RE: How to determine the speed of the PCI bus attached?

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

> Is there any bits in the configuration space that we
> can read to determine the speed of the PCI bus that a
> device is attached to? The 66MHz Capable Bit (Bit 5 in
> the Status register)from what I understand only
> showing the capability of the device as a 66MHz device
> but the PCI bus it attached not necessary to be
> running at 66MHz. So is there any way that we can
> determine the speed of the PCI bus that a PCI device
> is attached to?
 
Use the M66EN signal, pin 49B on the PCI connector.

See section 7.5.1 in the PCI specs, revision 2.2.

The M66EN pin tells "whether the bus segment is operating at 66 or 33 MHz"
(which really means 33-66 MHz mode or 0-33 MHz mode).  If any device on the
bus, including the host, is not 66MHz-capable, M66EN is grounded, and the
bus will be in 0-33 MHz mode.

"If a 66 MHz PCI agent requires clock speed information ... it is permitted
to use M66EN as an input."