skip to primary navigation skip to content

ECAD and Architecture Practical Classes

Tick 2 - Traffic Lights

We want to design a module that models the sequence of traffic lights at a cross roads. We suggest you base it around a counter for the eight states with the correct output being provided by decode logic.

traffic lights state transition diagram

Notes:

Red is the most significant bit, i.e. lightsA = {r,a,g} where {r,a,g} are the red, amber and green bits.
The very first output after reset needs to be for state 0 in the diagram.
Your output decoder needs to be combinational, i.e. inside an always_comb block.

Outline module

module twotrafficlights(
      input  logic clk,
      input  logic rst,
      output logic [2:0] lightsA, 
      output logic [2:0] lightsB
    );
  logic [2:0] state;
  // insert your code here

endmodule

Test bench

The ticker tests your design against a known-good golden model using the following test bench code. You may wish to simplify it to just instantiate your module and display the sequence produced.

module twotrafficlights_tester
  (
   output logic       clk,
   output logic       rst,
   output logic [2:0] lightsA,
   output logic [2:0] lightsB
   );

   logic [2:0]        correctA, correctB;

   initial
     begin
        clk = 1;
        rst = 1;
        #20 rst = 0;
        #165 $finish();
     end

   always #5 clk <= !clk;

   twotrafficlights dut(.clk(clk), .rst(rst), .lightsA(lightsA), .lightsB(lightsB));

   twotrafficlights_golden gold(.clk(clk), .rst(rst), .lightsA(correctA), .lightsB(correctB));

   always @(posedge clk)
     if(!rst)
       begin
          $write("%04d: rst=%1b  (lightsA,lightsB) = (%03b, %03b)   correct: (%03b, %03b)",
                 $time, rst, lightsA, lightsB, correctA, correctB);
          if({lightsA,lightsB} == {correctA,correctB})
            $display(" - pass");
          else
            $display(" - FAIL");
       end
endmodule