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 organization, and tests to verify its operation by means of simulation.

A Hardware Description Language (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.

HDLs are used to write executable specifications of some piece of hardware. A simulation program, designed to implement the underlying semantics of the language statements, coupled with simulating the progress of time, provides the hardware designer with the ability to model a piece of hardware before it is created physically. It is this executability that gives the illusion of HDLs being a programming language. Simulators capable of supporting discrete event (digital), and continuous time (analog) modeling exist and HDLs targeted for each are available.

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.

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.

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.
History of HDLs
The first hardware description languages were ISP, developed at Carnegie Mellon University, and KARL, developed at University of Kaiserslautern, both around 1977. ISP was, however, more like a software programming language used to describe relations between the inputs and the outputs of the design. Therefore, it could be used to simulate the design, but not to synthesize it. KARL included design calculus language features supporting VLSI chip floorplanning and Structured hardware design, which was also the basis of KARL's interactive graphic sister language ABL, implemented in the early 1980s as the ABLED graphic VLSI design editor, by the telecommunication research center CSELT at Torino, Italy. In the mid 80's, a VLSI design framework was implemented around KARL and ABL by an international consortium funded by the commission of the European Union (chapter in [1]). In 1983 Data-I/O introduced ABEL. It was targeted for describing programmable logical devices and was basically used to design finite state machines.

The first modern HDL, Verilog, was introduced by Gateway Design Automation in 1985. Cadence Design Systems later acquired the rights to Verilog-XL, the HDL-simulator which would become the de-facto standard (of Verilog simulators) for the next decade. In 1987, a request from the U.S. Department of Defense led to the development of VHDL (Very High Speed Integrated Circuit Hardware Description Language.) Initially, Verilog and VHDL were used to document and simulate circuit-designs already captured and described in another form (such as a schematic file.) HDL-simulation enabled engineers to work at a higher level of abstraction than simulation at the schematic-level, and thus increased design capacity from hundreds of transistors to thousands.

The introduction of logic-synthesis for HDLs pushed HDLs from the background into the foreground of digital-design. Synthesis tools compiled HDL-source files (written in a constrained format called "RTL") into a manufacturable gate/transistor-level netlist description. Writing synthesizeable RTL files required practice and discipline on the part of the designer; compared to a traditional schematic-layout, synthesized-RTL netlists were almost always larger in area and slower in performance. Circuit design by a skilled engineer, using labor-intensive schematic-capture/hand-layout, would almost always outperform its logically-synthesized equivalent, but synthesis's productivity advantage soon displaced digital schematic-capture to exactly those areas which were problematic for RTL-synthesis: extremely high-speed, low-power, or asynchronous circuitry. In short, logic synthesis not only propelled HDLs into a central role for digital design, it was a revolutionary technology for digital-circuit design industry.

Within a few years, both VHDL and Verilog emerged as the dominant HDLs in the electronics industry, while older and less-capable HDLs gradually disappeared from use. But VHDL and Verilog share many of the same limitations: neither HDL is suitable for analog/mixed-signal circuit simulation. Neither possesses language constructs to describe recursively-generated logic structures. Specialized HDLs (such as Confluence) were introduced with the explicit goal of fixing a specific Verilog/VHDL limitation, though none were ever intended to replace VHDL/Verilog.

Over the years, a lot of effort has gone into improving HDLs. The latest iteration of Verilog, formally known as IEEE 1800-2005 Systemverilog, introduces many new features (classes, random variables, and properties/assertions) to address the growing need for better testbench randomization, design hierarchy, and reuse. A future revision of VHDL is also in development, and is expected to match Systemverilog's improvements. Both VHDL and Verilog, with their continual refinements, are expected to remain in active use for years to come.


[edit] Design using HDL
The vast majority of modern digital circuit-design revolves around an HDL-description of the desired circuit, device, or subsystem.

