Debouncing as a state machine in Embedded C - Mini Project: Build & Apply

Debouncing as a State Machine

📖 Scenario: You are working on a small embedded system with a button input. When the button is pressed, the signal can be noisy and cause multiple unwanted triggers. To fix this, you will create a debouncing mechanism using a state machine in C.

🎯 Goal: Build a simple state machine in C to debounce a button input. The program will read a noisy button signal and output a clean, stable button press state.

📋 What You'll Learn

Create an enum for the debounce states

Create a variable to hold the current state

Write a function to update the state machine based on the button input

Print the debounced button state

💡 Why This Matters

🌍 Real World

Debouncing is used in embedded systems to ensure that noisy mechanical button presses do not cause multiple unwanted signals.

💼 Career

Understanding state machines and debouncing is important for embedded software engineers working on hardware interfaces and real-time systems.

1

Create debounce states enum

Create an enum called DebounceState with these exact states: BUTTON_UP, BUTTON_FALLING, BUTTON_DOWN, BUTTON_RISING.

Embedded C

# Your code here

Need a hint?

Use typedef enum { ... } DebounceState; to define the states.

2

Create current state variable

Create a variable called currentState of type DebounceState and initialize it to BUTTON_UP.

Embedded C

typedef enum {
    BUTTON_UP,
    BUTTON_FALLING,
    BUTTON_DOWN,
    BUTTON_RISING
} DebounceState;

// Your code here

Need a hint?

Declare currentState as DebounceState and set it to BUTTON_UP.

3

Write debounce state machine function

Write a function called debounceUpdate that takes an int buttonInput (0 or 1) and updates currentState using a switch statement on currentState. Implement these transitions:

- BUTTON_UP: if buttonInput == 0, stay BUTTON_UP, else go to BUTTON_FALLING
- BUTTON_FALLING: if buttonInput == 1, go to BUTTON_DOWN, else go to BUTTON_UP
- BUTTON_DOWN: if buttonInput == 1, stay BUTTON_DOWN, else go to BUTTON_RISING
- BUTTON_RISING: if buttonInput == 0, go to BUTTON_UP, else go to BUTTON_DOWN

Embedded C

typedef enum {
    BUTTON_UP,
    BUTTON_FALLING,
    BUTTON_DOWN,
    BUTTON_RISING
} DebounceState;

DebounceState currentState = BUTTON_UP;

// Your code here

Need a hint?

Use a switch on currentState and update it based on buttonInput as described.

4

Print debounced button state

Write a main function that calls debounceUpdate with these button inputs in order: 0, 1, 1, 0, 0. After each call, print the current debounced state as an integer using printf. Use printf("%d\n", currentState);.

Embedded C

typedef enum {
    BUTTON_UP,
    BUTTON_FALLING,
    BUTTON_DOWN,
    BUTTON_RISING
} DebounceState;

DebounceState currentState = BUTTON_UP;

void debounceUpdate(int buttonInput) {
    switch (currentState) {
        case BUTTON_UP:
            if (buttonInput == 0) {
                currentState = BUTTON_UP;
            } else {
                currentState = BUTTON_FALLING;
            }
            break;
        case BUTTON_FALLING:
            if (buttonInput == 1) {
                currentState = BUTTON_DOWN;
            } else {
                currentState = BUTTON_UP;
            }
            break;
        case BUTTON_DOWN:
            if (buttonInput == 1) {
                currentState = BUTTON_DOWN;
            } else {
                currentState = BUTTON_RISING;
            }
            break;
        case BUTTON_RISING:
            if (buttonInput == 0) {
                currentState = BUTTON_UP;
            } else {
                currentState = BUTTON_DOWN;
            }
            break;
    }
}

// Your code here

Need a hint?

Call debounceUpdate with each input and print currentState after each call.