M66EN and PRSNT signals

[Anand.Kuriakose@smartm.com]

Hi,

In sec 4.3.7 of PCI spec 2.2, we see that PRSNT1# and PRSNT2# signals have to be
individually decoupled with 0.01uF caps on the motherboard. It also says that
one or both of these pins provide an AC return path.

Again sec 7.7.7 says that we have to provide a 0.01uF decoupling cap for the
M66EN pin near the connector to provide an AC return path. Also the same is
repeated for the expansion boards that are 66-Mhz capable.

1>   Why these pins, PRSNT and M66EN, need this unique treatment?

2>   Also I do not understand how these pins provide AC return path?  when we
have power and ground planes on the motherboard and the expansion board     as
well and sufficient decoupling caps on these planes, we have taken care of
return paths.

Thanks in advance.


Regards,
Anand Kuriakose,
Design Engineer,
Force Computers (I) Pvt. Ltd.,
Bangalore.