Most designs begin on traditional pencil and paper, as written set of requirements or a high-level architectural diagram. The process of writing the HDL-description is highly dependent on the designer's background and the circuit's nature. The HDL is merely the 'capture language' -- designers often begin with a high-level algorithmic description (such as MATLAB or a C++ mathematical model.) Control and decision structures are often prototyped in flowchart applications, or entered in a state-diagram editor. Designers even use scripting-languages (such as PERL) to auto-generate repetitive circuit-structures in the HDL language. Advanced text-editors (such as EMACS) offer an editor template to auto-indent, color-highlight syntax keywords, and macro-expand entity/architecture/signal declaration.

As the design's implementation is fleshed out, the HDL-code invariably must undergo code review (i.e. auditing.) In preparation for synthesis, the HDL-description is subject to an array of automated checkers. The checkers enforce standardized-code guidelines (to identify ambiguous code-constructs before they can cause mis-interpretation by downstream synthesis) and check for common logical-coding errors (such as dangling ports or shorted outputs.)

In industry parlance, HDL-design generally ends at the synthesis stage. Once the synthesis-tool has mapped the HDL-description into a gate-netlist, the netlist is passed off to the 'back-end' stage. Depending on the physical technology (FPGA vs ASIC gate-array vs ASIC standard-cell), HDLs may or may not play a significant role in the back-end flow. In general, as the design-flow progresses toward a physically realizeable form, the design-database becomes progressively more laden with technology-specific information, which cannot be stored in a generic HDL-description. The end result is a silicon chip that would be manufactured in a fab.


[edit] Simulating and debugging HDL code
Essential to HDL-design is the ability to simulate HDL programs. Simulation allows a HDL-description of a design (called a model) to pass design verification, an important milestone that validates the design's intended function (specification) against the code-implementation (HDL-description.) It also permits architectural exploration. The engineer can experiment with design choices by writing multiple variations of a base design, then comparing their behavior in simulation. Thus, simulation is critical for successful HDL-design.

To simulate an HDL-model, the engineer writes a toplevel simulation environment (called a testbench.) At a minimum, the testbench contains an instantiation of the model (called the device-under-test or DUT), pin/signal declarations for the model's I/O, and a clock-waveform. The testbench-code is event-driven: the engineer writes HDL-statements to implement the (testbench-generated) reset-signal, to model interface-transactions (such as a host-bus read/write), and to monitor the DUT's output. The HDL-simulator, which is the program which executes the testbench, maintains the simulator-clock, the master reference for all events in the testbench. Events occur only at the instants dictated by the testbench-HDL (such as a reset-toggle coded into the testbench), or in reaction (by the model) to stimulus and triggering events. Modern HDL-simulators have a full-featured GUI (graphical user interface), complete with a suite of debug tools. These allow the user to stop/restart the simulation at any time, insert simulator breakpoints (independent of the HDL-code), and monitor/modify any element in the HDL-model's hierarchy. Modern-simulators can also link the HDL-environment to user-compiled libraries, through a defined PLI/VHPI interface. Linking is machine-dependent (Win32/Linux/SPARC), as the HDL-simulator and user-libraries are compiled and linked outside the HDL-environment.

Design verification is often the most time-consuming portion of the design process, due to the disconnect between a device's functional specification, designer interpretation of the specification, and imprecision of the HDL-language. The majority of the initial test/debug cycle is conducted in the HDL simulator environment, as the early stage of the design is subject to frequent and major circuit changes. An HDL-description can also be prototyped and tested in hardware -- programmable logic device are often used for this purpose. Hardware prototyping is comparatively more expensive than HDL-simulation, but offers a real-world view of the design. Prototyping is the best way to check interfacing against other hardware-devices, and hardware-prototypes, even those running on slow FPGAs, offer much faster simulation times than pure HDL-simulation.


[edit] Design Verification with HDLs
Historically, design verification was a laborious, repetitive loop of writing and running simulation testcases against the design-under-test. As chip designs have grown larger and more complex, the task of design verification has grown to the point where it now dominates the schedule of a design-team. Looking for ways to improve design productivity, the EDA industry developed the property specification language.

