Modelsim Tutorial

Modelsim Tutorial

A. Creating a Project in Modelsim

The goals for this lesson are:

- Create a project

A project is a collection entity for an HDL design under specification or test. Projects ease interaction

with the tool and are useful for organizing files and simulation settings. At a minimum, projects have a

work library and a session state that is stored in a .mpf file. A project may also consist of:

o HDL source files or references to source files

o other files such as READMEs or other project documentation

o local libraries

o references to global libraries

For more information about using project files, see the ModelSim User’s Manual.

1. Start ModelSim:

from a Windows shortcut icon, from the Start menu



Upon opening ModelSim for the first time, you will see the Welcome to ModelSim dialog. (If this

screen is not available, you can display it by selecting Help > Welcome Menu from the Main window.)


2. Select Create a Project


from the Welcome dialog, or File > New > Project (Main window). In the Create Project dialog box,

enter "test" as the Project Name and select a directory where the project file will be stored. Leave the

Default Library Name set to "work."




Upon selecting OK, you will see a blank Project tab in the workspace area of the Main window and

the Add Items to the Project dialog.





3. The next step is to add the files that contain your design units.


Click Add Existing File in the Add Items to Project dialog. For this exercise, we’ll add two Verilog

files. Click the Browse button in the Add file to Project dialog box and open the examples directory in

your ModelSim installation. Select tcounter.v and counter. v. Select Reference from current location and then click OK






4. Click your right mouse button

in the Project page and select Compile > Compile All.





5. The two files are compiled.

Click on the Library tab and expand the work library by clicking the "+" icon. You’ll see the compiled

design units listed.




6. Load one of the design units

The last step in this exercise is to load one of the design units. Double-click counter on the Library

page. You’ll see a new page appear in the Workspace that displays the structure of the counter design





At this point, you would generally run the simulation and analyze or debug your design. We’ll do just

that in the upcoming lessons. For now, let’s wrap up by ending the simulation and closing the project.

Select Simulate > End Simulation and confirm that you want to quit simulating. Next, select File >

Close > Project, confirm that you want to close the project, and select Yes to update your project file

with the changes you made during this session.


Note that a test.mpf file has been created in your working directory. This file contains information

about the project test that you just created. ModelSim will open this project automatically the next

time you invoke the tool.



1. Compiling the design

a) Create and change to a new directory to make it the current directory.

You can make the directory current by invoking ModelSim from the new directory or by using the

File > Change Directory command from the ModelSim Main window.

b) Copy the Verilog files (files with ".v" extension)

from the \<install_dir>\modeltech\examples directory into the current directory.

Before you can compile a Verilog design, you need to create a design library in the new directory.

Since ModelSim is a compiled Verilog simulator, it requires a target design library for the

compilation. ModelSim can compile both VHDL and Verilog code into the same library if desired.

c) Invoke ModelSim:

from a Windows shortcut icon, from the Start menu




Click Close if the Welcome dialog appears.


d) Create library

Before you compile any HDL code, you’ll need a design library to hold the compilation results. To

create a new design library, make this menu selection in the Main window: File > New > Library.

Make sure Create: a new library and a logical mapping to it is selected.


Type "work" in the Library Name field and then select OK. This creates a subdirectory named work -

your design library - within the current directory. ModelSim saves a special file named _info in the


(PROMPT: vlib work vmap work work)




In the next step you’ll compile the Verilog design. The example design consists of two Verilog source

files, each containing a unique module. The file counter.v contains a module called counter, which

implements a simple 8-bit binary up-counter. The other file, tcounter. v, is a testbench module

(test_counter) used to verify counter.


Under simulation you will see that these two files are configured hierarchically with a single instance

(instance name dut) of module counter instantiated by the testbench.

You'll get a chance to look at the structure of this code later. For now, you need to compile both files

into the work design library.


Note: Do not create a Library directory using Windows commands, because

the _info file will not be created. Always use the File menu or the vlib command from

either the ModelSim or UNIX/DOS prompt.)


e) Compile

Compile the counter.v, and tcounter.v files into the work library by selecting Compile > Compile

from the menu.

(PROMPT: vlog counter.v tcounter.v)

This opens the Compile HDL Source Files dialog box.





