Why Input capture mode in Embedded C? - Purpose & Use Cases

The Big Idea

What if your device could catch events the instant they happen, without you watching every second?

The Scenario

Imagine you want to measure the exact time when a button is pressed on a device. Without input capture mode, you have to constantly check the button state in a loop, hoping to catch the moment it changes.

The Problem

This manual checking wastes processor time and can miss the exact moment the button is pressed, especially if the processor is busy doing other tasks. It's like trying to catch a falling ball by looking away and back quickly--you might miss it.

The Solution

Input capture mode lets the hardware automatically record the exact time an input event happens, without the processor constantly watching. This means you get precise timing and free up the processor for other work.

Before vs After

✗ Before

while(!button_pressed()) { /* wait */ }
record_time();

✓ After

configure_input_capture();
// hardware records time automatically on button press

What It Enables

Input capture mode enables precise and efficient timing of external events without wasting processor resources.

Real Life Example

Measuring the speed of a rotating wheel by capturing the exact time each rotation sensor triggers, allowing accurate speed calculation.

Key Takeaways

Manual polling wastes time and can miss events.

Input capture mode records event times automatically and precisely.

This frees the processor and improves accuracy.

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