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.