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.

Practice

1. What is the main purpose of Input Capture Mode in embedded C timer modules?

easy

A. To record the timer value when a specific input event occurs

B. To generate PWM signals automatically

C. To reset the timer periodically

D. To output a fixed frequency signal

Solution

1. Step 1: Understand Input Capture Mode function

   Input Capture Mode records the timer count when an input event (like a signal edge) happens.

2. Step 2: Compare options with this function

   Only To record the timer value when a specific input event occurs describes recording the timer value on an input event, which matches the purpose.

3. Final Answer:

   To record the timer value when a specific input event occurs -> Option A

4. Quick Check:

   Input Capture = Record timer on event [OK]

Hint: Input Capture records timer on input events [OK]

Common Mistakes:

• Confusing Input Capture with PWM generation
• Thinking it resets the timer automatically
• Assuming it outputs signals instead of capturing

2. Which of the following is the correct way to configure an input capture to trigger on a rising edge in embedded C?

easy

A. Clear the timer counter register

B. Set the capture edge select bit to 0

C. Set the capture edge select bit to 1

D. Enable the PWM output mode

Solution

1. Step 1: Identify capture edge select bit meaning

   Usually, setting the capture edge select bit to 1 configures capture on rising edge; 0 is for falling edge.

2. Step 2: Match options with rising edge setting

   Set the capture edge select bit to 1 sets the bit to 1, which is correct for rising edge trigger.

3. Final Answer:

   Set the capture edge select bit to 1 -> Option C

4. Quick Check:

   Edge select = 1 for rising edge [OK]

Hint: Rising edge capture usually means edge bit = 1 [OK]

Common Mistakes:

• Confusing edge select bit values
• Trying to clear timer instead of setting edge
• Enabling unrelated PWM mode

3. Given this code snippet for input capture:

volatile uint16_t captured_value = 0;
void ISR_InputCapture() {
  captured_value = TIMER_CAPTURE_REG;
  CLEAR_CAPTURE_INTERRUPT_FLAG();
}

// Assume timer increments every 1us
// Input event occurs at 1500us

What will be the value of captured_value after the input event?

medium

A. 1500

B. 0

C. Timer overflow value

D. Undefined

Solution

1. Step 1: Understand timer and capture behavior

   The timer increments every 1 microsecond, so at 1500us the timer count is 1500.

2. Step 2: ISR captures timer value on input event

   The ISR stores the current timer count (1500) into captured_value.

3. Final Answer:

   1500 -> Option A

4. Quick Check:

   Timer count at event = captured value = 1500 [OK]

Hint: Captured value equals timer count at event time [OK]

Common Mistakes:

• Assuming captured value resets to zero
• Confusing overflow with current count
• Ignoring ISR capture assignment

4. Identify the error in this input capture interrupt service routine:

void ISR_InputCapture() {
  uint16_t value = TIMER_CAPTURE_REG;
  process(value);
  CLEAR_CAPTURE_INTERRUPT_FLAG();
}

What is the main problem?

medium

A. The process function should not be called inside ISR

B. The interrupt flag is cleared before reading the capture register

C. The ISR does not enable the timer

D. The captured value is stored in a local variable and lost after ISR ends

Solution

1. Step 1: Analyze variable scope in ISR

   The variable value is local to ISR and lost after ISR finishes.

2. Step 2: Consider data usage outside ISR

   If the captured value is needed elsewhere, it should be stored in a global or volatile variable.

3. Final Answer:

   The captured value is stored in a local variable and lost after ISR ends -> Option D

4. Quick Check:

   Local variable in ISR = lost data [OK]

Hint: Store captured data in global variable, not local [OK]

Common Mistakes:

• Clearing interrupt flag too early
• Not enabling timer before capture
• Avoiding function calls in ISR (not always wrong)

5. You want to measure the pulse width of a signal using input capture mode on a timer. Which approach correctly uses input capture to find the pulse width?

hard

A. Reset timer on every input event and read timer after fixed delay

B. Capture timer value on rising edge, then capture again on falling edge; subtract first from second

C. Capture timer value only on rising edges and multiply by 2

D. Use input capture only on falling edges and ignore rising edges

Solution

1. Step 1: Understand pulse width measurement

   Pulse width is the time between rising and falling edges of a signal.

2. Step 2: Use input capture on both edges

   Capture timer value at rising edge (start) and falling edge (end), then subtract to get pulse width.

3. Final Answer:

   Capture timer value on rising edge, then capture again on falling edge; subtract first from second -> Option B

4. Quick Check:

   Pulse width = falling edge time - rising edge time [OK]

Hint: Measure pulse by capturing rising and falling edges [OK]

Common Mistakes:

• Capturing only one edge and guessing width
• Resetting timer incorrectly
• Ignoring one edge of the pulse