Which HDL?

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View

I would like to start a serious adventure in FPGA development,
so which HDL would you recommend me? I can restrict myself
to a single chip vendor (i.e. Altera, because their chips are quite
cheap and very easy to obtain in small quantities in Poland),
thus the spectrum of alternatives should be wider. The most
important thing is good support of genericity, for instance:

    generic type vector where {const N : positive} is group of N bits
    x : vector(32); -- x : std_logic_vector(31 downto 0)

or even constants:

    generic const square : type of N where {const N} is N*N;
    const nine : int is square;

The languages I know about are:

VHDL: very disappointing, the nicest part of Ada has ben removed.
No support for anonymous types, stiff and annoying syntax, weak
interface inference (needs explicit component specifications).

Verilog: same as above + lack of generate statements, so how
does one specify generic pipelines (very useful e.g. in parallel
CORDIC specifications)?

AHDL: I don't know much about it, because I can't find a good manual.

    Best regards
    Piotr Wyderski

Re: Which HDL?
Apart from behavioral languages such as systemC thats about
it,vhdl,verilog,ahdl is rarely used now, so VHDL is about your best
bet,its not perfect but its better than a poke in the eye with a sharp

Re: Which HDL?

Quoted text here. Click to load it

I agree.

If you want to implement the hardware I recomment either verilog or VHDL.
AHDL is vendor specific while most vendor tools can deal with both
verilog or vhdl or mixed.

So now between verilog and vhdl, I can hardly guide you. I don't know
verilog but from what I've seen it's more concise than VHDL. The latter
tends to be very verbose. However, I work only with vhdl and I'm happy,
with generate you can write more "generic" code to be reused and it
still syntetize well.

Finally, what ever you choose, never forget that you must decribe the
hardware you want built and not the behavior you want implemented !


Re: Which HDL?

Quoted text here. Click to load it

Only *just*... ;)


Re: Which HDL?
Quoted text here. Click to load it

I thought VHDL was a poke in the eye with a sharp stick :->

Re: Which HDL?

|> VHDL: very disappointing, the nicest part of Ada has ben removed.
|> No support for anonymous types, stiff and annoying syntax, weak
|> interface inference (needs explicit component specifications).

Hm, you can define components in packages and just "use" them. So you don't need
to cut'n'paste the interface into each architecture. A lot of VHDL examples
(esp. from the FPGA vendors) don't use packages, as it hides the port names and
makes the instantiation examples harder to understand.

         Georg Acher, snipped-for-privacy@in.tum.de
We've slightly trimmed the long signature. Click to see the full one.
Re: Which HDL?

Quoted text here. Click to load it

Thanks, I didn't know about that. Looks like one of my problems has been

    Best regards
    Piotr Wyderski

Re: Which HDL?
I would say that everyone else seems to manage in producing working
designs without  rewritting either vhdl or verilog,niether of them are
meant to be a 'programming language' so aproaching them from the point
of view of a programmer isnt the best way of looking at them.

Re: Which HDL?
Quoted text here. Click to load it

Verilog and VHDL both tries to be many things at the same time, but at
the core are specification languages.  Programming languages are a
special class of specification languages.  That old "It's not a PL" is
getting old as an excuse of either. Any decent first year student of
programming languages looking at either would recognize many obvious

VHDL is excessively verbose and it's much praised type safety is
designed in a way that forces way too many explicit type casts.

Verilog (which I know far better) is less verbose but is often too loose
with what it allows.  It's a mix of two models, schematic and
behavioral, but it's impossible for any non-trivial design to be purely
behavioral as the abstraction unit (the module) must be instantiated as
a component and attached to wires and registers.

I've witnessed HW development at a Silly Valley startup and observed how
Verilog was mostly used as assembly, generated by big (and IMO nasty)
perl scripts.  I take that as a statement to the fact that we need
something better.

PS: For a certain class of problems, I like my own variant of
Timogriffer-C: Hard-C (http://not.meko.dk/Hacks/hacks.html ).

Re: Which HDL?

Quoted text here. Click to load it

Genericity is not a programming language approach. This is an
abstract concept, which is used by many programming languages,
but is not a part of them. Moreover, it is much more useful in
hardware specifications than in programming languages. However,
its support in VHDL and Verilog is very restricted and annoying.
So the only reasonable thing I can do is to create my own HDL
and a translator to VHDL, to reuse existing tools. :-)

    Best regards
    Piotr Wyderski

Re: Which HDL?
Before you create your own HDL, take a look at Confluence,
www.confluent.org. It's IMO much more advanced than Verilog or VHDL and
generates FNF, which can be converted to all kinds of stuff including
Verilog or VHDL. 3 lines of Confluence is equivalent to about 30 lines
of VHDL in some cases (ie. component instantiation). I think there are
problems with creating dual-ported RAMs though, but this is mostly
because other vendor tools (ie. Quartus, ISE) inference structures from
generic HDL is still pretty poor in some cases. This may no longer be
the case, it's been awhile since I checked.

-- Pete

Quoted text here. Click to load it

Re: Which HDL?
Quoted text here. Click to load it

You might also check EVHDL, available at www.entner-electronics.com

Thomas Entner

Re: Which HDL?
How well do these HDLs cope with asynchronous or high speed designs?
I find that Verilog/VHDL allows you to get the most out of the FPGA,
albeit with the use of vendor constraints ;)
Peter Sommerfeld wrote:
Quoted text here. Click to load it

Re: Which HDL?

Quoted text here. Click to load it

Verilog 2001 does support generate.  It also supports arrays of
instances which are often easier to use than generate for many
applications.  Verilog is also very popular in the U.S. although
it has fewer followers elsewhere.  If you already know VHDL the
more concise nature of Verilog may not be a big win, nor Verilog's
similarity to C.

