Standby mode behavior in Embedded C - Mini Project: Build & Apply

Standby Mode Behavior

📖 Scenario: You are working on a simple embedded system that needs to save power by entering standby mode when idle. In standby mode, the system stops most activities but can wake up on an external event.

🎯 Goal: Build a small program that simulates entering standby mode and waking up on an external event using flags and simple functions.

📋 What You'll Learn

Create a variable to represent the system state

Create a variable to represent an external wake-up event

Write a function to enter standby mode

Write a function to simulate an external event waking the system

Print the system state before and after standby mode

💡 Why This Matters

🌍 Real World

Embedded systems often need to save power by entering standby mode and waking up on events like button presses or sensor signals.

💼 Career

Understanding standby mode behavior is important for embedded software engineers working on low-power devices such as wearables, IoT sensors, and mobile electronics.

1

Create system state variable

Create an integer variable called system_state and set it to 0 to represent the system being active.

Embedded C

# Create an integer variable called system_state and set it to 0
# Your code here

Hint

Use int system_state = 0; to create the variable.

2

Create external event flag

Create an integer variable called wake_event and set it to 0 to represent no external event.

Embedded C

int system_state = 0; // 0 means active
// Create an integer variable called wake_event and set it to 0
# Your code here

Hint

Use int wake_event = 0; to create the variable.

3

Write standby and wake functions

Write a function called enter_standby() that sets system_state to 1 (standby). Write another function called wake_up() that sets wake_event to 1 and system_state back to 0 (active).

Embedded C

int system_state = 0; // 0 means active
int wake_event = 0; // 0 means no event
// Write enter_standby() and wake_up() functions
# Your code here

Hint

Define two functions: enter_standby() and wake_up() that change the variables as described.

4

Simulate standby and wake-up with output

Write a main() function that prints system_state, calls enter_standby(), prints system_state again, calls wake_up(), and finally prints system_state and wake_event.

Embedded C

int system_state = 0; // 0 means active
int wake_event = 0; // 0 means no event

void enter_standby() {
    system_state = 1; // 1 means standby
}

void wake_up() {
    wake_event = 1; // event occurred
    system_state = 0; // back to active
}

// Write main() to print states and call functions
# Your code here

Hint

Use printf to show the values and call the functions in order.

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