Mealy vs. Moore Machine: Verilog Code Examples

This page provides Verilog code examples for both Mealy and Moore Machines.

Mealy Machine Verilog Code

Here’s a look at the Mealy Machine, along with its corresponding Verilog code. In a Mealy machine, the output is determined by both the current state and the input.

module mealy_verilog_code(out, in, rst, clk);
  output out;
  input in;
  input clk, rst;

  reg out;
  reg[1:0] state;

  parameter s0 = 2'd0,
            s1 = 2'd1,
            s2 = 2'd2,
            s3 = 2'd3;

  always @(posedge clk or negedge rst)
    if(rst == 0) begin
      state = s0;
      out = 0;
    end
    else begin
      case (state)
        s0: if(in == 0) begin
              out = 0;
              state = s1;
            end
            else begin
              out = 0;
              state = s0;
            end
        s1: if(in == 0) begin
              out = 0;
              state = s1;
            end
            else begin
              out = 0;
              state = s2;
            end
        s2: if(in == 0) begin
              out = 0;
              state = s3;
            end
            else begin
              out = 0;
              state = s0;
            end
        s3: if(in == 0) begin
              out = 0;
              state = s1;
            end
            else begin
              out = 1;
              state = s2;
            end
        default: state = s0;
      endcase
    end
endmodule

Moore Machine Verilog Code

Now, let’s examine the Moore Machine and its Verilog implementation. In a Moore machine, the output is solely determined by the current state.

module moore_verilog_code(out, in, rst, clk);
  output out;
  input in;
  input clk, rst;

  reg out;
  reg[1:0] state;

  parameter s0 = 2'd0,
            s1 = 2'd1,
            s2 = 2'd2,
            s3 = 2'd3;

  always @(posedge clk or negedge rst)
    if(rst == 0) begin
      state = s0;
      out = 0;
    end
    else begin
      case (state)
        s0: begin
              out = 0;
              if(in == 0) state = s1;
              else state = s0;
            end
        s1: begin
              out = 0;
              if(in == 0) state = s1;
              else state = s2;
            end
        s2: begin
              out = 0;
              if(in == 0) state = s3;
              else state = s0;
            end
        s3: begin
              out = 1;
              if(in == 0) state = s1;
              else state = s2;
            end
        default: state = s0;
      endcase
    end
endmodule