[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Control line pull-ups on mixed 3v/5v system

> I am designing a system which will support either 3V or 5V PCI adapter cards (not at the same time).
 
Interesting.  Do you intend to manufacture your system boards with 5V-keyed connectors in one case, and 3.3V-keyed connectors in the other?

I hope you don't use non-standard connectors without the physical "key" that can accept either kind of card.  There is no such thing in PCI as a "Universal" motherboard or system board.  That would be problematic (nothing to prevent the customer from plugging a 3.3V card and a 5V card into the same bus at the same time!).

> On-board devices are 3.3V, 5V tolerant.  
 
If they are 5V tolerant, that implies they don't clamp above 3.3V, and are therefore inappropriate for 3.3V PCI.  To meet the PCI spec for 3.3V signaling, devices MUST clamp above 3.3V (below 5V).

Hopefully these devices have programmable clamps that can be set according to the type of connectors that manufacturing put on the board.

> Which rail should the control lines be pulled to?  
 
They should pull to VCC, which is 5V +/- 0.25V in one case, or 3.3V +/-0.3V in one case in the other.  Again, a function of which connector was used.

> More simply, will a 3V only device have problems if the control lines are pulled to the 5V rail and vise-versa? 
 
Yes.  A 3V-only device with pull-ups to 5V would be continually in clamp mode when signals are high.  While this might be alright, don't count on it.  For most devices, operating that way is beyond what they define as "normal" DC operation.  Also, if the device uses an active clamp circuit, I've seen some devices that behave strangely (oscillations) under similar conditions.

It is probably OK to have pull-ups to 3V on a 5V PCI bus.  You would need to recalculate the pull-up resistor values, though; don't use the formulas and values in the Spec.

Regards,
Andy