In this tutorial you will learn how to control your device with well organized code using state machine approach instead of hundred's of if-s embeded in each other.
Table of contents:
- State machine
- Simple example number 1
- Simple example number 2
- Simple example number 3
- Simple example number 4
State machine is defined by few elements:
- set of states,
- set of actions,
- set of transitions.
A state is a description of the current status of a system.
When one of a predefined conditions is fulfilled or when an event is received an action is generated.
In result a transition from current state to a next state occures.
A set of transitions can be defined in one of possible tabular form.
Tabular form 1:
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current | action ||next state | ||state --------+--------++------ ... | || state X |action Y||state Z ... | || |
Tabular form 2:
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| state 1 | ... | state X | ... | state N ---------+----------+-----+---------+-----+-------- action 1 |next state| | | | ---------+----------+-----+---------+-----+-------- ... ---------+----------+-----+---------+-----+-------- action Y | | | state Z | | ---------+----------+-----+---------+-----+-------- ... ---------+----------+-----+---------+-----+-------- action M | | | | | |
In practical application it is much more handy to have transition rules in different form where in each line one transition is defined:
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routes = [{"currentState": ..., "action": ..., "newState": ...}, <-- route number 1 ... {"currentState": ..., "action": ..., "newState": ...}] <-- route number N |
I call a transition given in this form a route.
It is usefull to represent a state machine as a graph with states as nodes and actions as arrows:
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In most cases one of states is distinguished from others: this is an initial state, the first state you start your system. There can be also a set of other special states -- every time you reach one of them, something unusual must have happened. For example STOP state may mean a contoled stop of your system, while TERMINATE may mean an uncontrolled stop of your system being a result of some unexpected data or component malfunction. I classify STOP as an unusual state because typically every system, when started is expected to run forever.
If you hava a definition of a state machine (you know a set of sates, actions and all possible transitions) you can easily "execute" it with te code similar to the follwing:
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initialState = START routes = [{...}, ... {...}] currentState = initialState while (currentState != STOP) { action = getAction() if(action != ACTION_NONE){ if (action == TERMINATE) { currentState = TERMINATE break } noRoute = TRUE for route in routes { if (currentState == route["currentState"] AND action == route["action"]) { currentState = route["newState"] executeState(currentState) noRoute = FALSE break } } if (currentState == TERMINATE || noRoute == TRUE) { break } } } |
In this example you will work with a very simple circuit:
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As you can see, there are:
- Arduino Uno board to control all the interactions.
- LED controlled by Arduino Uno in response to button press action.
- Button
ONconnected with green wire to the Arduino. You will use this button to turn LEDON. - Button
OFFconnected with orange wire to the Arduino. You will use this button to turn LEDOFF.
In the "classical" approach probably you will write code more o less similar to the following:
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#define PIN_LED 2 #define PIN_BTN_ON 8 #define PIN_BTN_OFF 9 #define BTN_INACTIVE 0 #define BTN_ON_PRESS 1 #define BTN_OFF_PRESS 2 int button; void setup() { pinMode(PIN_LED, OUTPUT); pinMode(PIN_BTN_ON, INPUT); pinMode(PIN_BTN_OFF, INPUT); digitalWrite(PIN_LED, LOW); button = BTN_INACTIVE; } int checkButtonState(){ if(digitalRead(PIN_BTN_ON) == HIGH){ delay(20); while(digitalRead(PIN_BTN_ON) == HIGH); delay(20); return BTN_ON_PRESS; } else if(digitalRead(PIN_BTN_OFF) == HIGH){ delay(20); while(digitalRead(PIN_BTN_OFF) == HIGH); delay(20); return BTN_OFF_PRESS; } return BTN_INACTIVE; } void loop() { button = checkButtonState(); if (button == BTN_ON_PRESS){ digitalWrite(PIN_LED, HIGH); } else if (button == BTN_OFF_PRESS){ digitalWrite(PIN_LED, LOW); } } |
