GtkWave isn't getting signals from IVerilog simulation

Question

I wrote the following test for my code:

module HalfAdder_Test;
wire sum;
wire carry;
reg a = 0;
reg b = 0;

initial begin
    $dumpfile("test.vcd");
    $dumpvars(0, HalfAdder_Test);

    # 10 a = 0;
    # 10 b = 0;

    # 30 a = 1;
    # 30 b = 0;

    # 50 a = 0;
    # 50 b = 1;

    # 70 a = 1;
    # 70 b = 1;

    # 90 $stop;
end

HalfAdder ha (a, b, sum, carry);
endmodule

I then compile it and open it in GTKWave using the following commands:

iverilog -o HalfAdder -c files.txt
vvp HalfAdder -lxt2
gtkwave output.vcd

When I do that, I get the following output though:

The problem is that my signals aren't going high even when they do in the simulation. What could be causing my simulation to not output?

When I open my .vcd file, I get the following output:

1380 0001 4000 0000 0800 0000 9a00 0000
1400 0000 4400 0000 3800 0000 2300 1f8b
0800 0000 0000 020b 6360 f048 cc49 734c
4949 2d8a 0f49 2d2e d14b 6460 e04f 02e2
e4c4 a2a2 4a20 5d5c 9a0b 2433 12f5 1c19
1884 9c80 d819 8883 1900 93ef 3630 3800
0000 1f8b 0800 0000 0000 020b 6360 a02a
e000 6246 343e 131a 9f19 990f 003c 2732
3f80 0000 0000 0000 6a00 0000 5800 0000
0000 0000 0000 0000 0000 0000 fa1f 8b08
0000 0000 0000 0b62 6065 4005 4650 ba09
4a6f 81d2 bf00 0000 00ff ff62 6460 6462
6604 0000 00ff ff62 6062 6262 0043 2000
0000 00ff ff62 0400 0000 ffff 83ca 8943
694e 282d 08a5 e5a1 3408 b000 003e bbb2
446a 0000 0000 0000 2900 0000 3f00 0000
0000 0001 9a00 0000 0000 0001 9a1f 8b08
0000 0000 0000 0b62 6064 0003 c659 0000
0000 ffff 0233 1801 0000 00ff ff02 0206
0000 0000 ffff 6204 0000 00ff ff83 08c1
6930 1b00 c899 14b8 2900 0000

Answer 1

The problem was with the time scale in viewing the simulation.

Answer 2

module tb_top
(
    top_clk,
    top_rst
);

    input top_clk;
    input top_rst;

    reg thirty;
    reg twenty;
    reg [31:0]SHOW_CLK;

    always @( posedge top_clk or negedge top_rst)
    begin
        if (top_rst==1'b0)
        begin
            SHOW_CLK <= 32'h0;
            twenty <= 1'h0;
            thirty <= 1'h0;
        end
        else
        begin
            SHOW_CLK <= (SHOW_CLK+32'h1);
            twenty <= 1'h0;
            thirty <= 1'h0;
            if ((SHOW_CLK==32'h14))
            begin
                twenty <= 1'h1;
            end 
            if ((SHOW_CLK==32'h1e))
            begin
                thirty <= 1'h1;
            end 
        end 
    end 

endmodule

this is what a vcd file looks like, they vary by tool as to what the shortcut name is (D is SHOW_CLK in this case)

$scope module tb_top0 $end
$var wire 32 D SHOW_CLK $end
$var wire 1 B thirty $end
$var wire 1 E top_rst $end
$var wire 1 C twenty $end
$enddefinitions $end
#0
b00000000000000000000000000000000 D
0B
0E
0C
#20
b00000000000000000000000000000001 D
1E
#22
b00000000000000000000000000000010 D
#24
b00000000000000000000000000000011 D
#26
b00000000000000000000000000000100 D
#28
b00000000000000000000000000000101 D
#30
b00000000000000000000000000000110 D
#32
b00000000000000000000000000000111 D
#34
b00000000000000000000000000001000 D
#36
b00000000000000000000000000001001 D
#38
b00000000000000000000000000001010 D
#40
b00000000000000000000000000001011 D
#42
b00000000000000000000000000001100 D
#44
b00000000000000000000000000001101 D
#46
b00000000000000000000000000001110 D
#48
b00000000000000000000000000001111 D
#50
b00000000000000000000000000010000 D
#52
b00000000000000000000000000010001 D
#54
b00000000000000000000000000010010 D
#56
b00000000000000000000000000010011 D
#58
b00000000000000000000000000010100 D
#60
b00000000000000000000000000010101 D
1C
#62
b00000000000000000000000000010110 D
0C
#64
b00000000000000000000000000010111 D
#66
b00000000000000000000000000011000 D
#68
b00000000000000000000000000011001 D
#70
b00000000000000000000000000011010 D
#72
b00000000000000000000000000011011 D
#74
b00000000000000000000000000011100 D
#76
b00000000000000000000000000011101 D
#78
b00000000000000000000000000011110 D
#80
b00000000000000000000000000011111 D
1B
#82
b00000000000000000000000000100000 D
0B
#84
b00000000000000000000000000100001 D
#86
b00000000000000000000000000100010 D
#88
b00000000000000000000000000100011 D
#90
b00000000000000000000000000100100 D
#92
b00000000000000000000000000100101 D
#94
b00000000000000000000000000100110 D
#96
b00000000000000000000000000100111 D
#98
b00000000000000000000000000101000 D

If you get to the end of definitions and maybe a #0 but no clocked data that could mean that your design didnt actually change state for any of the selected signals (which could be because of your design or because you didnt run the sim long enough).

VCD is an ascii file format so independent of your recent success you should try to figure out how to fiew text files (same text editor you use to edit your verilog for example).