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RE: 66MHz or 33MHz PCI Bus



Michael richardson wrote:

>   If M66EN is high, then the system may run the bus at up to 66Mhz.
>   If M66EN is low, then the system may only run the bus up to 33Mhz.
> 
>   The system in both cases may run the bus at any speed it wants. It can
> run a system that says it is capable of 66Mhz, and run it at 12Mhz if it
> wants to.
 
Not quite.  While the system can run at <33MHz even though other components
are 66MHz-capable, the system MUST pull M66EN low in that case.

>From section 2.2.7 of the PCI Spec, revision 2.2: "M66EN  The 66MHZ_ENABLE
pin indicates to a device whether the bus segment is operating at 66 or 33
MHz. Refer to Section 7.5.1. for details of this signal's operation."  Note
that it says "is operating at."  It doesn't say "is capable of."  The pin
indicates the actual operation of the bus.

>From section 7.5.1: "The 66 MHz PCI clock generation circuitry must connect
to M66EN to generate the appropriate clock for the segment (33 to 66 MHz if
M66EN is asserted, 0 to 33 MHz if M66EN is deasserted)."  Note that it must
use a 33 to 66 MHz clock if M66EN is asserted.  It cannot run at 12 MHz.

And: "If a 66 MHz PCI agent requires clock speed information (for example,
for a PLL bypass), it is permitted to use M66EN as an input."

Correcting your first statement:

  If M66EN is high, then the system may only run the bus in 66 MHz mode
(between 33 and 66 MHz).

Regards,
Andy