RE: PCI - decoupling of unused 3.3V connector pads

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

> "All unused 3.3 volt and 5 volt connector pads must
> be individually attached to a 0.01 uF high speed
> capacitor.... Several connector pads can share a
> capacitor if trace requirements are met".
> 
> For this design, I am actually not using the 3.3 volt
> from the PCI connector.  However, the above
> implies that I should still decouple these pads.
> 
> Is that correct?  
 
Yes.


> If so, what is the reasoning behind
> needing to decouple those pads if I am not using
> the 3.3 volt supply on this card?
 
Because the PCI Spec says so!

And the reason behind it, is that all the ground AND power pins in the
connector provide the high-speed signal return path for all the signal pins
on the bus.  If you don't decouple the 3.3V pads, it is just like
eliminating a bunch of GND pins from the connector.  They might not be
needed to handle the DC current, but they are ALWAYS needed for the AC
(switching) current.

If you check the PCI connector pinout, you'll see that almost every signal
pin is adjacent to a power or ground pin.  If you don't bypass the 3.3V
pins, you create an area with many fewer power/ground pins, and many signals
that are not adjacent to a power or ground pin anymore.  When all those
signals switch, the return current has no easy path; it may couple into the
other signals (causing crosstalk) or take a lousy, inductive path (causing
ground bounce at the board level).

Andy