Skip to content
Fetching contributors…
Cannot retrieve contributors at this time
115 lines (104 sloc) 3.82 KB
* Copyright (c) 1999 Stephen Williams (
* This source code is free software; you can redistribute it
* and/or modify it in source code form under the terms of the GNU
* General Public License as published by the Free Software
* Foundation; either version 2 of the License, or (at your option)
* any later version.
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* GNU General Public License for more details.
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
* This example program simulates a 16x1 ram, and is used as an
* example for using VCD output and waveform viewers.
* Like any other Verilog simulation, compile this program with the
* command:
* iverilog show_vcd.vl
* This will generate the show_vcd command in the current directory.
* When you run the command, you will see the output from all the
* calls to $display, but also there will be a dump file ``show_vcd.vcd''.
* The name of this file is set by the statement:
* $dumpfile("show_vcd.vcd");
* in the main module. The output file uses the standard VCD file format
* so can be viewed using off-the-shelf waveform viewers. The remaining
* steps describe how to use GTKWave to view the file. If you are using
* a different viewer, see the documentation for that tool.
* To view the output generated by running show_vcd, start the GTKWave
* viewer with the command:
* gtkwave show_vcd.vcd
* The GTKWave program will display its main window, and show in a small
* status box (upper left corner) that it succeeded in loading the dump
* file. However, there are no waveforms displayed yet. Select signals to
* add to the waveform display using the menu selection:
* "Search --> Signal Search Tree"
* This will bring up a dialog box that shows in directory tree format
* the signals of the program. Select the signals you wish to view, and
* click one of the buttons on the bottom of the dialog box to display
* the selected signals in the waveform window. Click "Exit" on the box
* to get rid of it.
* The magic that makes all this work is contained in the $dumpfile and
* $dumpvars system tasks. The $dumpfile task tells the simulation where
* to write the VCD output. This task must be called once before the
* $dumpvars task is called.
* The $dumpvars task tells the simulation what variables to write to
* the VCD output. The first parameter is how far to descend while
* scanning a scope, and the remaining parameters are signals or scope
* names to include in the dump. If a scope name is given, all the
* signals within the scope are dumped. If a wire or register name is
* given, that signal is included.
module ram16x1 (q, d, a, we, wclk);
output q;
input d;
input [3:0] a;
input we;
input wclk;
reg mem[15:0];
assign q = mem[a];
always @(posedge wclk) if (we) mem[a] = d;
endmodule /* ram16x1 */
module main;
wire q;
reg d;
reg [3:0] a;
reg we, wclk;
ram16x1 r1 (q, d, a, we, wclk);
initial begin
$dumpvars(1, main.r1);
wclk = 0;
we = 1;
for (a = 0 ; a < 4'hf ; a = a + 1) begin
d = a[0];
#1 wclk = 1;
#1 wclk = 0;
$display("r1[%x] == %b", a, q);
for (a = 0 ; a < 4'hf ; a = a + 1)
#1 if (q !== a[0]) begin
$display("FAILED -- mem[%h] !== %b", a, a[0]);
endmodule /* main */
Jump to Line
Something went wrong with that request. Please try again.