CODEWITHSUNDEEP
verilogdisplaymux
Saubhagya
Saubhagya

Reputation: 63

Verilog did not give the expected result

I wrote this Verilog code. The inner module is an 8-bit mux, and the top module is used to test the mux. It should display 11110000, but it displayed xxxxxxxx every time. How do I fix this?

module testbench;

reg CLK;
test mytest(CLK);
initial begin
CLK = 1'b0;
#10
CLK = 1'b1;
end
endmodule

module test(CLK);
input CLK;
reg [7:0] in0,in1;
reg sel;
wire [7:0] out;

mux myux(in0,in1,sel,out);

always @(posedge CLK) begin
    sel = 1'b0;
    in0 = 8'b11110000;
    $display("%b",out);
    end
endmodule

This is the mux module:

module mux(in0,in1,sel,out);
    input sel;
    input [7:0] in1,in0;
    output [7:0] out;
    reg out;

    always @(in0,in1,sel) begin

    if(sel == 1'b0) begin

        out = in0;

    end
    else begin

        out = in1;

    end
    end

endmodule

Upvotes: 0

Views: 66

Answers (1)

toolic
toolic

Reputation: 62236

The problem is that you did not run your simulation long enough. You only ran it for one clock cycle. Here is one way to change your testbench module to run many clock cycles:

module testbench;

reg CLK;
test mytest(CLK);

initial begin
    CLK = 1'b0;
    forever #10 CLK =~CLK;
end

initial #1000 $finish;

endmodule

I now see output like this:

xxxxxxxx
11110000
11110000
11110000
11110000

Also, I got a compile error with your code. In your mux module, you should change:

reg out;

to:

reg [7:0] out;

Upvotes: 2

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