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INTRODUCTION TO VHDL
What is VHDL?
VHDL stands for VHSIC Hardware Description Language, where VHSIC stands for Very High Speed Integrated Circuit. VHDL is a language for describing the behavior of digital hardware. VHDL is just another way of describing what outputs of a digital circuit are desired when it is given certain inputs. The critical difference between VHDL and these other languages are that it can be readily interpreted by software, enabling the computer to accomplish your design work for you.
As the size and complexity of digital systems increase, more computers aided design tools are introduced into the hardware design process. The early paper-and-pencil design methods have given way to sophisticated design entry, verification, and automatic hardware generation tools. The newest addition to this design methodology is the hardware description languages (HDL). Although the concept of HLDs is not new, their widespread use in digital system design is no more than a decade old. Based on HDLs, new digital system CAD tools have been developed and are now being utilized by hardware designers.
VHDL History:
In 1980 the US government developed the Very High Speed Integrated Circuit (VHSIC) project to enhance the electronic design process, technology, and procurement, spawning development of many advanced integrated circuit (IC) process technologies. This was followed by the arrival of VHSIC Hardware Description Language (VHDL).
Why We Use VHDL?
There are many reasons why it makes good design sense to use VHDL:
1. Portability:
Technology changes so quickly in the digital industry that discrete digital devices require constant rework in order to remain current. VHDL is designed to be device-independent, meaning that if you describe your circuit in VHDL, as opposed to designing it with discrete devices, changing hardware becomes a (relatively) trivial process.
2. Flexibility:
Most working engineers can recall a situation where they felt frustrated with their customer, supervisor, or team members because the design specification that they were working with was constantly changing. Sometimes these changes can't be helped. Design work is usually focused on creating small, easily maintainable components and then integrating these components into a larger device. On larger projects different teams of engineers will each design separate parts of the project at the same time. This can mean that if one component in the project changes, all of the components must change, even those being worked on by other engineering teams. Suppose you were told to design a simple counter that set an output bit after it had counted to 100. However, the software engineer working on this project discovered that the entire design could be radically simplified if your counter could count down from 300 instead of up to 100. If you had implemented your design in discrete circuits, you'd have to start over from scratch. But, if you'd designed using VHDL, all you'd have to do is change your code.
VHDL Features:
General features:
VHDL can be used for design documentation, high level design, simulation, synthesis, and testing of hardware and as a driver for a physical design tool.
Concurrency:
In VHDL the transfer statements, descriptions of components, and instantiations of gates or logical units can all be executed such that in the end they appear to have been executed simultaneously.
Support for design hierarchy:
In VHDL the operation of a system can be specified based on its functionality, or it can be specified structurally in terms of its smaller subcomponents.
Library support:
User and system defined primitives and descriptions reside in the library system. VHDL provides a mechanism for accessing various libraries. Moreover different designers can access these libraries.
Sequential statement:
VHDL provides mechanism for executing sequential statements. These statements provide an easy method for modeling hardware components based on their functionality. Sequential or procedural capability is only for convenience, and the overall structure of the VHDL language remains highly concurrent.
Type declaration and usage:
VHDL is not limited to just bit or Boolean types, but it also supports integer, floating-point, enumerated types and user-defined types. In addition, VHDL also allows array-type declarations and composite-type definitions.
Use of subprograms:
VHDL allows the use of functions and procedures which can be used in type conversions, logic unit definitions, operator redefinitions, new operation definitions, and other applications.
Timing control:
VHDL allows the designer to schedule values to signals and delay the actual assignment of values until a later time. It also allows the use of any number of explicitly defined clock signals. It provides features for edge detection, delay specification, setup and hold time specification, pulse width checking, and setting various time constraints.
Structural specification:
VHDL allows the designer to describe a generic 1-bit design and use it when describing multibit regular structures in one or more dimensions
Advantages of VHDL:
VHDL offers the following advantages for digital design:
Standard:
VHDL is an EKE standard. Just like any standard (such as graphics X- window standard, bus communication interface standard, high-level programming languages, and so on), it reduces confusion and makes interfaces between tools, companies, and products easier. Any development to the standard would have better chances of lasting longer and have less chance of becoming obsolete due to incompatibility with others.
Government support:
VHDL is a result of the VHSIC program; hence, it is clear that the US government supports the VHDL standard for electronic procurement. The Department of Defense (DOD) requires contractors to supply VHDL for all Application Specific Integrated Circuit (ASIC) designs.
Industry support:
With the advent of more powerful and efficient VHDL tools has come the growing support of the electronic industry. Companies use VHDL tools not only with regard to defense contracts, but also for their commercial designs.
Portability:
The same VHDL code can be simulated and used in many design tools and at different stages of the design process. This reduces dependency on a set of design tools whose limited capability may not be competitive in later markets. The VHDL standard also transforms design data much easier than a design database of a proprietary design tool.
Modelling capability:
VHDL was developed to model all levels of designs, from electronic boxes to transistors. VHDL can accommodate behavioral constructs and mathematical routines that describe complex models, such as queuing networks and analog circuits. It allows use of multiple architectures and associates with the same design during various stages of the design process. VHDL can describe low-level transistors up to very large systems.
Reusability:
Certain common designs can be described, verified, and modified slightly in VHDL for future use. This eliminates reading and marking changes to schematic pages, which is time consuming and subject to error. For example, a parameterized multiplier VHDL code can be reused easily by changing the width parameter so that the same VHDL code can do either 16 by 16 or 12 by 8 multiplications.
Technology and foundry independence: The functionality and behavior of the design can be described with VHDL and verified, making it foundry and technology independent. This frees the designer to proceed without having to wait for the foundry and technology to be selected.