PCI-X Bus loading in Pf

[Brent Barr <b.barr@f5.com>]

I have been unable to find a simple number that tells me how strong of a
driver to use in my ASIC for PCI-X. The old rule of thumb was to use a 5
load driver. Does that change for PCI-X? The spec gives me a bunch of shaped
curve rules, but the vendor just wants to know what the load is. They are
not about to create a custom I/O cell to perfectly match those curves for
us.

What is a realistic capacitive load to assume for a PCI-X bus?  Since at 133
MHz it is only a single slot, my guess is 8pf for the other end, 4pf for the
slot, plus the 8pf at the source puts me at about 20pf.  At 100Mhz with two
slots I get about 32pf.

Are those numbers realistic?


Brent Barr