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RE: 3.3V power rail / M66EN

Please read the section of the 2.1 spec again.  It does NOT say the 
system MUST supply 3.3V.  Here is the section you refer to:

"...Systems implementing the 5V signaling environment MAY either ship the 
3.3V supply with the system, or provide a means to add it afterward..."

What IS required is the bussing on the system board and decoupling caps on 
the expansion card of the 3.3V power connections.  NOT the actual supply.

From: 	jurry@csw.L-3com.com
Sent: 	Tuesday, June 20, 2000 10:53 AM
To: 	pci-sig@znyx.com
Subject: 	RE: 3.3V power rail / M66EN

Section 4.3.4 of the 2.1 spec states that systems MUST supply 3.3V.  What 
optional is the 3.3Vaux power.  This 3.3Vaux is supplied on pin 14A and is
used for powering logic that needs to remain active when the rest of the
system is unpowered (see the PCI Bus Power Management Interface
Specification).  The reason you may find motherboards that do not provide
3.3V is simply that older versions of the PCI spec did not require the 

John Urry

> -----Original Message-----
> From:	Austin Franklin [SMTP:austin@darkroom.com]
> Sent:	Tuesday, June 20, 2000 7:25 AM
> To:	'pci-sig@znyx.com'
> Subject:	RE: 3.3V power rail / M66EN
> 1) Does really every motherboard have a 3.3V power supply connected to
> the PCI connectors?
> I understand section 4.3.4 of the spec that every motherboard must
> provide 3.3V to the connectors.
> [Austin] My understanding is no, 4.3.4 of the 2.1 spec says it is
> 'basically' optional, but decoupling caps must be provided none the less.