Re: FPGA for PCI based servo control board

[jmkasunich-at-ra.rockwell.com]

John Sheahan wrote:



> On Tue, Apr 01, 2003 at 01:52:57AM -0500, Jon Elson wrote:
>>
>> That is NOT true 5 V I/O.  It is 5V TOLERANT I/O.
>> If you want to pull up above 3 V, you need low-value
>> pull-up resistors external to the chip.  If you
>> need really fast positive-going transitions to +5 V,
>> you can't get them.
>
>
> true - but 5v pci only needs to be 5v tolerant.
> driving 3v3 is in specification.
>
> is there much 5v PCI any more?
>

Repeat after me:
"The world is not a PC"
"The world is not a PC"

Just because you don't need 5V to talk to a PCI bus inside
a PC doesn't mean you don't need 5V.  The outputs of the
FPGA will have to drive opto-couplers, relay drivers, analog
filters, OPTO-22 modules, solid state relays, and all kinds
of other real world stuff.  For that stuff, 5V is FAR better
than 3.3V, and don't even mention 2.5V.

On a positive note, I believe that you could use HCT series
buffer chips running on 5V, feed them a signal from a 3.3V
FPGA, and get a good 5V signal on the buffer outputs.  Could
get bulky if you have lots of output pins to buffer.

John Kasunich