Simple state machine with switch-case in Embedded C - Mini Project: Build & Apply

Simple state machine with switch-case

📖 Scenario: You are programming a simple traffic light controller for a small intersection. The traffic light cycles through three states: RED, GREEN, and YELLOW. Each state lasts for a certain number of seconds before switching to the next.

🎯 Goal: Build a simple state machine using switch-case in C to cycle through the traffic light states and print the current state.

📋 What You'll Learn

Create an enum for the traffic light states: RED, GREEN, YELLOW

Create a variable called current_state to hold the current state

Use a switch-case statement to handle state transitions

Print the current state name in each case

💡 Why This Matters

🌍 Real World

Traffic lights and many devices use state machines to control behavior based on current conditions.

💼 Career

Understanding state machines and switch-case logic is essential for embedded systems programming and device control.

1

Define the traffic light states

Create an enum called TrafficLight with the states RED, GREEN, and YELLOW. Then create a variable called current_state of type enum TrafficLight and set it to RED.

Embedded C

enum TrafficLight { RED, GREEN, YELLOW };
enum TrafficLight current_state = RED;
// Your code here

Hint

Use enum TrafficLight { RED, GREEN, YELLOW }; to define the states and then enum TrafficLight current_state = RED; to set the initial state.

2

Add a variable for the timer

Create an int variable called timer and set it to 0. This will count seconds in each state.

Embedded C

enum TrafficLight { RED, GREEN, YELLOW };
enum TrafficLight current_state = RED;
int timer = 0;
// Your code here

Hint

Just add int timer = 0; below the current_state variable.

3

Implement the state machine logic with switch-case

Write a switch statement on current_state. For each case (RED, GREEN, YELLOW), print the state name using printf. Then update current_state to the next state in this order: RED -> GREEN -> YELLOW -> RED.

Embedded C

enum TrafficLight { RED, GREEN, YELLOW };
enum TrafficLight current_state = RED;
int timer = 0;

switch (current_state) {
    // Your code here
}

Hint

Use switch (current_state) and inside each case, print the state name and update current_state to the next state.

4

Print the current state

Add a printf statement after the switch-case block to print the text "Current state updated.".

Embedded C

enum TrafficLight { RED, GREEN, YELLOW };
enum TrafficLight current_state = RED;
int timer = 0;

switch (current_state) {
    case RED:
        printf("RED\n");
        current_state = GREEN;
        break;
    case GREEN:
        printf("GREEN\n");
        current_state = YELLOW;
        break;
    case YELLOW:
        printf("YELLOW\n");
        current_state = RED;
        break;
}

// Your code here

Hint

Use printf("Current state updated.\n"); after the switch-case block.

Practice

1.

What is the main purpose of using a switch-case statement in a simple state machine?

easy

A. To create loops that repeat forever

B. To handle different states clearly and execute code based on the current state

C. To declare variables for each state

D. To stop the program immediately

Solution

1. Step 1: Understand the role of switch-case in state machines

   The switch-case statement lets the program choose what to do based on the current state value.

2. Step 2: Recognize how states are handled

   Each case corresponds to a state, so the program runs code specific to that state.

3. Final Answer:

   To handle different states clearly and execute code based on the current state -> Option B

4. Quick Check:

   switch-case manages states = To handle different states clearly and execute code based on the current state [OK]

Hint: Switch-case matches states to actions clearly [OK]

Common Mistakes:

• Thinking switch-case creates loops
• Confusing variable declaration with state handling
• Assuming switch-case stops the program

2.

Which of the following is the correct syntax for a switch statement in embedded C?

switch(state) {
  case 0:
    // code
    break;
  case 1:
    // code
    break;
  default:
    // code
}

easy

A. switch state { case 0: ... break; case 1: ... break; default: ... }

B. switch(state) { case 0: ... case 1: ... default: ... }

C. switch(state) case 0: ... break; case 1: ... break; default: ...

D. switch(state) { case 0: ... break; case 1: ... break; default: ... }

Solution

1. Step 1: Check the syntax structure of switch-case

   The switch statement requires parentheses around the variable and braces around the cases.

2. Step 2: Verify the use of colons and breaks

   Each case must have a colon and usually ends with a break to prevent fall-through.

3. Final Answer:

   switch(state) { case 0: ... break; case 1: ... break; default: ... } -> Option D

4. Quick Check:

   Correct switch-case syntax = switch(state) { case 0: ... break; case 1: ... break; default: ... } [OK]

Hint: Remember switch needs parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses after switch
• Missing colons after case labels
• Forgetting break statements

3.

What will be the output of this code snippet?

int state = 1;
switch(state) {
  case 0:
    printf("State is zero\n");
    break;
  case 1:
    printf("State is one\n");
    break;
  default:
    printf("Unknown state\n");
}

medium

A. Unknown state

B. State is zero

C. State is one

D. No output

Solution

1. Step 1: Identify the value of state

   The variable state is set to 1 before the switch statement.

2. Step 2: Match the case with state value

   The switch matches case 1, so it prints "State is one" and then breaks.

3. Final Answer:

   State is one -> Option C

4. Quick Check:

   state=1 prints "State is one" [OK]

Hint: Match state value to case label to find output [OK]

Common Mistakes:

• Confusing case 0 with case 1
• Forgetting break causes fall-through
• Assuming default runs when case matches

4.

Find the error in this state machine code snippet:

int state = 0;
switch(state) {
  case 0:
    printf("Start\n");
  case 1:
    printf("Running\n");
    break;
  default:
    printf("Unknown\n");
}

medium

A. Missing break after case 0 causes fall-through

B. Switch statement missing parentheses

C. Default case should be first

D. Variable state is not initialized

Solution

1. Step 1: Check each case for break statements

   Case 0 does not have a break, so after printing "Start", it continues to case 1.

2. Step 2: Understand fall-through effect

   This causes both "Start" and "Running" to print when state is 0, which may be unintended.

3. Final Answer:

   Missing break after case 0 causes fall-through -> Option A

4. Quick Check:

   Missing break causes fall-through = Missing break after case 0 causes fall-through [OK]

Hint: Always add break after each case to avoid fall-through [OK]

Common Mistakes:

• Ignoring missing break leads to unexpected output
• Thinking default must be first case
• Assuming uninitialized variable error

5.

You want to design a simple state machine with three states: INIT (0), PROCESS (1), and DONE (2). The machine should start at INIT, then move to PROCESS, and finally to DONE. Which code snippet correctly updates the state inside the switch-case?

int state = 0;
switch(state) {
  case 0:
    // Move to PROCESS
    state = 1;
    break;
  case 1:
    // Move to DONE
    state = 2;
    break;
  case 2:
    // Finished
    break;
}

hard

A. Correctly updates state from INIT to PROCESS to DONE

B. Does not update state, stays in INIT forever

C. Skips PROCESS and goes directly to DONE

D. Causes infinite loop without state change

Solution

1. Step 1: Check state transitions in each case

   Case 0 sets state to 1 (PROCESS), case 1 sets state to 2 (DONE), case 2 does nothing (finished).

2. Step 2: Confirm the order of states

   The code moves from INIT to PROCESS to DONE in order, updating the state variable correctly.

3. Final Answer:

   Correctly updates state from INIT to PROCESS to DONE -> Option A

4. Quick Check:

   State updates follow sequence = Correctly updates state from INIT to PROCESS to DONE [OK]

Hint: Update state variable inside each case to move forward [OK]

Common Mistakes:

• Not updating state variable inside cases
• Skipping intermediate states
• Forgetting break causing unexpected behavior