RE: VIO pin connection

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

>   Are there any issues if we always clamp inputs
>   to 5v by connecting VIO of an ASIC to 5v rail
>   instead of VIO.
 
Yes.

If you plan to use this ASIC in both 3.3V and 5V Signaling Environments,
then the clamping MUST change, based on the current environment into which
the card (presumably this will go on a plug-in PCI card) is plugged.

In the 3.3V environment, if the inputs do not clamp starting above 3.3V,
then it does not comply with the PCI specs.  See "Ich" in Table 4-4, where
"Vcc" is the +3.3V supply in this case.

The only way to tell which environment a card is plugged into, is with the
+VIO pins on the connector.

Presumably, the only way your ASIC can tell which type of clamping to use,
is with its +VIO pins.

If the ASIC's +VIO pins are not connected to the PCI connector's +VIO pins,
then the ASIC won't know which environment it is being used in, and its
overshoot clamping would be incorrect.

Regards,
Andy