Last updated
Paradigm concurrent, reactive
First appeared1980s
Typing discipline strong
Influenced by
Ada, [1] Pascal
VHDL source for a signed adder. Vhdl signed adder source.svg
VHDL source for a signed adder.

VHDL (VHSIC-HDL) (Very High Speed Integrated Circuit Hardware Description Language) is a hardware description language used in electronic design automation to describe digital and mixed-signal systems such as field-programmable gate arrays and integrated circuits. VHDL can also be used as a general purpose parallel programming language.

In computer engineering, a hardware description language (HDL) is a specialized computer language used to describe the structure and behavior of electronic circuits, and most commonly, digital logic circuits.

Electronic design automation (EDA), also referred to as electronic computer-aided design (ECAD), is a category of software tools for designing electronic systems such as integrated circuits and printed circuit boards. The tools work together in a design flow that chip designers use to design and analyze entire semiconductor chips. Since a modern semiconductor chip can have billions of components, EDA tools are essential for their design.

Digital electronics Electronic circuits that utilize digital signals

Digital electronics, digital technology or digital (electronic) circuits are electronics that operate on digital signals. In contrast, analog circuits manipulate analog signals whose performance is more subject to manufacturing tolerance, signal attenuation and noise. Digital techniques are helpful because it is a lot easier to get an electronic device to switch into one of a number of known states than to accurately reproduce a continuous range of values.



In 1983, VHDL was originally developed at the behest of the U.S Department of Defense in order to document the behavior of the ASICs that supplier companies were including in equipment. The standard MIL-STD-454N [2] in Requirement 64 in section 4.5.1 "ASIC documentation in VHDL" explicitly requires documentation of "Microelectronic Devices" in VHDL.

United States Department of Defense United States federal executive department

The United States Department of Defense is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government directly related to national security and the United States Armed Forces. The DoD is the largest employer in the world, with nearly 1.3 million active-duty service members as of 2016. More employees include over 826,000 National Guard and Reservists from the armed forces, and over 732,000 civilians bringing the total to over 2.8 million employees. Headquartered at the Pentagon in Arlington, Virginia, just outside Washington, D.C., the DoD's stated mission is to provide "the military forces needed to deter war and ensure our nation's security".

Application-specific integrated circuit Integrated circuit customized (typically optimized) for a specific task

An application-specific integrated circuit is an integrated circuit (IC) customized for a particular use, rather than intended for general-purpose use. For example, a chip designed to run in a digital voice recorder or a high-efficiency bitcoin miner is an ASIC. Application-specific standard products (ASSPs) are intermediate between ASICs and industry standard integrated circuits like the 7400 series or the 4000 series.

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.

Logic simulation is the use of simulation software to predict the behavior of digital circuits and hardware description languages. Simulation can be performed at varying degrees of physical abstraction, such as at the transistor level, gate level, register-transfer level (RTL), electronic system-level (ESL), or behavioral level.

In electronics, logic synthesis is a process by which an abstract specification of desired circuit behavior, typically at register transfer level (RTL), is turned into a design implementation in terms of logic gates, typically by a computer program called a synthesis tool. Common examples of this process include synthesis of designs specified in hardware description languages, including VHDL and Verilog. Some synthesis tools generate bitstreams for programmable logic devices such as PALs or FPGAs, while others target the creation of ASICs. Logic synthesis is one aspect of electronic design automation.

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,[ citation needed ] VHDL borrows heavily from the Ada programming language in both concepts and syntax.

Ada (programming language) Programming language

Ada is a structured, statically typed, imperative, and object-oriented high-level computer programming language, extended from Pascal and other languages. It has built-in language support for design-by-contract, extremely strong typing, explicit concurrency, tasks, synchronous message passing, protected objects, and non-determinism. Ada improves code safety and maintainability by using the compiler to find errors in favor of runtime errors. Ada is an international standard; the current version is defined by ISO/IEC 8652:2012.

