RE: VIO pin connection

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

> Is it Ok to connect VIO pins of a 3.3v ASIC
> that has 5v input tolerant PCI pads to connect
> to +5v supply (through a resistor) instead of
> VIO pins of a PCI connector.
 
Since this ASIC has 5V tolerant PCI pads, it must be a 5V-only component
(which happens to be powered by 3.3V).  It should only be used in a 5V
Signaling Environment, and must never be used on a 3.3V PCI board or a
Universal PCI board.

Therefore, the +VIO pins of the PCI connector ARE +5V (in fact should be
physically bussed together with the +5V pins on the 5V card), so they are
the same.

Such an ASIC is not recommended for new designs, as all new designs ought to
use dual-voltage buffers and Universal plug-in cards.

(If I have misunderstood what you meant by "5v input tolerant PCI pads",
then the answer may be different.  A PCI pad that can work in a 3.3V
Signaling Environment must have overshoot clamps that conduct when signals
go much above 3.3V.  While such an ASIC might "tolerate" +5V on its pins
without blowing up, it would have to sink something like 100mA at each I/O
pin that is brought to +5V.  In my mind, that doesn't meet the requirements
of being 5V tolerant.)

Regards,
Andy