Hardware description language

In electronics, a hardware description language or HDL is any language from a class of computer languages for formal description of electronic circuits. It can describe the circuit's operation, its design, and tests to verify its operation by means of simulation.

Related Topics:
Electronics - Electronic circuit - Simulation

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An HDL is a standard text-based expression of the temporal behaviour and/or (spatial) circuit structure of an electronic system. In contrast to a software programming language, an HDL's syntax and semantics include explicit notations for expressing time and concurrency which are the primary attributes of hardware. Languages whose only characteristic is to express circuit connectivity between a hierarchy of blocks are properly classified as netlist languages.

Related Topics:
Programming language - Netlist

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Frequently (and incorrectly), the term programming is used to be synonymous with writing a hardware description. This often arises because of the goal that HDLs be executable specifications of some piece of hardware. A simulation program, designed to implement the underlying semantics of the language statements and the progression of time, provides the hardware designer with the ability to model a piece of hardware before it is created physically. This execution capability makes it seem like the language is used to program something. Simulators capabable of supporting discrete event (digital), and continuous time (analog) modeling exist and HDL's targeted for each are available.

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It is certainly possible to represent hardware semantics using traditional programming languages such as C++ (and augmented with extensive and unwieldy class libraries.) However, the C++ language does not include any capability for expressing time explicitly and consequently is not a proper hardware description language.

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Using the proper subset of virtually any (hardware description or software programming) language a software program called a synthesizer can infer hardware logic operations from the language statements and produce an equivalent netlist of generic hardware primitives to implement the specified behaviour. This typically (as of 2004) requires the synthesizer to ignore the expression of any timing constructs in the text. The ability to have a synthesizable subset of the language does not itself make a hardware description language.

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Designing a system in HDL is generally much harder and more time consuming than writing a program that would do the same thing using a programming language like C. Consequently, there has been much work done on automatic conversion of C code into HDL, but this has not reached a high level of commercial success as of 2004.

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