'1011' Overlapping (Moore) Sequence Detector in Verilog

Question

I'm designing a "1011" overlapping sequence detector, using Moore Model in Verilog .

The FSM that I am trying to implement is as shown below :-

Verilog Module :-

`timescale 1ns / 1ps

module seq_detector(
input x,clk,reset,
output reg z
);

parameter S0 = 0 , S1 = 1 , S2 = 2 , S3 = 3 , S4 = 4;
reg [1:0] PS,NS ;

    always@(posedge clk or posedge reset)
        begin
            if(reset)
                PS <= S0;   
            else    
                PS <= NS ;
        end             

    always@(PS or x)
        begin 
            
            case(PS)
                S0 : begin 
                            z <= 0 ;
                            NS <= x ? S1 : S0 ;
                            $display(PS);
                        end
                S1 : begin 
                            z <= 0 ;
                            NS <= x ? S1 : S2 ;
                            $display(PS);
                        end
                S2 : begin 
                            z <= 0 ;
                            NS <= x ? S3 : S0 ;
                            $display(PS);
                        end 
                S3 : begin 
                            z <= 0;
                            NS <= x ? S4 : S2 ;
                            $display(PS);
                        end
                S4 : begin 
                            z <= 1; 
                            NS <= x ? S1 : S2 ;
                            $display(PS);
                        end

            endcase
        end
endmodule

Testbench :-

`timescale 1ns / 1ps

module testbench;
    // Inputs
    reg x;
    reg clk;
    reg reset;
    // Outputs
    wire z;
    // Instantiate the Unit Under Test (UUT)
    seq_detector uut (
        .x(x), 
        .clk(clk), 
        .reset(reset), 
        .z(z)
    );
    
initial
    begin
        clk = 1'b0;
        reset = 1'b1;
        #15 reset = 1'b0;
    end

always #5 clk = ~ clk;  

initial begin
        #12 x = 0;#10 x = 0 ; #10 x = 1 ; #10 x = 0 ;
        #12 x = 1;#10 x = 1 ; #10 x = 0 ; #10 x = 1 ;
        #12 x = 1;#10 x = 0 ; #10 x = 0 ; #10 x = 1 ;
        #12 x = 0;#10 x = 1 ; #10 x = 1 ; #10 x = 0 ;
        #10 $finish;
    end
      
    
endmodule

Simulation Output :-

The issue is that, the output 'z' is staying low always, even when I've applied an input sequence which has three '1011' patterns in it . What's the possible modification that I'd have to do, so as to eliminate this error ?

Answer 1

In Moore Machines the output depends only on the current state. So when you are changing your output, (z in this case), the sensitivity list should be only the current state.

You should add the default case so that your FSM remains idle when there is no change in the current state.

In your combinational block, you should use blocking statements to prevent your simulation from running into infinite loops and getting locked up.

Simulate the circuit here on my eda playground:

Design:

`timescale 1ns / 1ps

module seq_detector(
input x,clk,reset,
output reg z
);

parameter S0 = 0 , S1 = 1 , S2 = 2 , S3 = 3 , S4 = 4;
reg [2:0] PS,NS ;

always @(posedge clk or posedge reset)
    begin
        if(reset)
            PS <= S0;   
        else    
            PS <= NS ; 
    end
always @(PS, x)
begin          
case(PS)
            S0 : begin
                        NS = x ? S1 : S0 ;
                        $display(PS);
                    end
            S1 : begin 
                        NS = x ? S1 : S2 ;
                        $display(PS);
                    end
            S2 : begin 
                        NS = x ? S3 : S0 ;
                        $display(PS);
                    end 
            S3 : begin 
                        NS = x ? S4 : S2 ;
                        $display(PS);
                    end
            S4 : begin 
                        NS = x ? S1 : S2 ;
                        $display(PS);
                    end
            default: NS = S0; 
        endcase
    end
always @(PS)
begin
  case(PS)
    S4: z = 1;
    default: z = 0;
  endcase 
end 
endmodule

