PCI 3.3V Supply Rail Tolerance (resend)

[Richard Blackborow <richard@alacritech.com>]

I suspect my original request for help got lost in the post-holiday
catch-up, since I got no replies.  Can anyone please help: (sorry for
the length of this):

The PCI 3.3V supply rail tolerance is specified as +/- 0.3V (Table 4-10,

Version 2.2).  This equates to about +/- 9% tolerance on the 3.3V rail
(compared with +/- 5% on the 5V and 12V rails).

As more PCI board designs migrate to components using 3.3V power, it
would be nice to run them directly from the 3.3V rail, but there is a
problem if the components can only tolerate 5% on their supplies.  One
solution is to regulate down from 5V, but this costs more, and is
wasteful of power if linear regulators are used.

In the PC world, it may be that the problem does not exist in practice,
since the ATX power supply specifies the 3.3V output at +/- 4% (see
ftp://download.intel.com/design/motherbd/atx_201.pdf ).  In fact, if
this specification for a 3Com Server NIC is to be believed:
http://www.3com.com/products/nics/3cr990svrsp.html , at least one PCI
product intended for the PC market has been developed that runs off
3.3V, but requires a tighter spec than +/- 9%  (+/- 5%, in this example)

My questions are:

1. What are platform manufacturers (PC and others such as Sun) actually
providing for the tolerance of their PCI’s 3.3V rail?

2. How do people view the approach that 3Com is apparently using (i.e.
claiming PCI 2.2 compliance, but specifying a tighter 3.3V tolerance)?
Can anyone confirm that they are actually doing this?

3. Is anyone aware of other brand-name, high-volume PCI products that
run off 3.3V and require a tighter tolerance than the PCI spec?

4. Is anyone aware of a movement to change this tolerance spec in the
future?

Thanks,

Richard