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.