State Machines Part 1 - Overview

State Machines

ITP 348 - Introduction to Physical Computing

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Learning Objectives

  • Understand what a state machine is
  • Describe situations where a state machine is useful
  • Analyze a real-world situation to create states, transitions, and input
  • Implement a state machine in Photon 2 firmware

What’s the main point?

  • Our devices should operate remotely without constant user intervention
  • Our devices can receive various sensor data (input) in different situations (states), which results in different actions (outputs)
  • Tracking and organizing all these possibilities can become very complicated
  • State machines provide a way of thinking about and designing for the various situations that arise

State Machine

  • A state machine (or finite state machine) is mental model to help us make decisions
    • It is not just hardware or just software code
  • Examine
    • What has happened previously (state)
    • What is happening currently (input)
  • Then generate
    • Next output
    • Next state (transition)

Example #1 Stoplight

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Example #1 Stoplight

  • Consider a simplified stoplight without any pedestrians
  • Lights changed after a fixed time
  • What states are there?
  • How do we transition to new states?

Example #1 Stoplight Diagram

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Example #1 Stoplight

State Output (Stoplight Color)
Traffic Flows Green
Traffic Stopping Yellow
Traffic Stopped Red

Example #2 Sprinkles Cupcake ATM

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Example #2 Sprinkles Cupcake ATM Diagram

  • Similar to soft drink vending machine
  • What states are there?
  • How do we transition to new states?

Example #2 Sprinkles Cupcake ATM

State Input Next State
Idle None Idle
Idle Start button Display choices
Display choices Choice available Process credit card
Display choices Choice unavailable Display choices
Display choices Cancel Idle
Process credit card Card valid Dispense cupcake
Process credit card Card invalid Process credit card
Dispense cupcake Cupcake taken Idle
Dispense cupcake Cupcake remains Dispense cupcake

State Machines in Photon 2

  • In order to implement this in our firmware, what elements / functions do we need to create?
    • inputs
    • outputs
    • state
    • state transition logic
  • How could we implement these in firmware?

State Machines in Photon 2

  • inputs (e.g. millis() timer, digitalRead, analogRead)

  • outputs (e.g. LED, speaker, OLED screen)

  • state (create enum to track current state and next state)

  • state transition logic

    • typically a function with conditional logic like switch or if
    • use current state and any input to determine the next state

State Transition Logic Example

  • Let’s look at just one piece of the Sprinkles vending machine, namely processing valid and invalid credit cards
State Input Next State
Process credit card Card valid Dispense cupcake
Process credit card Card invalid Process credit card

State Transition Logic Example

  • Input: credit card valid or invalid
    • global variable bool cardValid
  • States: describe all possible states and track current state
    • enum State {...}
    • State currentState; 
      //Global variables
bool cardValid;   //is customer card valid
enum State {IDLE, DISPLAY, PROCESS, DISPENSE }; //all possible states
State currentState;   //state the machine is in right now

State Transition Logic Example

  • State transition: conditional logic to move to next state based on currentState and cardValid 
    void updateNextState() {
 switch(currentState) { 
    case PROCESS:                 //current state is processing credit card
       if (cardValid == true) {   //credit cart is valid so start dispensing
          currentState = DISPENSE;
       }
       //if cardValid == false, remain in PROCESS state
       //no else block is needed
 }
}

Credits

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