Simplifying FPGA Complexity

Can I use HDL code provided by Campera "as is" in my safety critical application?

Yes, but with care. We follow a rigorous design flow and every HDL module we develop could be used (as we did) in a safety critical application. Remember that each standard has a different V&V flowbut we can provide you the safety case for each module. There is, however, to take into account that perhaps you would need to include techniques to reduce MTBF, as ECC on memories or safe FSM (which can be done in different ways), and this would results in unnecessary high resource usage.

That's why we don't include such features in standard IPs, but only on a customer request.

Anyway other V&V stages, usually employed in safety critical verification, such as version control, coding standard, self checking testbenches with 100% coverage and much more are done on every module we release

What is the difference between VHDL and Verilog?

Fundamentally speaking, not a lot. You can produce robust designs and comprehensive test environments with both langauges, for both ASIC and FPGA. However, the two langauges approach the task from different directions; VHDL, intended as a specification langauge, is very exact in its nature and hence very verbose. Verilog, intended as a simulation langauge, it much closer to C in style, in that it is terse and elegant to write but requires much more care to avoid nasty bugs. VHDL doesn't let you get away with much; Verilog assumes that whatever you wrote was exactly what you intended to write. If you get a VHDL architecture to compile, it's probably going to approximate to the function you wanted. For Verilog, successful compilation merely indicates that the syntax rules were met, nothing more. VHDL has some features that make it good for system-level modelling, whereas Verilog is much better than VHDL at gate-level simulation. To confuse the situation more, see SystemVerilog...

How must I write VHDL to make it synthesisable?

Because large parts of the language make no sense in a hardware context, synthesisable VHDL is a relatively small subset of VHDL. You must stick to this subset, and understand exactly how the synthesis tool you use interprets that code. For FPGA in particular you must also develop a good understanding of the structure of your chip, and know how your code must reflect the most efficient use of that structure. Fundamentally, never forget that you are designing a circuit, not writing a program. Forgetting this simply but important fact will only lead to pain later.

How many versions of VHDL are there?

There are four. The original release of the VHDL language occured in 1987 with the adoption of the Language Reference Manual as an IEEE standard. In 1993, the IEEE-1076 standard was modified and ratified and became known as VHDL'93. This is now widely supported. In 2000, the VHDL 1076 2000 Edition appeared - this fixed shared variables by introducing the idea of protected types. Finally, VHDL 1076-2002 appeared. This includes protected mode types, but also changes ports of mode buffer to make them more usable, along with some other small changes. In practise, VHDL 1076-1993 is the current flavour of VHDL which is widely supported by tool vendors.

Is VHDL going to be developed further?

You might have heard a lot about System Verilog, and wondered if VHDL is going to also be developed? There is an activity to develop an improved VHDL, and VHDL-2008 was released in January 2009. You will find more information here.

A VHDL design can be moved to any tool or technology. Right?

On the face of it, this is true. VHDL was designed to be and is a technology independent design language. However, there is less of a compliance issue between different simulators than there is for synthesis tools. Generally speaking, moving VHDL code from one simulator to another involves one or two minor changes to the VHDL. Two different synthesis tools may have broad agreement of what constitutes sythesisable code, but may interpret that code in different ways. This is particularly an issue for us at Doulos in developing our training courses, because we like to present a reasonably generic approach to writing VHDL for synthesis. This means that the VHDL we teach you is guaranteed to be more transportable between synthesis tools than it otherwise would be. Our pain is your gain! In addition, because we are so aware of the differences between synthesis tools this means that we emphasise the best way of writing VHDL to get the best from your synthesis tool. For FPGA, code that uses one technology in the most efficient way is unlikely to be as efficient when sythesised to a different device family.

Are there any tools to generate VHDL test benches automatically?

The basic answer is no. Writing a testbench can be a complex task, and can be more complex than the design being tested. If you mean "Can I get a code framework for a simple testbench", then a number of tools provide simple "testbench templates"; even the Emacs editor VHDL mode can do this! For more advanced ways of writing testbenches, you might want to look at the so-called "Testbench Automation" tools, such as SystemVerilog, SystemC Verification Library, Cadence Specman, and Synopys Vera. These tools involve learning another language of course.

Can you give me a measure of the productivity improvements I should expect from VHDL?

Well, do you believe the hype! Yes, ultimately there are considerable productivity gains to be had from using high-level design techniques in conjunction with synthesis technology, providing that your designs are: complex, amenable to synthesis, not dependent upon the benefits of a particular technology
Obviously, complex means different things to different people, but a good rule of thumb is that complex means the implementation part of the design process is considerably more awkward than the specification phase. Let's face it, if the specification phase is significantly longer than the implementation phase, you need to put effort here, not into HLD. Of course, once you are benefiting from HLD productivity gains, the specification phase becomes more significant. OK, that's HLD: VHDL is a HLD design entry language, so we would expect the use of VHDL with synthesis technology to improve productivity in the design process. However, you won't get those benefits immediately. Your first VHDL-based project will probably take slightly longer than if you had used your previous design process. Where you really win out is second time around. In order to reduce the time spent on your first project and to ensure that subsequent projects benefit from good VHDL design practices, you need to ensure that your engineers are well trained (well, we would say that wouldn't we!)

I can see how to write abstract behavioural descriptions in VHDL, but how do you describe and simulate the actual hardware?

