From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Thu Jan 06 2000 - 08:32:42 PST
>> >> The "VCC5REF" node is suspicious ... what would this be for in
>> >> 3.3V PCI signaling environment?
>> >High-side clamping. 5-v tolerant 3.3v devices need to bias the N-well
>> >to 5 volts to keep the parasitic drain-well diodes of the P-channel
>> >from forward-biasing. (The gate logic behind them is -- ahem! --
>> Well, then this IS a "5V" PCI device (despite making use of 3.3V power),
>> the 0.8V, 2.0V input thresholds ARE the correct ones.
>Not really. ...
If it has a way to not clamp to 3.3V on a 5V bus, AND to clamp to 3.3V on a
3.3V bus, then it's OK, and I guess it should have TWO different IBIS spec
sheets covering the two cases.
But if it is simply 5V-tolerant, all the time, then it can't be used on a
3.3V bus. I thought that was the case with this part (with its VCC5REF pin
... unless that pin is part of an adjustable clamp and is connected to the
PCI bus's +Vi/o pins).
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