Standby mode behavior in Embedded C - Time & Space Complexity

Time Complexity: Standby mode behavior

O(n)

Understanding Time Complexity

When working with standby mode in embedded systems, it is important to understand how the program's execution time changes as it waits or checks for events.

We want to know how the time spent in standby grows as the system waits for input or conditions.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

void standby_mode(int timeout) {
    int counter = 0;
    while(counter < timeout) {
        if(check_event()) {
            break;
        }
        counter++;
    }
}
    

This code waits in standby, checking for an event repeatedly until a timeout or event occurs.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The while loop that checks for an event repeatedly.
• How many times: Up to timeout times, or fewer if the event occurs early.

How Execution Grows With Input

As the timeout value increases, the number of checks grows roughly the same amount.

Input Size (timeout)	Approx. Operations (event checks)
10	About 10 checks
100	About 100 checks
1000	About 1000 checks

Pattern observation: The number of operations grows directly with the timeout value.

Final Time Complexity

Time Complexity: O(n)

This means the time spent waiting grows linearly with the timeout value.

Common Mistake

[X] Wrong: "The standby loop runs instantly and does not depend on timeout."

[OK] Correct: The loop actually runs once per check until timeout or event, so more timeout means more checks and more time.

Interview Connect

Understanding how loops behave in standby or waiting code helps you reason about system responsiveness and efficiency in real embedded projects.

Self-Check

"What if we replaced the while loop with an interrupt-driven event check? How would the time complexity change?"

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