Select counter.v and tcounter.v (use Ctrl + click) and then choose Compile and then Done.


Note: The order in which you compile the two Verilog modules is not important (other than the

source-code depende ncies created by compiler directives). So it doesn’t matter here if you choose to

compile counter.v before or after tcounter. v.


2. Loading the design


a) Load the design

by selecting Simulate > Simulate from the menu:


(PROMPT: vsim test_counter)

The Simulate dialog appears. Click the "+" sign next to ’work’ to see the counter and test_counter

design units. (You won’t see this dialog box if you invoke vsim with test_counter from the command




The Simulate dialog allows you to select a design unit to load from the specified library. You can also

select the resolution limit for the simulation. The default resolution is 1 ns.

Select test_counter and click Load to accept these settings.


b) Bring up the Signals, Source, and Wave windows

by entering the following command at the VSIM prompt within the Main window : view signals

source wave

(Main MENU: View > <window name>)

c) Add signals

Now let’s add signals to the Wave window with ModelSim’s drag and drop feature. In the Signals

window, select Edit > Select All to select the three signals. Drag the signals to either the pathname or

the values pane of the Wave window.





HDL items can also be copied from one window to another (or within the Wave and List windows)

with the Edit > Copy and Edit > Paste menu selections.


d) Structure pane

You may have noticed when you loaded the design in Step 1 that a new tab appeared in the workspace

area of the Main window.



The Structure tab shows the hierarchical structure of the design. By default, only the top level of the

hierarchy is expanded. You can navigate within the hierarchy by clicking on any line with a "+"


(expand) or "-" (contract) symbol. The same navigation technique works anywhere you find these

symbols within ModelSim.

By clicking the "+" next to dut: counter you can see all three hierarchical levels: test_counter,

counter and a function called increment. (If test_counter is not displayed you simulated counter

instead of test_counter.)

Click on Function increment and notice how other ModelSim windows are automatically updated as

appropriate. Specifically, the Source window displays the Verilog code at the hierarchical level you

selected in the Structure window, and the Signals window displays the appropriate signals. Using the

Structure tab in this way is analogous to scoping commands in interpreted Verilogs.

For now, make sure the test_counter module is showing in the Source window by clicking on the top

line in the Structure pane.



3. Running the simulation



Now you will exercise different Run functions from the toolbar.

a) Run

Select the Run button on the Main window toolbar. This causes the simulation to run and then stop

after 100 ns (the default simulation length).

(PROMPT: run) (MENU: Simulate > Run > Run 100 ns)

b) Specify run length

Next change the run length to 500 on the Run Length selector and select the Run button again.





Now the simulation has run for a total of 600ns (the default 100ns plus the 500 you just asked for).

The status bar at the bottom of the Main window displays this information.

c) Run until specified time

The last command you executed (run 500) caused the simulation to advance for 500ns. You can also

advance simulation to a specific time. Type: run @ 3000

This advances the simulation to time 3000ns. Note that the simulation actually ran for an additional

2400ns (3000 - 600).



d) Run until breakpoint

Now select the Run -All button from the Main window toolbar. This causes the

simulator to run until the stop statement in tcounter.v.

(PROMPT: run -all) (MENU: Simulate > Run > Run -All)






You can also use the Break button to interrupt a run.



4. Debugging



Next we’ll take a brief look at an interactive debugging feature of the ModelSim environment.

a) Set a breakpoint

Let’s set a breakpoint at line 30 in the counter.v file (which contains a call to the Verilog function

increment). To do this, select dut: counter in the Structure pane of the Workspace. Move the cursor to

the Source window and scroll the window to display line 30. Click on or near line number 30 to set a

breakpoint. You should see a red dot next to the line number where the breakpoint is set.

The breakpoint can be toggled between enabled and disabled by clicking it. When a breakpoint is

disabled, the dot appears open. To delete the breakpoint, click the line number with your right mouse

button and select Remove Breakpoint.


b) Restart

Select the Restart button to reload the design elements and reset the simulation time to zero.



(Main MENU: Simulate > Run > Restart) (PROMPT: restart)


Note: Breakpoints can be set only on executable lines, denoted by blue line numbers




Make sure all items in the Restart dialog box are selected, then click Restart.




