Re: Is 3.3v supplied by motherboards?

["Andy Ingraham" <ingraham@wrksys.ENET.dec.com>]

> I am designing an expansion card that uses a mix of 5v and 3.3v parts. My
> questions are:
>
> 1) Should I derive 3.3v through a regulator on my board or is 3.3v
available
> from the motherboard?
>
> 2) If 3.3v is available, is it available on all motherboards or only
some? 
>
> 3) If 3.3v is available, which type of connector do I use? 5v, 3v, or
universal?

Section 4.3.4.1 in the PCI spec version 2.1 makes the +3.3V power supply
optional in systems with a 5V PCI bus.  Because it might not be present,
you should derive it on your card.

Both +3.3V and +5V power supplies must be present in systems with a 3.3V
PCI bus.  But 3.3V PCI busses may not exist yet (except for "Mobile PCI",
with a different form factor).

The type of PCI connector you use depends entirely on the kind of
electrical interface you provide to the PCI bus.  If your interface chip
has the right drive and input capabilities and meets both 5V and 3.3V
bus specs, you can use the UNIVERSAL connector.  It has nothing to do
with the power used by your board.

Regards,
Andy Ingraham


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