Testbench:

 `timescale 1ns / 1ps

 module testbench;
// Inputs
reg x;
reg clk;
reg reset;
// Outputs
wire z;
// Instantiate the Unit Under Test (UUT)
seq_detector uut (
    .x(x), 
    .clk(clk), 
    .reset(reset), 
    .z(z)
);

always #5 clk = ~ clk;  

initial begin
 $dumpfile("dump.vcd");
 $dumpvars(1, testbench);

    fork
    clk = 1'b0;
    reset = 1'b1;
    #15 reset = 1'b0;
    begin 
    #12 x = 0;#10 x = 0 ; #10 x = 1 ; #10 x = 0 ;
    #12 x = 1;#10 x = 1 ; #10 x = 0 ; #10 x = 1 ;
    #12 x = 1;#10 x = 0 ; #10 x = 0 ; #10 x = 1 ;
    #12 x = 0;#10 x = 1 ; #10 x = 1 ; #10 x = 0 ;
    #10 $finish;
    end
    join  
end 
endmodule

`

Waveform:

https://www.edaplayground.com/w/x/kk

---

There are totally 32 possibilities to test for this Overlapping FSM because there are 5 states. Alternative: Use a formal property to verify the FSM.

property CHK_SEQ_DETECT;
  @(posedge clk) disable iff (reset) x ##1 !x ##1 x[*2] |=> z;
endproperty;


ASSERT_CHK_SEQ_DETECT: assert property (CHK_SEQ_DETECT);

Add these lines in the design module, complete setup of formal tool and run it.

Note:

As mentioned in Coding And Scripting Techniques For FSM Designs With Synthesis-Optimized, Glitch-Free Outputs by Clifford Cummings, Sunburst Design, Inc,

The combinational outputs generated by these two coding styles (Example 1 and Example 2) suffer two principal disadvantages:

1. Combinational outputs can glitch between states.
2. Combinational outputs consume part of the overall clock cycle that would have been available to the block of logic that is driven by the FSM outputs.

When module outputs are generated using combinational logic, there is less time for the receiving module to pass signals through inputs and additional combinational logic before they must be clocked.

Hence, coding an overlapping Moore with registered output, in SystemVerilog, and with better signal names:

module seq_detector(
   input seq_in, clk, reset,
   output logic detect_out
);

   //one-hot encoding of FSM
   enum logic [4:0] {S0 = 5'b00001, S1 = 5'b00010, S2 = 5'b00100, S3 = 5'b01000, S4 = 5'b10000}  state, next;

   //state registers
   always_ff @(posedge clk or posedge reset)
     if (reset) state <= S0;  
     else       state <= next;

   // Next state assignment logic
   always_comb begin: set_next_state       
     next = state;
     unique case (state)
       S0 : if (seq_in) next = S1; else next = S0;
       S1 : if (seq_in) next = S1; else next = S2;    
       S2 : if (seq_in) next = S3; else next = S0;
       S3 : if (seq_in) next = S4; else next = S2;
       S4 : if (seq_in) next = S1; else next = S2;
     endcase 
     $monitor(state);
   end: set_next_state

   // Registered output logic
   always_ff @(posedge clk, posedge reset)
     if (reset) detect_out <= 1'b0;
     else       detect_out <= (state == S4);

property CHK_SEQ_DETECT;
   @(posedge clk) disable iff (reset) seq_in ##1 !seq_in ##1 seq_in[*2] |=> ##1 detect_out;
endproperty;

ASSERT_CHK_SEQ_DETECT: assert property (CHK_SEQ_DETECT);

endmodule

As the input is not directly connected to the output, the FSM is free from being affected by glitches at the input.

Answer 2

Your state variable is too small. You have 5 states, but your variable is only 2 bits wide. It must be at least 3 bits wide. Change:

reg [1:0] PS,NS ;

to:

reg [2:0] PS,NS ;

Now I see z go high 3 times.

---

Now that you have unused states (5-7), you should add a default to your case statement.