CSC270  Lab #7

Experiment 1: A traffic light sequencer

Design a traffic light sequencer that has 4 states. The Green light stays on for two periods of the clock, while the Yellow and Red lights for one only.

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






• 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 same values we used in class today.

  State	QA	QB

• Put together a State Transition Table, where you will 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

• Noting that "Next Q_A" is what Q_A will be at the next clock tick, then it must be that its value is already waiting on 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 = ___________________
  
  

• Implement this circuit with a SN74LS74 D-flipflop circuit. It 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.

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

• Connect the two outputs Q_A and Q_B to two LEDs. Observe that the circuit you have builts cycles through the three states correctly.

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

  Figure 1. Timing diagram for first sequencer.

Experiment 2: Adding 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.

Your new design should not be close enough to the previous one that you might be able to use most of your wiring!

Record the 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).





Figure 2. Timing diagram for second sequencer.