State machine for protocol handling in Embedded C - Step-by-Step Execution

Concept Flow - State machine for protocol handling

Start

↓

Wait for Event

↓

Check Event Type

↓

Event A

↓

Handle A

↓

Update State

↓

Wait for Event

The state machine waits for events, checks their type, handles them accordingly, updates state, and loops back to wait for the next event.

Execution Sample

Embedded C

typedef enum {IDLE, RECEIVING, PROCESSING, DONE} State;
State current_state = IDLE;

void handle_event(char event) {
  switch(current_state) {
    case IDLE: if(event == 'S') current_state = RECEIVING; break;
    case RECEIVING: if(event == 'E') current_state = PROCESSING; break;
    case PROCESSING: current_state = DONE; break;
    case DONE: current_state = IDLE; break;
  }
}

This code shows a simple state machine that changes states based on events 'S' and 'E'.

Execution Table

Step	Current State	Event	Condition Checked	State Change	Next State
1	IDLE	'S'	event == 'S' is True	current_state = RECEIVING	RECEIVING
2	RECEIVING	'E'	event == 'E' is True	current_state = PROCESSING	PROCESSING
3	PROCESSING	any	no condition, move to DONE	current_state = DONE	DONE
4	DONE	any	no condition, move to IDLE	current_state = IDLE	IDLE
5	IDLE	'X'	event == 'S' is False	no change	IDLE

💡 Execution stops after step 5 because event 'X' does not trigger a state change.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
current_state	IDLE	RECEIVING	PROCESSING	DONE	IDLE	IDLE

Key Moments - 3 Insights

Why does the state not change when event 'X' is received in IDLE state?

Because the condition event == 'S' is False at step 5 in the execution_table, so no state change happens.

Why does the PROCESSING state move to DONE without checking an event?

At step 3, the code moves from PROCESSING to DONE unconditionally, as shown in the execution_table under Condition Checked.

What happens after the DONE state is reached?

At step 4, the state machine resets to IDLE unconditionally, preparing for new events.

Visual Quiz - 3 Questions

Look at the execution_table, what is the current_state after step 2?

A PROCESSING

B DONE

C RECEIVING

D IDLE

💡 Hint

Check the 'Next State' column at step 2 in the execution_table.

At which step does the state machine return to IDLE?

A Step 3

B Step 4

C Step 5

D Step 2

💡 Hint

Look at the 'Next State' column for when current_state becomes IDLE.

If event 'S' was received at step 5 instead of 'X', what would be the next state?

A PROCESSING

B IDLE

C RECEIVING

D DONE

💡 Hint

Refer to step 1 where event 'S' causes transition from IDLE to RECEIVING.

Concept Snapshot

State machine uses states and events to control flow.
Use enum for states and switch-case for transitions.
Check events in each state to decide next state.
Unconditional transitions are possible.
Loop back to wait for new events after handling.

Full Transcript

This example shows a simple state machine in embedded C for protocol handling. It starts in the IDLE state and waits for events. When event 'S' is received in IDLE, it moves to RECEIVING. Then, on event 'E' in RECEIVING, it moves to PROCESSING. From PROCESSING, it moves unconditionally to DONE, and from DONE back to IDLE. If an unknown event like 'X' is received in IDLE, no state change occurs. The execution table traces each step, showing conditions checked and state changes. The variable tracker shows how current_state changes after each step. Key moments clarify why some transitions happen unconditionally and why some events do not cause changes. The visual quiz tests understanding of state changes at specific steps. This pattern helps manage protocol states clearly and predictably in embedded systems.

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