Exercise an ESP32 device state machine

Verify normal, timeout, wrong-state, fault, and reset transitions with buttons, LEDs, and structured serial proof.

Gateway Gus: I want every button, timeout, guard, retry, and reset to leave a state transition another engineer can audit., your practice guide

Gateway Gus: I want every button, timeout, guard, retry, and reset to leave a state transition another engineer can audit.
Predict the reading, then compare it with the measurement.

Wokwi ESP32

Third party Tool

Verify normal, timeout, wrong-state, fault, and reset transitions with buttons, LEDs, and structured serial proof.

Tier 1 · Web · No account

Version tested: Compiled and booted by the Wokwi gate (ESP32 Arduino core 3.3.11); captured in the Wokwi web editor on 2026-09-10. Date: 2026-09-10.

Open the ESP32 editor, paste diagram.json, then paste sketch.ino.

Open Wokwi to paste in the files (new tab)

Get the files

Use both prepared files. This is a paste-in setup; saving a project requires a Wokwi account.

diagram.json

2,318 bytes · Circuit connections

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{
  "version": 1, "author": "IoTClass", "editor": "wokwi",
  "parts": [
    { "type": "wokwi-esp32-devkit-v1", "id": "esp", "top": 35, "left": 0, "attrs": {} },
    { "type": "wokwi-pushbutton", "id": "sample", "top": -125, "left": -150, "attrs": { "color": "blue", "label": "Sample", "bounce": "0" } },
    { "type": "wokwi-pushbutton", "id": "ack", "top": 45, "left": -150, "attrs": { "color": "green", "label": "Ack", "bounce": "0" } },
    { "type": "wokwi-pushbutton", "id": "reset", "top": 215, "left": -150, "attrs": { "color": "red", "label": "Fault / Reset", "bounce": "0" } },
    { "type": "wokwi-led", "id": "idle", "top": -120, "left": 235, "attrs": { "color": "white", "label": "IDLE" } },
    { "type": "wokwi-resistor", "id": "r1", "top": -65, "left": 235, "attrs": { "value": "220" } },
    { "type": "wokwi-led", "id": "sampling", "top": 10, "left": 235, "attrs": { "color": "yellow", "label": "SAMPLE" } },
    { "type": "wokwi-resistor", "id": "r2", "top": 65, "left": 235, "attrs": { "value": "220" } },
    { "type": "wokwi-led", "id": "transmit", "top": 140, "left": 235, "attrs": { "color": "green", "label": "TRANSMIT" } },
    { "type": "wokwi-resistor", "id": "r3", "top": 195, "left": 235, "attrs": { "value": "220" } },
    { "type": "wokwi-led", "id": "fault", "top": 270, "left": 235, "attrs": { "color": "red", "label": "FAULT" } },
    { "type": "wokwi-resistor", "id": "r4", "top": 325, "left": 235, "attrs": { "value": "220" } }
  ],
  "connections": [
    [ "esp:TX0", "$serialMonitor:RX", "", [] ], [ "esp:RX0", "$serialMonitor:TX", "", [] ],
    [ "sample:1.l", "esp:D12", "blue", [] ], [ "sample:2.l", "esp:GND.1", "black", [] ],
    [ "ack:1.l", "esp:D13", "green", [] ], [ "ack:2.l", "esp:GND.1", "black", [] ],
    [ "reset:1.l", "esp:D14", "red", [] ], [ "reset:2.l", "esp:GND.1", "black", [] ],
    [ "esp:D2", "idle:A", "white", [] ], [ "idle:C", "r1:1", "black", [] ], [ "r1:2", "esp:GND.1", "black", [] ],
    [ "esp:D4", "sampling:A", "yellow", [] ], [ "sampling:C", "r2:1", "black", [] ], [ "r2:2", "esp:GND.1", "black", [] ],
    [ "esp:D5", "transmit:A", "green", [] ], [ "transmit:C", "r3:1", "black", [] ], [ "r3:2", "esp:GND.1", "black", [] ],
    [ "esp:D18", "fault:A", "red", [] ], [ "fault:C", "r4:1", "black", [] ], [ "r4:2", "esp:GND.1", "black", [] ]
  ]
}

sketch.ino

2,345 bytes · ESP32 program

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#include <Arduino.h>

enum State { IDLE, SAMPLE, TRANSMIT, FAULT };
State state = IDLE;
const int SAMPLE_BUTTON=12, ACK_BUTTON=13, FAULT_RESET_BUTTON=14;
const int LEDS[] = {2, 4, 5, 18};
bool priorSample=HIGH, priorAck=HIGH, priorReset=HIGH;
unsigned long enteredAt=0;
int retries=0;

const char* stateName(State value) {
  static const char* names[] = {"IDLE", "SAMPLE", "TRANSMIT", "FAULT"};
  return names[value];
}

void enterState(State next, const char* event, const char* proof) {
  State before=state; state=next; enteredAt=millis();
  for (int i=0;i<4;i++) digitalWrite(LEDS[i], i==state);
  Serial.printf("TRANSITION %s + %s -> %s proof=%s retries=%d\n",
    stateName(before), event, stateName(state), proof, retries);
}

void reject(const char* event) {
  Serial.printf("EVENT_REJECTED state=%s event=%s reason=wrong_state\n", stateName(state), event);
}

