Re: [oc] Modular FPGA board (PCI)

[Richard Herveille <richard@asics.ws>]

> >
> >The pci connector has a keyed section, which must mate up with a
> >corresponding slot in the plug in card. This key slot can be in one of two
> >positions (actually the connector is just rotated 180) which indicate
> >whether the motherboard uses 5v or 3v3 signalling.  Many plug in pc cards
> >are 'universal' which means they have two slots and can fit in any hole -
> >this is possible by powering the ASIC's IO ring with the VIO supply on the
> >PCI connector.
> >This is not possible using virtex FPGA's as VCC_IO cannot be 5V.
>
>Doesn't matter. As long as the IO's can handle the over-voltage. Some FPGAs
>use 3v3 IOrings, but have dedicated IOpads which can handle 5v signals.
>This is normally taken care of by terminating diodes in the IOpads.
>
>MIKEJ : agreed - I was attempting to point out that universal cards should
>power
>the output drivers from VCC_IO  which can only be 3v3 in a bus which
>contains
>3v3 slots or the 3v3 power rail itself. (pci spec 2.2 page 114).
>Therefore a card in a 3v3 slot does not need to be 5v tolerant - in theory
>at least.
>Anyone know any different?
>As you say the important point for universal cards is that the inputs are
>tolerant upto
>5v as in the case of virtex 1 and sparten2.
>
> >If you can guarantee the dev board will only go in 3v3 slots (and these are
> >still quite rare) then everything will be fine.
>
>Not true. PCI has set a course where eventually 5v devices won't be
>supported anymore, but there's still a long time to go. This means even
>though your board is a pure 3v3 design, PCI signals can still reach 5v.
>This is caused by other boards in the system (or even the system) which use
>5v output drivers.
>
>MIKEJ : disagree, see above

OK, agree. In a purely 3v3 design you don't have to expect any 5v signals :-)

> >I'm not sure you can get away with external buffers around the FPGA for
>PCI.
> >as was suggested :
> >1, as the signals are bi-directional you would have terrible timing
>problems
> >and two loads per net - which is illegal
>
>
>the rest has been trimmed as we got told off !
>
>Cheers,
>MikeJ
>
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