Standby mode behavior in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when MCU enters standby mode?

Consider the following embedded C code snippet for a microcontroller. What will be the output on the debug console before the MCU enters standby mode?

Embedded C

#include <stdio.h>
#include "mcu.h"  // hypothetical MCU header

int main() {
    printf("System initializing...\n");
    MCU_EnterStandbyMode();
    printf("This line after standby\n");
    return 0;
}

A System initializing...\nThis line after standby\n

B System initializing...\n

C No output at all

D System initializing...\nError: Standby mode failed

💡 Hint

Think about what happens to the CPU after entering standby mode.

When the MCU enters standby mode, it stops executing code until a wake-up event occurs. Therefore, the line after entering standby mode is never reached, so only the first print appears.

🧠 Conceptual

intermediate

Which event wakes the MCU from standby mode?

In embedded systems, what kind of event typically wakes the microcontroller from standby mode?

A An external interrupt or a configured wake-up source

B A reset triggered by the watchdog timer

C Completion of the current instruction

D A software command executed after entering standby

💡 Hint

Think about what can physically interrupt the MCU when it is in low power.

Standby mode stops the CPU until a wake-up event occurs. Usually, this is an external interrupt or a specific internal event configured as a wake-up source.

🔧 Debug

advanced

Why does the MCU not wake up after standby mode?

Given this code snippet, the MCU never wakes up from standby mode. Identify the bug.

Embedded C

#include "mcu.h"

int main() {
    MCU_EnableWakeupPin();
    MCU_EnterStandbyMode();
    while(1) {
        // MCU should wake up and run here
    }
    return 0;
}

A MCU_EnterStandbyMode() is called before enabling the wakeup pin

B Missing a delay before entering standby mode

C The infinite loop prevents the MCU from waking up

D Wakeup pin is enabled but not configured as an interrupt source

💡 Hint

Enabling a pin alone may not be enough to wake the MCU.

Enabling the wakeup pin without configuring it as an interrupt or wake-up source means the MCU will not detect the event to wake up.

📝 Syntax

advanced

Identify the syntactically correct standby mode code

Which option does NOT contain a syntax error preventing compilation?

Embedded C

void enter_standby() {
    MCU_EnterStandbyMode()
    printf("Entered standby\n");
}

A

void enter_standby() {
    MCU_EnterStandbyMode();
    printf("Entered standby\n");
}

B

void enter_standby() {
    MCU_EnterStandbyMode()
    printf("Entered standby\n");
}

C

void enter_standby() {
    MCU_EnterStandbyMode();
    printf("Entered standby\n")
}

D

void enter_standby() {
    MCU_EnterStandbyMode;
    printf("Entered standby\n");
}

💡 Hint

Check for missing semicolons.

Option A is correct with proper semicolons. Options B and C miss semicolons causing syntax errors. Option A uses MCU_EnterStandbyMode without parentheses, which is invalid.

🚀 Application

expert

How many bytes of RAM remain powered in standby mode?

An MCU has 64KB RAM. In standby mode, only the backup SRAM of 4KB remains powered. How many bytes of RAM are lost (powered off) during standby?

A 4,096 bytes

B 60,000 bytes

C 61,440 bytes

D 64,000 bytes

💡 Hint

Remember 1KB = 1024 bytes.

64KB = 64 * 1024 = 65,536 bytes. Backup SRAM = 4KB = 4 * 1024 = 4,096 bytes. Powered off RAM = 65,536 - 4,096 = 61,440 bytes (B). This tests understanding of memory units and subtraction.

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