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CHAPTER 3 FINITE STATE MACHINES
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break; case STATE_3: ClearScreen(); Meter(3); event = WaitEvent(); if (event == TOUCH) state = STATE_4; if (event == TIMER_ELAPSE) state = STATE_2; ResetTimer(); break; case STATE_4: ClearScreen(); Meter(4); event = WaitEvent(); if (event == TOUCH) state = STATE_5; if (event == TIMER_ELAPSE) state = STATE_3; ResetTimer(); break; case STATE_5: ClearScreen(); Meter(5); event = WaitEvent(); if (event == TOUCH) state = STATE_5; if (event == TIMER_ELAPSE) state = STATE_4; ResetTimer(); break; } Wait(100); } }
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Note The constants STATE_0, STATE_1, and so on that you find in the program are just aliases defined at
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the top of the program using the preprocessor directive #define STATE_0 0. The aliases mean that when the compiler finds STATE_0 later in the program, it will view it as if it were the number 0. It is not essential, but it s a commonly used practice in programming to write constants with all capital letters.
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An FSM code skeleton is a switch statement nested inside an infinite loop. The switch variable is the state variable itself (called no surprise state); it s the variable that contains the actual state value (defined at the top of the program). In our example, the states have unimaginative names; for every state we find a case <value>: label. The code to be executed in this state is included between the case <value>: label and the break statement.
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CHAPTER 3 FINITE STATE MACHINES
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The switch statement is equivalent to a series of if/else statements, resulting in more efficient Tip
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compiled code. The break keyword makes the program flow out of the switch statement body once the part of code that s of interest has been executed. Consult the NXC Programming Guide for details.
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The code can perform output actions, and also change the internal state, by simply assigning a new value (among the ones defined in the program) to the state variable. Confused Let s see an example, say state 4, in detail. case STATE_4: Meter(4); event = WaitEvent(); if (event == TOUCH) state = STATE_5; if (event == TIMER_ELAPSE) state = STATE_3; ResetTimer(); break; At the actual loop iteration, the switch(state) brought us to the case labeled STATE_4. This means that in the previous loop iteration, the state variable was assigned the STATE_4 value. The whole switch body is equivalent to this: while(true) { if (state == 0) //case STATE_0 { //do something in state 0 } //break else if (state==1) { //do something in state 1 } //break else if (state==2) { //do something in state 2 } //break [...] //as many states as you want } First, you invoke the Meter(4) function to draw a bar with four black squares on the NXT display (the code of the Meter function is in Listing 3-3), then you use the WaitEvent() function to wait for the timer to elapse or for the button to be clicked (pressed and released). This function returns a value to let the main task know which event occurred.
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Note I omit here and later, on purpose, the code to read and reset the timer, to focus your attention better
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on the current topic. You ll learn how to use the timer in 6, on the NXT Turtle.
CHAPTER 3 FINITE STATE MACHINES
If the timer had elapsed, the state variable would be assigned the value of the lower STATE_3 (remember we are in state 4). But if the button was clicked, the next state would be the highest, STATE_5. At this point, the timer is reset and the program flow breaks out the switch, ready for another ride. On the next loop, the switch statement executes the code corresponding to the case labeled with the value just assigned to the state variable. In Figure 3-4, you can see the two transitions described in the last paragraph: both the transition from state 4 to 5 (the button was pressed), and the transition from state 4 back to state 3 (the timer has elapsed).
Figure 3-4. Some NXT display outputs of the saturating counter The program starts with a blank screen; if you don t do anything, you ll see the message Elapse Reset flashing periodically. This means you are in state 0, and when the timer elapses, it is reset, while the state remains at 0. If you try clicking the NXT orange button quickly (the interval between two clicks has to be less than the program-defined timeout), you ll see the number of squares increase. The message Button Reset indicates that action R (see Figure 3-3) is performed, as a result of a button click event B. When in state 5, pressing frequently on the button maintains the counter in its top state. If you stop pressing the button, the bar decreases every time the timer elapses, back to state 0. The NXC language and NXT firmware don t manage events directly (using interrupts), so you can implement waiting for events with the WaitEvent() function (see Listing 3-2). Listing 3-2. The WaitEvent() Function Code short WaitEvent() { short event = 0;
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