science

VHDL

VHDL is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand VHDL rather than just read about it. In short: VHDL (VHSIC hardware description language) is a hardware description language that can model the behavior and structure of digital systems at multiple levels of abstraction, ranging from the system level down to that of logic gates. It can be used for design entry, documentation, and verification.

Key takeaways

VHDL belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect VHDL to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of VHDL from memory before moving on to harder problems.

Reference excerpt

VHDL (VHSIC hardware description language) is a hardware description language that can model the behavior and structure of digital systems at multiple levels of abstraction, ranging from the system level down to that of logic gates. It can be used for design entry, documentation, and verification. The language was developed for the US military VHSIC program in the 1980s, and has been standardized by the Institute of Electrical and Electronics Engineers (IEEE) as IEEE Std 1076; the latest version of which is IEEE Std 1076-2019. To model analog and mixed-signal systems, an IEEE-standardized HDL based on VHDL called VHDL-AMS (officially IEEE 1076.1) has been developed.

History In 1983, VHDL was originally developed at the behest of the U.S. Department of Defense in order to document the behavior of the ASIC that supplier companies were including in equipment. The standard MIL-STD-454N in Requirement 64 in section 4.5.1 "ASIC documentation in VHDL" explicitly requires documentation of "Microelectronic Devices" in VHDL. The idea of being able to simulate the ASICs from the information in this documentation was so obviously attractive that logic simulators were developed that could read the VHDL files. The next step was the development of logic synthesis tools that read the VHDL and output a definition of the physical implementation of the circuit. Due to the Department of Defense requiring as much of the syntax as possible to be based on Ada, in order to avoid re-inventing concepts that had already been thoroughly tested in the development of Ada, VHDL borrows heavily from the Ada programming language in both concept and syntax. The initial version of VHDL, designed to IEEE standard IEEE 1076–1987, included a wide range of data types, including numerical (integer and real), logical (bit and Boolean), character and time, plus arrays of bit called bit_vector and of character called string. A problem not solved by this edition, however, was multi-valued logic, where a signal's drive strength (none, weak or strong) and unknown values are also considered. This required IEEE standard 1164, which defined the 9-value logic types: scalar std_logic and its vector version std_logic_vector. Being a resolved subtype of its std_Ulogic parent type, std_logic-typed signals allow multiple driving for modeling bus structures, whereby the connected resolution function handles conflicting assignments adequately. The updated IEEE 1076, in 1993, made the syntax more consistent, allowed more flexibility in naming, extended the character type to allow ISO-8859-1 printable characters, added the xnor operator, etc. Minor changes in the standard (2000 and 2002) added the idea of protected types (similar to the concept of class in C++) and removed some restrictions from port mapping rules. In addition to IEEE standard 1164, several child standards were introduced to extend functionality of the language. IEEE standard 1076.2 added better handling of real and complex data types. IEEE standard 1076.3 introduced signed and unsigned types to facilitate arithmetical operations on vectors. IEEE standard 1076.1 (known as VHDL-AMS) provided analog and mixed-signal circuit design extensions. Some other standards support wider use of VHDL, notably VITAL (VHDL Initiative Towards ASIC Libraries) and microwave circuit design extensions. In June 2006, the VHDL Technical Committee of Accellera (delegated by IEEE to work on the next update of the standard) approved so-called Draft 3.0 of VHDL-2006. While maintaining full compatibility with older versions, this proposed standard provides numerous extensions that make writing and managing VHDL code easier. Key changes include incorporation of child standards (1164, 1076.2, 1076.3) into the main 1076 standard, an extended set of operators, more flexible syntax of case and generate statements, incorporation of VHPI (VHDL Procedural Interface) (interface to C/C++ languages) and a subset of PSL (Property Specification Language). These changes should improve quality of synthesizable VHDL code, make testbenches more flexible, and allow wider use of VHDL for system-level descriptions. In February 2008, Accellera approved VHDL 4.0, also informally known as VHDL 2008, which addressed more than 90 issues discovered during the trial period for version 3.0 and includes enhanced generic types. In 2008, Accellera released VHDL 4.0 to the IEEE for balloting for inclusion in IEEE 1076–2008. The VHDL standard IEEE 1076-2008 was published in January 2009.

Standardization The IEEE Standard 1076 defines the VHSIC Hardware Description Language, or VHDL. It was originally developed under contract F33615-83-C-1003 from the United States Air Force awarded in 1983 to a team of Intermetrics, Inc. as language experts and prime contractor, Texas Instruments as chip design experts and IBM as computer-system design experts. The language has undergone numerous revisions and has a variety of sub-standards associated with it that augment or extend it in important ways. IEEE Standard 1076 was and continues to be a milestone in the design of electronic systems.

Revisions IEEE 1076–1987 First standardized revision of version 7.2 of the language from the United States Air Force. IEEE 1076–1993 (also published with ISBN 1-55937-376-8). Significant improvements resulting from several years of feedback. Probably the most widely used version with the greatest vendor tool support. IEEE 1076–2000. Minor revision. Introduces the use of protected types. IEEE 1076–2002. Minor revision of 1076–2000. Rules with regard to buffer ports are relaxed. IEC 61691-1-1:2004. IEC adoption of IEEE 1076–2002. IEEE 1076c–2007. Introduced VHPI, the VHDL procedural interface, which provides software with the means to access the VHDL model. The VHDL language required minor modifications to accommodate the VHPI. IEEE 1076–2008 (previously referred to as 1076–200x). Major revision released on 2009-01-26. Among other changes, this standard incorporates a basic subset of PSL, allows for generics on packages and subprograms and introduces the use of external names. IEC 61691-1-1:2011. IEC adoption of IEEE 1076–2008. IEEE 1076–2019. Major revision.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with VHDL

Start with the simplest possible case. Write down what VHDL claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to VHDL before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about VHDL ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of VHDL

In research: VHDL appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses VHDL in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: VHDL is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ada programming language family, Domain-specific programming languages, Hardware description languages, so understanding it makes those chapters shorter.
In everyday life: Look for VHDL outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study VHDL in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what VHDL means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain VHDL out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is VHDL in simple terms?

Why does VHDL matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study VHDL?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on VHDL.