Select the Run -All button to re-start the simulation run





(PROMPT: run -all) (Main MENU: Simulate > Run > Run -All)

When the simulation hits the breakpoint, it stops running, highlights the line with an arrow in the

Source window, and issues a Break message in the Main window.

c) Reading signal values

When a breakpoint is reached, typically you will want to know one or more signal values. You have

several options for checking values :

- look at the values shown in the Signals window

- hover your mouse pointer over the count variable in the Source window and a "balloon" will

pop up with the value



- select the count variable in the Source window, right-click it, and select Examine from the

context menu

- use the examine command to output the value to the Main window transcript: examine count

d) Step

Let’s move through the Verilog source functions with ModelSim’s Step command.

Click Step on the toolbar.




This command single-steps the debugger.

e) Hands-on

Experiment by yourself for awhile. Set and clear breakpoints and use the Step and Step Over

commands until you feel comfortable with their operation. When you’re done, quit the simulator by

entering the command: quit –force

C. Exhaustive simulation using Verilog


  1. Test bench for the counter module


As you have seen, along with the actual module is another Verilog file, called a test bench for

this module. This module creates the test vector that enables us to check that the program

actually performs the right function.

In this example, the test bench is pretty short, since the only input is the clock, but other

systems might have more inputs and you might want to simulate all possible realizations of

these inputs. (For example, for a 3-to8 decoder, you want to generate all possible 3 bit inputs)



We will now analyse the structure of the program to understand the different commands.


module test_counter;

reg clk, rst;

wire [7:0] count;

Regular module declaration : there is no inputs nor outputs


In th is code fragment, the stimulus and response capture are going to be coded using

a couple of initial blocks. An initial block can contain sequential statements that can

be used to describe the behaviour of signals in a test bench. In the Stimulus initial

block, we need to generate waveforms on the clock and reset inputs. Thus:

initial // Clock generator


clk = 0;

#10 forever #10 clk = !clk;


initial // Test stimulus


rst = 0;

#5 rst = 1;

#4 rst = 0;

#50000 $stop;


Everytime there is a pound sign followed by a number n, it means that the simulator

advances by n times of simulation and then does whatever assignment is specified.

The $stop command is a Verilog built-in command that stops the simulation. In this

case, the simulation will stop after 50009 simulation times.

Concerning the clock, its behavior is easy to understand. At the beginning, its value is

set to zero ; then, every 10 seconds and for ever, its value is inverted.


counter #(5,10) dut (count,clk,rst);

The test bench has to instantiate an instance of the module counter.




$monitor($stime,, rst,, clk,,, count);

The Response initial block can be described very easily in Verilog as we can benefit

from a built-in Verilog system task. Indeed, $monitor is a system task that is part of

the Verilog language. Its mission in life is to print values to the screen. The values it

prints are those corresponding to the arguments that you pass to the task when it is

executed. The $monitor task is executed whenever any one of its arguments changes,

with one or two notable exceptions :

$time is a system function (as opposed to a system task). It returns the current

simulation time. In the above example, $time is an argument to $monitor. However,

$time changing does not cause $monitor to execute — $monitor is clever enough to

know that you wouldn't really want to print to the screen the values of all of the

arguments every time the simulation time changed.

The succession of two commas in the argument list ensures that a space is printed to

the screen after the value of $time each time $monitor is executed. This is a simple

method of formatting the screen output.

Finally we come to the signal arguments themselves. Each time one of these signals

changes value, $monitor will execute. When $monitor executes it will print all of the

argument values to the screen, including $time. This is the output created by $monitor

in our counter test bench:

# 0 0 0 x

# 5 1 0 x

# 9 0 0 x

# 15 0 0 0

# 20 0 1 0

# 25 0 1 1

# 30 0 0 1

# 40 0 1 1

# 45 0 1 2

# 50 0 0 2

# 60 0 1 2

# 65 0 1 3

# 70 0 0 3

# 80 0 1 3

# 85 0 1 4

# 90 0 0 4


A last note on this test bench ; to access and monitor a signal which is in a lower hierarchical

level, you just have to call the signal by the module name, followed by a dot, followed by the

signal name.