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Re: Live PCI interface in a powered down system

Chas Horvath <chas@isis.com>  writes:
| We'd like to design a PCI board that has power applied to it even when
| the rest of the system it is in in powered off.  The board would be
| responsible for a variety of system management functions including
| powering the rest of the system on and off.  It would be connected to
| an external power source and an external terminal.
| Does the PCI spec address how a powered up PCI interface should behave
| in a powered down PCI bus?  Two areas of concern are:
| 1) low level electrical considerations, e.g. can the drivers handle this
|    situation
| 2) PCI protocol considerations, e.g. will an interface seeing a powered
|    down PCI bus go off into the weeds because all PCI inputs are in
|    bogus state (all inputs low).
| Our software would ensure we are not trying to use the interface when
| the system is powered down.
| In glancing at the 2.1 PCI spec, PLX PCI9060 data sheets, and Intel
| 80960RP data sheets the issues above are not addressed.  I assume that
| means we are out of luck.

The suggestion of FET switches might be the way to go, but I question the
ability to stick extra components between a "normal" PCI interface chip
and the connector and keep everything in spec. I'm not saying it can't
be done, but you'd start to have to worry about low-level signal integrity
and timing things that you might not be able to get enough info on.

Another, probably simpler option might be to use a generic interface 
chip (I saw one advertised recently that claimed to be a way to recycle
ISA designs, by having a simple ISA-like bus on the backside) and put the
FET switches on the backside of the PCI interface. If you want a PCI bus
on your side of the card interface, then put the FET switches in *your*
PCI bus. Since you control all of the componentry on the card side, you'll
have a better chance of controlling the specs. And you draw a big line
across the card for "my power" and "your power".

Another alternative might be the chipsets intended to extending PCI
busses to docking stations. These have to deal with one side or the other
not being powered up when the device is plugged. Not exactly what you
have in mind, but it does include some of the same problems.

Jeff Carter
Interware, Inc.
451 Great Road
Bedford MA