VHDL Compilation and Simulation with ModelSim

1. What you will learn

1.1 How to create a working directory for the VHDL tools.
1.2 How to compile a VHDL description.
1.3 How to simulate the VHDL description.

2. Getting started

This tutorial and those that follow it assume you are familiar with the Unix environment and commands within it to create directories, list, copy, and move files, etc. Knowledge of a text editor available on Unix systems like vi or jove is also required. If you are not familiar with Unix, it is recommended that you obtain the relevant take-outs from University Computing Services on these subjects and study them. or see the Basic Introduction to Unix available on-line.

This tutorial also assumes that you are familiar with the VHDL language itself, or are in the process of learning it. Consult the VHDL tutorial available from the tutorial web page if you are unfamiliar with VHDL.

2.1 First, you should create a separate directory under your home directory to hold the designs for this tutorial:

>> mkdir tutorial

>> cd tutorial

The ModelSim tool needs a work directory (VHDL library) for the compiled VHDL files. Create this directory with the following command:

>> vlib work

3. Examine and compile the code for this lab

3.1 Using your favorite text editor, create a file called package.vhd with the following contents:

-- user defined enumerated type
TYPE level IS ('X', '0', '1', 'Z');
-- type for vectors (buses)
TYPE level_vector IS ARRAY (NATURAL RANGE <>) OF level;
-- subtype used for delays
SUBTYPE delay IS time;

END resources;

3.2 Compile the VHDL code:

>> vcom package.vhd

The compiler should display the following message with no errors:

// ModelSim EE/VHDL 5.1h Oct 1 1998 SunOS 5.6
//
// Copyright (c) Mentor Graphics Corporation, 1982-1998, All Rights Reserved.
// UNPUBLISHED, LICENSED SOFTWARE.
// CONFIDENTIAL AND PROPRIETARY INFORMATION WHICH IS THE
// PROPERTY OF MENTOR GRAPHICS CORPORATION OR ITS LICENSORS.
//
// Copyright (c) Model Technology Incorporated 1990-1998, All Rights Reserved.
//
-- Loading package standard
-- Compiling package resources

3.3 Using a text editor, create a file called and2.vhd that contains the following:

USE work.resources.all;

ENTITY and2 is

GENERIC(trise : delay := 10 ns;

PORT(a : IN level;

c : OUT level);

END and2;

ARCHITECTURE behav OF and2 IS

ELSIF (a = '0' OR b = '0') THEN

ELSE

END IF;

END PROCESS one;

END behav;

3.4 Compile the VHDL code:

>> vcom and2.vhd

4. Simulate the compiled code

4.1 Start up the ModelSim simulator

>> vsim and2 &

This will bring up the following window:

Next, select the signals for viewing. Using the left mouse button, select View->Signals from the menus at the top of the window. This will bring up a signals window that looks like this:

4.2 View the waveforms of the selected signals by selecting, in the signals window, . Use the left mouse button to pull down the menu. This will create a wave window that looks like a graph, with the signals on the vertical axis and the time along the horizontal axis as shown below:

Note that in the main ModelSim window, the following commands appeared when you selected the menu items:

In addition to using the menus,commands can also be typed directly into the command window like this, or they can be executed from a script file, as will be shown later.

4.3 Stimulate the inputs to view circuit's response. Bring the Signals window to the top again if necessary and use the cursor and left mouse button to select the a signal. Use the Edit->Force... menu item to bring up the force window and fill it out so that it looks like the one below:

This command sets the a input to 0 after 5 ns. Notice when you pressed the Force button in the dialog box, the following line comes up in the ModelSim main window:

4.4 Now run the simulator for sufficient time by typing the following command in the ModelSim main window:

This command will run the simulation for 20 ns and update the wave window. Use the menu item in the wave window to expand the waveform display. Use the command to create two new cursors in the wave window and move them to the point where the a and c signals change. The resulting window should look like this:

4.5 Add some additional forces and run the simulation by typing the following commands into the main window:

VSIM 6>force -freeze /and2/b 1 0
VSIM 7>run 40

After using the command in the wave window, it should look like the one below:

4.6 Quit the simulator by typing quit in the main window and confirming yes in the dialog box.

5. Create, compile and simulate the D Flip Flop code

5.1 Using a text editor, create a file called dff.vhd which contains the following code:

ENTITY dff is

PORT(d : IN level;

clk : IN level;
enable : IN level;
q : OUT level;
qn : OUT level);

END dff;

ARCHITECTURE behav OF dff IS

IF ((clk = '1' AND clk'LAST_VALUE = '0') AND -- rising clock edge
enable = '1') THEN -- ff enabled

IF (d'STABLE(tsu)) THEN -- check setup

IF (d = '0') THEN -- check valid input data

q <= '0' AFTER tprop;
qn <= '1' AFTER tprop;

ELSIF (d = '1') THEN

q <= '1' AFTER tprop;
qn <= '0' AFTER tprop;

ELSE -- else invalid date

qn <= 'X';

END IF;

ELSE -- else no setup

qn <= 'X';

END IF;

END PROCESS one;

END behav;

5.2 Compile the dff.vhd file:

>>vcom dff.vhd

5.3 Simulate the DFF:

>>vsim dff &

Bring up the signals window as before using and the wave window using

5.4 Set up forces to generate a clock signal for the design:

VSIM 3>force -freeze /dff/clk 1 10 -repeat 20

This will setup a clock that is '0' for 10 ns, '1' for the next 10 ns, and then repeats.

5.5 Set up forces for the enable and d inputs and run the simulation:

VSIM 5>force -freeze /d 0, 1 15, 0 49
VSIM 6>run 100

After using and adding some cursors, the result should look like the one below:

5.6 Quit the simulator by typing quit in the main ModelSim window.

6.0 Run the dff simulation again, using a do file

As mentioned previously, you can use a script file, called a do file, to automate tasks like repetitive simulations for debugging purposes. The syntax of the commands for each action you want the tool to do can be looked up in the ModelSim documentation, but a better approach is to simply execute the command once in the actual tool, and then capture the resulting command from the main window into the do file.

6.1 Using a text editor, create a text file, called dff.do , that contains the following commands:

force -freeze /dff/clk 0 -repeat 20
force -freeze /dff/clk 1 10 -repeat 20
force -freeze /dff/enable 1
force -freeze /dff/d 0, 1 15, 0 49
run 100

6.2 Start the simulation of the dff again:

>>vsim dff &

6.3 Execute the dff.do file by either using the item from the main menu and selecting the dff.do file in the dialog box that comes up, or typing the following into the main window:

A wave window with the same simulation results you got above (minus the cursors) should appear.

6.4 Quit the simulator by typing quit in the main ModelSim window.