Why state machines are used in embedded in Embedded C - Challenge Your Understanding

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why use state machines in embedded systems?

Which of the following best explains why state machines are commonly used in embedded programming?

A They replace the need for interrupts and timers in embedded systems.

B They automatically optimize the code for faster execution without programmer intervention.

C They allow embedded devices to run multiple operating systems simultaneously.

D They help organize complex behavior into clear states and transitions, making code easier to understand and maintain.

💡 Hint

Think about how breaking down tasks into steps helps manage complexity.

State machines break down complex device behavior into simple states and clear rules for moving between them. This makes the code easier to read, debug, and maintain.

❓ Predict Output

intermediate

Output of a simple embedded state machine code

What will be the output of this embedded C code simulating a state machine?

Embedded C

typedef enum {IDLE, RUNNING, ERROR} State;

State current_state = IDLE;

void run_state_machine() {
    switch(current_state) {
        case IDLE:
            printf("Idle state\n");
            current_state = RUNNING;
            break;
        case RUNNING:
            printf("Running state\n");
            current_state = ERROR;
            break;
        case ERROR:
            printf("Error state\n");
            current_state = IDLE;
            break;
    }
}

int main() {
    for(int i=0; i<3; i++) {
        run_state_machine();
    }
    return 0;
}

A

Idle state
Idle state
Idle state

B

Idle state
Running state
Error state

C

Running state
Error state
Idle state

D

Error state
Running state
Idle state

💡 Hint

Look at how the state changes after each print.

The code starts in IDLE, prints "Idle state", then moves to RUNNING. Next, it prints "Running state" and moves to ERROR. Finally, it prints "Error state" and returns to IDLE.

🔧 Debug

advanced

Identify the bug in this embedded state machine code

What error will this embedded C code cause when run?

Embedded C

typedef enum {START, PROCESS, STOP} State;

State current_state = START;

void run_state_machine() {
    switch(current_state) {
        case START:
            printf("Start\n");
            current_state = PROCESS;
            break;
        case PROCESS:
            printf("Process\n");
            // Missing break here
        case STOP:
            printf("Stop\n");
            current_state = START;
            break;
    }
}

int main() {
    for(int i=0; i<4; i++) {
        run_state_machine();
    }
    return 0;
}

A

The output will be:
Start
Process
Stop
Start
Process
Stop

B

The output will be:
Start
Process
Process
Start
Process
Process

C The program will crash with a segmentation fault.

D

The output will be:
Start
Stop
Start
Stop

💡 Hint

Check what happens when a break is missing in a switch-case.

Because the break is missing after PROCESS, the code falls through to STOP case, printing both "Process" and "Stop" each time PROCESS is reached.

📝 Syntax

advanced

Which option has correct syntax for a state machine transition?

Which of the following code snippets correctly updates the state variable in an embedded C state machine?

A current_state == RUNNING;

B current_state => RUNNING;

C current_state = RUNNING;

D current_state := RUNNING;

💡 Hint

Remember the assignment operator in C.

In C, '=' is used to assign a value. '==' is a comparison, ':=' and '=>' are invalid syntax.

🚀 Application

expert

How many states are in this embedded state machine?

Given this embedded C code, how many unique states does the state machine have?

Embedded C

typedef enum {
    INIT = 0,
    WAIT = 1,
    RUN = 2,
    PAUSE = 3,
    STOP = 4
} State;

State current_state = INIT;

void run_state_machine() {
    switch(current_state) {
        case INIT:
            current_state = WAIT;
            break;
        case WAIT:
            current_state = RUN;
            break;
        case RUN:
            current_state = PAUSE;
            break;
        case PAUSE:
            current_state = STOP;
            break;
        case STOP:
            current_state = INIT;
            break;
    }
}

A 5

B 4

C 6

D 3

💡 Hint

Count all the enum values representing states.

The enum defines 5 states: INIT, WAIT, RUN, PAUSE, and STOP.