Wake-up sources configuration in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a wake-up source in embedded systems?

A wake-up source is a hardware or software event that brings the microcontroller out of a low-power or sleep mode to resume normal operation.

beginner

Name three common wake-up sources in microcontrollers.

Common wake-up sources include external interrupts (like a button press), timers (RTC alarms), and communication peripherals (UART, I2C activity).

intermediate

How do you typically configure an external interrupt as a wake-up source in embedded C?

You enable the interrupt line, configure its trigger condition (rising/falling edge), and enable the microcontroller's wake-up capability on that interrupt line.

beginner

Why is it important to configure wake-up sources correctly?

Correct configuration ensures the device wakes up only on intended events, saving power and avoiding unwanted wake-ups that drain battery.

intermediate

Show a simple embedded C code snippet to enable a timer as a wake-up source.

// Enable timer interrupt as wake-up source Timer_EnableInterrupt(); Power_EnableWakeupSource(TIMER_INTERRUPT); Power_EnterSleepMode();

Which of the following can be a wake-up source in embedded systems?

A External interrupt

B Random number generator

C Display refresh

D Internal temperature sensor reading

External interrupts are common wake-up sources because they signal an event from outside the microcontroller.

What is the main purpose of configuring wake-up sources?

A To save power by waking up only on specific events

B To increase CPU speed

C To disable interrupts

D To reset the microcontroller

Wake-up sources help the device save power by waking only when needed.

Which trigger condition is commonly used for an external interrupt wake-up?

A Constant high level

B Rising edge

C No trigger

D Timer overflow

Rising edge triggers when the signal changes from low to high, commonly used for interrupts.

What happens if no wake-up source is configured before entering sleep mode?

A The device speeds up

B The device wakes up immediately

C The device resets

D The device may never wake up automatically

Without a wake-up source, the device stays in sleep mode indefinitely.

Which function might you call to enable a wake-up source in embedded C?

A UART_Close()

B Timer_Disable()

C Power_EnableWakeupSource()

D GPIO_SetLow()

Power_EnableWakeupSource() is typically used to enable a specific wake-up source.

Explain what a wake-up source is and why it is important in embedded systems.

Think about how devices save battery by sleeping and waking only when needed.

Describe the steps to configure an external interrupt as a wake-up source in embedded C.

Consider how you prepare the microcontroller to listen for an event before sleeping.

Practice

1. Which of the following is a common wake-up source in embedded C devices?

easy

A. GPIO pin interrupt

B. Floating point calculation

C. String concatenation

D. File system read

Solution

1. Step 1: Understand wake-up sources

   Wake-up sources are hardware events that can wake a device from sleep mode, such as GPIO interrupts or timers.

2. Step 2: Identify valid wake-up sources

   GPIO pin interrupts are commonly used as wake-up sources because they detect external signals.

3. Final Answer:

   GPIO pin interrupt -> Option A

4. Quick Check:

   Wake-up source = GPIO pin interrupt [OK]

Hint: Wake-up sources are hardware events like GPIO or timers [OK]

Common Mistakes:

• Confusing software operations with hardware wake-up sources
• Thinking file operations can wake embedded devices
• Assuming calculations can trigger wake-up

2. Which syntax correctly enables a GPIO pin as a wake-up source in embedded C?

easy

A. enable_wakeup(GPIO_PIN_5);

B. GPIO_WAKEUP_ENABLE(5);

C. wakeup_enable(GPIO5);

D. set_wakeup_pin(5);

Solution

1. Step 1: Recognize common embedded C macros

   Macros often use uppercase and underscores, like GPIO_WAKEUP_ENABLE(pin).

2. Step 2: Match syntax style

   GPIO_WAKEUP_ENABLE(5); matches typical embedded C macro style for enabling wake-up on GPIO pin 5.

