RE: Universal PCI signaling

[Andrew Ingraham <Andrew.Ingraham@digital.com>]

>Does anyone know if it is possible for a PCI chip to be completely
>compatible with both 5V and 3.3V PCI signaling if it has absolutely no
>connection to a 5V source and no external glue logic?  If so, is there
>one commercially available?

Yes it is possible, depending on how strict you are about "no connection
to a 5V source."

You must change the overshoot clamping between 3.3V and 5V environments.
The clamping level could change, or it could be simply disabled for the
5V environment.

On a PCI expansion board, the only way to tell which environment you are
in, is by way of the "+Vio" pins on the PCI connector.  These pins have
+5V when the bus uses 5V signaling.  So technically, if you connect +Vio
directly to the chip, you would have the system's +5V source connected
to it.  Or you could use external glue logic to sense +Vio and have it
signal the chip.

Intel's (formerly DEC's) PCI chips with dual-voltage capability, come
close.  They use 3.3V power only and require no 5V power.  But there is
that connection to +Vio to control the overshoot clamps.  On these
chips, it is a low-current input pin, not a power pin.

Regards,
Andy Ingraham