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A previous example explored a simple sequence detector. Here is another example for a pattern detector which detects a slightly longer pattern.

Design

  
  
module det_110101 ( input clk,
                  	 input rstn,
                  	 input in,
                  	 output out );
  
  parameter IDLE 	= 0,
  			S1 		= 1,
  			S11 	= 2,
  			S110 	= 3,
  			S1101 	= 4,
  			S11010 	= 5,
  			S110101 = 6;
  
  reg [2:0] cur_state, next_state;
  
  assign out = cur_state == S110101 ? 1 : 0;
  
  always @ (posedge clk) begin
    if (!rstn)
      	cur_state <= IDLE;
     else 
     	cur_state <= next_state;
  end
  
  always @ (cur_state or in) begin
    case (cur_state)
      IDLE : begin
        if (in) next_state = S1;
        else 	next_state = IDLE;
      end
      
      S1: begin
        if (in) next_state = S11;
        else 	next_state = IDLE;
      end

      S11: begin
        if (!in) next_state = S110;
        else 	next_state = S11;
      end
      
      S110 : begin
        if (in) next_state = S1101;
        else 	next_state = IDLE;
      end
      
      S1101 : begin
        if (!in) next_state = S11010;
        else 	next_state = IDLE;
      end
      
      S11010: begin
        if (in) next_state = S110101;
        else 	next_state = IDLE;
      end
      
      S110101: begin
        if (in) next_state = S1;
        else 	next_state = IDLE; 		// Bug 2
      end      
    endcase
  end
endmodule

  

Testbench

  
  
module tb;
  reg clk, in, rstn;
  wire out;
  integer l_dly;
  
  always #10 clk = ~clk;
  
  det_110101 u0 ( .clk(clk), .rstn(rstn), .in(in), .out(out) );
  
  initial begin
  	clk <= 0;
    rstn <= 0;
    in <= 0;
    
    repeat (5) @ (posedge clk);
    rstn <= 1;

    @(posedge clk) in <= 1;
    @(posedge clk) in <= 1;
    @(posedge clk) in <= 0;
    @(posedge clk) in <= 1;
    @(posedge clk) in <= 0;
    @(posedge clk) in <= 1;
    @(posedge clk) in <= 1;
    @(posedge clk) in <= 1;
    @(posedge clk) in <= 0;
    @(posedge clk) in <= 1;
    @(posedge clk) in <= 0;
    @(posedge clk) in <= 1;
    
    #100 $finish;
  end
endmodule

  
Simulation Log
ncsim> run
T=10 in=0 out=0
T=30 in=0 out=0
T=50 in=0 out=0
T=70 in=0 out=0
T=90 in=0 out=0
T=110 in=1 out=0
T=130 in=1 out=0
T=150 in=0 out=0
T=170 in=1 out=0
T=190 in=0 out=0
T=210 in=1 out=0
T=230 in=1 out=1
T=250 in=1 out=0
T=270 in=0 out=0
T=290 in=1 out=0
T=310 in=0 out=0
T=330 in=1 out=0
T=350 in=1 out=1
T=370 in=1 out=0
T=390 in=1 out=0
T=410 in=1 out=0
Simulation complete via $finish(1) at time 430 NS + 0

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