State Machines for Embedded Logic

States, events and transitions

As projects grow, nested ifs and boolean flags become unmanageable. A finite state machine names each mode the device can be in (IDLE, HEATING, ALARM), the events that matter (button press, timeout, sensor threshold) and the transitions between states. In C/C++ use an enum for states and a switch in loop(); each case does its non-blocking work and decides whether to change state. Record when the state was entered with millis() to implement timeouts. State machines combine naturally with non-blocking timing and interrupt flags, and they are easy to draw, review and test.

From sketch to dependable device

Real devices combine clear structure, careful networking and systematic debugging.

A traffic light with a pedestrian button

Arduino C++; pins are examples for an Uno R3 with LEDs through 330 Ω resistors.

enum class Light : uint8_t { Green, Yellow, Red };

const uint8_t RED = 10, YELLOW = 11, GREEN = 12, BUTTON = 2;
Light state = Light::Green;
unsigned long enteredAt = 0;
bool requested = false;

void enter(Light s) {
  state = s;
  enteredAt = millis();
  digitalWrite(GREEN,  s == Light::Green);
  digitalWrite(YELLOW, s == Light::Yellow);
  digitalWrite(RED,    s == Light::Red);
}

void setup() {
  pinMode(RED, OUTPUT); pinMode(YELLOW, OUTPUT); pinMode(GREEN, OUTPUT);
  pinMode(BUTTON, INPUT_PULLUP);
  enter(Light::Green);
}

void loop() {
  if (digitalRead(BUTTON) == LOW) requested = true;
  unsigned long inState = millis() - enteredAt;

  switch (state) {
    case Light::Green:
      if (requested && inState >= 5000) enter(Light::Yellow);
      break;
    case Light::Yellow:
      if (inState >= 2000) enter(Light::Red);
      break;
    case Light::Red:
      if (inState >= 6000) { requested = false; enter(Light::Green); }
      break;
  }
}

A board game

A state machine is like a board game: you are always on exactly one square, and the rules say which squares you can move to next and what makes you move.

Quick check: What is the main benefit of a state machine in embedded code?

• It replaces all interrupts
• It removes the need for any timing
• It makes the code run in parallel on several cores
• Behaviour is organised into explicit states and transitions that are easy to reason about