Implementing a Xilinx Project

Tom Kelliher, CS 240

Apr. 4, 2008

So, you've synthesized your VHDL design and now it's time to implement it, generate the bitstream file, and download the bitstream into the FPGA to configure it according to your design. This document will take you through that process using the Xilinx and XSTOOLS software:

• Required VHDL code for clock signals. Do this after synthesizing and simulating.

• Adding pin constraints to your design.

• Implementing your design and generating a bitstream file.

• Simulating your design after adding the VHDL clock signal code.

• Testing the FPGA board (XSTOOLS).

• Downloading a bitstream into the FPGA board (XSTOOLS).

I assume you have a synthesized design upon which you've performed behavioral simulation.

Required VHDL Code for Clock Signals

You can hold off on adding the buffers below until you're ready to download your design into a Xilinx board. However, I recommend you use the clock naming convention described below.

First off, you'll need to verify that you're using the correct device. In the Sources for pane, verify that the device you're using is xc2s50-5tq144-XST VHDL. If not, right-click on the line, choose Properties, and set the properties as follows:

• Device Family: Spartan2
• Device: xc2s50
• Package: tq144
• Speed Grade: -6

Following this, open the Project menu and select Cleanup Project Files to clear out any old, erroneous information. (Cleaning up the project files will often correct ``weird'' errors you may be getting.)

Due to their importance, clock signals require special handling. You will need to add the following to your VHDL code. I will assume that you named your external clock signal (the name given in the port list of the entity declaration) clk_ext and that you are using the signal clk internally to clock your flip-flops. (Otherwise, modify the following for your naming convention.) Initially, you should tie clk to clk_ext like so:

signal clk: std_logic;

...

begin

clk <= clk_ext;

When it's time to add the buffers, comment-out your assignment to clk and add the following to the declarations section of your architecture body:

  component ibuf port (
    i : in  std_logic;
    o : out std_logic);
  end component;

  component bufg port (
    i : in  std_logic;
    o : out std_logic);
  end component;

  -- clk is the internal clock name.  Modify as necessary.
  signal clk_pad, clk : std_logic;

Add this to the body of your architecture:

  u1 : ibuf port map (
    i => clk_ext,   -- External clock name.  Modify as necessary.
    o => clk_pad);

  u2 : bufg port map (
    i => clk_pad,
    o => clk);   -- Internal clock name.  Modify as necessary.

The clock signal that you will now use to control your flip-flops is clk.

Adding Pin Constraints

1. Add pin constraints after implementing your project once (see the next Section).

2. With your project open, create a new source file. For the type of source file, choose
   Implementation Constraints File. Once you've created the new file, highlight it in the Sources for window and double-click Assign Package Pins in the Processes for Current Source window.

3. The Xilinx PACE tool will run.

4. Within the Design Object List -- I/O Pins window, you will see the names of all your project's port signals listed. To constrain these I/O signals to specific FPGA pins, you will enter values for the Location field. The values to be used for the problems will be given to you separately.

   Be careful when filling in this information. Doing this incorrectly can result in damage to the FPGA board, the PC, or both. Not to mention what it will do to your wallet!

5. Save the constraints and exit the tool.

6. Re-implement your design.

Note: Any time you modify the constraints, you will need to re-implement the design and generate a new bitstream file.

Implementing a Design and Generating a Bitstream File

1. In the Sources for window, highlight your VHDL file. Then, in the Processes for Current Source window, double-click Implement Design. Your design will be implemented. Correct any errors. (If you neglect to add the VHDL for clock signals, you will receive several fatal errors at this point.)

2. Expand the Implement Design tree by clicking on the + symbol before it. Find and open the Pad Report and confirm that your I/O signals were indeed constrained to the correct FPGA pins. If they were not, go back and correct your constraints.

   Do not proceed until the Pad Report is correct.

3. Contract the Implement Design tree.

4. Once your design has been successfully implemented, double-click Generate Programming File to generate your design's bitstream file.

Simulating Your Design After Adding the VHDL Clock Signal Code

Once you've added the VHDL for the clock signal, the behavioral VHDL simulation will no longer work properly. However, the Post-Translate VHDL simulation will, so use that instead if you need to perform further simulation.

Testing the FPGA Board

You can do this to verify that the board is functioning correctly.

• From the Start menu, go to Programs, then XSTOOLS, then choose GXSTEST.

• Set Board Type to XSA-50 and Port to LPT1.

• Press the TEST button. GXSTEST will report the result of the test.

Downloading a Bitstream

1. Open your design folder and locate your bitstream file. It will have a .bit extension.

2. From the Start menu, go to Programs, then XSTOOLS, then choose GXSLOAD.

3. Make sure that the Board Type is set to XSA-50 and that Port is set to LPT1.

4. Drag your bitstream file into the FPGA/CPLD area of GXSLOAD. GXSLOAD will download the highlighted bitstream file into the FPGA when you click the Load button.

   If the bitstream file was successfully downloaded, the decimal point on the seven-segment display will be lit. Anytime the decimal point is not lit, the FPGA is not correctly configured.

5. You're now ready to use the FPGA.

Tom Kelliher