Wake-up sources configuration in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Wake-up source bitmask configuration output

What is the output of the following code snippet configuring wake-up sources using bitmasks?

Embedded C

/* Define wake-up sources as bitmasks */
#define WAKEUP_TIMER   (1 << 0)
#define WAKEUP_GPIO    (1 << 1)
#define WAKEUP_RTC     (1 << 2)

unsigned int wakeup_sources = 0;

void enable_wakeup_sources() {
    wakeup_sources |= WAKEUP_TIMER;
    wakeup_sources |= WAKEUP_GPIO;
}

int main() {
    enable_wakeup_sources();
    printf("0x%X", wakeup_sources);
    return 0;
}

A 0x4

B 0x1

C 0x2

D 0x3

💡 Hint

Remember that bitwise OR combines bits set in both masks.

WAKEUP_TIMER is bit 0 (value 1), WAKEUP_GPIO is bit 1 (value 2). ORing them sets bits 0 and 1, resulting in 0x3.

❓ Predict Output

intermediate

Wake-up source configuration with conditional flags

What is the output of this code that conditionally enables wake-up sources?

Embedded C

#include <stdio.h>
#define WAKEUP_TIMER (1 << 0)
#define WAKEUP_GPIO  (1 << 1)
#define WAKEUP_RTC   (1 << 2)

unsigned int configure_wakeup(int enable_timer, int enable_gpio, int enable_rtc) {
    unsigned int sources = 0;
    if (enable_timer) sources |= WAKEUP_TIMER;
    if (enable_gpio) sources |= WAKEUP_GPIO;
    if (enable_rtc) sources |= WAKEUP_RTC;
    return sources;
}

int main() {
    unsigned int result = configure_wakeup(1, 0, 1);
    printf("0x%X", result);
    return 0;
}

A 0x5

B 0x3

C 0x6

D 0x7

💡 Hint

Check which bits correspond to enabled sources.

enable_timer=1 sets bit 0 (1), enable_gpio=0 skips bit 1, enable_rtc=1 sets bit 2 (4). 1 + 4 = 5 (0x5).

🔧 Debug

advanced

Identify the error in wake-up source configuration

What error does this code produce when compiling?

Embedded C

#define WAKEUP_TIMER (1 << 0)
#define WAKEUP_GPIO (1 << 1)

unsigned int wakeup_sources = 0;

void enable_sources() {
    wakeup_sources = wakeup_sources | WAKEUP_TIMER & WAKEUP_GPIO;
}

int main() {
    enable_sources();
    return 0;
}

A No error, compiles successfully

B Syntax error: missing semicolon

C Logical error: no bits are set (remains unchanged)

D Type error: incompatible types in bitwise operation

💡 Hint

Check operator precedence between | and &.

Bitwise AND (&) has higher precedence than OR (|), so expression evaluates as wakeup_sources | (WAKEUP_TIMER & WAKEUP_GPIO). Since WAKEUP_TIMER & WAKEUP_GPIO is 0, only wakeup_sources remains unchanged.

📝 Syntax

advanced

Find the syntax error in wake-up source configuration

Which option contains the correct syntax to enable both WAKEUP_TIMER and WAKEUP_GPIO bits?

A wakeup_sources |= (WAKEUP_TIMER & WAKEUP_GPIO);

B wakeup_sources |= WAKEUP_TIMER | WAKEUP_GPIO;

C wakeup_sources =+ WAKEUP_TIMER | WAKEUP_GPIO;

D wakeup_sources = wakeup_sources | WAKEUP_TIMER & WAKEUP_GPIO;

💡 Hint

Check operator precedence and assignment operators.

Option B correctly uses bitwise OR to set both bits. Option B uses AND which results in 0. Option B uses invalid operator '=+'. Option B has operator precedence issue, setting no bits.

🚀 Application

expert

Calculate number of enabled wake-up sources

Given the wakeup_sources variable holds enabled wake-up bits, what is the number of enabled sources after this code runs?

Embedded C

#define WAKEUP_TIMER (1 << 0)
#define WAKEUP_GPIO  (1 << 1)
#define WAKEUP_RTC   (1 << 2)
#define WAKEUP_USB   (1 << 3)

unsigned int wakeup_sources = 0;

void enable_sources() {
    wakeup_sources |= WAKEUP_TIMER;
    wakeup_sources |= WAKEUP_GPIO;
    wakeup_sources |= WAKEUP_RTC;
    wakeup_sources &= ~WAKEUP_GPIO;
}

int count_enabled_sources(unsigned int sources) {
    int count = 0;
    while (sources) {
        count += sources & 1;
        sources >>= 1;
    }
    return count;
}

int main() {
    enable_sources();
    int enabled = count_enabled_sources(wakeup_sources);
    printf("%d", enabled);
    return 0;
}

A 2

B 3

C 4

D 1

💡 Hint

Check which bits remain set after clearing WAKEUP_GPIO.

Initially, TIMER, GPIO, RTC bits are set (bits 0,1,2). Then GPIO bit (bit 1) is cleared. So only TIMER and RTC remain set, total 2 bits.

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