void setup() {
  Serial.begin(115200);
  pinMode(SAMPLE_BUTTON,INPUT_PULLUP); pinMode(ACK_BUTTON,INPUT_PULLUP); pinMode(FAULT_RESET_BUTTON,INPUT_PULLUP);
  for (int pin:LEDS) pinMode(pin,OUTPUT);
  digitalWrite(LEDS[IDLE],HIGH);
  Serial.println("ESP32 STATE MACHINE READY");
  Serial.println("STATE=IDLE retries=0");
}

void loop() {
  bool s=digitalRead(SAMPLE_BUTTON), a=digitalRead(ACK_BUTTON), r=digitalRead(FAULT_RESET_BUTTON);
  if (priorSample==HIGH && s==LOW) {
    if (state==IDLE) { retries=0; enterState(SAMPLE,"EV_SAMPLE_REQUEST","sample_timer_started"); }
    else reject("EV_SAMPLE_REQUEST");
  }
  if (priorAck==HIGH && a==LOW) {
    if (state==TRANSMIT) { retries=0; enterState(IDLE,"EV_ACK_RECEIVED","message_closed"); }
    else reject("EV_ACK_RECEIVED");
  }
  if (priorReset==HIGH && r==LOW) {
    if (state==FAULT) { retries=0; enterState(IDLE,"EV_RESET","manual_reset_safe"); }
    else if (state==SAMPLE) enterState(FAULT,"EV_SAMPLE_INVALID","fault_reason_recorded");
    else reject("EV_RESET");
  }
  priorSample=s; priorAck=a; priorReset=r;

if (state==SAMPLE && millis()-enteredAt>=500) enterState(TRANSMIT,"EV_SAMPLE_READY","message_id=42");
  if (state==TRANSMIT && millis()-enteredAt>=1200) {
    retries++;
    if (retries<3) {
      enteredAt=millis();
      Serial.printf("TRANSITION TRANSMIT + EV_TIMEOUT -> TRANSMIT proof=retry retries=%d\n",retries);
    } else enterState(FAULT,"EV_TIMEOUT","ack_timeout_radio_off");
  }
  delay(10);
}

Use the launch button above to open the ESP32 editor in Wokwi.
Select the editor’s diagram.json tab and replace all its text with the supplied diagram.json.
Select the sketch.ino tab, replace all its text with the supplied sketch.ino, then click Start Simulation.

Steps

Screens captured against Wokwi ESP32 Compiled and booted by the Wokwi gate (ESP32 Arduino core 3.3.11); captured in the Wokwi web editor on 2026-09-10 on 2026-09-10; the tool may have moved on — the text steps are the contract.

Step 1
Do
Paste the supplied diagram and sketch into Wokwi; identify the three event buttons and four state LEDs.
You will see
Sample, Ack, and Fault / Reset buttons feed GPIO12-14; white, yellow, green, and red LEDs represent IDLE, SAMPLE, TRANSMIT, and FAULT.
Why it matters
Named inputs and outputs make the state contract observable.
Step 2
Do
Inspect the State enum, enterState function, and wrong-state reject function.
You will see
The code names four states and logs every accepted or rejected event with context.
Why it matters
The transition owner should be one dispatcher, not unrelated callback side effects.
Step 3
Do
Start the simulation and open Serial Monitor at 115200 baud.
You will see
ESP32 STATE MACHINE READY appears followed by STATE=IDLE retries=0, with the white LED on.
Why it matters
A known initial state makes every later event traceable.
Step 4
Do
Press Sample once and wait for the simulated sample to complete.
You will see
IDLE + EV_SAMPLE_REQUEST -> SAMPLE is followed by SAMPLE + EV_SAMPLE_READY -> TRANSMIT and message_id=42.
Why it matters
This exercises the normal request and completion rows from the page's transition table.
Step 5
Do
Press Ack before the transmit timeout expires.
You will see
TRANSMIT + EV_ACK_RECEIVED -> IDLE appears with proof=message_closed and retries=0.
Why it matters
The acknowledgement closes the pending work and returns to a known state.
Step 6
Do
Press Sample again, do not acknowledge, and wait through three transmit timeouts.
You will see
Retries increment to 1 and 2, then TRANSMIT + EV_TIMEOUT -> FAULT reports ack_timeout_radio_off at retries=3.
Why it matters
A retry guard prevents the page's explicit unbounded-retry failure.
Step 7
Do
Press Ack while the machine is in FAULT.
You will see
EVENT_REJECTED state=FAULT event=EV_ACK_RECEIVED reason=wrong_state appears and the red LED stays on.
Why it matters
Late or wrong-state events must be explicit and must not silently change device state.
Step 8
Do
Press Fault / Reset once while FAULT is active.
You will see
FAULT + EV_RESET -> IDLE appears with proof=manual_reset_safe and retries=0.
Why it matters
Recovery should record why reset was allowed and return outputs and context to a known state.

Chapter checks

These questions refer to the chapter’s examples. Use the return links to review their answers.

A sensor alternates between SLEEP, SAMPLE, TRANSMIT, and FAULT. Why is this a good fit for a finite state machine?
Return to the chapter’s knowledge check
A TRANSMIT state retries forever after acknowledgement timeouts. Which transition-table change most directly fixes the design?
Return to the chapter’s knowledge check
A firmware design handles radio callbacks, sensor completions, button presses, and timer expirations. What is the safest state-machine pattern?
Return to the chapter’s knowledge check

Return to State Machine Fundamentals · Browse Labs

Wokwi ESP32

Get the files

diagram.json

sketch.ino

Steps

1 Step 1

2 Step 2

3 Step 3

4 Step 4

5 Step 5

6 Step 6

7 Step 7

8 Step 8

Chapter checks

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Step 7

Step 8