Introduction to VHDL

 VHDL Programming

Introduction : 

The VHSIC Hardware Description Language is an industry standard language used to describe hardware from the abstract to the concrete level. VHDL resulted from work done in the ’70s and early ’80s by the U.S. Department of Defense. Its roots are in the ADA language, as will be seen by the overall structure of VHDL as well as other VHDL statements.
“VHDL” usage has risen rapidly since its inception and is used by literally tens of thousands of engineers around the globe to create sophisticated electronic products. VHDL is a powerful language with numerous language constructs that are capable of describing very complex behavior.

In 1986, VHDL was proposed as an IEEE standard. It went through a number of revisions and changes until it was adopted as the IEEE 1076 standard in December 1987. The IEEE 1076-1987 standard VHDL is the VHDL used in this book. (Appendix D contains a brief description of VHDL 1076-1993.) All the examples have been described in IEEE 1076 VHDL, and compiled and simulated with the VHDL simulation environment from Model Technology Inc. The synthesis examples were synthesized with the Exemplar Logic Inc. synthesis tools.

VHDL Terms : 
Before we go any further, let’s define some of the terms that we use throughout the book. These are the basic VHDL building blocks that are used in almost every description, along with some terms that are redefined in VHDL to mean something different to the average designer.
Entity:  All designs are expressed in terms of entities. An entity is the most basic building block in a design. The uppermost level of the design is the top-level entity. If the design is hierarchical, then the top-level description will have lower-level descriptions contained in it. These lower-level descriptions will be lower-level entities contained in the top-level entity description.
Architecture :  All entities that can be simulated have an architecture description. The architecture describes the behavior of the entity. A single entity can have multiple architectures. One architecture might be behavioral Zhile another might be a structural description of the design.
Configuration : A configuration statement is used to bind a component instance to an entity-architecture pair. A configuration can be considered like a parts list for a design. It describes which behavior to use for each entity, much like a parts list describes which part to use for each part in the design.
Package : A package is a collection of commonly used data types and subprograms used in a design. Think of a package as a toolbox that contains tools used to build designs. 

Driver :  This is a source on a signal. If a signal is driven by two sources, then when both sources are active, the signal will have two drivers.

Bus :  The term “bus” usually brings to mind a group of signals or a particular method of communication used in the design of hardware. In VHDL, a bus is a special kind of signal that may have its drivers turned off.

Attribute :  An attribute is data that are attached to VHDL objects or predefined data about VHDL objects. Examples are the current drive capability of a buffer or the maximum operating temperature of the device.

Generic :  A generic is VHDL’s term for a parameter that passes information to an entity. For instance, if an entity is a gate level model with a rise and a fall delay, values for the rise and fall delays could be passed into the entity with generics.

Process:  A process is the basic unit of execution in VHDL. All operations that are performed in a simulation of a VHDL description are broken into single or multiple processes.

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