In linguistics, syntax is the set of rules, principles, and processes that govern the structure of sentences in a given language, usually including word order. The term syntax is also used to refer to the study of such principles and processes. The goal of many syntacticians is to discover the syntactic rules common to all languages.

The initial version of VHDL, designed to IEEE standard IEEE 1076-1987, [3] 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.

Integer Number in {..., –2, –1, 0, 1, 2, ...}

An integer is a number that can be written without a fractional component. For example, 21, 4, 0, and −2048 are integers, while 9.75, 5+1/2, and 2 are not.

A real data type is a data type used in a computer program to represent an approximation of a real number. Because the real numbers are not countable, computers cannot represent them exactly using a finite amount of information. Most often, a computer will use a rational approximation to a real number.

The bit is a basic unit of information in information theory, computing, and digital communications. The name is a portmanteau of binary digit.

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 IEEE 1164 standard is a technical standard published by the IEEE in 1993. It describes the definitions of logic values to be used in electronic design automation, for the VHDL hardware description language. It was sponsored by the Design Automation Standards Committee of the Institute of Electrical and Electronics Engineers (IEEE). The standardization effort was based on the donation of the Synopsys MVL-9 type declaration.

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.[ specify ]

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 [4] was published in January 2009.


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 with Intermetrics, Inc. as language experts and prime contractor, with 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.

1076 was and continues to be a milestone in the design of electronic systems.[ citation needed ]


  • IEEE 1076-1987 [3] First standardized revision of ver 7.2 of the language from the United States Air Force.
  • IEEE 1076-1993 [5] (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 [6] Minor revision. Introduces the use of protected types.
  • IEEE 1076-2002 [7] Minor revision of 1076-2000. Rules with regard to buffer ports are relaxed.
    • IEC 61691-1-1:2004 [8] IEC adoption of IEEE 1076-2002
  • IEEE 1076-2008 [9] (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 [10] IEC adoption of IEEE 1076-2008
  • IEEE 1076.1 VHDL Analog and Mixed-Signal (VHDL-AMS)
  • IEEE 1076.1.1 VHDL-AMS Standard Packages (stdpkgs)
  • IEEE 1076.2 VHDL Math Package
  • IEEE 1076.3 VHDL Synthesis Package (vhdlsynth) (numeric_std)
  • IEEE 1076.3 VHDL Synthesis Package - Floating Point (fphdl)
  • IEEE 1076.4 Timing (VHDL Initiative Towards ASIC Libraries: vital)
  • IEEE 1076.6 VHDL Synthesis Interoperability (withdrawn in 2010) [11]
  • IEEE 1164 VHDL Multivalue Logic (std_logic_1164) Packages


VHDL is generally used to write text models that describe a logic circuit. Such a model is processed by a synthesis program, only if it is part of the logic design. A simulation program is used to test the logic design using simulation models to represent the logic circuits that interface to the design. This collection of simulation models is commonly called a testbench.

A VHDL simulator is typically an event-driven simulator. [12] This means that each transaction is added to an event queue for a specific scheduled time. E.g. if a signal assignment should occur after 1 nanosecond, the event is added to the queue for time +1ns. Zero delay is also allowed, but still needs to be scheduled: for these cases Delta delay is used, which represent an infinitely small time step. The simulation alters between two modes: statement execution, where triggered statements are evaluated, and event processing, where events in the queue are processed.

VHDL has constructs to handle the parallelism inherent in hardware designs, but these constructs (processes) differ in syntax from the parallel constructs in Ada (tasks). Like Ada, VHDL is strongly typed and is not case sensitive. In order to directly represent operations which are common in hardware, there are many features of VHDL which are not found in Ada, such as an extended set of Boolean operators including nand and nor.

VHDL has file input and output capabilities, and can be used as a general-purpose language for text processing, but files are more commonly used by a simulation testbench for stimulus or verification data. There are some VHDL compilers which build executable binaries. In this case, it might be possible to use VHDL to write a testbench to verify the functionality of the design using files on the host computer to define stimuli, to interact with the user, and to compare results with those expected. However, most designers leave this job to the simulator.

