Standby mode behavior in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enter standby mode.

Embedded C

void enter_standby() {
    [1]();
}

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

Aenable_standby

Benter_standby_mode

Cstandby_mode_enable

Dstandby_enter

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function that only enables standby but does not enter it.

Confusing enabling with entering standby mode.

The function enter_standby_mode() is used to put the device into standby mode.

2fill in blank

medium

Complete the code to clear the standby flag after waking up.

Embedded C

void clear_standby_flag() {
    [1](STANDBY_FLAG);
}

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

Aflag_clear

Breset_flag

Cclear_flag

Dremove_flag

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function that resets all flags instead of clearing a specific one.

Using a function that does not exist in the API.

The function clear_flag() is used to clear specific flags like the standby flag.

3fill in blank

hard

Fix the error in the code to correctly configure the wakeup source before standby.

Embedded C

void configure_wakeup() {
    [1](WAKEUP_PIN);
}

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

Aenable_wakeup_source

Bwakeup_enable_source

Cset_wakeup_pin

Dactivate_wakeup

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function that sets the pin but does not enable the wakeup source.

Using a function name that is not part of the standard API.

The correct function to enable a wakeup source is enable_wakeup_source().

4fill in blank

hard

Fill both blanks to correctly check and clear the standby flag after wakeup.

Embedded C

if ([1](STANDBY_FLAG)) {
    [2](STANDBY_FLAG);
}

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

Ais_flag_set

Bclear_flag

Ccheck_flag

Dreset_flag

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same function for both checking and clearing.

Using functions that do not exist or have wrong parameters.

Use check_flag() to test if the standby flag is set, then clear_flag() to clear it.

5fill in blank

hard

Fill all three blanks to create a dictionary mapping wakeup pins to their status after standby.

Embedded C

wakeup_status = {{BLANK_4}}: [2] for [3] in wakeup_pins if check_flag([3])

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

Apin

Bget_pin_status()

Ddict

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Not calling the function to get pin status properly.

This dictionary comprehension maps each pin to its status using get_pin_status() for pins where check_flag(pin) is true.

Practice

1. What is the main purpose of putting a microcontroller into standby mode?

easy

A. To save power by entering a deep sleep state

B. To increase the clock speed temporarily

C. To reset all registers to default values

D. To enable all peripherals for faster processing

Solution

1. Step 1: Understand standby mode function

   Standby mode is designed to reduce power consumption by putting the device into a very low power state.

2. Step 2: Compare options with standby purpose

   Only To save power by entering a deep sleep state correctly describes saving power by deep sleep. Other options describe unrelated functions.

3. Final Answer:

   To save power by entering a deep sleep state -> Option A

4. Quick Check:

   Standby mode = power saving [OK]

Hint: Standby mode means deep sleep to save power [OK]

Common Mistakes:

• Confusing standby with reset or speed boost
• Thinking standby enables all peripherals
• Assuming standby clears registers

2. Which of the following code snippets correctly sets the microcontroller to enter standby mode in embedded C?

easy

A. PWR->CR &= ~PWR_CR_PDDS; SCB->SCR &= ~SCB_SCR_SLEEPDEEP_Msk; __WFI();

B. PWR->CR |= PWR_CR_PDDS; SCB->SCR &= ~SCB_SCR_SLEEPDEEP_Msk; __WFI();

C. PWR->CR |= PWR_CR_CWUF; SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk; __WFE();

D. PWR->CR |= PWR_CR_PDDS; SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk; __WFI();

Solution

1. Step 1: Identify correct register settings for standby

   Setting PWR_CR_PDDS bit and SLEEPDEEP bit enables standby mode deep sleep.

