Timing The Transitions

Part of the Fundamentals section of Coddy's Verilog journey. Lesson 81 of 90.

Challenge

In this lesson, you will add the counter logic that controls how long each light stays on. The counter counts down from a preset value to zero, then triggers the next state change.

Timing Requirements

State	Duration	Counter Value
Green	30 seconds	30
Yellow	10 seconds	10
Red	40 seconds	40

How the Counter Works

When a state begins, the counter is loaded with the duration value
On each clock tick, the counter decreases by 1
When the counter reaches 0, it is time to change to the next state

Your task is to add the missing counter logic to the state machine. The next input from the previous step is no longer used: the counter now decides when the state changes, and the testbench no longer drives next.

What to do:

Remove input next, from the module header and replace the else if (next) condition with the counter logic below
Add a reg [5:0] counter declaration (keep it if you already declared it in the first step)
On reset, set counter to 0
When counter == 0:
- Load the next state's duration into the counter
- Change to the next state
Otherwise, decrement the counter by 1 on each clock

Try it yourself

module traffic_light (
  input clk,
  input reset,
  output reg red,
  output reg yellow,
  output reg green
);

  reg [1:0] state;
  // TODO: Declare counter register (6 bits)
  

  // Output assignments
  always @(*) begin
    case (state)
      0: begin green = 1; yellow = 0; red = 0; end
      1: begin green = 0; yellow = 1; red = 0; end
      2: begin green = 0; yellow = 0; red = 1; end
      default: begin green = 0; yellow = 0; red = 1; end
    endcase
  end

  // State machine with timing
  always @(posedge clk or posedge reset) begin
    if (reset) begin
      state <= 2;      // Start at Red
      // TODO: Set counter to 0
      
    end else begin
      if (counter == 0) begin
        case (state)
          0: begin
            state <= 1;
            // TODO: Load counter for Yellow (10 seconds)
            
          end
          1: begin
            state <= 2;
            // TODO: Load counter for Red (40 seconds)
            
          end
          2: begin
            state <= 0;
            // TODO: Load counter for Green (30 seconds)
            
          end
        endcase
      end else begin
        // TODO: Decrement counter by 1
        
      end
    end
  end

endmodule