This is probably the biggest hurdle that many hardware engineers face when moving to VHDL. After all, sometimes we need to be able to describe actual implementation as well as abstract functionality. The way to describe "physical" hardware in VHDL is to write VHDL models of those components. This is supported in VHDL through the use of instantiation. VHDL does not allow you to physically simulate your hardware. You can only simulate a model of that component in a VHDL simulation. Historically, gate-level simulation using VHDL has been notoriously slow. This led to the creation of the 1076.4 working group to provide a mechanism to allow faster gate-level simulation using VHDL. Their effort became known as the VITAL standard. VITAL is not a VHDL issue for you, but an EDA vendor/ASIC supplier issue. A simulator is VITAL compliant if it implements the VITAL package in its kernel (this is faster than simulating the VITAL primitives in the VITAL package). You don't need to change your VHDL netlist; your ASIC vendor needs to have a VITAL compliant library though, in order for you to take advantage of the simulation speed up. Thus the ASIC vendor's library elements need to be implemented entirely in VITAL primitives. Note that many companies use Verilog for gate-level simulations as it is still faster than VHDL, even with the improvemens from VITAL. The reason is that Verilog was designed from the start as a gate-level simulation langauge.

I've heard that VHDL is very inefficient for FPGAs. Is that true?

It might be. If the code in question was written with no thought for how the FPGA would implement the circuit, then it's entirely possible that it was inefficient. If the code is written with consideration of the FPGA resources available and the synthesis tool being used, then no, it's not inefficient.

Are there any VHDL source code libraries available to save me having to re-invent common code fragments and functions?

There are a few libraries available for most levels of VHDL design. The IEEE library contains very low-level type-and-function packages. The std_logic_1164 package is an industry standard, and practically every piece of VHDL you ever write will use this package; the types std_logic and std_logic_vector are the overwhelmingly dominant types for anything related to digital design. For arithmetic, numeric_std (from the same IEEE library) is a collection of functions that work on std_logic and its derivatives. For other libraries of components, have a look in the comp.lang.vhdl FAQ.

Are freeware / shareware VHDL tools available?

There are a few; generally these are limited in some way (for instance they have no graphical interface, or simulate only certain sizes of model).

Are there any inexpensive VHDL tools available?

Yes, there are one or two. The main FPGA vendors including Altera and Xilinx provide simple toolsets, including a VHDL simulator. These toolsets are normally limited to small devices, and will also have a limit on the number of lines of code you can write; and of course the target technology for synthesis is fixed to a particular manufacture's chips.

What is Synthesis?

Synthesis is the stage in the design flow which is concerned with translating your VHDL code into gates - and that's putting it very simply! First of all, the VHDL must be written in a particular way for the target technology that you are using. Of course, a synthesis tool doesn't actually produce gates - it will output a netlist of the design that you have synthesised that represents the chip which can be fabricated through an ASIC or FPGA vendor.

What is the difference between Verilog and VHDL?

On the surface, not that much. Both are IEEE standards and are supported by all the major EDA vendors. Both can be used for designing ASICs and simulating systems. However, VHDL is altogether a grander language. Its support for system level modeling and simulation is far more comprehensive than Verilog, despite some Verilog built-in functions for stochastic modeling. However, VHDL requires longer to learn and is not so amenable to quick coding. As a final thought many engineers need to know both languages due to the use of IP (Intellectual Property) blocks written in "the other" language.

What versions of Verilog are there?

The original Verilog was standardised by the IEEE as Verilog 95; there was then an improved version released, Verilog 2001. This includes things like generate statements. Work is now ongoing on SystemVerilog (see the SystemVerilog website). If you want to know more about SystemVerilog, have a look at our 1 day Introduction to SystemVerilog.

Can I use Verilog for the analog part of a design?

No. You can't design analog circuitry in Verilog, however you can model analogue circuitry in Verilog (said this digital designer!). In theory, Verilog can be used to model the behaviour of any system or component. However, Verilog does not offer the same level of modeling accuracy as say Spice, without an awful lot of work.

How must I write Verilog to make it synthesisable?

Writing Verilog for synthesis is not particularly difficult, but you need to be disciplined, not only in your use of Verilog syntax but also your approach to writing Verilog for synthesis. It is this latter aspect which many engineers overlook; thorough training is really the only way to avoid making poor strategy decisions in writing synthesisable Verilog.

A Verilog design can be moved to any tool or technology. Right?

On the face of it, this is true. Verilog was designed to be and is a technology independent design language. However, there is less of a compliance issue between different simulators than there is for synthesis tools. Generally speaking, moving Verilog code from one simulator to another involves one or two minor changes to the Verilog, assuming you don't use a lot of system tasks (which tend to be proprietary) in your code. Two different synthesis tools may support two quite different Verilog subsets. This is particularly an issue for us at Doulos in developing our training courses, because we like to present a reasonably generic approach to writing Verilog for synthesis. This means that the Verilog we teach you is guaranteed to be more transportable between synthesis tools than it otherwise would be. Our pain is your gain! In addition because we are so aware of the differences between synthesis tools, this means that we emphasise the best way of writing Verilog to get the best from your synthesis tool.

I can see how to write abstract behavioural descriptions in Verilog, but how do you describe and simulate the actual hardware?

This is probably the biggest hurdle that many hardware engineers face when moving to Verilog. After all, sometimes we need to be able to describe actual implementation as well as abstract functionality. The way to describe "physical" hardware in Verilog is to write Verilog models of those components. This is supported in Verilog through the use of instantiation. Verilog does not allow you to physically simulate your hardware. You can only simulate a model of that component in a Verilog simulation.

What is Synthesis?

Synthesis is the stage in the design flow which is concerned with translating your Verilog code into gates - and that's putting it very simply! First of all, the Verilog must be written in a particular way for the synthesis tool that you are using. Of course, a synthesis tool doesn't actually produce gates - it will output a netlist of the design that you have synthesised that represents the chip which can be fabricated through an ASIC or FPGA vendor.