Standby mode behavior in Embedded C - Step-by-Step Execution

Concept Flow - Standby mode behavior

Start Normal Operation

↓

Check Standby Request?

No→Continue Normal Operation

Yes↓

Save Context & Peripherals

↓

Enter Standby Mode

↓

Wait for Wake-up Event

↓

Wake-up Event Detected

↓

Restore Context & Peripherals

↓

Resume Normal Operation

The system checks if standby is requested, saves state, enters low power standby, waits for wake-up, then restores state and resumes.

Execution Sample

Embedded C

void enter_standby() {
  save_context();
  standby_mode_enable();
  wait_for_wakeup();
  restore_context();
}

This code saves the system state, enters standby mode, waits for a wake-up event, then restores the state.

Execution Table

Step	Action	Function Called	System State	Notes
1	Check if standby requested	N/A	Normal operation	Decision point
2	Save context and peripherals	save_context()	Context saved	Prepare for standby
3	Enable standby mode	standby_mode_enable()	Entering standby	CPU and peripherals powered down
4	Wait for wake-up event	wait_for_wakeup()	Standby mode active	System halted, low power
5	Wake-up event detected	N/A	Wake-up triggered	Interrupt or event detected
6	Restore context and peripherals	restore_context()	Context restored	System ready to resume
7	Resume normal operation	N/A	Normal operation	Back to main program

💡 Wake-up event detected, standby mode exited, system resumes normal operation

Variable Tracker

Variable	Start	After Step 2	After Step 6	Final
context_saved	false	true	true	true
standby_enabled	false	true	false	false
system_state	normal	pre-standby	post-standby	normal

Key Moments - 3 Insights

Why do we save the context before entering standby?

Because the CPU and peripherals power down in standby, saving context (see step 2 in execution_table) preserves the system state to restore after wake-up.

What causes the system to exit standby mode?

A wake-up event like an interrupt or external signal (step 5 in execution_table) triggers exit from standby mode.

Why is the system state 'pre-standby' after saving context but before standby mode?

Because context is saved but the system has not yet powered down (step 2), so it is preparing to enter standby.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the system actually in low power standby mode?

A Step 4 - Wait for wake-up event

B Step 3 - Enable standby mode

C Step 2 - Save context

D Step 6 - Restore context

💡 Hint

Check the 'System State' column for when 'Standby mode active' is shown.

According to variable_tracker, what is the value of 'standby_enabled' after restoring context?

A true

B undefined

C false

D null

💡 Hint

Look at the 'After Step 6' column for 'standby_enabled' variable.

If the wake-up event never occurs, what happens according to the execution flow?

A System resumes normal operation immediately

B System stays in standby mode indefinitely

C System restarts automatically

D System saves context again

💡 Hint

Refer to the concept_flow where system waits for wake-up event before resuming.

Concept Snapshot

Standby mode behavior:
- Save system context before standby
- Enter low power standby mode
- Wait for wake-up event (interrupt or signal)
- Restore context after wake-up
- Resume normal operation

Full Transcript

This visual execution trace shows how an embedded system enters standby mode. First, it checks if standby is requested. If yes, it saves the current context and peripheral states to preserve system information. Then it enables standby mode, which powers down CPU and peripherals to save energy. The system waits in this low power state until a wake-up event occurs, such as an interrupt or external signal. Upon wake-up, the system restores the saved context and peripherals, then resumes normal operation. Variables like 'context_saved' and 'standby_enabled' track the system state changes. Key moments include understanding why context saving is necessary and what triggers wake-up. The quizzes test knowledge of when the system is in standby, variable states, and behavior if no wake-up occurs.

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