CSC 270   Lab 7

Experiment #1: A Traffic Light Sequencer

Design a traffic light sequencer as in the homework except now design the sequence so that the Green light stays on for two periods of the clock, while the Yellow and Red lights for one only period. How many states does this require (and so, how many flip flops)?

You will be using 7474 D-Flip-Flops to implement this sequencer



• Pins 1 and 13 = clear
• Pins 3 and 11 = clock
• Pins 4 and 10 = set


• Create a state table, where you indicate the status of the outputs as a function of the current state:

  Current State	Green	Yellow	Red

• Figure out the number of states and the number of flip-flops

        Number of states = N = _______
  
        Number of flip-flops = F =______
  
        Make sure that the following property holds: 
  
            2F >= N
  
  

• Associate flip-flop output values to each state. For example, you may want Q_A and Q_B equal to (0, 0) when the system is in State 0. In general, select values of Q_A and Q_B that change in only one bit during state to state transitions. You do not need to select the states so the transition in the binary sequence, but instead, select a sequence that will result in simpler (more efficient) combinational logic.

  State	QA	QB

• Combining these tables, put together a State Transition Table, to show the present states, the future states, and the (current) outputs.

  Current QA	Current QB	Next QA	Next QB	Current Green	Current Yellow	Current Red

• Recall that "Next Q_A" is what Q_A will be at the next clock tick, so it must be that its value is already present on the input, D_A. Thus, you can form the boolean expressions for "Next Q_A" and "Next Q_B" as a function of "Current Q_A" and "Current Q_B", and assign them to D_A and D_B.

            DA = Next QA = ___________________
  
            DB = Next QB = ___________________
  
  

• Also, form the output expressions for each output, Green, Yellow and Red

            G = ______________________
  
            Y = ______________________
  
            R = ______________________
  

• Implement this circuit with a SN74LS74 D-flipflop circuit. This chip contains 2 D-flipflops. Make sure you connect the Preset (S_D on the datasheet) and Clear (C_D) inputs of both flipflops to V_cc (5 Volts).

• Connect the 1Hz clock signal to the two clock inputs.

• Now, connect the two outputs Q_A and Q_B to two logic indicators (or you can use LEDs if you'd like to try something new and different). Observe that the circuit you have built cycles through the states correctly.
• Next, connect the three outputs representing Green, Yellow and Red to logic indicators and confirm that your circuit is producing the intended output sequence.

• Record the timing diagram of the variation of the clock signal, Q1, Q2, Green, Yellow, and Red.

• Be sure to also include your circuit diagram and observations in your write-up.
  

  Figure 1. Timing diagram for first sequencer.

Experiment #2: A Traffic Light Sequencer with an input switch

Same experiment, but this time add a switch that is normally 0. When it is pressed, though, the sequencer remains stuck in the state that activates the Red light. When the switch is released, the sequencer goes back through its regular cycle. Think about what state each of the flip flops must be in to ensure a 'red' output, and what logic gates you need to add to achieve this.

Your new design might be close enough to the previous one that you might be able to use most of your wiring, so do not remove any wires yet!

Record the state table, circuit diagram (additions), and timing diagram of the variation of the clock signal, Q1, Q2, Green, Yellow, and Red, when the switch is activated (first timing diagram), and when it is not (second timing diagram).

What might this "stuck at red" switch represent for an actual traffic light?






Figure 2. Timing diagram for second sequencer.