RE: PCI-X Compatibility

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

I didn't say this quite the way I meant it, so I'll re-state....

A PCI-X device on a plug-in card can indeed be "Universal."

When it is operating in PCI-X mode (pre- PCI-X Revision 2.0), or when operating in 66MHz PCI mode, it uses 3.3V signaling only, never 5V.

When operating in 33MHz PCI mode, it can be compatible with 5V signaling in addition to 3.3V signaling.  That is, it can be either 3.3V only, or "Universal."  But 5V signaling could only be used in 33MHz PCI mode.

If it is "Universal," it needs to connect to Vi/o from the connector, so it can set its overshoot clamps accordingly.

I don't know how feasible it is for a 0.18u process ASIC to be 5V compatible.  Note that it needs to meet the "Maximum AC Ratings and Device Protection" requirements in PCI section 4.2.1.3.  While this spec doesn't require applying +11V directly to the IC pins, it is not unusual to see clamped overshoots reaching +6 or +7V at the IC pins ... not only in this test, but in real operation when sharing a PCI bus with a 5V card.

Regards,
Andy