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	TABLE OF CONTENTS Introduction Why Bother? Designing a Statechart The Next Step Coding a Statechart in C It's All About Me Declaring State Machine Objects Initializing State Machine Objects Deploying the Application on the Toolstick

Designing a Statechart

Considering the limited capabilities of the Toolstick, I was constrained with the choice of a compelling example. Basically, the Toolstick can only blink its two LEDs (see Figure 1), or at most change the LED intensity using the built-in PWM generators. To me this resembled the operation of a PEdestrian LIght CONtrolled (PELICAN) crossing that I've used before in "Deja Vu".

The PELICAN crossing controller starts with cars enabled (green light for cars) and pedestrians disabled (don't-walk signal for pedestrians). To activate the traffic light change, a pedestrian must push the button at the crossing, which generates the PEDS_WAITING event. In response, the cars get the yellow light, which after a few seconds changes to red light. Next, pedestrians get the walk signal, which shortly thereafter changes to the flashing don't-walk signal. When the don't-walk signal stops flashing, cars get the green light again. After this cycle, the traffic lights don't respond to the PEDS_WAITING button press immediately, although the button "remembers" that it has been pressed. The traffic light controller always gives the cars a minimum of several seconds of green light before repeating the traffic light change cycle. One additional feature (coming late into the project) is that at any time an operator can take the PELICAN crossing offline (by providing the OFF event). In the "offline" mode, the cars get a flashing yellow and pedestrians flashing don't-walk signal. At any time the operator can turn the crossing back online (by providing the ON event).

Due to the hierarchical character of statecharts, you can approach the design from the top down or bottom up. Here, I use a combination of the two.

The limitation on the number of diagrams I can use here does not permit me to show a series of progressively elaborated statecharts, which would be perhaps the most educational method of explaining the design process. Instead, Figure 2 shows the complete PELICAN crossing statechart.

I start the design with just two states: carsEnabled and pedsEnabled. This pair of states realizes the main function of the PELICAN crossing, which is to alternate between enabling cars and enabling pedestrians. Obviously, both states need some substates to realize the details of the specification, but I ignore this at first. At this stage, I only make sure that the design guarantees mutually exclusive access to the crossing, which is the main safety concern here. The exit action from the pedsEnabled state disables pedestrians, and the exit action from carsEnabled disables cars. Now, the UML semantics of state transitions guarantees that these exit actions execute whichever way the states happen to be exited, so I can be sure that the pedestrians have always don't-walk signal outside the pedsEnabled state and cars have the red light outside the carsEnabled state.

Previous Page | 1 Introduction | 2 Why Bother? | 3 Designing a Statechart | 4 The Next Step | 5 Coding a Statechart in C | 6 It's All About Me | 7 Declaring State Machine Objects | 8 Initializing State Machine Objects | 9 Deploying the Application on the Toolstick Next Page

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