INTRODUCTION
VHDL is an acronym for VHSlC Hardware
Description Language (VHSIC is an acronym for Very High Speed Integrated
Circuits), is a hardware description language that can be used to model a
digital system at many levels of abstraction ranging from the algorithmic level
to the gate level.
The complexity of the digital system
being modeled could vary from that of a simple gate to a complete digital
electronic system, or anything in between. The digital system can also be
described hierarchically. Timing can also be explicitly modeled in the same
description.
The VHDL language can be regarded as
an integrated amalgamation of the following languages:
Sequential language + concurrent
language + net-list language + timing specifications + waveform generation
language => VHDL
The language has constructs that
enable you to express the concurrent or sequential behavior of a digital system
with or without timing. It also allows you to model the system as an
interconnection of components. Test waveforms can also be generated using the
same constructs. All the above constructs may be combined to provide a
comprehensive description of the system in a single model. The language not
only defines the syntax but also defines very clear simulation semantics for
each language construct. Therefore, models written in this language can be
verified using a VHDL simulator. It is a strongly typed language and is often
verbose to write. It inherits many of its features, especially the sequential
language part, from the Ada programming language. Because VHDL provides an
extensive range of modeling capabilities, it is often difficult to understand.
CAPABILITIES
·
The VHDL language can be used as an exchange medium between
chip vendors and CAD tool users. Different chip vendors can provide VHDL
descriptions of their components to system designers. CAD tool users can use it
to capture the behavior of the design at a high level of abstraction for
functional simulation.
·
The language can also be used as a communication medium between
different CAD and CAE tools, Eg., a schematic capture program may be used
to generate a VHDL description for the design which can be used as an input to
a simulation program.
·
The language supports hierarchy, that is, a digital system
can be modeled as a set of interconnected components; each component, in turn,
can be modeled as a set of interconnected subcomponents.
·
The language supports flexible design methodologies: top-down,
bottom-up, or mixed.
·
The language is not technology-specific, but is capable of
supporting technology-specific features. It can also support various hardware
technologies, eg., we may define new logic types and new components, you may
also specify technology-specific attributes. By being technology independent,
the same behavior model can be synthesized into different vendor libraries.
·
It supports both synchronous and asynchronous timing models.
·
Various digital modeling techniques such as finite-state machine
descriptions, algorithmic descriptions, and boolean equations can be modeled
using the language.
·
The language is publicly available, human readable, machine
readable, and above all, it is not proprietary.
·
It is an IEEE and ANSI standard, and therefore, models
described using this language are portable. The government also has a strong
interest in maintaining this as a standard so that re-procurement and
second-sourcing may become easier.
·
The language supports three basic different description styles:
structural, dataflow, and behavioral. A design may also be expressed in any
combination of these three descriptive styles.
·
It supports a wide range, of abstraction levels ranging
from abstract behavioral descriptions to very precise gate-level descriptions.
It does not, however, support modeling at or below the transistor level. It
allows a design to be captured at a mixed level using a single coherent
language.
·
Arbitrarily large designs can be modeled using the language
and there are no limitations that are imposed by the language on the size of a
design.
·
The language has elements that make large scale design modeling
easier, for example, components, functions, procedures, and packages.
·
There is no need to learn a different language for simulation
control. Test benches can be written using the same language to test
other VHDL models.
·
Nominal propagation delays, min-max delays, setup and hold timing,
timing constraints, and spike detection can all be described very naturally in
this language.
·
The use of generics and attributes in the models facilitate
back-annotation of static information such as timing or placement
information.
·
Generics and attributes are also useful in describing parameterized
designs.
·
A model can not only describe the functionality of a design, but
can also contain information about the design itself 'in terms of
user-defined attributes, for example, total area and speed.
·
A common language can be used to describe library components from
different vendors. Tools that understand VHDL models will have no difficulty in
reading models from a variety of vendors since the language is a standard.
·
Models written in this language can be verified by simulation
since precise simulation semantics are defined for each language
construct.
·
Behavioral models that conform to a certain synthesis description
style are capable of being synthesized to gate-level descriptions.
·
The capability of defining new data types provides the
power to describe and simulate a new design technique at a very high level of
abstraction without any concern about the implementation details.
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