VHDL (VHSIC hardware description language) is commonly used as a design-entry language for field-programmable gate arrays and application-specific integrated circuits in electronic design automation of digital circuits.
VHDL was originally developed at the behest of the US Department of Defense in order to document the behavior of the ASICs that supplier companies were including in equipment. That is to say, VHDL was developed as an alternative to huge, complex manuals which were subject to implementation-specific details.
The idea of being able to simulate 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. Modern synthesis tools can extract RAM, counter, and arithmetic blocks out of the code, and implement them according to what the user specifies. Thus, the same VHDL code could be synthesized differently for lowest area, lowest power consumption, highest clock speed, or other requirements.
VHDL borrows heavily from the Ada programming language in both concepts (for example, the slice notation for indexing part of a one-dimensional array) and syntax. 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 case insensitive. There are many features of VHDL which are not found in Ada, such as an extended set of Boolean operators including nand and nor, in order to represent directly operations which are common in hardware. VHDL also allows arrays to be indexed in either direction (ascending or descending) because both conventions are used in hardware, whereas Ada (like most programming languages) provides ascending indexing only. The reason for the similarity between the two languages is that the Department of Defense required 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.
The initial version of VHDL, designed to IEEE standard 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_ulogic and its vector version std_ulogic_vector.
The second issue of 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
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, VHDL Technical Committee of Accellera (delegated by IEEE to work on 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 (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 plans to release VHDL 4.0 to the IEEE for balloting for inclusion in IEEE 1076-2008.
Although background in a computer programming language (such as C) is helpful, it is not essential. Free VHDL simulators are readily available, and although these are limited in functionality compared to commercial VHDL simulators, they are more than sufficient for independent study. If the user's goal is to learn RTL coding, (that is, design hardware circuits in VHDL, as opposed to simply document or simulate circuit behavior), then a synthesis/design package is also needed.
As with VHDL simulators, free FPGA synthesis tools are readily available, and are more than adequate for independent study. Feedback from the synthesis tool gives the user a feel for the relative efficiencies of different coding styles. A schematic/gate viewer shows the user the synthesized design as a navigable netlist diagram. Many FPGA design packages offer alternative design input methods, such as block-diagram (schematic) and state-diagram capture. These provide a useful starting template for coding certain types of repetitive structures, or complex state-transition diagrams. Finally, the included tutorials and examples are valuable aids.
Nearly all FPGA design and simulation flows support both Verilog and VHDL, allowing the user to learn either or both languages.
Free design & simulation packages for VHDL/Verilog:
|Vendor||Trial Software||License||Simulator||Synthesizer||RTL view||Gate view|
|Actel||Libero gold||one year free license||ModelSim Actel Edition||Synplify Actel Edition||No||yes**|
|Aldec||Active-HDL (Student Edition)||one year free license||Aldec (mixed language) Student||All Synthesis (interfaces)||yes||yes|
|Altera||Quartus II web edition||6 months renewable free license||ModelSim Altera Edition||Altera Quartus II||yes||yes**|
|Lattice||ispLever starter||6 months renewable free license||Precision/Synplify Lattice Edition||No||yes|
|Dolphin||none||free seduction license||SMASH||no||?||?|
|Mentor||none||6 months renewable free license||ModelSim PE Student Edition||no||yes||no|
|Xilinx||ISE webpack||free license||ISE Simulator*||Xilinx XST||yes||yes**|
|Blue Pacific||BlueHDL||free license||BlueSim||?||?||?|
A simple AND gate in VHDL would look something like this:
While the example above may seem very verbose to HDL beginners, one should keep in mind that many parts are either optional or need to be written only once. And 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 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.
Originally synthesizers, which would correspond to compilers in the HDL world, used a set of templates to identify the common hardware constructs in the HDL code (recall that VHDL is a hardware 'description' language, not a programming language). These templates can still be used today, although the HDL tools are far more sophisticated these days. Due to their usually one-to-one mapping to well known digital circuits, the templates are usually what an otherwise experienced hardware designer would use when entering the HDL world. But they are also useful for those who are completely new to digital design.
Some digital components have multiple templates, consider for instance the multiplexer example that follows:
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:
The three last templates make use of what VHDL calls 'sequential' code. The sequential sections are always placed inside a process and have a slightly different syntax which may resemble more traditional programming languages.
A transparent latch is basically one bit of memory which is updated when an enable signal is raised:
A SR-latch uses a set and reset signal instead:
Template 2 has an implicit "else Q <= Q;" which may be explicitly added if desired.
Some flip-flops also have Enable signals and asynchronous or synchronous Set and Reset signals:
A common beginner mistake is to have a set or reset input but not use it. For example, the following two snippets are not equal, the first one is a simple D-type flip-flop, while the second one is a DFF with a feedback MUX.
This is very similar to the 'transparent latch' mistake mentioned earlier.
The following example is an up-counter with asynchronous reset, parallel load and configurable width. It demonstrates the use of the 'unsigned' type and VHDL generics. The generic are very close to arguments or templates in other traditional programming languages like C or C++.
While not recommended for new designs, the std_logic_vector type can be used instead of the unsigned type.
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.
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 the 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!
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 synthesizable logic that will be implemented in future.