This system can be also control with the following state machine:
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State machine depicetd on the above image is implemented below:
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#define TRUE 1 #define FALSE 0 #define PIN_LED 2 #define PIN_BTN_ON 8 #define PIN_BTN_OFF 9 #define BTN_INACTIVE 0 #define BTN_ON_PRESS 1 #define BTN_OFF_PRESS 2 #define STATE_TERMINATE (-2) #define STATE_STOP (-1) #define STATE_START 0 #define STATE_STATE_LED_ON 1 #define STATE_STATE_LED_OFF 2 #define INITIAL_STATE STATE_START #define ACTION_TERMINATE (-1) #define ACTION_BTN_ON_PRESS 1 #define ACTION_BTN_OFF_PRESS 2 #define ACTION_NONE 3 #define NUMBER_OF_ROUTES 4 #define INDEX_STATE_CURRENT 0 #define INDEX_ACTION 1 #define INDEX_STATE_NEXT 2 // Warning!!! // Remember to update NUMBER_OF_ROUTES when changing routing table int smRoutes[NUMBER_OF_ROUTES][3] = { {STATE_START, ACTION_BTN_ON_PRESS, STATE_STATE_LED_ON}, {STATE_START, ACTION_BTN_OFF_PRESS, STATE_STATE_LED_OFF}, {STATE_STATE_LED_ON, ACTION_NONE, STATE_START}, {STATE_STATE_LED_OFF, ACTION_NONE, STATE_START} }; int button; void setup() { pinMode(PIN_LED, OUTPUT); pinMode(PIN_BTN_ON, INPUT); pinMode(PIN_BTN_OFF, INPUT); digitalWrite(PIN_LED, LOW); button = BTN_INACTIVE; } int checkButtonState() { if(digitalRead(PIN_BTN_ON) == HIGH){ delay(20); while(digitalRead(PIN_BTN_ON) == HIGH); delay(20); return BTN_ON_PRESS; } else if(digitalRead(PIN_BTN_OFF) == HIGH){ delay(20); while(digitalRead(PIN_BTN_OFF) == HIGH); delay(20); return BTN_OFF_PRESS; } return BTN_INACTIVE; } int getAction() { button = checkButtonState(); if (button == BTN_ON_PRESS){ return ACTION_BTN_ON_PRESS; } else if (button == BTN_OFF_PRESS){ return ACTION_BTN_OFF_PRESS; } return ACTION_NONE; } void smExecuteSTATE_STATE_LED_ON(){ digitalWrite(PIN_LED, HIGH); } void smExecuteSTATE_STATE_LED_OFF(){ digitalWrite(PIN_LED, LOW); } void smExecuteState(int currentState) { switch (currentState){ case STATE_START: { break; } case STATE_STATE_LED_ON: { smExecuteSTATE_STATE_LED_ON(); break; } case STATE_STATE_LED_OFF: { digitalWrite(PIN_LED, LOW); break; } } } void smExecute(int initalState) { int currentState = initalState; int action; int noRoute; int r; while (currentState != STATE_STOP) { action = getAction(); if(action != ACTION_NONE){ if (action == ACTION_TERMINATE) { currentState = STATE_TERMINATE; break; } noRoute = TRUE; for(r=0; r<NUMBER_OF_ROUTES; ++r){ if (currentState == smRoutes[r][INDEX_STATE_CURRENT] && action == smRoutes[r][INDEX_ACTION]) { currentState = smRoutes[r][INDEX_STATE_NEXT]; smExecuteState(currentState); noRoute = FALSE; break; } } } if (currentState == STATE_TERMINATE || noRoute == TRUE) { break; } } } void loop() { smExecute(INITIAL_STATE); } |
At this moment this may seem like an unnecessary complication of simple things, but I hope you will appreciate this approach in subsequent, more and more complex versions of this system.
In this example you will use sligtly modfied version of circuit from example 1: there is only one button to turn led on or off.
Susprisingly, one button simplifies the design a lot:
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However I propose you to work with the state machine given below. There are two reasons for that:
- You will see how to pass control from one state to another with variables.
- It would be easier to extend this state machine in next modiffication of my example.
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In this example you will use sligtly modfied version of circuit from example 2: you add one more led and second button to select the number of active LEDs -- one or two.
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In this example you will use exactly the same circuit as in example 3 but will chane LEDs behaviour from constant ligt to blinking.
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