Standby mode behavior in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when MCU enters standby mode?

Consider the following embedded C code snippet for a microcontroller. What will be the output on the debug console before the MCU enters standby mode?

Embedded C

#include <stdio.h>
#include "mcu.h"  // hypothetical MCU header

int main() {
    printf("System initializing...\n");
    MCU_EnterStandbyMode();
    printf("This line after standby\n");
    return 0;
}

A System initializing...\nThis line after standby\n

B System initializing...\n

C No output at all

D System initializing...\nError: Standby mode failed

💡 Hint

Think about what happens to the CPU after entering standby mode.

When the MCU enters standby mode, it stops executing code until a wake-up event occurs. Therefore, the line after entering standby mode is never reached, so only the first print appears.

🧠 Conceptual

intermediate

Which event wakes the MCU from standby mode?

In embedded systems, what kind of event typically wakes the microcontroller from standby mode?

A An external interrupt or a configured wake-up source

B A reset triggered by the watchdog timer

C Completion of the current instruction

D A software command executed after entering standby

💡 Hint

Think about what can physically interrupt the MCU when it is in low power.

Standby mode stops the CPU until a wake-up event occurs. Usually, this is an external interrupt or a specific internal event configured as a wake-up source.

🔧 Debug

advanced

Why does the MCU not wake up after standby mode?

Given this code snippet, the MCU never wakes up from standby mode. Identify the bug.

Embedded C

#include "mcu.h"

int main() {
    MCU_EnableWakeupPin();
    MCU_EnterStandbyMode();
    while(1) {
        // MCU should wake up and run here
    }
    return 0;
}

A MCU_EnterStandbyMode() is called before enabling the wakeup pin

B Missing a delay before entering standby mode

C The infinite loop prevents the MCU from waking up

D Wakeup pin is enabled but not configured as an interrupt source

💡 Hint

Enabling a pin alone may not be enough to wake the MCU.

Enabling the wakeup pin without configuring it as an interrupt or wake-up source means the MCU will not detect the event to wake up.

📝 Syntax

advanced

Identify the syntactically correct standby mode code

Which option does NOT contain a syntax error preventing compilation?

Embedded C

void enter_standby() {
    MCU_EnterStandbyMode()
    printf("Entered standby\n");
}

A

void enter_standby() {
    MCU_EnterStandbyMode();
    printf("Entered standby\n");
}

B

void enter_standby() {
    MCU_EnterStandbyMode()
    printf("Entered standby\n");
}

C

void enter_standby() {
    MCU_EnterStandbyMode();
    printf("Entered standby\n")
}

D

void enter_standby() {
    MCU_EnterStandbyMode;
    printf("Entered standby\n");
}

💡 Hint

Check for missing semicolons.

Option A is correct with proper semicolons. Options B and C miss semicolons causing syntax errors. Option A uses MCU_EnterStandbyMode without parentheses, which is invalid.

🚀 Application

expert

How many bytes of RAM remain powered in standby mode?

An MCU has 64KB RAM. In standby mode, only the backup SRAM of 4KB remains powered. How many bytes of RAM are lost (powered off) during standby?

A 4,096 bytes

B 60,000 bytes

C 61,440 bytes

D 64,000 bytes

💡 Hint

Remember 1KB = 1024 bytes.

64KB = 64 * 1024 = 65,536 bytes. Backup SRAM = 4KB = 4 * 1024 = 4,096 bytes. Powered off RAM = 65,536 - 4,096 = 61,440 bytes (B). This tests understanding of memory units and subtraction.