Digital Circuit Design with an Introduction to CPLDs and FPGAs

Appendix B: Introduction to VHDL

Overview

This appendix provides a brief overview of the VHSIC Hardware Description Language brieflyreferred to as VHDL. This language was developed to be used for documentation, verification, andsynthesis of large digital designs.

B.1 Introduction

The VHSIC [*]Hardware Description Language, henceforth referred to as VHDL is a powerful programming language used todescribe hardware designs much the same way we use schematics. It was developed by the Institute ofElectrical and Electronics Engineers (IEEE) as Standard VHDL-1076. It was introduced in 1987 as Std1076 1987, and was upgraded in 1993 as Std 1076 1993. Its popularity stems from the fact that it can be used fordocumentation, verification, and synthesis of large digital designs.

[*] VHSIC is an acronym for Very High Speed Integrated Circuits

B.2 The VHDL Design Approach

Consider a combinational digital circuit such as a full adder. Such a circuit is often referred toas a module, and it is described in terms of its inputs and outputs. For instance,the full adder module has three inputs and two outputs. In VHDL terminology, the module is referred toas a design entity, or simply design, and the inputs andoutputs are called ports. A typical design consists of two or more blocks. Thus,the blocks in a full adder are gates and inverters. In other words, blocks are connected together toform a design.

In VHDL it is not necessary to provide a description of the function performed by a block, e.g., agate or inverter. However, we must describe the function performed by the...