State machine for protocol handling in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the initial state of the protocol.

Embedded C

enum State { IDLE, RECEIVING, PROCESSING, [1] };

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

ADONE

BWAITING

CERROR

DSENDING

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a state that does not represent sending data.

Using a state that is not part of the protocol flow.

The state SENDING is commonly used to represent the state where the protocol is sending data.

2fill in blank

medium

Complete the code to transition to the PROCESSING state when data is received.

Embedded C

if (event == DATA_RECEIVED) {
    current_state = [1];
}

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

AIDLE

BPROCESSING

CSENDING

DRECEIVING

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the state to IDLE instead of PROCESSING.

Using RECEIVING which is the previous state.

When data is received, the protocol should move to the PROCESSING state to handle the data.

3fill in blank

hard

Fix the error in the state transition to IDLE after sending is done.

Embedded C

if (event == SEND_COMPLETE) {
    current_state = [1];
}

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

AIDLE

BRECEIVING

CERROR

DPROCESSING

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the state to PROCESSING or RECEIVING incorrectly.

Using ERROR state without an error event.

After sending is complete, the protocol should return to the IDLE state to wait for new events.

4fill in blank

hard

Fill both blanks to complete the state machine switch-case for handling events.

Embedded C

switch (current_state) {
    case IDLE:
        if (event == START_RECEIVE) {
            current_state = [1];
        }
        break;
    case RECEIVING:
        if (event == DATA_RECEIVED) {
            current_state = [2];
        }
        break;
}

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

ARECEIVING

BPROCESSING

CIDLE

DSENDING

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the states in the blanks.

Using IDLE or SENDING incorrectly in these transitions.

From IDLE, on START_RECEIVE, move to RECEIVING. From RECEIVING, on DATA_RECEIVED, move to PROCESSING.

5fill in blank

hard

Fill all three blanks to complete the dictionary-like state transition table.

Embedded C

State next_state(State current, Event event) {
    if (current == [1] && event == START_SEND) {
        return [2];
    } else if (current == [3] && event == SEND_COMPLETE) {
        return IDLE;
    }
    return current;
}

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

APROCESSING

BIDLE

CSENDING

DRECEIVING

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the states for sending and idle.

Returning wrong states on events.

When in IDLE and START_SEND event occurs, move to SENDING. When in SENDING and SEND_COMPLETE event occurs, return to IDLE.

Practice

1. What is the main purpose of using a state machine in protocol handling in embedded C?

easy

A. To reduce the size of the compiled program

B. To speed up the processor clock

C. To avoid using any conditional statements

D. To clearly control the sequence of protocol steps and handle events safely

Solution

1. Step 1: Understand the role of a state machine

   A state machine organizes different states and transitions to manage complex sequences clearly.

2. Step 2: Relate to protocol handling

   Protocols require strict step-by-step control and safe event handling, which state machines provide.

3. Final Answer:

   To clearly control the sequence of protocol steps and handle events safely -> Option D

4. Quick Check:

   State machine controls protocol steps safely = C [OK]

Hint: State machines organize steps and events clearly [OK]

Common Mistakes:

• Thinking state machines speed up hardware
• Believing they reduce program size automatically
• Assuming they remove all conditionals

2. Which of the following is the correct way to define states for a protocol using enums in embedded C?

easy

A. enum ProtocolState {IDLE, WAIT_ACK, SEND_DATA, ERROR};

B. enum ProtocolState = {IDLE, WAIT_ACK, SEND_DATA, ERROR};

C. enum ProtocolState {IDLE; WAIT_ACK; SEND_DATA; ERROR};

D. enum ProtocolState (IDLE, WAIT_ACK, SEND_DATA, ERROR);

Solution

1. Step 1: Recall enum syntax in C

   Enums use the syntax: enum Name {value1, value2, ...}; with commas separating values.

2. Step 2: Check each option

   enum ProtocolState {IDLE, WAIT_ACK, SEND_DATA, ERROR}; uses correct syntax with commas and no assignment. Others use invalid syntax like '=' or semicolons.

