RE: [SI-LIST] : Input switching threshold & CPCI

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Wed Jan 05 2000 - 09:52:09 PST


>> However, are you certain this model is for a part with 3.3V signaling
>> levels? It's possible that this part is designed for "5V" switching
levels
>> but is powered by 3.3V (has 5V-compatible 3.3V outputs and 5V-tolerant
>> inputs). The "VCC5REF" node is suspicious ... what would this be for in
a
>> 3.3V PCI signaling environment?
>
>High-side clamping. 5-v tolerant 3.3v devices need to bias the N-well up
>to 5 volts to keep the parasitic drain-well diodes of the P-channel devices
>from forward-biasing. (The gate logic behind them is -- ahem! -- exotic.)
 
Well, then this IS a "5V" PCI device (despite making use of 3.3V power), and
the 0.8V, 2.0V input thresholds ARE the correct ones.

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