Verilog testbench problem: waveform shows Unknown

Question

I have problem with the clock signal in a Verilog testbench.

I've tried almost every possible way to create the clock, but in the waveform it is U, meaning "Unknown".

Here is my code for clock (i_SysCLK is my input clock):

`timescale 1ps/100fs   

 module sim_tb_top;  
   parameter CLKIN_PERIOD          = 5000;  
  reg                     sys_clk_i;  

  reg clk_ref_i;  
  wire                    i_SysCLK;  
      initial  
    sys_clk_i = 1'b0;  
  always  
    sys_clk_i = #(CLKIN_PERIOD/2.0) ~sys_clk_i;  
    
  initial  
    clk_ref_i = 1'b0;  
  always  
    clk_ref_i = #REFCLK_PERIOD ~clk_ref_i;  
  initial  
    i_SysCLK = 1'b0;  
  always  
    i_SysCLK = #(REFCLK_PERIOD/2.0) ~i_SysCLK;  
endmodule

Answer 1

I get 2 compile errors when I run your code on 2 different simulators. Did you try your code on edaplayground?

You didn't define REFCLK_PERIOD and you declared i_SysCLK as a wire. After making these 2 corrections, and assuming some value for REFCLK_PERIOD, I get output which clearly demonstrates that all 3 clock signals are toggling between 0 and 1. Un-comment any ONE of the $monitor lines:

`timescale 1ps/100fs

module sim_tb_top;
    parameter REFCLK_PERIOD = 1000;
    parameter CLKIN_PERIOD  = 5000;
    reg sys_clk_i;
    reg clk_ref_i;
    reg i_SysCLK;

initial sys_clk_i = 1'b0;

always sys_clk_i = #(CLKIN_PERIOD/2.0) ~sys_clk_i;

initial clk_ref_i = 1'b0;
  always
    clk_ref_i = #REFCLK_PERIOD ~clk_ref_i;
  initial
    i_SysCLK = 1'b0;
  always
    i_SysCLK = #(REFCLK_PERIOD/2.0) ~i_SysCLK;

initial begin
//    $monitor($time, " %b", sys_clk_i);
//    $monitor($time, " %b", clk_ref_i);
    $monitor($time, " %b", i_SysCLK);
    #10_000 $finish;
end
endmodule