Hour 04: VHDL Intro and IDE

What is VHDL?

VHDL stands for VHSIC Hardware Description Language

                                                         VHSIC = Very High Speed IC

First we need to know what is a hardware description language (HDL) and why it was used in the first place. A few reasons to use HDLs are:

1. Time needed for a digital circuit design reduces dramatically.

2. Its painful to draw schematic diagrams.

3. No need of optimization using Karnaugh Maps etc. 

4. Design is portable.

5. TECHNOLOGY INDEPENDENT. 

VHDL is a HDL/programming language to build digital electronic circuits. We can write VHDL  codes to create digital circuits, for example, from simple ripple-carry adder to complicated processor. 

Level of abstraction

Its important to understand in which abstraction level are we gonna use VHDL. The typical level of abstractions for any design is:

1. Algorithmic level

2. System level.

3. Register Transfer level.

4. Gate level.

5. Transistor level. 

We are going to use RTL level designs in this tutorial site, Where exactly are we going to design can be explained by some nice diagram drawn by Professor Christopher Batten,

Originally designers used manual translation and breadboards for verification

Hardware description languages enabled gate-level verification via simulation

Designers began to use HDLs for RTL level verification and design exploration

Tools

If you are looking for a site to learn VHDL in short time, that means, you are not very keen to learn the history of HDLs. Instead, we will talk about tools and how to use them in brief here.

The link to download modelsim student edition is

http://www.mentor.com/company/higher_ed/modelsim-student-edition

A good tutorial about using the modelsim tool can be found here:

http://www.tkt.cs.tut.fi/tools/public/tutorials/mentor/modelsim/getting_started/gsms.html

Starting with ModelSim 

First download this software from the given link, then after set up there will be pop up window on the browser which recommends to register before using. The registration is necessary if you want to use it for free.

When you will run the software after installing and registration you will see an window like below.

Click on the Jumpstart button. After that there will be a small window asking to start new project of open existing project. For the first time you have to choose "Create a Project". (See the below figure)

It will be then asked to give a project name and location on another window as below.

Give a name to your project. There may be more than one file of a project. Those file must be created under the same project. So you have to give separate names for your work convenient. That's why select "Create New File" after determining the project name.

By the above procedure the new project will be opened. For example the project name is given as example.vhd. You will notice this on the left portion of the ModelSim window.

Double click on the project file then there will be a new window on the right side for writing necessary VHDL codes. (See the below figure)

Write your codes on the right portion line by line then compile to check if the codes are ok. To compile your codes, go to the menu bar and select the menu "Compile" and from the list select "compile all".

If there are some errors you will see that on the "Transcript" window at the bottom portion of the ModelSim window. Otherwise it will show that compile was successful.

In this way you can run a simple VHDL code using ModelSim.

Or you can download the free Xilinx ISE design suite from here

http://www.xilinx.com/support/download/index.html/content/xilinx/en/downloadNav/design-tools.html