RE: Capacitors on Power Pins

[David Black <DavidB@anchorchips.com>]

On Monday, September 07, 1998 10:45 PM, William Benner wrote, in part:
> ...
> I thought that I had read somewhere that each PCI plug in board must have
> capacitors on the power pins, even if these pins are not used by that
> particular board.  ...

Page 153 of the PCI 2.1 spec mentions coupling requirements to ground, even 
if they are not delivering power.  Also, there is a line for documenting 
this in the PCI Compliance Checklist which is in the public section of the 
SIG's website at http://www.pcisig.com/docs.html

> Another question, while designing my "Universal" 3V/5V board, I believe that
> the so called VIO pins will go unused by my design because I am using a 3.3V
> PCI chip that is 5V tolerant. I believe that the Gods that put together PCI
> did not anticipate such "5V tolerant but 3.3V powered" devices when they
> created the spec. Can I simply leave these VIO pins unconnected, or must I
> connect them and use a capacitor?

Actually, the gods (or whoever was standing around at the time) did put a 
footnote in on this subject.  It is footnote 37 on page 153.  From what I 
understand of your design, you may use the 3.3V rail to power your chip's 
I/O buffers.  You don't need to use the VIO pins.  However, the VIO pins, 
used or unused, still need decoupling per the spec.

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David Black               Anchor Chips Incorporated
davidb@anchorchips.com    www.anchorchips.com
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