Input capture mode in Embedded C

Introduction

Input capture mode helps you measure the exact time when an event happens on a pin, like when a button is pressed.

You want to measure how long a button is pressed.

You need to record the time of a signal change, like a sensor output.

You want to measure the speed of a rotating wheel using a sensor.

You want to detect the exact moment a signal goes high or low.

Syntax

Embedded C

void InputCapture_Init(void) {
    // Configure timer for input capture
    // Set capture edge (rising/falling)
    // Enable input capture interrupt
}

void InputCapture_ISR(void) {
    // Read captured timer value
    // Process the captured time
    // Clear interrupt flag
}

Input capture uses a timer peripheral to record the timer value when an input event occurs.

You usually set which edge (rising or falling) triggers the capture.

Examples

Initialize input capture to record time when signal rises.

Embedded C

void InputCapture_Init(void) {
    // Set timer prescaler
    // Configure input capture on channel 1
    // Capture on rising edge
    // Enable interrupt
}

Interrupt service routine reads the captured timer value and clears the interrupt.

Embedded C

void InputCapture_ISR(void) {
    uint16_t capturedValue = TIMER_CAPTURE_REGISTER;
    // Use capturedValue to calculate pulse width
    CLEAR_INTERRUPT_FLAG();
}

Sample Program

This program simulates input capture by initializing, then simulating an event that triggers the interrupt and prints the captured timer value.

Embedded C

#include <stdint.h>
#include <stdio.h>

volatile uint16_t captured_time = 0;

// Simulated timer capture register
uint16_t TIMER_CAPTURE_REGISTER = 1234;

// Simulated interrupt flag
int INTERRUPT_FLAG = 0;

void InputCapture_Init(void) {
    // Normally hardware setup here
    printf("Input capture initialized.\n");
}

void InputCapture_ISR(void) {
    captured_time = TIMER_CAPTURE_REGISTER;
    INTERRUPT_FLAG = 0; // Clear interrupt
    printf("Captured time: %u\n", captured_time);
}

int main(void) {
    InputCapture_Init();
    // Simulate an input capture event
    INTERRUPT_FLAG = 1;
    if (INTERRUPT_FLAG) {
        InputCapture_ISR();
    }
    return 0;
}

Important Notes

Input capture depends on hardware timers and interrupts, so it may look different on various microcontrollers.

Always clear the interrupt flag inside the ISR to avoid repeated triggers.

Summary

Input capture mode records the timer value when an input event happens.

It helps measure timing of signals without missing events.

You set which edge triggers the capture and handle the event in an interrupt.

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