3. Final Answer:

   GPIO_WAKEUP_ENABLE(5); -> Option B

4. Quick Check:

   Macro style = GPIO_WAKEUP_ENABLE(5) [OK]

Hint: Look for uppercase macro style for hardware config [OK]

Common Mistakes:

• Using lowercase function names not defined in embedded C
• Confusing parameter formats
• Missing parentheses or semicolons

3. What will be the output of the following code snippet?

void configure_wakeup() {
    WAKEUP_ENABLE_TIMER(1000); // 1000 ms
    WAKEUP_ENABLE_GPIO(3);
}

int main() {
    configure_wakeup();
    if (is_wakeup_source(GPIO_3)) {
        printf("Wakeup by GPIO 3\n");
    } else if (is_wakeup_source(TIMER)) {
        printf("Wakeup by Timer\n");
    } else {
        printf("No wakeup source\n");
    }
    return 0;
}

medium

A. Wakeup by GPIO 3

B. Wakeup by Timer

C. No wakeup source

D. Compilation error

Solution

1. Step 1: Analyze function and macros usage

   The code uses WAKEUP_ENABLE_TIMER and WAKEUP_ENABLE_GPIO macros but does not define or include them, causing compilation errors.

2. Step 2: Check function calls and conditions

   Functions like is_wakeup_source and constants GPIO_3, TIMER are undefined, leading to compilation failure.

3. Final Answer:

   Compilation error -> Option D

4. Quick Check:

   Undefined macros/functions cause compilation error [OK]

Hint: Undefined macros or functions cause compile errors [OK]

Common Mistakes:

• Assuming macros are predefined without headers
• Ignoring missing function definitions
• Confusing runtime output with compile errors

4. Identify the error in this wake-up source configuration code:

void setup_wakeup() {
    WAKEUP_ENABLE_GPIO(2);
    WAKEUP_ENABLE_GPIO(2);
    WAKEUP_ENABLE_TIMER(500);
}

medium

A. Duplicate GPIO wake-up enable causes conflict

B. Timer value 500 is invalid

C. Missing return statement

D. No error, code is correct

Solution

1. Step 1: Review GPIO wake-up configuration

   The code enables GPIO pin 2 twice, which can cause conflicts or redundant configuration.

2. Step 2: Check timer configuration and function structure

   Timer value 500 ms is valid; no return needed for void function.

3. Final Answer:

   Duplicate GPIO wake-up enable causes conflict -> Option A

4. Quick Check:

   Duplicate GPIO enable = conflict [OK]

Hint: Avoid enabling same GPIO wake-up twice [OK]

Common Mistakes:

• Thinking timer value is invalid
• Expecting return in void function
• Ignoring duplicate hardware config issues

5. You want to configure a device to wake up either by GPIO pin 4 or after 2 seconds using a timer. Which code snippet correctly sets both wake-up sources?

hard

A. WAKEUP_CONFIG(GPIO4 | TIMER_2S);

B. WAKEUP_ENABLE_TIMER(2);
WAKEUP_ENABLE_GPIO(4);

C. WAKEUP_ENABLE_GPIO(4);
WAKEUP_ENABLE_TIMER(2000);

D. WAKEUP_ENABLE_GPIO(4, 2000);

Solution

1. Step 1: Understand separate wake-up source configuration

   GPIO and timer wake-up sources are usually enabled with separate calls specifying pin and time in milliseconds.

2. Step 2: Evaluate options for correctness

   WAKEUP_ENABLE_GPIO(4); WAKEUP_ENABLE_TIMER(2000); correctly enables GPIO pin 4 and timer with 2000 ms (2 seconds). WAKEUP_ENABLE_TIMER(2); WAKEUP_ENABLE_GPIO(4); uses 2 (seconds) instead of milliseconds. WAKEUP_ENABLE_GPIO(4, 2000); incorrectly combines parameters. WAKEUP_CONFIG(GPIO4 | TIMER_2S); uses an undefined macro.

3. Final Answer:

   WAKEUP_ENABLE_GPIO(4); WAKEUP_ENABLE_TIMER(2000); -> Option C

4. Quick Check:

   Separate calls with correct units = WAKEUP_ENABLE_GPIO(4); WAKEUP_ENABLE_TIMER(2000); [OK]

Hint: Enable GPIO and timer separately with correct units [OK]

Common Mistakes:

• Using seconds instead of milliseconds for timer
• Combining GPIO and timer in one call
• Using undefined macros