Verilog-volgordedetector

Een veelvoorkomend voorbeeld van een FSM is die van een sequentiedetector waarbij het hardwareontwerp naar verwachting detecteert wanneer een vast patroon wordt gezien in een stroom binaire bits die erin worden ingevoerd.

Voorbeeld

  
  
module det_1011 ( input clk,
                  input rstn,
                  input in,
                  output out );
  
  parameter IDLE 	= 0,
  			S1 		= 1,
  			S10 	= 2,
  			S101 	= 3,
  			S1011 	= 4;
  
  reg [2:0] cur_state, next_state;
  
  assign out = cur_state == S1011 ? 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 = IDLE;
        else 	next_state = S10;
      end
      
      S10 : begin
        if (in) next_state = S101;
        else 	next_state = IDLE;
      end
      
      S101 : begin
        if (in) next_state = S1011;
        else 	next_state = IDLE;
      end
      
      S1011: begin
        next_state = IDLE;
      end
    endcase
  end
endmodule

  

Testbank

  
  
module tb;
  reg 			clk, in, rstn;
  wire 			out;
  reg [1:0] l_dly;
  reg 			tb_in;
  integer 	loop = 1;
  
  always #10 clk = ~clk;
  
  det_1011 u0 ( .clk(clk), .rstn(rstn), .in(in), .out(out) );
  
  initial begin
  	clk <= 0;
    rstn <= 0;
    in <= 0;
    
    repeat (5) @ (posedge clk);
    rstn <= 1;

		// Generate a directed pattern
    @(posedge clk) in <= 1;
    @(posedge clk) in <= 0;
    @(posedge clk) in <= 1;
    @(posedge clk) in <= 1; 		// Pattern is completed
    @(posedge clk) in <= 0;
    @(posedge clk) in <= 0;
    @(posedge clk) in <= 1;
    @(posedge clk) in <= 1;
    @(posedge clk) in <= 0;
    @(posedge clk) in <= 1;
    @(posedge clk) in <= 1; 	 // Pattern completed again
    
    // Or random stimulus using a for loop that drives a random
    // value of input N times
    for (int i = 0 ; i < loop; i ++) begin
      l_dly = $random;
      repeat (l_dly) @ (posedge clk);
      tb_in = $random;
      in <= tb_in;
    end
    
    // Wait for sometime before quitting simulation
    #100 $finish;
  end
endmodule

  

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=0 out=0
T=150 in=1 out=0
T=170 in=1 out=0
T=190 in=0 out=1
T=210 in=0 out=0
T=230 in=1 out=0
T=250 in=1 out=0
T=270 in=0 out=0
T=290 in=1 out=0
T=310 in=1 out=0
T=330 in=1 out=1
T=350 in=1 out=0
T=370 in=1 out=0
T=390 in=1 out=0
Simulation complete via $finish(1) at time 410 NS + 0

Er zit een fout in het ontwerp. Kun je het vinden?