RE: 33/66MHz operation.

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

> I have designed a PCI expansion card which utilises an Oxford
> Semiconductor
> OX9160 bridge chip. This is a 33 MHz device, fully compatible with version
> 2.2 of the PCI Spec. The PCI card itself has the 5V footprint. According
> to
> the specification, this makes my card incompatible with 66MHz operation.
> They have only ever been installed into PCs with 33MHz PCI buses.
> 
> However, inadvertantly, my colleagues have intalled my boards into PCs
> with
> 66MHz buses. The boards are now not operational. Is it likely that
> installing my boards into a 66MHz bus would fry the PCI chips?
 
It is possible, but I can't say if it's likely.  If the Vcc (+Vi/o) pins on
your card were connected to +3.3 V, and your chip was designed only for +5
V, there is a chance it might have done something bad to it.  Or, if your
chip was fed a 66 MHz clock, it might have been over-clocked to the point of
failure.

In theory, it should have been impossible to physically plug your board into
a 66 MHz PCI bus slot, because all 66 MHz PCI buses are supposed to be 3.3 V
only and the motherboard connectors should be keyed to only accept 3.3 V or
Universal plug-in cards.  However, some motherboards were made which had one
or more "66 MHz" slots that had the wrong keying ... a clear violation of
the PCI spec.  Maybe your board was plugged into one of those non-compliant
systems.  All bets are off when designers take such liberties.

Regards,
Andy