fpga laboratory assignment 1 due date: 19/09/2012

Department of Electrical and Computer Engineering, University of Cyprus

ECE408 -Digital Design with FPGAs 1

FPGA Laboratory Assignment 1

Due Date: 19/09/2012

Aim

The purpose of this lab is to introduce you to the basic FPGA design and

programming tools. For the purposes of this lab you will first perform an installation

of the tools and then implement an adder circuit using FPGA design flow.

Objectives

Verify that the Xilinx tools are up and running

Introduce you to the Xilinx ISE software.

Become familiar with VHDL coding and use of the ISE simulator (ISim).

Be able to synthesize and implement VHDL designs to FPGAs using ISE.

Equipment

Xilinx ISE 12.XX

ISE Simulator (Isim)

Introduction

This is a step-by-step tutorial for building a 1-bit full adder and a D flip-flop in Xilinx

ISE 12.4, a Design Suite software that provides designers with the ability to code

designs in a hardware description language such as VHDL or Verilog. The ISE

Design Suite also provides the ability to apply FPGA pin and timing constraints,

analyse for errors and violations, and synthesize to generate configuration bit file

formats for FPGAs.

By the end of this tutorial, you should be able to:

• Create a new design by VHDL coding.

• Verify the function of a design by behavioural simulation.

• Map a design to an FPGA device through placement and routing procedures.

• Estimate the performance of the design by timing analysis.



• Use the 1-bit full adder created in this tutorial to implement and simulate a 4-bit

adder.

Step-by-step 1-bit Full Adder Design

Coding the Full Adder Circuit

An example 1-bit full adder block diagram and Boolean circuit are shown in Fig.1 (a)

and (b), respectively. The objective of the lab is to code the Boolean circuit in VHDL

and test its functionality using VHDL testbench.

Fig.1 (a)

Fig.1 (b)

Tasks:

(1) Create a project file in Xilinx Project Navigator.

To launch the Project Navigator run Start Programs Xilinx ISE Design Suite

12.4 ISE Design Tools Project Navigator. Create a project folder by selecting

FileNew Project. Use the Project Name and Location tabs to name and store the

A B Cin Cout S

0 0 0 0 0

0 0 1 0 1

0 1 0 0 1

0 1 1 1 0

1 0 0 0 1

1 0 1 1 0

1 1 0 1 0

1 1 1 1 1



project. (No spaces in the file path or file name – Xilinx ISE does not support such

ASCII characters)

In the Property tab set the Device to “XC3S1600E-FG320” (Spartan 3 device) and

the Design Flow to VHDL and Click NEXT. The next two windows ask for file

sources, these are not relevant at the moment so to continue press Next to both

windows.

Enter file name and path

(no spaces in the path)



(2) Create a VHDL module.

To create a VHDL module right-click on the icon xc3s1600e-4fg320 (shown

below) and select New Source.

Enter

FPGA

device

details and

required

simulator



Now select VHDL Module from the left-side of the menu and enter a name for

the file (for example “Full_Adder”) and click on Next (as shown below).

Create a

new VHDL

Module



In the Architecture Name tab write “Behavioral” and enter the appropriate inputs and

outputs of the 1-bit Full Adder in the Port Name tab. Select Next and Finish to create

the VHDL module. (Click through the remaining menu to complete). Select

FileSave-all. (Remember to regularly save your work)

A VHDL module should now appear in project navigator, as shown below. This

provides the entity and architecture layout for your circuit.

(3) Edit the VHDL file to realise the 1-bit full-adder circuit depicted in Fig.1 (b). The

VHDL code is highlighted in the red box of Fig 2(a). When you have coded the circuit

in VHDL run the syntax checker: This is available in the process window under

Synthesize-XST Check Syntax, see Fig. 2(b) below. When the design is error-

free, synthesize the VHDL file: Double click on Synthesize-XST.

Specify the

signal

names and

directions

for the new

VHDL

Module



Fig.2 (a)

Adder VHDL

Code



Fig. 2 (b)

Synthesis

Process

Synthesis

report



Fig. 2 (c)

Now examine the synthesis report – this is available by right-clicking Synthesize-

XST and select View Text Report, see Fig. 2(b). Scroll down the file to find the

section entitled Final Report, as shown in Fig. 2(c). Note the number of LUTs and

IBUF/OBUFs that have been synthesised to create the circuit.

Behavioral Simulation of a Full Adder

In this section, we will introduce the concept of test bench and show how to verify the

function of our full adder by behavioural simulation using the Xilinx Isim tool.

What is a test bench?

A test bench is an entity (usually a VHDL/Verilog program) which is used to verify

the correctness of a design. The design to be verified is called Unit Under Test (UUT).

The test bench supplies stimuli to the design, observes the outputs of the design, and

compares the observed outputs with the expected values. If any mismatch happens,

Synthesis

report



the test bench issues certain messages signifying that there are errors in the design.

Fig. 3 shows the concept of test bench.

Fig.3: Conceptual Diagram of a test bench.

Advanced EDA tools such as Xilinx ISE usually have the capability to automatically

generate the test bench. All the users need to do is specifying the waveforms of the

stimuli and the expected outputs; the software produces the test bench program which

can be tailored later on.

Build the test bench by specifying waveforms

(1) In the Design Panel, select “Simulation”, click ‘Full_Adder - Behavioral’, and

then double-click ‘Simulate Behavioural Model’. You may need to hit the ‘+’ beside

‘ISim Simulator’.



