Re: Questions about PCI

[Ivor Bowden <ivor@peritek.com>]

At 06:16 AM 8/25/00 GMT, you wrote:
>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? 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 doing a system motherboard you can put a fixed freq clock on it.
They you can use your pll clock driver no problem. If you are making an add
in card you can not use a pll clock driver and be compliant. Unless 66MHz
only.

>2. According to PCI spec. , any unused +5V and 3.3V pins on the PCI edge 
>connector provide an AC return path, and must be coupled to ground plane. 
>THE DECOUPLING MUST AVERAGE AT LEAST 0.01 microF PER VCC PIN. What does this 
>means? Does it means there is no need for a decoupling capacitor for each of 
>the 3.3V pin(there are 6 3.3v pins on each side) as long as the average is 
>0.01.  I have looked at some PCI add-in cards, there only have two 
>decoupling capacitors to decouple the 3.3V pins.

It is best to put a cap next to each pin. You could also bus the pins
together and use a few caps. But the bus should be at least .02" and
the distance to the cap less than .25". The idea is to have
a low impedance AC path to ground to maintain signal integrity.
See v2.2 sec 4.4.2.1

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

Don't connect the pins to ground! If you are making a mother board, +12
(.5A) and -12 (.1A) are required. (v2.2 sec 4.3.4.1) If an add on card,
you can leave pins floating.

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