Free Range VHDL

The no-frills guide to writing powerful code for your digital implementations

Bryan Mealy Fabrizio Tappero

Free Range VHDL

VHDL (VHSIC 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.
(From http://en.wikipedia.org/wiki/VHDL)

VHDL has a rich and interesting history1. But since knowing this history
is probably not going to help you write better VHDL code, it will only be
briey mentioned here. Consulting other, lengthier texts or search engines
will provide more information for those who are interested. Regarding the
VHDL acronym, the V is short for yet another acronym: VHSIC or Very
High-Speed Integrated Circuit. The HDL stands for Hardware Description
Language. Clearly, the state of technical a airs these days has done away
with the need for nested acronyms. VHDL is a true computer language
with the accompanying set of syntax and usage rules. But, as opposed to
higher-level computer languages, VHDL is primarily used to describe hard-
ware. The tendency for most people familiar with a higher-level computer
language such as C or Java is to view VHDL as just another computer lan-
guage. This is not altogether a bad approach if such a view facilitates the
understanding and memorization of the language syntax and structure.
The common mistake made by someone with this approach is to attempt
to program in VHDL as they would program a higher-level computer lan-
guage. Higher-level computer languages are sequential in nature; VHDL
is not.
VHDL was invented to describe hardware and in fact VHDL is a con-
current language. What this means is that, normally, VHDL instructions
are all executed at the same time (concurrently), regardless of the size ofyour implementation. Another way of looking at this is that higher-level
computer languages are used to describe algorithms (sequential execu-
tion) and VHDL is used to describe hardware (parallel execution). This
inherent di erence should necessarily encourage you to re-think how you
write your VHDL code. Attempts to write VHDL code with a high-level
language style generally result in VHDL code that no one understands.
Moreover, the tools used to synthesize2 this type of code have a tendency
to generate circuits that generally do not work correctly and have bugs
that are nearly impossible to trace. And if the circuit does actually work,
it will most likely be inefficient due to the fact that the resulting hardware
was unnecessarily large and overly complex. This problem is compounded
as the size and complexity of your circuits becomes greater.

Submitted by sasvari on Feb. 14, 2012, 8:54 a.m.
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