. What is the main purpose of deep sleep mode in ARM architecture?

easy

A. To reset the system to factory settings

B. To save power by shutting down most of the system

C. To increase the processing speed temporarily

D. To enable debugging features

Solution

1. Step 1: Understand the function of deep sleep mode

   Deep sleep mode is designed to reduce power consumption by turning off most parts of the system when not in use.

2. Step 2: Compare options with this function

   Only To save power by shutting down most of the system correctly describes saving power by shutting down most of the system.

3. Final Answer:

   To save power by shutting down most of the system -> Option B

4. Quick Check:

   Deep sleep mode = Power saving [OK]

Hint: Deep sleep means power saving by shutting down [OK]

Common Mistakes:

• Confusing deep sleep with speed boost
• Thinking it resets the system
• Assuming it enables debugging

2. Which of the following is the correct step to enter deep sleep mode in ARM architecture?

easy

A. Clear the SLEEPDEEP bit and execute a WFE (Wait For Event) instruction

B. Enable the debug mode and halt the CPU

C. Set the SLEEPONEXIT bit and reset the processor

D. Set the SLEEPDEEP bit and execute a WFI (Wait For Interrupt) instruction

Solution

1. Step 1: Identify the control bit for deep sleep

   The SLEEPDEEP bit in the system control register must be set to enter deep sleep mode.

2. Step 2: Identify the instruction to enter sleep

   The WFI (Wait For Interrupt) instruction is used to put the processor into sleep after setting SLEEPDEEP.

3. Final Answer:

   Set the SLEEPDEEP bit and execute a WFI (Wait For Interrupt) instruction -> Option D

4. Quick Check:

   SLEEPDEEP + WFI = Deep sleep entry [OK]

Hint: Set SLEEPDEEP and use WFI to enter deep sleep [OK]

Common Mistakes:

• Using WFE instead of WFI
• Clearing SLEEPDEEP instead of setting it
• Confusing SLEEPONEXIT with SLEEPDEEP

3. Consider this ARM assembly snippet to enter deep sleep mode:

    MRS R0, SCR
    ORR R0, R0, #0x4
    MSR SCR, R0
    WFI

What is the effect of this code?

medium

A. The processor enters deep sleep mode

B. The processor enables debug mode

C. The processor resets immediately

D. The processor enters normal sleep mode

Solution

1. Step 1: Analyze the SCR register modification

   The code reads the System Control Register (SCR), sets bit 2 (value 0x4) which is the SLEEPDEEP bit, then writes it back.

2. Step 2: Understand the WFI instruction effect

   After setting SLEEPDEEP, executing WFI puts the processor into deep sleep mode.

3. Final Answer:

   The processor enters deep sleep mode -> Option A

4. Quick Check:

   SLEEPDEEP set + WFI = Deep sleep [OK]

Hint: Setting SCR bit 2 and WFI means deep sleep [OK]

Common Mistakes:

• Thinking it enters normal sleep
• Assuming it resets processor
• Confusing SCR bits

4. A developer writes this code to enter deep sleep mode but the processor does not enter it:

    MRS R0, SCR
    BIC R0, R0, #0x4
    MSR SCR, R0
    WFI

What is the likely error?

medium

A. The SLEEPDEEP bit is cleared instead of set

B. The WFI instruction is missing

C. The SCR register is not readable

D. The processor is in debug mode

Solution

1. Step 1: Examine the bit operation on SCR

   The BIC instruction clears bit 2 (0x4) of SCR, which disables deep sleep mode.

2. Step 2: Check the effect on sleep mode

   Since SLEEPDEEP bit is cleared, WFI will only enter normal sleep, not deep sleep.

3. Final Answer:

   The SLEEPDEEP bit is cleared instead of set -> Option A

4. Quick Check:

   SLEEPDEEP cleared = No deep sleep [OK]

Hint: Clearing SLEEPDEEP disables deep sleep mode [OK]

Common Mistakes:

• Assuming WFI is missing
• Thinking SCR is unreadable
• Blaming debug mode incorrectly

5. A device must save maximum power but wake quickly on an external interrupt. Which ARM deep sleep mode feature helps achieve this?

hard

A. Resetting the device periodically

B. Using normal sleep mode without interrupts

C. Using deep sleep mode with WFI and enabling wakeup interrupts

D. Disabling all interrupts before sleep

Solution

1. Step 1: Identify power saving with quick wakeup

   Deep sleep mode saves power by shutting down most components but can wake quickly on interrupts.

2. Step 2: Match the feature to the requirement

   Using WFI with deep sleep and enabling wakeup interrupts allows low power and fast wakeup.

3. Final Answer:

   Using deep sleep mode with WFI and enabling wakeup interrupts -> Option C

4. Quick Check:

   Deep sleep + interrupts = Power save + quick wake [OK]

Hint: Deep sleep + wakeup interrupts = power save and fast wake [OK]

Common Mistakes:

• Choosing normal sleep which saves less power
• Resetting device wastes power and time
• Disabling interrupts prevents wakeup