Why won't EDA Playground show all eight times I set my clock to zero?

[kvn0smnsn]

I'm trying to create a module that stores the contents of a queue, with two parameters, (nmElems) for the number of elements in the queue and (nmBits) for the number of bits in each element. My Verilog code for module (sQueue) is:

// (c) Kevin Simonson 2024

module sQueue #( nmBits = 2, nmElems = 1)
              ( head, clock, tail, shift, reset);
  localparam         maxBit  = nmBits  - 1;
  localparam         maxElem = nmElems - 1;
  output [ maxBit:0] head;
  input              clock;
  input  [ maxBit:0] tail;
  input              shift;
  input              reset;
  reg    [ maxBit:0] data [ maxElem:0];
  var                elem;

  always @( posedge clock)
  begin
    for (elem = 0; elem < nmElems; elem = elem + 1)
    begin
      if      (reset)
        data[ elem] <= 0;
      else if (shift)
      begin
        if (elem < maxElem)
          data[ elem] <= data[ elem + 1];
        else
          data[ maxElem] <= tail;
      end
    end
  end

  assign head = data[ 0];

endmodule

and my Verilog code for (t_sQueue) is:

// (c) Kevin Simonson 2024

module t_sQueue;
  reg  [ 7:0] insertee;
  reg         clock;
  reg         shift;
  reg         reset;
  wire [ 7:0] extractee;

  sQueue #( 8, 4) sq_8_4( extractee, clock, insertee, shift, reset);

  initial
  begin
    reset       = 1'b1;
    shift       = 1'b0;
    insertee    = 8'b00001000;
    clock       = 1'b0;            // Queue gets reset to all zeroes.
    #2 clock    = 1'b1;
    #2 reset    = 1'b0;
    clock       = 1'b0;            // 8 doesn't get inserted.
    #2 shift    = 1'b1;
    insertee    = 8'b00001010;
    clock       = 1'b1;            // Insert  10.
    #2 clock    = 1'b0;
    #2 shift    = 1'b0;
    clock       = 1'b1;            // (shift) is zero, so 10 only gets inserted
    #2 clock    = 1'b0;            // once.
    #2 shift    = 1'b1;
    insertee    = 8'b01000101;
    clock       = 1'b1;            // Insert  69.
    #2 clock    = 1'b0;
    #2 insertee = 8'b00000011;
    clock       = 1'b1;            // Insert   3.
    #2 clock    = 1'b0;
    #2 insertee = 8'b10001000;
    clock       = 1'b1;            // Insert 136. 10 should be visible.
    #2 clock    = 1'b0;
    #2 insertee = 8'b10100101;
    clock       = 1'b1;            // Insert 165. 69 should be visible.
    #2 clock    = 1'b0;
    shift       = 1'b0;
    #2 clock    = 1'b1;
  end

  always @( clock)
  begin
    if (clock == 1'b0)
      $display
        ( "time: %2t, reset: %1d, shift: %1d, insertee: %3d, extractee: %3d."
        , $time     , reset     , shift     , insertee     , extractee       );
  end

endmodule

When I go to EDA Playground and put (t_sQueue) in the left window and (sQueue) in the right window, and click on (Run), I get:

CPU time: .251 seconds to compile + .255 seconds to elab + .286 seconds to link
Chronologic VCS simulator copyright 1991-2021
Contains Synopsys proprietary information.
Compiler version S-2021.09; Runtime version S-2021.09; Jan 4 17:21 2024
time: 0, reset: 1, shift: 0, insertee: 8, extractee: X.
Execution interrupted or reached maximum runtime.
Exit code expected: 0, received: 137
Done

What am I doing wrong here? Why does it only show the time (clock) is zero at time 0, and doesn't show any of the other seven times (clock) gets set to zero?

A poster here suggested I post articles like this to the forum at "https://groups.google.com/forum/#!forum/eda-playground". I did that two hours ago, and nobody has responded to it yet, so I'm posting it here too.

 

[ThisIsNotSam]

the way you define the clock is a mess, that is why. your delay assignments are not doing what you think they do.

try this structure:
clock = 0;
#2;
clock = 1;
#2;

(note: this is still terribly unnecessary code for clock generation)

 

[dpaul]

Use something like the following for clock generation in test-benches. Depending on the intended clock frequency, use an appropriate value for CLKIN_PERIOD.

initial
sys_clk_i = 1'b0;
always
sys_clk_i = #(CLKIN_PERIOD/2.0) ~sys_clk_i;

 

[FvM]

The testbench is o.k., there's however a trivial fault in sQueue causing stuck of the for loop

var elem must be integer elem

# time:  4, reset: 0, shift: 0, insertee:   8, extractee:   0.
# time:  8, reset: 0, shift: 1, insertee:  10, extractee:   0.
# time: 12, reset: 0, shift: 0, insertee:  10, extractee:   0.
# time: 16, reset: 0, shift: 1, insertee:  69, extractee:   0.
# time: 20, reset: 0, shift: 1, insertee:   3, extractee:   0.
# time: 24, reset: 0, shift: 1, insertee: 136, extractee:  10.
# time: 28, reset: 0, shift: 0, insertee: 165, extractee:  69.