[SI-LIST] Re: PCI at 66 Mhz
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxxxxxx>
- To: SI <si-list@xxxxxxxxxxxxx>
- Date: Thu, 30 Aug 2001 09:36:24 -0400
The PCI Specification does not define a limit on number of loads, for either
33MHz or 66MHz.
The system (motherboard) designer must make a compromise, between number of
loads, how they are routed, and the PCI clock speed at which he wishes to
run the bus.
From a practical point of view, I doubt anyone could get PCI to work at 66
MHz with eight loads. The PCI Spec does hint that it may be impossible to
have more than one PCI card per 66 MHz PCI bus, but I am aware that people
have made 66 MHz PCI work with two card slots. How well they work, I can't
say. "It ain't easy."
PCI-X at 66 MHz is easier to do than PCI at 66 MHz.
> Take your load capacitance, spec sheet and then it is simple.
I doubt it's that simple. The bus is not a very good approximation to a
lumped capacitive load. The actual topography (whether devices are on the
motherboard or on plug-in cards, which means stubs, etc.) makes a difference
too. PCI buses tend to have some amount of ringing, and the ringback often
sets the maximum Tprop.
Regards,
Andy
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