Debouncing as a state machine in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the initial state of the debounce state machine.

Embedded C

typedef enum { BUTTON_[1], BUTTON_PRESSED, BUTTON_RELEASED } ButtonState;

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AIDLE

BPRESSED

CRELEASED

DWAIT

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a state that implies the button is already pressed.

Using a state name not defined in the enum.

The initial state is usually IDLE, meaning no button press detected yet.

2fill in blank

medium

Complete the code to check if the button input is pressed (active low).

Embedded C

if (button_input == [1]) {

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A0

B1

CHIGH

DTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of 0 for pressed state.

Confusing HIGH and LOW logic levels.

For active low buttons, pressed means the input reads 0.

3fill in blank

hard

Fix the error in the debounce timer reset code.

Embedded C

debounce_timer = [1];

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A0

BDEBOUNCE_DELAY

C1

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Setting timer to 0 resets it immediately.

Using negative values causes errors.

The timer should be set to the debounce delay constant to start counting down.

4fill in blank

hard

Fill both blanks to update the state machine when the button is pressed and debounce timer expires.

Embedded C

if (debounce_timer == 0 && button_state == [1]) {
    button_state = [2];
}

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ABUTTON_IDLE

BBUTTON_PRESSED

CBUTTON_RELEASED

DBUTTON_WAIT

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong current state in condition.

Setting state to RELEASED instead of PRESSED.

When timer expires and state is IDLE, update state to PRESSED.

5fill in blank

hard

Fill all three blanks to implement the debounce state machine update logic.

Embedded C

switch (button_state) {
    case BUTTON_IDLE:
        if (button_input == [1]) {
            debounce_timer = [2];
            button_state = [3];
        }
        break;
    // other states omitted for brevity
}

Drag options to blanks, or click blank then click option'

A0

BDEBOUNCE_DELAY

CBUTTON_WAIT

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of 0 for pressed input.

Setting wrong state after starting timer.

When button input is 0 (pressed), start debounce timer and move to WAIT state.

Practice

1.

What is the main purpose of using debouncing in embedded C button handling?

easy

A. To avoid false triggers caused by mechanical noise

B. To speed up button press detection

C. To reduce power consumption

D. To increase button press sensitivity

Solution

1. Step 1: Understand mechanical noise in buttons

   Mechanical buttons can create multiple rapid on/off signals when pressed or released, called noise.

2. Step 2: Purpose of debouncing

   Debouncing filters out these rapid changes to detect a single clean press or release.

3. Final Answer:

   To avoid false triggers caused by mechanical noise -> Option A

4. Quick Check:

   Debouncing = Avoid false triggers [OK]

Hint: Debouncing stops noisy signals from causing multiple presses [OK]

Common Mistakes:

• Thinking debouncing speeds up detection
• Confusing debouncing with power saving
• Assuming it increases sensitivity

2.

Which of the following is the correct way to declare a state variable for a debounce state machine in embedded C?

?

easy

A. char state = 0;

B. int state = "IDLE";

C. enum ButtonState {IDLE, DEBOUNCE, PRESSED}; ButtonState state = IDLE;

D. bool state = false;

Solution

1. Step 1: Identify proper state variable type

   Using an enum clearly names states and improves code readability.

2. Step 2: Check syntax correctness

   enum ButtonState {IDLE, DEBOUNCE, PRESSED}; ButtonState state = IDLE; correctly declares an enum and initializes the state variable.

3. Final Answer:

   enum ButtonState {IDLE, DEBOUNCE, PRESSED}; ButtonState state = IDLE; -> Option C

4. Quick Check:

   Enum for states = enum ButtonState {IDLE, DEBOUNCE, PRESSED}; ButtonState state = IDLE; [OK]

Hint: Use enum for clear, readable state variables [OK]

Common Mistakes:

• Assigning string to int variable
• Using bool for multiple states
• Using char without clear meaning

3.

