HDL vs. schematics (was Re: ebay - cardamatic)

From: Eric Smith <eric_at_brouhaha.com>
Date: Wed Feb 16 17:44:55 2005

woodelf wrote:
> However FPGA fast ripple adders really don't give you the real idea of
> the FPGA logic speeds

It gives you information on the speed of adders/subtractors, and some
kinds of counters, but nothing else.

> since this is a trick on the FPGA venders to lie
> about the speed of the system

I strongly disagree. The hardware carry chain is a Very Good Thing,
not a cheat. Certainly it doesn't solve all problems, but in terms
of value for the amount of extra silicon it requires, it's extremely
cost effective.

If the vendors claimed that all user logic when implemented in an
FPGA operated at the speed of the carry chain, that would be a lie.
But I've never seen them make such a claim.

> If I could figure out what is generated in VHDL since I find it too
> verbose
> for my likeing

If you want terse (and cryptic, IMNSHO) you can use Verilog instead.

> and a free compiler I think I would have more positive
> things to say about VHDL.

Xilinx Webpack is a no-charge download, supports both Verilog and VHDL,
and targets all of the Xilinx CPLDs and most of the FPGAs up to a certain
size. Nowdays that size limit is actually quite large, an XC3S1500 is
supported, which is 1.5 million "marketing gates". In that part
you get 29,952 logic cells, each of which is a four-bit LUT (lookup
table) and a flipflop, as well as 32 18-bit multipliers, and 576 Kbits
of block RAM.

The small FPGAs like the XC3S50 (1728 logic cells, 4 multipliers, and
72 Kbits block RAM) cost only $10.10 direct from Xilinx in quantity

For small stuff, I like the Xilinx CPLDs, and some of them are quite
inexpensive. The XC9536XL, a 3.3V 36-macrocell part, is only around
$1 from Digikey in small quantities. Even the fairly big CPLDs aren't
too expensive. And they have 5V and 2.5V parts as well.

Sorry if I sound like a Xilinx shill. I don't have any relationship
with them other than being a satisfied customer.

Received on Wed Feb 16 2005 - 17:44:55 GMT

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