3. Final Answer:

   enum ProtocolState {IDLE, WAIT_ACK, SEND_DATA, ERROR}; -> Option A

4. Quick Check:

   Enum syntax with commas = B [OK]

Hint: Enums use commas, not equals or semicolons [OK]

Common Mistakes:

• Using '=' after enum name
• Separating values with semicolons
• Using parentheses instead of braces

3. Given the following code snippet, what will be the output printed?

typedef enum {IDLE, WAIT_ACK, SEND_DATA} State;

void handle_event(State *state, int event) {
    switch (*state) {
        case IDLE:
            if (event == 1) {
                *state = SEND_DATA;
                printf("Sending data\n");
            }
            break;
        case SEND_DATA:
            if (event == 2) {
                *state = WAIT_ACK;
                printf("Waiting for ACK\n");
            }
            break;
        case WAIT_ACK:
            if (event == 3) {
                *state = IDLE;
                printf("ACK received, back to IDLE\n");
            }
            break;
    }
}

int main() {
    State current_state = IDLE;
    handle_event(¤t_state, 1);
    handle_event(¤t_state, 2);
    handle_event(¤t_state, 3);
    return 0;
}

medium

A. Sending data
Waiting for ACK
ACK received, back to IDLE

B. Waiting for ACK
Sending data
ACK received, back to IDLE

C. Sending data
ACK received, back to IDLE
Waiting for ACK

D. No output

Solution

1. Step 1: Trace first event with state IDLE and event 1

   State is IDLE, event 1 triggers transition to SEND_DATA and prints "Sending data".

2. Step 2: Trace second event with state SEND_DATA and event 2

   State is SEND_DATA, event 2 triggers transition to WAIT_ACK and prints "Waiting for ACK".

3. Step 3: Trace third event with state WAIT_ACK and event 3

   State is WAIT_ACK, event 3 triggers transition to IDLE and prints "ACK received, back to IDLE".

4. Final Answer:

   Sending data Waiting for ACK ACK received, back to IDLE -> Option A

5. Quick Check:

   Events 1,2,3 cause correct state prints = A [OK]

Hint: Follow state changes step-by-step with events [OK]

Common Mistakes:

• Mixing order of printed lines
• Ignoring state changes after events
• Assuming all events print output

4. Identify the bug in this state machine code snippet and select the fix:

typedef enum {IDLE, SEND, WAIT} State;

void process_event(State *state, int event) {
    switch (*state) {
        case IDLE:
            if (event == 1) {
                *state = SEND;
                printf("Start sending\n");
            }
        case SEND:
            if (event == 2) {
                *state = WAIT;
                printf("Waiting for response\n");
            }
            break;
        case WAIT:
            if (event == 3) {
                *state = IDLE;
                printf("Back to idle\n");
            }
            break;
    }
}

medium

A. Change enum values to strings

B. Add a break after the IDLE case to prevent fall-through

C. Remove the break after SEND case

D. Replace switch with if-else statements

Solution

1. Step 1: Analyze switch-case structure

   The IDLE case lacks a break, so after executing IDLE code, it falls through to SEND case unintentionally.

2. Step 2: Identify fix to prevent fall-through

   Adding a break after IDLE case stops execution from continuing into SEND case.

3. Final Answer:

   Add a break after the IDLE case to prevent fall-through -> Option B

4. Quick Check:

   Missing break causes fall-through = A [OK]

Hint: Always add break after each case unless intentional fall-through [OK]

Common Mistakes:

• Ignoring missing break causing bugs
• Changing enum types unnecessarily
• Removing needed breaks

5. You want to extend a protocol state machine to handle a timeout event that resets the state to IDLE from any state. Which is the best way to implement this in embedded C?

hard

A. Use goto statements inside switch-case to jump to IDLE

B. Add a new state TIMEOUT and transition only from WAIT state

C. Add a check for timeout event before the switch-case and reset state to IDLE if triggered

D. Ignore timeout events in the state machine and handle separately

Solution

1. Step 1: Understand requirement for timeout handling

   Timeout should reset to IDLE from any state immediately when detected.

2. Step 2: Choose implementation approach

   Checking timeout before switch-case allows immediate reset regardless of current state.

3. Step 3: Evaluate other options

   Adding TIMEOUT state limits reset to one state only; ignoring timeout misses requirement; goto is poor style and error-prone.

4. Final Answer:

   Add a check for timeout event before the switch-case and reset state to IDLE if triggered -> Option C

5. Quick Check:

   Pre-check timeout for immediate reset = D [OK]

Hint: Check global events before state-specific logic [OK]

Common Mistakes:

• Adding unnecessary states for global events
• Ignoring timeout in state machine
• Using goto for flow control