In formal verification terms, a property is a factual statement about the expected or assumed behavior of another object. Ideally, for a given HDL-design description, a property (or properties) can be proven true or false using formal mathematical methods. In practical terms, many properties cannot be proven because they occupy an unbounded solution space. However, if provided a set of operating assumptions or constraints, a property-checker tool can prove (or disprove) more properties, over the narrowed solution space.

The assertions do not model circuit activity, but rather, capture and document the "designer's intent" in the HDL code-listing. In a simulation environment, the simulator evaluates all specified assertions, reporting the location and severity of any violations. In a synthesis environment, the synthesis tool would probably take the policy of halting synthesis on any violation. Assertion-based verification is still in its infancy, but is expected to become an integral part of the HDL design-toolset.


[edit] HDL and programming languages
An HDL is analogous to a software programming language, but with major differences. Programming languages are inherently procedural (single-threaded), with limited syntactical and semantic support to handle concurrency. HDLs, on the other hand, can model multiple parallel processes (such as flipflops, adders, etc.) that automatically execute independently of one another. Any change to the process's input automatically triggers an update in the simulator's process stack. Both programming languages and HDLs are processed by a compiler (usually called a synthesizer in the HDL case), but with different goals. For HDLs, 'compiler' refers to synthesis, a process of transforming the HDL code-listing into a physically-realizable gate netlist. The netlist-output can take any of many forms: a "simulation" netlist with gate-delay information, a "handoff" netlist for post-synthesis place&route, or a generic industry-standard EDIF format (for subsequent conversion to a JEDEC-format file.)

On the other hand, a software compiler converts the source-code listing into a microprocessor-specific object-code, for execution on the target microprocessor. As HDLs and programming-languages borrow concepts and features from each other, the boundary between them is becoming less distinct. However, pure HDLs are unsuitable for general purpose software application development, just as general-purpose programming languages are undesirable for modeling hardware. However, as electronic systems grow increasingly complex, and reconfigurable systems become increasingly mainstream, there is growing desire in the industry for a single language that can perform some tasks of both hardware-design and software-programming. SystemC is an example of such -- embedded system hardware can be modeled as non-detailed architectural blocks (blackboxes with modeled signal-inputs and output drivers.) The target application is written in C/C++, and natively-compiled for the host-development system (as opposed to the targeting the embedded CPU, requiring host-simulation of the embedded CPU.) SystemC model's high-level of abstraction is well suited for early architecture exploration, as the architect can quickly evaluate architectural modifications, with little concern about signal-level implementation issues.


[edit] Languages

[edit] Digital circuit design
The two most widely-used and well-supported HDL varieties used in industry are:

VHDL
Verilog
Others include:

Advanced Boolean Expression Language (ABEL)
AHDL (Altera HDL, a proprietary language from Altera)
Atom (behavioral synthesis and high-level HDL based on Haskell)
Bluespec (high-level HDL originally based on Haskell, now with a SystemVerilog syntax)
Confluence (a functional HDL; has been discontinued)
CUPL (a proprietary language from Logical Devices, Inc.)
HDCaml (based on Objective Caml)
Hardware Join Java (based on Join Java)
HML (based on SML)
Hydra (based on Haskell)
JHDL (based on Java)
Lava (based on Haskell)
Lola (a simple language used for teaching)
MyHDL (based on Python)
PALASM (for Programmable Array Logic (PAL) devices)
Ruby (hardware description language)
RHDL (based on the Ruby programming language)
CoWareC, a C-based HDL by CoWare. Now discontinued in favor of SystemC
SystemVerilog, a superset of Verilog, with enhancements to address system-level design and verification
SystemC, a standardized class of C++ libraries for high-level behavioral and transaction modeling of digital hardware at a high level of abstraction, i.e. system-level