2. Step 2: Check the wait instruction

   __WFI() (Wait For Interrupt) is used to enter sleep mode properly.

3. Final Answer:

   PWR->CR |= PWR_CR_PDDS; SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk; __WFI(); -> Option D

4. Quick Check:

   Set PDDS + SLEEPDEEP + __WFI() = standby [OK]

Hint: Set PDDS and SLEEPDEEP bits, then call __WFI() [OK]

Common Mistakes:

• Clearing PDDS or SLEEPDEEP bits disables standby
• Using __WFE() instead of __WFI() incorrectly
• Not setting SLEEPDEEP bit before sleep

3. Given the code below, what will be the output after the microcontroller wakes up from standby mode?

volatile int flag = 0;
void enter_standby() {
  PWR->CR |= PWR_CR_PDDS;
  SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk;
  __WFI();
  flag = 1;
}

int main() {
  enter_standby();
  if(flag) {
    printf("Woke up from standby\n");
  } else {
    printf("Still running\n");
  }
  return 0;
}

medium

A. Woke up from standby

B. Compilation error due to volatile variable

C. Still running

D. No output (program stuck in standby)

Solution

1. Step 1: Understand standby mode effect on program flow

   Standby mode resets the device on wake-up, so code after __WFI() is not executed.

2. Step 2: Analyze flag variable and output

   Since flag is set after __WFI(), but device resets, flag remains 0 and main restarts, so output will be "Still running" because flag is 0 after reset.

3. Final Answer:

   Still running -> Option C

4. Quick Check:

   Standby resets device, code after __WFI() not run, flag remains 0 [OK]

Hint: Standby resets device; code after __WFI() won't run [OK]

Common Mistakes:

• Assuming code after __WFI() runs after wake-up
• Ignoring device reset on standby wake-up
• Confusing volatile with persistence across reset

4. Identify the error in the following code snippet that prevents the microcontroller from entering standby mode:

void enter_standby() {
  PWR->CR |= PWR_CR_PDDS;
  SCB->SCR &= ~SCB_SCR_SLEEPDEEP_Msk;
  __WFI();
}

medium

A. PDDS bit is not set correctly

B. SLEEPDEEP bit is cleared, so device does not enter deep sleep

C. __WFI() is the wrong instruction to enter standby

D. Missing enabling of power clock before setting registers

Solution

1. Step 1: Check SLEEPDEEP bit usage

   SLEEPDEEP bit must be set to enter deep sleep (standby). Clearing it disables deep sleep.

2. Step 2: Verify other settings

   PDDS bit is set correctly; __WFI() is correct; power clock enabling is not shown but not always mandatory here.

3. Final Answer:

   SLEEPDEEP bit is cleared, so device does not enter deep sleep -> Option B

4. Quick Check:

   SLEEPDEEP cleared = no deep sleep [OK]

Hint: SLEEPDEEP bit must be set for standby mode [OK]

Common Mistakes:

• Clearing SLEEPDEEP bit disables standby
• Confusing __WFI() with __WFE()
• Forgetting to set PDDS bit

5. You want to configure a microcontroller to enter standby mode but wake up only when a specific external interrupt occurs. Which sequence correctly achieves this behavior?

hard

A. Enable external interrupt line; configure wake-up pin; set PDDS and SLEEPDEEP bits; call __WFI()

B. Set PDDS and SLEEPDEEP bits; call __WFI(); enable external interrupt after wake-up

C. Call __WFI(); then set PDDS and SLEEPDEEP bits; enable external interrupt

D. Enable external interrupt; call __WFE(); clear PDDS bit; set SLEEPDEEP bit

Solution

1. Step 1: Configure wake-up source before standby

   External interrupt line and wake-up pin must be enabled before entering standby.

2. Step 2: Set standby mode bits and enter sleep

   Set PDDS and SLEEPDEEP bits to enable standby, then call __WFI() to enter standby mode.

3. Final Answer:

   Enable external interrupt line; configure wake-up pin; set PDDS and SLEEPDEEP bits; call __WFI() -> Option A

4. Quick Check:

   Setup wake-up + set bits + __WFI() = standby with wake-up [OK]

Hint: Setup wake-up source before standby, then set bits and __WFI() [OK]

Common Mistakes:

• Enabling interrupts after entering standby
• Clearing PDDS bit disables standby
• Using __WFE() without proper wake-up config