Event-driven state machine in Embedded C - Step-by-Step Execution

Concept Flow - Event-driven state machine

Start

↓

Wait for Event

↓

Event Received?

No→Wait for Event

Yes↓

Process Event

↓

Change State if Needed

↓

Perform Actions

↓

Back to Wait for Event

The state machine waits for events, processes them, changes state if needed, performs actions, then waits for the next event.

Execution Sample

Embedded C

typedef enum {IDLE, RUNNING, ERROR} State;
State current_state = IDLE;

void handle_event(int event) {
  switch(current_state) {
    case IDLE:
      if(event == 1) current_state = RUNNING;
      break;
    case RUNNING:
      if(event == 2) current_state = ERROR;
      break;
    case ERROR:
      if(event == 0) current_state = IDLE;
      break;
  }
}

This code changes the state based on events: from IDLE to RUNNING on event 1, RUNNING to ERROR on event 2, and ERROR to IDLE on event 0.

Execution Table

Step	Current State	Event	Condition Checked	State Change	New State
1	IDLE	1	event == 1 (true)	current_state = RUNNING	RUNNING
2	RUNNING	2	event == 2 (true)	current_state = ERROR	ERROR
3	ERROR	0	event == 0 (true)	current_state = IDLE	IDLE
4	IDLE	2	event == 1 (false)	no change	IDLE
5	IDLE	0	event == 1 (false)	no change	IDLE

💡 No more events to process; state machine waits for next event.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
current_state	IDLE	RUNNING	ERROR	IDLE	IDLE	IDLE
event	N/A	1	2	0	2	0

Key Moments - 2 Insights

Why does the state not change when event 2 is received in IDLE state (Step 4)?

Because in IDLE state, only event 1 triggers a state change. Step 4 shows event 2 does not meet the condition event == 1, so state stays IDLE.

How does the state machine return to IDLE from ERROR state?

At Step 3, when event 0 is received in ERROR state, the condition event == 0 is true, so current_state changes back to IDLE.

Visual Quiz - 3 Questions

Look at the execution table, what is the current_state after Step 2?

A ERROR

B IDLE

C RUNNING

D Undefined

💡 Hint

Check the 'New State' column at Step 2 in the execution_table.

At which step does the state machine change from ERROR to IDLE?

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Look for the row where current_state is ERROR and new state is IDLE.

If event 1 is received while in RUNNING state, what happens?

A No state change

B State changes to IDLE

C State changes to ERROR

D State machine resets

💡 Hint

Refer to the switch-case logic and see which events cause state changes in RUNNING state.

Concept Snapshot

Event-driven state machine in embedded C:
- Define states as enum.
- Use a variable to track current state.
- On event, check current state and event.
- Change state if conditions match.
- Repeat waiting for next event.

Full Transcript

An event-driven state machine waits for events and changes its state based on the current state and the event received. The example code uses an enum to define states and a switch-case to decide state transitions. The execution table shows step-by-step how the state changes with each event. Variables track the current state and event values. Key moments clarify why some events do not cause state changes and how the machine returns to the initial state. The quiz tests understanding of state transitions and event handling.