Another thing to remember about Verilog is that it is less rigid
about type-checking.  For example you can build designs where the
size of a vector applied to a module port doesn't match the size
in the module.  This will only generate a warning - not an error,
and warnings are easy to miss when the tools produce 100's of them.

Just my 2 cents.


Quoted text here. Click to load it

Re: Which HDL?

Quoted text here. Click to load it

So I'll have a look at it.

Quoted text here. Click to load it

Yes, this is a big disadvantage.

    Best regards
    Piotr Wyderski

Re: Which HDL?
I find Verilog a heck of a lot easier and results in more compact code
vs. VHDL. Verilog-2001 seems to have what was missing between the last
version and VHDL (generate, configurations, good file I/O, lots of
other new stuff). AHDL was the easiest to write, but supported only by
Altera, therefore no coded testbenching.

-- Pete

Re: Which HDL?

Quoted text here. Click to load it
Verilog 2001 still lacks the ability to define constants  inside
generates that are dependent on the generate index variable.  Structural
placement, eg assigning values to the RLOC constraints for placed
instances, remains difficult to do with Verilog 2001, while it has been
available with VHDL for quite a while.  Also,  I find that the type
checking in VHDL is a life saver for designs that have a lot of
arithmetic.  If you are already comfortable with VHDL, you may very well
find Verilog to be frustrating to use for designs that are more
structural than behavioral.

--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
We've slightly trimmed the long signature. Click to see the full one.
Re: Which HDL?
One other criterion to consider - is there an easily installable
platform independant free implementation of a simulator available?
Verilog wins on this one with Icarus.

And do your generic/generation magic with a scripting language that
generates verilog as output. Then you are in control of your toolchain.

... apart from the synthesis tool of course :-)



Re: Which HDL?
When will the FPGA vendors support system verilog?  System Verilog adds
structures to Verilog 2001, so you can say: bus.data, bus.addr, etc.

When will VCS support Verilog 2001?  It lacks the ability to make
parameterized modules if you use the new "ANSI" style declaration syntax:

module foo #(parameter WIDTH10%) // <-- this part not supported in VCS
  input clk,
  input reset_l,
  input [WIDTH-1:0] in,
  output reg [WIDTH-1:0] out



Also most tools support 'always @*', but not vcs :-(

Most designers still code in pre-v2001 style because of inconsistent vendor
support for the standard.

When will Altera Verilog support the 'wor' data type?  I use this to make
or-tree buses:

wor [7:0] foo;
assign foo = enable_1 ? data_1 : 0;
assign foo = enable_2 ? data_2 : 0;

When will Xilinx fix its buggy casex in xst?  (it works in 5.2i)

The complaint about verilog's lack of type checking never bothered me
because I used Synplicity- it was just slightly stricter than vcs.  Now I
use xst and altera's- both seem to be more lax, but I can't remember the
details right now.

Anyway, I still prefer Verilog over VHDL's horrible syntax.

Quoted text here. Click to load it

/*   snipped-for-privacy@world.std.com ( */               /* Joseph H. Allen */
int a[1817];main(z,p,q,r){for(p80%;q+p-80;p-=2*a[p])for(z=9;z--;)q=3&(r=time(0)
We've slightly trimmed the long signature. Click to see the full one.
Re: Which HDL?
Whichever way you go, get the revised classic

HDL Chip Design, A Practical Guide for Designing, Synthesizing and
Simulating ASICs and FPGAs using VHDL or Verilog by Douglas J Smith,
published by Doone Publications.

I have the much earlier ASIC edition which compares 100 or more typical
small problems such as datapaths, FSMs etc in V & V and with well drawn
(hand, not machine IIRC) schematics of what's synthesized. It seems its
been brought upto date with added C & FPGA stuff, must treat myself.

I prefer Verilog too inspite of its own wierdness and shortcomings,
after all it was designed more at the wirehead level where VHDL came
from the DOD as a committee language. Both langs are very bloated and
you will likely use <20% of either.

As for the noted Verilog C likeness, thats only true for expressions
which can sometimes be almost C compiled but then you would lose most
of the expressive HW power. If only C expressions could handle =
, very large size expressions, logic reductions, continuous &
registered <= assignments etc. The rest of the language is much more
Pascal'ish as is VHDL. VHDL certainly enforces more rigourous type
checking in the ADA style by multiple descriptions as in the C++ way of
prototypes and implementations. Note that Java moved away from that
idea since multiple compiler passes can resolve all the forward
backward references of declare before or after useage..

Also on a syntax note, C is far more complex to describe from a
compiler pt of view, HDLs in general have always had powerfull but
consistant BNF structures where C allows trully lazy coding style (10
ways to say the exact same thing), Verilog's BNF is way easier to
describe than C, its in the back of most Verilog books.

Certainly for synthesis about 20% or less can be synthesized either
way, the rest is there for modelling which came long before synthesis
(early 90s). Indeed Verilog contains a chunk of language (UDPs) usefull
only to the distant past of gate level table driven modelling.

I'd like to see a unified Verilog C language which looks like either &
both but with most of the useless stuff thrown out, ie a cycle/event C
with Verilogs expressiveness that is synthesizeable when using that
syntax and not when using the C syntax. (I'm not too keen on

Career wise though Verilog in USA, Japan & ASICs, VHDL in EU, Mil, and
FPGAs although most ASIC guys are gonna end up doing FPGAs so the war
will continue. Anyway both languages are now under the same roof!


my 2c


johnjakson at usa dot com

Site Timeline