6.111 Lab #2
Goal: implement simple circuits in Verilog; download and run a
sample circuit on the labkit.
Exercise 1: Writing Verilog code
In this exercise you'll design a Verilog module that implements a
74LS163. Here are the steps:
- Log into one of the workstations in the Digital Lab. Your
username is your Athena login name and your initial password is
- Download lab2_1.v by right-clicking on
the link and select "Save As" (or "Save Link As"), specify your home
directory (U:) as the destination.
The steps below describe how to use our Verilog simulator Modelsim
as a standalone application. One can also run the simulator from the
Xilinx ISE toolkit -- see the labkit documentation Simulating
with Modelsim for details on how to do this. Feel free to use
either approach for this part of the lab.
- Start Modelsim (look in the Programs listing under the Start
menu). If it complains about a missing license file, go to Start
-> Programs -> Modelsim -> Licensing Wizard. Click the
Continue button and specify email@example.com as the location of
the licensing file. If it offers to add the appropriate environment
variable, let it! The wizard will complete, click OK when it's done.
Now restart Modelsim and everything should be happy.
- At the bottom of the Modelsim window there's a frame labeled
"Transcript" where you can type in commands and see various
messages from the simulator. Type in "cd u:" to change to
your home directory.
- Type "vlib work" to set up the simulator's working
- Type "vlog lab2_1.v" to compile the verilog file you
downloaded above. Output from the compiler is displayed in the
- Type "vsim test" to start simulating the test module
found in lab2_1.v.
- Type "run 2000ns" to run the simulation for 2000ns.
You should see the following printout in the Transcript window
# Starting test of LS163...
# clear was asserted low, but counter didn't clear
# out = xxxx, expected 0000
# Break at lab2_1.v line 48
These messages were generated by code inside the test module as it
runs through various tests of the LS163 module. The LS163 module
supplied in lab2_1.v is empty which is why the test failed. Your job is
to fill in the body for the LS163 module, implementing the correct
functionality. Refer to the 74LS163 datasheet to see what
functionality your code needs to implement.
You can use the editor of your choice to edit lab2_1.v
appropriately; Modelsim has a simple built-in editor which should be
displaying lab2_1.v after you completed step 8. As you edit lab2_1.v,
repeat steps 6 through 8 above to test your code. When you're
successful you'll see
# Starting test of LS163...
# Finished test of LS163...
- When your code passes the tests, have a staff member check
you off. They may ask a simple question or two, but mostly they
just want to see it working under Modelsim.
- After checkoff, please upload your Verilog
file using the "Submit Verilog" page on the course website. We'll
review your code and post some comments to help you improve your
Verilog style. We'll be looking for proper use of comments and
formatting to make your code easy to understand.
Exercise 2: Compiling and running Verilog on the labkit
In this exercise you'll design a Verilog module that reads a 4-bit
value from labkit's switches and displays the appropriate hex digit
on a 7-segment display.
- To learn more about the Xilinx FPGA tools please read the
section of the Labkit documentation. You'll follow the steps
outlined there whenever creating a new project for the labkit.
- Download labkit.v and
labkit.ucf by right-clicking on the links and
selecting "Save As" (or "Save Link As"), specify your home directory
(U:) as the destination. Note that the browser may save your downloads
with a ".txt" extension -- you'll have to rename the files to have
".v" and ".ucf" extensions in order for the Xilinx tools to recognize
The labkit module (defined in labkit.v)
has port declarations for all the labkit peripherals as well as
supplying default values for all the output ports. This is the top-level
module for all labkit projects -- you should make a copy of it using
a meaningful file name (eg, lab2_2.v) and modify the copy to implement
the circuitry for your project. labkit.ucf (which you'll never need to
modify) specifies which FPGA pin is connected to which named port in
- Start the Xilinx ISE tool and create a new project following
the steps outlined in the Getting Started document. The Xilinx tools
create a very large number of files, so to keep things neat and tidy I
recommend keeping your .v files separate from the project directory.
For example, if you keep your .v files in U:, specify U: as the
location for your project directories, and when you supply a name for
the project (eg, lab2_2) a directory of that name will be created in
U: and used to store all the Xilinx-created files. When you add
verilog source files to the project, just go up one directory level
to locate your source files.
- We'll be using the 7-segment display from your kit of parts
(this is the display you used in Exercise 6 of Lab 1), this time wired
to the FPGA via the labkit's breadboard (see photo below). Wire up
the display connecting its ground pins to the appropriate columns of
the breadboard, and the signal pins to the User 1 connector at the
top of the labkit (I used pins 0 through 7).
- Add Verilog code to the labkit module using four of the
labkit's slide switches to specify which hex digit to display on a
7-segment display. The switch port of the labkit module is
an 8-bit value reflecting the current settings of the labkit's slide
switches. Use switch[3:0] as the 4-bit hex digit to be
displayed. Here's the appropriate pattern of segments for each
Compute the appropriate value for each of segment control signals
and drive them onto the appropriate FPGA output pins (I used
user1[7:0]). Note that you'll have modify or comment-out the
existing line in the code that sets a default value for the output
pins you're using.
Synthesize and implement your design. Generate a programming
file and configure the FPGA. When your circuit is working, ask
a staff member to check you off. For checkoff be prepared to show
your circuit in operation, displaying different digits as the switches
are turned on and off.
- After checkoff, please upload your Verilog file using the
"Submit Verilog" page on the course website. We'll review your code
and post some comments to help you improve your Verilog style.