Low-power design patterns in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of low-power mode entry code

What is the output of this embedded C code snippet that simulates entering a low-power mode and waking up?

Embedded C

#include <stdio.h>
#include <stdbool.h>

volatile bool wakeup_flag = false;

void enter_low_power_mode() {
    printf("Entering low power mode...\n");
    // Simulate low power mode by waiting for wakeup_flag
    while (!wakeup_flag) {
        // CPU sleeps here
    }
    printf("Woke up from low power mode!\n");
}

int main() {
    enter_low_power_mode();
    wakeup_flag = true;
    return 0;
}

A

Woke up from low power mode!
Entering low power mode...

B No output (infinite loop)

C

Entering low power mode...
Woke up from low power mode!

D Entering low power mode...

💡 Hint

Think about when wakeup_flag is set and how the while loop behaves.

The code enters a while loop waiting for wakeup_flag to become true. However, wakeup_flag is set to true only after enter_low_power_mode() returns, which never happens because of the infinite loop. So the program never prints the second message and never exits the loop.

🧠 Conceptual

intermediate

Best low-power design pattern for periodic sensor reading

Which low-power design pattern is best suited for a device that needs to read a sensor every 10 seconds and sleep otherwise?

A Use a timer interrupt to wake device every 10 seconds and sleep in between

B Busy-wait loop checking sensor every millisecond

C Keep CPU running at full speed and poll sensor continuously

D Disable all interrupts and run sensor reading in main loop

💡 Hint

Think about how to save power by sleeping and waking only when needed.

Using a timer interrupt to wake the device every 10 seconds allows the CPU to sleep most of the time, saving power. Busy-waiting or continuous polling wastes power.

🔧 Debug

advanced

Identify the bug causing high power consumption

This embedded C code is intended to put the device into low-power mode but the device never wakes up. What is the bug?

Embedded C

void low_power_sleep() {
    // Disable interrupts
    __disable_irq();
    // Enter sleep mode
    __WFI();
    // Enable interrupts
    __enable_irq();
}

int main() {
    while (1) {
        low_power_sleep();
        // Do some work after waking
    }
    return 0;
}

A The __WFI() instruction is missing a semicolon

B Interrupts are disabled before sleep and enabled after, so wakeup interrupts never happen

C The while loop should be a for loop

D The function low_power_sleep() is never called

💡 Hint

Think about how interrupts and sleep instructions interact.

Disabling interrupts before entering sleep prevents any interrupt from waking the CPU. The CPU stays asleep indefinitely.

📝 Syntax

advanced

Identify the syntax error in low-power mode code

Which option contains the correct syntax for entering low-power mode using a macro?

Embedded C

#define ENTER_LOW_POWER() __asm__("WFI")

void sleep_mode() {
    ENTER_LOW_POWER()
}

A

#define ENTER_LOW_POWER() __asm__("WFI")

void sleep_mode() {
    ENTER_LOW_POWER();
}

B

#define ENTER_LOW_POWER() __asm__("WFI")

void sleep_mode() {
    ENTER_LOW_POWER()
}

C

#define ENTER_LOW_POWER() __asm__("WFI")

void sleep_mode() {
    ENTER_LOW_POWER
}

D

#define ENTER_LOW_POWER() __asm__("WFI");

void sleep_mode() {
    ENTER_LOW_POWER();
}

💡 Hint

Check macro usage and semicolon placement.

Option A correctly defines the macro without a trailing semicolon and calls it with parentheses and a semicolon. Option A adds an extra semicolon in the macro causing double semicolons. Option A misses semicolon in call. Option A misses parentheses in call.

🚀 Application

expert

Calculate total sleep time in a low-power cycle

An embedded device sleeps for 5 seconds, wakes for 1 second to perform tasks, then repeats. After 10 cycles, what is the total time spent sleeping?

A 10 seconds

B 55 seconds

C 50 seconds

D 60 seconds

💡 Hint

Multiply sleep time per cycle by number of cycles.

Each cycle has 5 seconds sleep and 1 second awake. Total sleep time after 10 cycles is 5 * 10 = 50 seconds.