It is relatively easy for an inexperienced developer to produce code that simulates successfully but that cannot be synthesized into a real device, or is too large to be practical. One particular pitfall is the accidental production of transparent latches rather than D-type flip-flops as storage elements. [13]

One can design hardware in a VHDL IDE (for FPGA implementation such as Xilinx ISE, Altera Quartus, Synopsys Synplify or Mentor Graphics HDL Designer) to produce the RTL schematic of the desired circuit. After that, the generated schematic can be verified using simulation software which shows the waveforms of inputs and outputs of the circuit after generating the appropriate testbench. To generate an appropriate testbench for a particular circuit or VHDL code, the inputs have to be defined correctly. For example, for clock input, a loop process or an iterative statement is required. [14]

A final point is that when a VHDL model is translated into the "gates and wires" that are mapped onto a programmable logic device such as a CPLD or FPGA, then it is the actual hardware being configured, rather than the VHDL code being "executed" as if on some form of a processor chip.


The key advantage of VHDL, when used for systems design, is that it allows the behavior of the required system to be described (modeled) and verified (simulated) before synthesis tools translate the design into real hardware (gates and wires).

Another benefit is that VHDL allows the description of a concurrent system. VHDL is a dataflow language, unlike procedural computing languages such as BASIC, C, and assembly code, which all run sequentially, one instruction at a time.[ clarification needed ]

A VHDL project is multipurpose. Being created once, a calculation block can be used in many other projects. However, many formational and functional block parameters can be tuned (capacity parameters, memory size, element base, block composition and interconnection structure).

A VHDL project is portable. Being created for one element base, a computing device project can be ported on another element base, for example VLSI with various technologies.

A big advantage of VHDL compared to original Verilog is that VHDL has a full type system. Designers can use the type system to write much more structured code (especially by declaring record types). [15]

Design examples

In VHDL, a design consists at a minimum of an entity which describes the interface and an architecture which contains the actual implementation. In addition, most designs import library modules. Some designs also contain multiple architectures and configurations.

A simple AND gate in VHDL would look something like

-- (this is a VHDL comment)-- import std_logic from the IEEE librarylibraryIEEE;useIEEE.std_logic_1164.all;-- this is the entityentityANDGATEisport(I1:instd_logic;I2:instd_logic;O:outstd_logic);endentityANDGATE;-- this is the architecturearchitectureRTLofANDGATEisbeginO<=I1andI2;endarchitectureRTL;

(Notice that RTL stands for Register transfer level design.) While the example above may seem verbose to HDL beginners, many parts are either optional or need to be written only once. Generally simple functions like this are part of a larger behavioral module, instead of having a separate module for something so simple. In addition, use of elements such as the std_logic type might at first seem to be an overkill. One could easily use the built-in bit type and avoid the library import in the beginning. However, using this 9-valued logic (U,X,0,1,Z,W,H,L,-) instead of simple bits (0,1) offers a very powerful simulation and debugging tool to the designer which currently does not exist in any other HDL.

In the examples that follow, you will see that VHDL code can be written in a very compact form. However, the experienced designers usually avoid these compact forms and use a more verbose coding style for the sake of readability and maintainability. Another advantage to the verbose coding style is the smaller amount of resources used when programming to a Programmable Logic Device such as a CPLD. [16]

Synthesizable constructs and VHDL templates

VHDL is frequently used for two different goals: simulation of electronic designs and synthesis of such designs. Synthesis is a process where a VHDL is compiled and mapped into an implementation technology such as an FPGA or an ASIC. Many FPGA vendors have free (or inexpensive) tools to synthesize VHDL for use with their chips, where ASIC tools are often very expensive.

