Strange testbench behavior in gtkwave

Question

I'm trying to design an elevator controller in SystemVerilog. The code compiles well but the testbench is completely different when I put it in gtkwave. The controller receives 3 different inputs (one for each of the 3 floors, like buttons of a proper elevator) and passes its output to the elevator engine, which can stay on the same floor, go up or go down by one or two floors.

This is the code for the module:

 enum logic [4:0] {fermo, sale, sale2, scende, scende2} motore;

    module ascx (input logic x0, x1, x2, clk, reset,
              output logic [4:0] y);
          
          enum logic [1:0] {pianoterra, primopiano, secondopiano} piani;
                    logic  [1:0] pianoatt, pianoprox;
                  
                  always_ff@(posedge clk, posedge reset)
                  if(reset) pianoatt <= pianoterra;
                  else pianoatt <= pianoprox;

              always_comb begin
                          if(x1) pianoprox = primopiano;
                          else if(x0) pianoprox = pianoterra;
                          else if(x2) pianoprox = secondopiano;     
              end                                                                      
              
              always @(*) begin
                case(pianoatt)
                    pianoterra:   begin 
                                    if(x0) assign y = fermo;    /*assign y = 5'b00001;*/
                                    if(x1) assign y = sale;     /*assign y = 5'b00010;*/
                                    if(x2) assign y = sale2;    /*assign y = 5'b00100;*/
                                  end
                    primopiano:   begin 
                                    if(x0) assign y = scende;   /*assign y = 5'b01000;*/
                                    if(x1) assign y = fermo;    /*assign y = 5'b00001;*/
                                    if(x2) assign y = sale;     /*assign y = 5'b00010;*/
                                  end 
                    secondopiano: begin
                                    if(x0) assign y = scende2;  /*assign y = 5'b10000;*/
                                    if(x1) assign y = scende;   /*assign y = 5'b01000;*/
                                    if(x2) assign y = fermo;    /*assign y = 5'b00001;*/
                                  end
                    default       assign y = fermo;
                  endcase
              end         
endmodule

Here's the testbench:

module tst_ascx();
       logic clk, reset, x0, x1, x2;
       logic [4:0] y;

       ascx dut(clk, reset, x0, x1, x2, y);

       always begin
           clk=0; #10;
           clk=1; #10;
       end

       initial begin 
           $dumpfile("ascx.vcd");
           $dumpvars;

          reset=1; x0=0; x1=0; x2=0; #10;

          reset=0; x0=1; x1=0; x2=0; #10;
                   x0=0; x1=1;       #10;
                         x1=0; x2=1; #10;
                   x0=1;       x2=0; #10;
                   x0=0;       x2=1; #10;
                         x1=1; x2=0; #10;
                   x0=1; x0=0;       #10;
       end
endmodule

And here the gtkwave display:

The clock, as shown in the image, is not correct.
The input x0 should not be periodic, it simply represents a button being pressed at some time. I can't say if for the rest the module is working properly, because of these two problems.

toolic · Accepted Answer · 2021-02-10T13:18:12.997

You connected signals to your dut incorrectly. The 1st signal in the dut instance (clk) is connected to the 1st port in the module declaration (x0), etc.

To avoid this type of common mistake, use connection-by-name instead of connection-by-position. Refer to IEEE Std 1800-2017, section 23.3.2.2 Connecting module instance ports by name.

Change:

ascx dut(clk, reset, x0, x1, x2, y);

to:

ascx dut (
    .clk    (clk),
    .reset  (reset),
    .x0     (x0),
    .x1     (x1),
    .x2     (x2),
    .y      (y)
);

Unrelated to your problem: you should not use the assign keyword in the always block. Refer to this explanation of a procedural continuous assignment

Strange testbench behavior in gtkwave

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