Event-driven state machine in Embedded C - Mini Project: Build & Apply

Event-driven state machine

📖 Scenario: You are programming a simple device that changes its behavior based on events like button presses. This device has three states: OFF, ON, and BLINKING. It starts in the OFF state.

🎯 Goal: Build a small event-driven state machine in C that changes the device state when events occur. You will create the states, define events, write the logic to handle events and update the state, and finally print the current state.

📋 What You'll Learn

Define an enum called State with values OFF, ON, and BLINKING

Define an enum called Event with values BUTTON_PRESS and TIMER_EXPIRE

Create a variable current_state of type State initialized to OFF

Write a function handle_event that takes an Event and updates current_state based on the rules:

OFF + BUTTON_PRESS -> ON

ON + BUTTON_PRESS -> BLINKING

BLINKING + BUTTON_PRESS -> OFF

BLINKING + TIMER_EXPIRE -> ON

Print the current state as a string after handling an event

💡 Why This Matters

🌍 Real World

Event-driven state machines are used in embedded devices like remote controls, appliances, and IoT gadgets to manage behavior based on user input or timers.

💼 Career

Understanding event-driven state machines is essential for embedded software engineers working on firmware for hardware devices.

1

Define states and initial state

Define an enum called State with values OFF, ON, and BLINKING. Then create a variable called current_state of type State and set it to OFF.

Embedded C

// Define enum State and variable current_state
// Your code here

Need a hint?

Use typedef enum { OFF, ON, BLINKING } State; and then State current_state = OFF;.

2

Define events

Define an enum called Event with values BUTTON_PRESS and TIMER_EXPIRE.

Embedded C

typedef enum {
    OFF,
    ON,
    BLINKING
} State;

State current_state = OFF;

// Define enum Event
// Your code here

Need a hint?

Use typedef enum { BUTTON_PRESS, TIMER_EXPIRE } Event;.

3

Write event handler function

Write a function called handle_event that takes an Event parameter named event. Inside, use switch on current_state and nested switch on event to update current_state following these rules: OFF + BUTTON_PRESS -> ON, ON + BUTTON_PRESS -> BLINKING, BLINKING + BUTTON_PRESS -> OFF, BLINKING + TIMER_EXPIRE -> ON. Do nothing for other cases.

Embedded C

typedef enum {
    OFF,
    ON,
    BLINKING
} State;

State current_state = OFF;

typedef enum {
    BUTTON_PRESS,
    TIMER_EXPIRE
} Event;

// Write handle_event function
// Your code here

Need a hint?

Use nested switch statements to check current_state and event, then update current_state accordingly.

4

Print current state after event

Write a main function that calls handle_event with BUTTON_PRESS, then prints the current state as a string. Then call handle_event with BUTTON_PRESS again and print the state. Finally, call handle_event with TIMER_EXPIRE and print the state. Use printf and a switch on current_state to print "OFF", "ON", or "BLINKING".

Embedded C

typedef enum {
    OFF,
    ON,
    BLINKING
} State;

State current_state = OFF;

typedef enum {
    BUTTON_PRESS,
    TIMER_EXPIRE
} Event;

void handle_event(Event event) {
    switch (current_state) {
        case OFF:
            switch (event) {
                case BUTTON_PRESS:
                    current_state = ON;
                    break;
                default:
                    break;
            }
            break;
        case ON:
            switch (event) {
                case BUTTON_PRESS:
                    current_state = BLINKING;
                    break;
                default:
                    break;
            }
            break;
        case BLINKING:
            switch (event) {
                case BUTTON_PRESS:
                    current_state = OFF;
                    break;
                case TIMER_EXPIRE:
                    current_state = ON;
                    break;
                default:
                    break;
            }
            break;
    }
}

// Write main function to test events and print states
// Your code here

Need a hint?

Call handle_event with the events in order, then print the state after each call using a switch and printf.