RE: Questions about PCI

[Richard Walter <rwalter@Brocade.COM>]

> From: Jim Chan [mailto:tan264@hotmail.com]
> Sent: Thursday, August 24, 2000 11:16 PM
>
> Hi guys
>
> I just have a few questions regarding PCI bus.
>
> 1. Is there any way to fix the PCI frequency? According to the spec, 
> components designed for use on the system motherboard may operate at any 
> single fixed frequency up to 33 MHz, and may enforce a policy of no 
> frequency changes. How could we do this?

Put a line in the datasheet (hopefully in a prominent place) that reads:
"This device only operates at a fixed freqency and does not support
frequency changes." so that the motherboard designer using your part
knows not to do that.  (Or knows not to pick your part if he needs to
do that.)

> I am trying to drive the PCI clock  with a zero delay PLL clcok driver,
> so i need to make sure the PCI frequency doesn't change.

If you are trying to do this on an add-in card, you are out of luck; it
is not PCI compliant.

<snip question 2, which I'll allow others to answer, since I'm a motherboard
guy and have never done a plug-in card design.>

> 3. Related to Question2, what about the +/- 12 V pins.Do they need any 
> decoupling, or i just connect them to ground.

Whether you decouple or not, I hope that you're not going to connect +/-
12V to ground !  That would be bad.  If your card doesn't use +/- 12V,
then I'd say that you don't need to do anything to them; just leave the
fingers as no connects.  They are at the end of the connector, next to
some JTAG signals and isolated from the high-speed signals by several pins
of power & gnd, so I'd say that the amount of AC switching current that
runs through them to be negilgible.  Also, they should be decoupled on
the motherboard.  If you do use them, then you ought to decouple them near
their load.

> Thank you for your time
>
> Regards
>
> Jimmy tan
>

-Richard Walter
Hardware Engineer
Brocade Communications Systems
rwalter@brocade.com
Note: I speak for myself, not for Brocade.