Not all constructs in VHDL are suitable for synthesis. For example, most constructs that explicitly deal with timing such as wait for 10 ns; are not synthesizable despite being valid for simulation. While different synthesis tools have different capabilities, there exists a common synthesizable subset of VHDL that defines what language constructs and idioms map into common hardware for many synthesis tools. IEEE 1076.6 defines a subset of the language that is considered the official synthesis subset. It is generally considered a "best practice" to write very idiomatic code for synthesis as results can be incorrect or suboptimal for non-standard constructs.

MUX template

The multiplexer, or 'MUX' as it is usually called, is a simple construct very common in hardware design. The example below demonstrates a simple two to one MUX, with inputs A and B, selector S and output X. Note that there are many other ways to express the same MUX in VHDL.


Latch template

A transparent latch is basically one bit of memory which is updated when an enable signal is raised. Again, there are many other ways this can be expressed in VHDL.

-- latch template 1:Q<=DwhenEnable='1'elseQ;-- latch template 2:process(D,Enable)beginifEnable='1'thenQ<=D;endif;endprocess;

D-type flip-flops

The D-type flip-flop samples an incoming signal at the rising (or falling) edge of a clock. This example has an asynchronous, active-high reset, and samples at the rising clock edge.


Another common way to write edge-triggered behavior in VHDL is with the 'event' signal attribute. A single apostrophe has to be written between the signal name and the name of the attribute.


VHDL also lends itself to "one-liners" such as




Which can be useful if not all signals (registers) driven by this process should be reset.

Example: a counter

The following example is an up-counter with asynchronous reset, parallel load and configurable width. It demonstrates the use of the 'unsigned' type, type conversions between 'unsigned' and 'std_logic_vector' and VHDL generics. The generics are very close to arguments or templates in other traditional programming languages like C++.

libraryIEEE;useIEEE.std_logic_1164.all;useIEEE.numeric_std.all;-- for the unsigned typeentityCOUNTERisgeneric(WIDTH:innatural:=32);port(RST:instd_logic;CLK:instd_logic;LOAD:instd_logic;DATA:instd_logic_vector(WIDTH-1downto0);Q:outstd_logic_vector(WIDTH-1downto0));endentityCOUNTER;architectureRTLofCOUNTERissignalCNT:unsigned(WIDTH-1downto0);beginprocess(RST,CLK)isbeginifRST='1'thenCNT<=(others=>'0');elsifrising_edge(CLK)thenifLOAD='1'thenCNT<=unsigned(DATA);-- type is converted to unsignedelseCNT<=CNT+1;endif;endif;endprocess;Q<=std_logic_vector(CNT);-- type is converted back to std_logic_vectorendarchitectureRTL;

More complex counters may add if/then/else statements within the rising_edge(CLK) elsif to add other functions, such as count enables, stopping or rolling over at some count value, generating output signals like terminal count signals, etc. Care must be taken with the ordering and nesting of such controls if used together, in order to produce the desired priorities and minimize the number of logic levels needed.

Simulation-only constructs

A large subset of VHDL cannot be translated into hardware. This subset is known as the non-synthesizable or the simulation-only subset of VHDL and can only be used for prototyping, simulation and debugging. For example, the following code will generate a clock with a frequency of 50 MHz. It can, for example, be used to drive a clock input in a design during simulation. It is, however, a simulation-only construct and cannot be implemented in hardware. In actual hardware, the clock is generated externally; it can be scaled down internally by user logic or dedicated hardware.


The simulation-only constructs can be used to build complex waveforms in very short time. Such waveform can be used, for example, as test vectors for a complex design or as a prototype of some synthesizer logic that will be implemented in the future.

processbeginwaituntilSTART='1';-- wait until START is highforiin1to10loop-- then wait for a few clock periods...waituntilrising_edge(CLK);endloop;foriin1to10loop-- write numbers 1 to 10 to DATA, 1 every cycleDATA<=to_unsigned(i,8);waituntilrising_edge(CLK);endloop;-- wait until the output changeswaitonRESULT;-- now raise ACK for clock periodACK<='1';waituntilrising_edge(CLK);ACK<='0';-- and so on...endprocess;

VHDL simulators



See also

Related Research Articles

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Further reading