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Circuit Description

Unfortunately, this demo is currently broken (missing resource files). Sorry.

This applet demonstrates the farm-road traffic light, used as an example for state-machine design in many textbooks. To motivate and illustrate the traffic-light controller design, the applet contains a special simulation component that visualizes the traffic on the highway (main road, horizontal) and the farm-road (vertical).

Use the popup-menu ('edit') on the traffic-light component to open the window with the traffic visualization. Press the 'send car' button a few times to trigger the farm-road contact and watch the behavior of the traffic-light controller. You can also open the state-machine editor to watch and edit the controller. The following image shows a screenshot of the applet with both the traffic-light animation and the state-machine editor open at the same time:

"A busy highway is intersected by a little-used farmroad. Detectors are placed along the farmroad to raise the signal C as long as a vehicle is waiting to cross the highway. The traffic light controller should operate as follows. As long as no vehicle is detected on the farmroad, the lights should remain green in the highway direction. If a vehicle is detected on the farmroad, the highway lights should change from yellow to red, allowing the farmroad lights to become green. The farmroad lights stay green only as long as a vehicle is detected on the farmroad and never longer than a set interval to allow the traffic to flow along the highway. If these conditions are met, the farmroad lights change from green to yellow to red, allowing the highway lights to return to green. Even if vehicles are waiting to cross the higway, the highway should remain green for a set interval.[From KATZ, Randy H., Contemporary Logic Design. University of California, Berkeley. 1994]"

Inputs:

C : detects vehicle on farmroad

ST: Restart Time counter (to TS and TL)

Outputs:

TS : short timer interval has expired (Yellow light)

TL : long timer interval has expired (Green or Red light)

HG, HY, HR : green, yellow, red highway lights

FG, FY, FR : green, yellow, red farmroad lights

Component written by Ulisses Chippe, Giliardo Freitas and Ricardo Ferreira, DPI, Universidade Federal Vicosa, Brazil, cacau@dpi.ufv.br

Note: the traffic-light shown here generates the light sequence (green, yellow, red, green) used in Brazil. Changing the state-machine to the central European light sequence (green, yellow, red, red+yellow, green) makes for a good exercise. You can also try to add extra all-red states (both roads red), which are used for increased safety.

To edit the state-machine, activate the popup-menu on the FSM symbol and select the edit menu item. This opens the editor window for the FSM, which uses a mode-oriented user-interface.

• move-mode: Click-and-drag any one of the state symbols (circles) to a new location on the editor canvas. The currently selected state is highlighted in red. Use the textfield on the lower left to set the name of the selected state. To change the output values for Moore-type outputs in the selected state, first select that output in the list on the lower left and then select the new output-value (0 or 1) in the radio-button on the bottom.
• state-mode: Click the mouse to create a new state and set its name and the default output values via the GUI controls on the lower left part of the editor window.
• transition-mode: To create a new transition between states, first click of the initial state, then on the final state. Enter the logical condition that activates the transition in the textfield on the lower left, e.g. "*" for an always active transition, or "A&B" for a transition that is only active if the "A" and "B" inputs are both active (1).
• comment-mode: Click the mouse to specify the position for a new text comment on the editor canvas; next enter the comment into the text-area in the lower left part of the editor window and finally press "apply" to create the comment.
• delete-mode: Click the mouse to delete a state, transition, or comment.
• starting state-mode: Click the mouse on a state symbol to select that state as the initial starting-state of the FSM.
• Click the test FSM button to run a simple selftest that checks for each state whether transitions are active for any combination of inputs.
• Use the load and save as operations to load or save the FSM design file.