Sleep modes overview in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of MCU entering sleep mode

What will be the output of this embedded C code snippet when the microcontroller enters sleep mode?

Embedded C

#include <stdio.h>
#include <avr/sleep.h>

int main() {
    printf("Before sleep\n");
    set_sleep_mode(SLEEP_MODE_PWR_DOWN);
    sleep_enable();
    sleep_cpu();
    printf("After sleep\n");
    return 0;
}

A Before sleep\nAfter sleep

B Before sleep

C After sleep

D No output

💡 Hint

Consider what happens after sleep_cpu() in this code.

The microcontroller prints "Before sleep", then enters sleep mode. After waking up, it continues and prints "After sleep".

🧠 Conceptual

intermediate

Purpose of different sleep modes

Which sleep mode is best suited for the lowest power consumption while keeping the RAM contents intact?

A Power-down mode

B Standby mode

C Idle mode

D Extended standby mode

💡 Hint

Think about which mode stops most clocks but preserves RAM.

Power-down mode stops almost all clocks, giving the lowest power consumption while preserving RAM contents.

🔧 Debug

advanced

Identify the error in sleep mode usage

What is the error in this code that prevents the microcontroller from entering sleep mode?

Embedded C

#include <avr/sleep.h>

void enter_sleep() {
    set_sleep_mode(SLEEP_MODE_IDLE);
    sleep_enable();
    sleep_cpu();
    sleep_disable();
}

A sleep_disable() should be called before sleep_cpu()

B sleep_enable() is not needed

C Missing semicolon after set_sleep_mode line

D SLEEP_MODE_IDLE is not a valid mode

💡 Hint

Check syntax carefully line by line.

The line set_sleep_mode(SLEEP_MODE_IDLE) is missing a semicolon, causing a syntax error.

❓ Predict Output

advanced

Effect of sleep_disable() after sleep_cpu()

What is the output of this code snippet?

Embedded C

#include <stdio.h>
#include <avr/sleep.h>

int main() {
    printf("Start\n");
    set_sleep_mode(SLEEP_MODE_STANDBY);
    sleep_enable();
    sleep_cpu();
    sleep_disable();
    printf("End\n");
    return 0;
}

A No output

B Start\nEnd

C Start

D End

💡 Hint

Consider the order of sleep_disable() and sleep_cpu() calls.

The MCU prints "Start", enters standby sleep, wakes up, disables sleep, then prints "End".

🧠 Conceptual

expert

Wake-up sources in different sleep modes

Which wake-up source can bring the microcontroller out of Power-down mode?

A External interrupt

B Watchdog timer interrupt

C Timer/Counter interrupt

D All of the above

💡 Hint

Think about which interrupts are active in Power-down mode.

In Power-down mode, external interrupts, watchdog timer, and some timer interrupts can wake the MCU.