Input capture mode in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this input capture code snippet?

Consider the following embedded C code that captures input from a timer input capture register. What will be the printed output?

Embedded C

#include <stdio.h>

volatile unsigned int IC_Value1 = 0;
volatile unsigned int IC_Value2 = 0;
volatile unsigned char CaptureDone = 0;

void InputCapture_ISR(void) {
    static unsigned char count = 0;
    if(count == 0) {
        IC_Value1 = 1000; // Simulated capture value
        count++;
    } else if(count == 1) {
        IC_Value2 = 3000; // Simulated capture value
        CaptureDone = 1;
        count = 0;
    }
}

int main() {
    InputCapture_ISR();
    InputCapture_ISR();
    if(CaptureDone) {
        printf("Pulse width: %u\n", IC_Value2 - IC_Value1);
    } else {
        printf("Capture not done\n");
    }
    return 0;
}

A Pulse width: 2000

B Pulse width: 4000

C Capture not done

D Pulse width: 3000

💡 Hint

Look at how the ISR updates the capture values and when the flag is set.

The ISR simulates two captures: first at 1000, second at 3000. The pulse width is the difference: 3000 - 1000 = 2000.

🧠 Conceptual

intermediate

Which statement about input capture mode is correct?

Choose the correct statement about input capture mode in embedded systems.

A Input capture mode is used to output analog voltages from a microcontroller.

B Input capture mode generates PWM signals automatically without software intervention.

C Input capture mode measures the time between two external events using a timer.

D Input capture mode disables all interrupts during operation.

💡 Hint

Think about what input capture mode is designed to measure.

Input capture mode records the timer value when an external event occurs, allowing measurement of time intervals.

🔧 Debug

advanced

What error does this input capture code cause?

Analyze the following code snippet. What error will occur when it runs?

Embedded C

volatile unsigned int IC_Value1 = 0;
volatile unsigned int IC_Value2 = 0;
volatile unsigned char CaptureDone = 0;

void InputCapture_ISR(void) {
    static unsigned char count = 0;
    if(count == 0) {
        IC_Value1 = 1000;
        count++;
    } else if(count == 1) {
        IC_Value2 = 3000;
        CaptureDone = 1;
        count = 0;
    }
}

int main() {
    InputCapture_ISR();
    InputCapture_ISR();
    if(CaptureDone) {
        printf("Pulse width: %u\n", IC_Value2 - IC_Value1);
    } else {
        printf("Capture not done\n");
    }
    return 0;
}

A The code causes an infinite loop due to incorrect count increment.

B The condition 'if(count = 0)' assigns zero instead of comparing, so IC_Value1 is never set properly.

C The code runs correctly and prints 'Pulse width: 2000'.

D The code causes a compilation error due to missing semicolons.

💡 Hint

Check the if condition inside the ISR carefully.

The condition uses assignment '=' instead of comparison '==', so it always sets count to 0 and the first block always runs, preventing proper capture.

📝 Syntax

advanced

Which option fixes the syntax error in this input capture code?

Identify the correct fix for the syntax error in this input capture code snippet.

Embedded C

volatile unsigned int IC_Value1 = 0;
volatile unsigned int IC_Value2 = 0;
volatile unsigned char CaptureDone = 0;

void InputCapture_ISR(void) {
    static unsigned char count = 0;
    if(count == 0) {
        IC_Value1 = 1000;
        count++;
    } else if(count == 1) {
        IC_Value2 = 3000;
        CaptureDone = 1;
        count = 0;
    }
}

A Change 'void InputCapture_ISR(void)' to 'void InputCapture_ISR()'

B Replace 'volatile' with 'static' for IC_Value1 and IC_Value2

C Remove the else if block entirely

D Add a semicolon after 'static unsigned char count = 0;'

💡 Hint

Look for missing punctuation in variable declarations.

The declaration of 'count' is missing a semicolon, causing a syntax error.

🚀 Application

expert

How many valid pulse widths can this input capture code measure?

Given the following input capture ISR that stores two capture values before setting a done flag, how many distinct pulse widths can be measured before the flag resets?

Embedded C

volatile unsigned int IC_Value1 = 0;
volatile unsigned int IC_Value2 = 0;
volatile unsigned char CaptureDone = 0;

void InputCapture_ISR(void) {
    static unsigned char count = 0;
    if(count == 0) {
        IC_Value1 = TIMER_REG;
        count++;
    } else if(count == 1) {
        IC_Value2 = TIMER_REG;
        CaptureDone = 1;
        count = 0;
    }
}

A Only one pulse width can be measured before CaptureDone resets.

B Two pulse widths can be measured before CaptureDone resets.

C An unlimited number of pulse widths can be measured without resetting CaptureDone.

D No pulse widths can be measured because count never increments.

💡 Hint

Consider how the count variable and CaptureDone flag control measurement cycles.

The ISR captures two values per cycle and sets CaptureDone once. After that, it resets count, so only one pulse width is measured per CaptureDone flag set.