Button debouncing in software in Embedded C - Time & Space Complexity

Time Complexity: Button debouncing in software

O(n)

Understanding Time Complexity

When we write code to handle button presses, we want to know how long it takes as the input changes.

Here, we ask: how does the time to check and debounce a button grow with the number of checks?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

#define DEBOUNCE_TIME 5

int debounceButton(int *buttonState, int (*readPin)(void)) {
    int count = 0;
    while (count < DEBOUNCE_TIME) {
        if (readPin() == *buttonState) {
            count++;
        } else {
            count = 0;
            *buttonState = readPin();
        }
    }
    return *buttonState;
}
    

This code checks a button state repeatedly until it reads the same value several times in a row to confirm the press.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The while loop that reads the button state repeatedly.
• How many times: Up to DEBOUNCE_TIME times in a row with the same reading.

How Execution Grows With Input

Each time the button state changes, the code resets the count and starts over.

Input Size (n)	Approx. Operations
5 (DEBOUNCE_TIME)	5 reads if stable
10 (twice stable)	10 reads if stable twice
100 (many changes)	Up to 100 reads if bouncing

Pattern observation: The time grows linearly with the number of reads needed to confirm stability.

Final Time Complexity

Time Complexity: O(n)

This means the time to debounce grows directly with the number of button state checks performed.

Common Mistake

[X] Wrong: "Debouncing always takes constant time regardless of input changes."

[OK] Correct: If the button bounces, the code resets counting and takes longer, so time depends on input stability.

Interview Connect

Understanding how input affects code timing shows you can think about real hardware behavior and write reliable software.

Self-Check

"What if we changed the debounce count to depend on a variable input instead of a fixed constant? How would the time complexity change?"

Practice

1. What is the main purpose of button debouncing in software?

easy

A. To ignore rapid, unwanted signals caused by mechanical button noise

B. To speed up the button press detection process

C. To increase the voltage of the button signal

D. To disable the button after first press

Solution

1. Step 1: Understand button signal noise

   Mechanical buttons often create multiple quick on/off signals when pressed, called noise or bouncing.

2. Step 2: Purpose of debouncing

   Debouncing waits for the signal to stabilize before confirming a press, ignoring these unwanted rapid signals.

3. Final Answer:

   To ignore rapid, unwanted signals caused by mechanical button noise -> Option A

4. Quick Check:

   Debouncing = Ignore noise [OK]

Hint: Debouncing filters noisy signals to detect one press [OK]

Common Mistakes:

• Thinking debouncing speeds up detection
• Confusing debouncing with hardware voltage changes
• Assuming debouncing disables buttons

2. Which of the following code snippets correctly implements a simple software debounce delay in embedded C?

easy

A. if(button_pressed()) { delay_ms(0); if(button_pressed()) { /* valid press */ } }

B. if(button_pressed()) { delay_ms(50); if(button_pressed()) { /* valid press */ } }

C. if(button_pressed()) { delay_ms(-50); if(button_pressed()) { /* valid press */ } }

D. if(button_pressed()) { delay_ms(5000); if(button_pressed()) { /* valid press */ } }

Solution

1. Step 1: Understand debounce delay

   A typical debounce delay is around 20-50 milliseconds to wait for signal stabilization.

2. Step 2: Analyze code options

   if(button_pressed()) { delay_ms(50); if(button_pressed()) { /* valid press */ } } uses 50 ms delay, which is correct. if(button_pressed()) { delay_ms(0); if(button_pressed()) { /* valid press */ } } uses 0 ms (no delay), C uses negative delay (invalid), D uses 5000 ms (too long).

3. Final Answer:

   if(button_pressed()) { delay_ms(50); if(button_pressed()) { /* valid press */ } } -> Option B

4. Quick Check:

   Debounce delay ~50ms = if(button_pressed()) { delay_ms(50); if(button_pressed()) { /* valid press */ } } [OK]

Hint: Use ~50ms delay to confirm button press [OK]

Common Mistakes:

• Using zero or negative delay
• Using too long delay causing slow response
• Not checking button state after delay

3. Consider this code snippet for debouncing a button press:

int debounce_button() {
  if (read_button_pin() == 0) {
    delay_ms(40);
    if (read_button_pin() == 0) {
      return 1;
    }
  }
  return 0;
}

What will debounce_button() return if the button is pressed and held steadily?

medium

A. 0

B. Random value

C. 1

D. Function will not return

Solution

1. Step 1: Check initial button state

   The function checks if the button pin reads 0 (pressed).

2. Step 2: Wait and re-check

   After 40 ms delay, it checks again. If still 0, it returns 1 indicating a valid press.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   Stable press returns 1 [OK]

Hint: Stable press returns 1 after delay check [OK]

Common Mistakes:

• Assuming immediate return without delay
• Confusing 0 and 1 as pressed/released
• Thinking function loops infinitely

4. This code is intended to debounce a button press but sometimes registers multiple presses. What is the likely problem?

if (read_button_pin() == 0) {
  delay_ms(10);
  if (read_button_pin() == 0) {
    button_pressed_flag = 1;
  }
}

medium

A. Delay of 10 ms is too short to filter bouncing

B. Button pin reading logic is inverted

C. Missing a delay after setting the flag

D. Using delay_ms instead of delay_us

Solution

1. Step 1: Analyze debounce delay duration

   10 ms delay is often too short to filter mechanical bouncing which can last up to 50 ms.

2. Step 2: Effect of short delay

   Short delay allows noise to pass, causing multiple press detections.

3. Final Answer:

   Delay of 10 ms is too short to filter bouncing -> Option A

4. Quick Check:

   Too short delay causes multiple presses [OK]

Hint: Use at least 20-50ms delay to debounce [OK]

Common Mistakes:

• Assuming logic inversion causes multiple presses
• Thinking delay after flag needed
• Confusing delay units (ms vs us)

5. You want to implement a software debounce that counts how many times a button is pressed without counting bounces. Which approach below best achieves this?

hard

A. Increment count immediately on button press without delay

B. Increment count only when button is released

C. Use a 1ms delay and increment count on every read

D. Check button state, wait 50ms, check again; if still pressed, increment count and wait until release

Solution

1. Step 1: Understand debounce counting logic

   To count presses without bounces, confirm stable press with delay, then count once.

2. Step 2: Avoid multiple counts during one press

   Waiting until release before next count prevents multiple increments from one press.

3. Final Answer:

   Check button state, wait 50ms, check again; if still pressed, increment count and wait until release -> Option D

4. Quick Check:

   Stable press + wait release = correct count [OK]

Hint: Confirm press, count once, wait release before next count [OK]

Common Mistakes:

• Counting immediately without debounce
• Using too short delay causing multiple counts
• Counting on release only misses presses