(2) The Isim window will open

Signals

in the

Design Unit

Signals

used the

simulation

waveform

Input and

Output

Values



(3) You can rename the inputs/outputs by right-clicking them and hitting ‘rename’. If

you forget which ports are which, you can look back in ISE Project Navigator (not the

simulator). You can also right-click on a particular signal to initialize it to a constant

value, or define it as a clock.

Right click on a signal to initialize it to a constant value or set it as a clock

To initialize a signal select the desire radix and set the value

Select

Radix

Enter Signal

Value Here



To define a clock signal simple set the edge parameters and clock period.

(4) Using the waveform display, fill in the truth table. First, specify a running time

period, then enter the input values and press run. Do this until all the values have been

assigned. You may need to scroll the waveform to start at time 0. You can click on

different times in the waveform and just read S/Cout directly off. Fill in the

observations based on this.

Basic Simulation Controls. Keep in mind the clock period when defining the simulation run time

Select Time to run

the simulation

e.g. 10ns

Run the simulation

for the specified

amount of time

Reset and Restart

the simulation

Specify

leading edge

value

e.g. 1

Specify

leading edge

value

e.g. 0

Specify the

period of the

clock

e.g. 10ns

Zoom in and Out.

View full waveform



(5) Close the ISim window, it will ask if you want to exit the application, hit “Yes”.

Build the test bench using VHDL coding

Another way to simulate the circuit to verify its expected Boolean behaviour is by

manually creating a Testbench for the circuit.

Tasks

(1) Right click on xc3s1600e-fg320 icon in the sources window and select New

Source, then select VHDL Test Bench and enter a filename and follow through to

finish

Create

Testbench



(2) You should see a similar screen shot as shown in Fig.4.

Fig.4 VHDL Testbench

Copy the following code into the Testbench file and select FileSave. Note: You

will need to remove the term <clock>_period in the file and replace it with

clock_period. You will also need to comment the lines of the process that generates

the clock, as our circuit is a combinational one. The screen shots above and below

provide a few examples of where these changes are required – complete all of them.

Appendix A provides the complete Testbench code.

Edit the

Testbench

Make sure to

select

Simulation in

the view!!



(3) Now select the Testbench file Full_Adder_test, in the top window in the left; the

following process will appear in the bottom window. Select the process Simulate

Behavioral Model and click-on RUN. This will invoke the ISim simulator and

execute the Testbench file Full_Adder_test.

(4) You should now have a similar screen shot as shown below with the Testbench

waveform to the right and its file equivalent to the left in the Sources window. The

simulation plot shown below shows the output from the circuit when the following

input values for A, B and Cin are stimulated. The values on S and Cout appear after

the simulation run.

------- insert stimulus here ---------- A <= '1'; B <= '1';

Cin <= '1'; wait for clock_period*2;

A <= '0'; B <= '1';


A <= '1'; B <= '0';


----------------------------------------------

Make sure to

select

Simulation in

the view!!

Select the

testbench file

Select

Simulate

Behavioral

Model process



A <= '0'; B <= '1'; Cin <= '1'; S <= ‘0’; Cout <= ‘1’;

A <= '1'; B <= '1'; Cin <= '1'; S <= ‘1’; Cout <= ‘1’;

A <= '1'; B <= '0'; Cin <= '0'; S <= ‘1’; Cout <= ‘0’;

Correct simulation outputs

shows S=1 and Cout=0 when

A=1, B=0 and Cin=0



Implement a 4-bit Adder Design

The next step of this lab is to implement a 4-bit adder using the 1-bit full adder as the

basic component. You should follow the process outlined for the 1-bit adder.

Specifically, you are required to synthesize the 4-bit adder and simulate it either using

a testbench or manually, to verify that it functions correctly.

Synthesize the 4-bit adder design and report resource utilization from the

synthesis text report. How does it compare to the 1-bit full adder?

Deliverables

The ISE project folder containing the code and simulation files for both the 1-bit and

4-bit adders, and a lab report will be submitted electronically through email

([email protected]). Please use compression software to (winRar or 7zip) to

keep the file size at a minimum. If the files are too large to be submitted via email

please use the UCY file sending service https://filesender.ucy.ac.cy/filesender/.

Please notify me in advance if you are going to delay delivering the lab

Also consider

This lab briefly introduced you to the ISE design suite tools. You are

advised to look at the tutorials posted on the lab web site for a more

comprehensive overview and detailed explanation of the tools.

You are encourage to visit the Xilinx user community forums

(http://forums.xilinx.com/). The community forum enables Xilinx FPGA

users to share, discuss, and resolve issues related to the Xilinx tools, HDL,

FPGA programming and more.

GOOD LUCK

mailto:[email protected]

https://filesender.ucy.ac.cy/filesender/



Appendix A

The ISE® Language Templates provide predefined pieces of code and code syntax for

use in your source files. These templates enable easy insertion of pre-built text

structures into your VHDL, Verilog, Tcl, or UCF source file. There are several types

of pre-built templates available, such as common language structures or instantiation

templates for synthesis.

To Use Language Templates:

1. Select Edit > Language Templates, or click the Language Templates toolbar

button .

2. In the Language Templates window that opens in the Project Navigator

Workspace, click the plus (+) icon next to one of the following

folders: Tcl, UCF, VHDL, Verilog.

3. Click the plus (+) icon to expand the folders until you find the template you

want to use.

4. Select the template to display it in the right pane.

5. Insert the code in your source file. Note that you will have to modify the port

names to that of your own source file.

VHDL

Code

Examples

VHDL Code

Examples

for

Different

Components

Language

toolbar

button

fpga laboratory assignment 1 due date: 19/09/2012

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