Given this snippet of a debounce state machine, what will be the value of state after 3 calls if the button is pressed and held?

enum State {IDLE, DEBOUNCE, PRESSED};
State state = IDLE;
int timer = 0;

void update(bool buttonPressed) {
  switch(state) {
    case IDLE:
      if(buttonPressed) {
        state = DEBOUNCE;
        timer = 2;
      }
      break;
    case DEBOUNCE:
      if(timer > 0) timer--;
      else state = PRESSED;
      break;
    case PRESSED:
      if(!buttonPressed) state = IDLE;
      break;
  }
}

// Calls:
update(true);
update(true);
update(true);

medium

A. PRESSED

B. Undefined

C. IDLE

D. DEBOUNCE

Solution

1. Step 1: Trace state changes through calls

   First call: state IDLE -> DEBOUNCE, timer=2. Second call: DEBOUNCE, timer=2>0, timer-- to 1, still DEBOUNCE. Third call: DEBOUNCE, timer=1>0, timer-- to 0, still DEBOUNCE (else not executed).

2. Step 2: Confirm final state after 3 calls

   Button is still pressed, state=DEBOUNCE (timer=0 now, changes to PRESSED on next call).

3. Final Answer:

   DEBOUNCE -> Option D

4. Quick Check:

   Timer counts down, state changes only if timer==0 at start of DEBOUNCE [OK]

Hint: Count timer ticks before state changes to PRESSED [OK]

Common Mistakes:

• Thinking PRESSED after exactly 3 calls
• Not decrementing timer
• Resetting state incorrectly

4.

Identify the bug in this debounce state machine code snippet:

enum State {IDLE, DEBOUNCE, PRESSED};
State state = IDLE;
int timer = 0;

void update(bool buttonPressed) {
  switch(state) {
    case IDLE:
      if(buttonPressed) {
        state = DEBOUNCE;
        timer = 3;
      }
      break;
    case DEBOUNCE:
      if(timer == 0) state = PRESSED;
      else timer--;
      break;
    case PRESSED:
      if(!buttonPressed) state = IDLE;
      break;
  }
}

What is the problem?

medium

A. State never changes from DEBOUNCE

B. Timer decrements after checking zero, causing one extra cycle

C. Timer is never initialized

D. Button press is ignored in PRESSED state

Solution

1. Step 1: Analyze timer decrement logic

   The code checks if timer == 0 before decrementing, so timer reaches 0 but state changes only next call.

2. Step 2: Identify effect on debounce timing

   This causes the debounce to last one extra cycle longer than intended.

3. Final Answer:

   Timer decrements after checking zero, causing one extra cycle -> Option B

4. Quick Check:

   Check timer before decrement causes delay [OK]

Hint: Decrement timer before zero check to fix timing [OK]

Common Mistakes:

• Thinking state never changes
• Ignoring timer initialization
• Assuming button ignored in PRESSED

5.

You want to implement a debounce state machine that only confirms a button press if the button is held for at least 5 cycles. Which approach below correctly implements this behavior?

enum State {IDLE, DEBOUNCE, PRESSED};
State state = IDLE;
int timer = 0;

void update(bool buttonPressed) {
  switch(state) {
    case IDLE:
      if(buttonPressed) {
        state = DEBOUNCE;
        timer = 5;
      }
      break;
    case DEBOUNCE:
      if(buttonPressed) {
        if(timer > 0) timer--;
        else state = PRESSED;
      } else {
        state = IDLE;
      }
      break;
    case PRESSED:
      if(!buttonPressed) state = IDLE;
      break;
  }
}

What happens if the button is pressed for 3 cycles then released?

hard

A. State returns to IDLE without reaching PRESSED

B. State changes to PRESSED after 3 cycles

C. State stays in DEBOUNCE indefinitely

D. State immediately changes to PRESSED on press

Solution

1. Step 1: Trace timer decrement with button held 3 cycles

   Timer starts at 5, decrements each cycle while buttonPressed is true, so after 3 cycles timer = 2.

2. Step 2: Button released before timer reaches zero

   On release, state immediately returns to IDLE without reaching PRESSED.

3. Final Answer:

   State returns to IDLE without reaching PRESSED -> Option A

4. Quick Check:

   Hold less than 5 cycles resets state [OK]

Hint: Hold button full timer cycles to reach PRESSED [OK]

Common Mistakes:

• Assuming PRESSED reached early
• Thinking DEBOUNCE lasts indefinitely
• Believing immediate PRESSED on press