Polling vs interrupt-driven execution in Embedded C - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of polling loop checking a flag

What will be the output of this embedded C code simulating a polling loop checking a flag variable?

Embedded C

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

volatile bool flag = false;

int main() {
    int count = 0;
    while (count < 5) {
        if (flag) {
            printf("Flag set!\n");
            flag = false;
            count++;
        } else {
            printf("Waiting...\n");
            count++;
        }
    }
    return 0;
}

A

Waiting...
Flag set!
Waiting...
Flag set!
Waiting...

B

Waiting...
Waiting...
Waiting...
Waiting...
Waiting...

C

Waiting...
Waiting...
Waiting...
Waiting...
Flag set!

D

Flag set!
Flag set!
Flag set!
Flag set!
Flag set!

💡 Hint

The flag variable is never set to true in this code, so the polling loop never detects it as true.

Since the flag variable is never changed to true, the if condition is never met. The program prints "Waiting..." five times and then exits.

❓ Predict Output

intermediate

Output of interrupt-driven flag set simulation

Given this simplified embedded C code simulating an interrupt setting a flag, what is the output?

Embedded C

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

volatile bool flag = false;

void interrupt_handler() {
    flag = true;
}

int main() {
    int count = 0;
    while (count < 3) {
        if (flag) {
            printf("Interrupt detected!\n");
            flag = false;
            count++;
        } else {
            printf("No interrupt\n");
        }
        if (count == 1) {
            interrupt_handler();
        }
    }
    return 0;
}

A

No interrupt
Interrupt detected!
Interrupt detected!

B

No interrupt
No interrupt
Interrupt detected!

C

No interrupt
No interrupt
No interrupt

D

Interrupt detected!
Interrupt detected!
Interrupt detected!

💡 Hint

The interrupt_handler sets the flag only after the first loop iteration.

Initially, flag is false, so "No interrupt" prints. After count == 1, interrupt_handler sets flag true. Next two loops detect the flag and print "Interrupt detected!" twice.

🧠 Conceptual

advanced

Why is interrupt-driven execution more efficient than polling?

Which of the following best explains why interrupt-driven execution is generally more efficient than polling in embedded systems?

A Interrupt-driven execution requires the CPU to check flags continuously, increasing CPU load.

B Polling uses less power because it constantly checks the flag, making it more efficient.

C Interrupt-driven execution allows the CPU to perform other tasks until an event occurs, reducing wasted cycles.

D Polling allows immediate response to events without delay, making it more efficient.

💡 Hint

Think about what the CPU does while waiting for an event in both methods.

Interrupt-driven execution lets the CPU do other work or sleep until an event triggers an interrupt, saving CPU cycles and power. Polling wastes CPU time checking repeatedly.

🔧 Debug

advanced

Identify the bug in this interrupt-driven flag example

What is the main problem with this embedded C code simulating an interrupt-driven flag?

Embedded C

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

volatile bool flag = false;

void interrupt_handler() {
    flag = true;
}

int main() {
    int count = 0;
    while (count < 3) {
        if (flag) {
            printf("Interrupt detected!\n");
            flag = false;
            count++;
        }
    }
    return 0;
}

A The count variable is not incremented inside the if statement.

B The interrupt_handler function is missing a return type.

C The main function should call interrupt_handler() inside the loop.

D The flag variable should be declared volatile to prevent compiler optimization issues.

💡 Hint

Consider how the compiler might optimize access to the flag variable without volatile.

Without volatile, the compiler may optimize away repeated reads of flag, causing the loop to never detect changes made by the interrupt handler.

🚀 Application

expert

Choosing between polling and interrupt-driven execution

You are designing an embedded system that must respond to a button press within 1 millisecond and also perform heavy background data processing. Which approach is best and why?

A Use interrupt-driven execution for the button press to ensure fast response, and polling for background processing to simplify design.

B Use polling for the button press to guarantee immediate detection, and interrupts for background processing to save power.

C Use polling for both button press and background processing to keep the system simple and predictable.

D Use interrupt-driven execution for background processing and polling for the button press to prioritize tasks.

💡 Hint

Consider which tasks require immediate response and which can be done in the background.

Interrupt-driven execution allows immediate response to the button press without wasting CPU cycles, while polling can be used for background tasks that are less time-critical.