Input capture mode in Embedded C - Step-by-Step Execution

Concept Flow - Input capture mode

Timer running

↓

Input signal edge detected

↓

Capture timer value

↓

Store captured value

↓

Process or read captured value

↓

Wait for next edge or event

The timer runs continuously until an input signal edge is detected, then the timer value is captured and stored for processing.

Execution Sample

Embedded C

volatile int captured_value = 0;
volatile int flag = 0;

void input_capture_ISR() {
  captured_value = TIMER_REG;
  flag = 1;
}

int main() {
  while(1) {
    if(flag) { process(captured_value); flag=0; }
  }
}

This code captures the timer value on an input event interrupt and processes it in the main loop.

Execution Table

Step	Event	Timer Value	Captured Value	Flag	Action
1	Timer running	1000	-	0	Waiting for input edge
2	Input edge detected	1025	-	0	ISR triggered
3	ISR execution	1025	1025	1	Captured timer value, set flag
4	Main loop check	1025	1025	1	Flag is set, process value
5	Processing done	1025	1025	0	Flag cleared, wait next edge
6	Timer running	1050	1025	0	Waiting for next input edge

💡 Loop runs indefinitely, capturing timer values on input edges

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
Timer Value	1000	1025	1025	1025	1025	1050
Captured Value	-	-	1025	1025	1025	1025
Flag	0	0	1	1	0	0

Key Moments - 2 Insights

Why does the captured value not change until the input edge occurs?

Because the timer runs continuously but the capture only happens inside the ISR when the input edge triggers it, as shown in steps 2 and 3 of the execution table.

Why do we use a flag variable after capturing the timer value?

The flag signals the main loop that a new captured value is ready to process, preventing missed or repeated processing. This is shown in steps 3 to 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the captured value immediately after the ISR executes?

A 1000

B 1025

C -

D 1050

💡 Hint

Check the 'Captured Value' column at step 3 in the execution_table.

At which step does the flag get cleared?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Look at the 'Flag' column in the execution_table and see when it changes from 1 to 0.

If the input edge never occurs, what happens to the captured value?

A It stays unchanged

B It keeps updating with timer value

C It resets to zero

D It causes an error

💡 Hint

Refer to the variable_tracker for 'Captured Value' before any input edge event.

Concept Snapshot

Input capture mode uses a timer running continuously.
When an input signal edge occurs, the timer value is captured and stored.
An interrupt service routine (ISR) handles the capture.
A flag signals the main program to process the captured value.
This allows precise timing measurement of input events.

Full Transcript

Input capture mode works by running a timer continuously. When an input signal edge is detected, an interrupt triggers and captures the current timer value. This value is stored in a variable. A flag is set to tell the main program that a new captured value is ready. The main program then processes this value and clears the flag. The timer keeps running, waiting for the next input edge. This method helps measure the exact time when an input event happens.

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