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

From: woodelf <bfranchuk_at_jetnet.ab.ca>
Date: Wed Feb 16 19:01:20 2005

Eric Smith wrote:

>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.
I say it is a cheat because if you impliment carry logic and the software
decides to use the hardware carry chain you don't really know the real
timing of your design. Using altera 10K FPGA software that has
a similar carry chain many years ago, I found when I got to 80%
of the resources the cycle time really got long.

>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.
Not directy , but you often see XYZ RISC machine has a ABC cycle time.
Most of the speed is because of the block ram and fast carry of Xilinix

>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
I got bit with the fact the FPGA proms are expensive already so that is
why I went with
CPLD's with in system programing. $99 US got me the cable, software and
FPGA demo
board from atmel.

>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.
Since the amtel software was smaller, I downloaded that first. Since my
project was using about
90 macro-cells the amtel CPLD's were cheaper ( 5Volt logic - 128 cells )
than Xilinx chose them.
At that time I was planning on doing a 24/12 bit cpu but have downsized
to a 18/9 bit cpu.

>Sorry if I sound like a Xilinx shill. I don't have any relationship
>with them other than being a satisfied customer.
Well looking at the canadian prices - digikey I find them pricey if I
want to stay with
5 volt logic. I need 7 CPLD's for a 24 bit processor plus a few glue
CPLD's for I/O
Ben Alias woodelf
Received on Wed Feb 16 2005 - 19:01:20 GMT

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