Re: gEDA-user: CPLDs and other high-density logic chips...

[John Sheahan <jrsheahan@xxxxxxxxxxxxxxxx>]

Samuel A. Falvo II wrote:
> On 9/3/06, Bob Paddock <bob.paddock@xxxxxxxxx> wrote:
> > > > For example, if you were to pick xilinx, and with what you have
> > > > mentioned, you might want to drop the cpld,
> >
> > Xilinx CoolRunner family of parts are still worth looking at if low power
> > is the overriding goal.
>
> If I were to drop the CPLD and just go with an FPGA, how would I be
> able to couple this to 5V logic?
>
> I suppose I could use 74ALVT16244 chips feeding all digital inputs,
> and just let the 3.3V logic direct-drive the 5V CMOS logic on the main
> circuit.  However, 3-state buses would need to be split into distinct
> input and output buses, thus sucking up some pins on the FPGA.  I was
> just wondering if anyone had any other suggestions.

I guess this depends on the amount of logic you have, and the sort of 
pin numbers you need to 5v logic.

old fpga's and many cpld's were/are happy with 5v. A 15 (20?) year old 
xilinx 4000 was 5v and would be big enough (but perhaps not fast enough) 
for the things you mentioned. Cannot remember the Altera
eqv, we used Xilinx 3000 and 4000 then. Hand route and all to make it 
fit.  Tweak the circuitry on the fly in the layout editor, to come up 
with something route-able.   (what was that about licking the road clean?)

The newer devices on 90 nm (or whatever) rules are amazing as to the 
amount of quick logic you can fit in.   You do need a bunch of core and 
IO power supplies, and 3v3 interfacing is about the upper limit though.
Need a ram interface, cpu, custom cores  -they fit in a corner...

If you don't need the fpga size, then 5v flash cplds (big pal things) 
may still be a valid solution. I wonder how long they will still be 
purchasable for though.  Feels just wrong putting them in a new design.
For me - fpga's are just generally nicer than cpld's. Fewer 
architectural limitations.
5v. leaving that behind was hard. Well worth it , though.


The big guys have eval boards in the order hundred dollar range.
FPGA + config support + programming connection to a PC via something.
It could be worth getting / borrowing one of those - playing flashing 
leds for a day or two - to push yourself over the entry.
Some distributors will lend you one if you can promise an order later
You probably won't need it for long.
john

> Thanks for the feedback you've given.  It's very useful.



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