VHDL Tutorial : Intoduction

What Is VHDL?

   VHDL is an acronym for VHSlC Hardware Description Language (VHSIC is an acronym for Very High Speed Integrated Circuits). It 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

   Therefore, 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. Fortunately, it is possible to quickly assimilate a core subset of the language that is both easy and simple to understand without learning the more complex features. This subset is usually sufficient to model most applications. The complete language, however, has sufficient power to capture the descriptions of the most complex chips to a complete electronic system.

   

History

   The requirements for the language were first generated in 1981 under the VHSIC program. In this program, a number of U.S. companies were involved in designing VHSIC chips for the Department of Defense (DoD). At that time, most of the companies were using different hardware description languages to describe and develop their integrated circuits. As a result, different vendors could not effectively exchange designs with one another. Also, different vendors provided DoD with descriptions of their chips in different hardware description languages. Reprocurement and reuse was also a big issue. Thus, a need for a standardized hardware description language for design, documentation, and verification of digital systems was generated.

A team of three companies, IBM, Texas Instruments, and Intermetrics, were first awarded the contract by the DoD to develop a version of the language in 1983. Version 7.2 of VHDL was developed and released to the public in 1985. There was a strong industry participation throughout the VHDL language development process, especially from the companies that were developing VHSIC chips. After the release of version 7.2, there was an increasing need to make the language an industry-wide standard. Consequently, the language was transferred to the IEEE for standardization in 1986. After a substantial enhancement to the language, made by a team of industry, university, and DoD representatives, the language was standardized by the IEEE in December 1987; this version of the language is now known as the IEEE Std 1076-1987. The official language description appears in the IEEE Standard VHDL Language Reference Manual made available by the IEEE. The language described in this book is based on this standard. The language has since also been recognized as an American National Standards Institute (ANSI) standard.

The Department of Defense, since September 1988, requires all its digital Application-Specific Integrated Circuit (ASIC) suppliers to deliver VHDL descriptions of the ASICs and their subcomponents, at both the behavioral and structural levels. Test benches that are used to validate the ASIC chip at all levels in its hierarchy must also be delivered in VHDL. This set of government requirements is described in military standard 454.

Capabilities

    The following are the major capabilities that the language provides along with the features that differentiate it from other hardware description languages.

• The 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, for example, 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 supportsflexible 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, for example, you 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.