Re: Problem with "Universal" PCI & PCI-X support: VIO

["Ingraham, Andrew" <Andrew.Ingraham@compaq.com>]

There may be some misunderstanding here.  I hope.

If a connector uses 5 volt keying, it MUST use 5 volt VIO.  And
vice-versa.

If a connector uses 3.3V keying, it MUST use 3.3 volt VIO.  And
vice-versa.

If you find any motherboards that don't follow these rules, PLEASE make
a lot of noise about them.  Scream and holler.  Announce that the
motherboards are not PCI-compliant.  Complain to the companies that
makes them.  Get them taken off the market.  Tell everyone you know not
to touch them with a ten foot pole.

Only 5 volt connectors are allowed to be 5 volt tolerant, and every 5
volt connector must be 5 volt tolerant.  ("5 volt tolerant" includes
tolerance of overshoot levels that might reach or exceed 7 volts.  If a
device calls itself "5 volt tolerant" but the absolute maximum input
level is 5.5V, it probably won't cut it for PCI!)

Now the confusion probably comes about because the "5 volt" signaling
environment doesn't necessarily mean that the SIGNALS actually switch
between 0 and 5 volts.  If you look at the electrical specs for 5 volt
PCI, the minimum Voh is only 2.4 volts.  It's just like TTL, and TTL
signals rarely reached a full 5V.  Typically they were more like 3-4V,
even though the TTL supply voltage was indeed 5 volts.

Since 3.3 > 2.4, it is totally acceptable to drive what amounts to 3.3
volt CMOS levels on a "5 volt" PCI bus.  But, while you do this, (a) you
must be 5 volt tolerant, and (b) VIO is still 5V!  This means that all
the "VIO" pins on the PCI connector are still connected to the 5 volt
supply.  You just don't use it directly for driving your outputs.

If you drive 3.3V levels, and VIO = 3.3V, then you're a 3.3 volt PCI bus
and the connector must use the 3.3V keying.

Unfortunately, you cannot capitalize on the fact that most "5 volt" PCI
devices today drive only 3.3 volt CMOS levels.  If you plug your card
into the 5 volt bus, you will (or should!) see VIO = 5V.  Even if you do
like most everyone else and use a 3.3V supply for your chip, remember
that anyone could plug an older (or even a newer!) PCI card that drives
full 5 volt rail-to-rail levels, into the same bus with yours, and then
your chip would be toast.

> A) We could make our PCI-X card 3.3 only; unfortunately I have yet to
> see 
> a 3.3 volt only PCI-33 motherboard, so the market is apparently very
> small. :-)
 
True, but I expect this to change in the near future.  The world needs a
"critical mass" of Universal plug-in cards, before making that leap to
having 3.3 volt motherboard slots.  Hopefully we have reached that
critical mass.  I don't have any inside information that the change to
3.3 volt slots will happen now; in fact I thought it would have started
much sooner.

Andy