RE: 3.3V PCI

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

> We are developing new 64 bit/66 MHz PCI boards on which most of the
> components rely on 3.3/2.5V supplies. Due to the speed and density of
> these products, I am very reluctant to derive these supplies from the 5V
> rail. However, I cannot find any motherboards with more than two 3.3V
> PCI slots. Would anybody care to comment on the approach they are taking
> with their current product development (requiring 3.3V supplied by the
> host).
 
I hope you are aware of the difference between 3.3V signaling and the
presence of 3.3V power.  According to PCI rev. 2.2, all PCI slots must
provide 3.3V power rails, whether they use 3.3V signaling or 5V signaling.
Thus, all modern systems should have 3.3V power available at every PCI slot.
This was not true under PCI rev. 2.1 (pre- Dec. 1998), so many older systems
lacked connections to the 3.3V power rails.

33 MHz PCI slots that use 3.3V signaling haven't appeared in large
quantities because of the chicken-and-egg problem.  Until most PCI adapter
cards are "Universal", it was risky for motherboard vendors to introduce
systems that are physically and electrically incompatible with older, 5V
(non-Universal) PCI cards.

Unfortunately, the state of 66 MHz PCI is really sad.  A number of
motherboard vendors apparently chose to willfully violate the PCI Spec by
building their 66 MHz PCI bus using 5V signaling!  Some others did follow
the spec.  You can tell by checking the keying of the connectors they used
in the 66 MHz positions.  So beware: your 66 MHz PCI board might not work in
some systems that supposedly have 66 MHz PCI slots.  Don't believe it when
someone tells you that 66 MHz slots are 3.3V slots.  The Spec says so, but
some people didn't follow the Spec.

When 66 MHz is available, generally you can get only one or two slots per
bus.  This is for timing and electrical (signal integrity) reasons.  In
fact, the PCI Spec (section 7.7.7 footnote 54) implies that only one
connector might be possible at 66 MHz.  Very careful design was hopefully
used in those systems that provide two of them.  (Well, I can be
optimistic!)

Regards,
Andy