[SI-LIST] Re: Vih for PCI
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 10 Jul 2002 18:39:44 -0400
> Just curious - How are the two different input voltage treshold levels
> managed in the chip when it's 3.3V and 5V compatible? Does the pin
> have actually two input cells?
Having two input cells is not necessary, especially when you realize
that the 3.3V and 5V PCI threshold regions overlap. It could be done
with two input cells, but that would be overkill.
The Vih(min) spec means, Thou Shalt Recognize Voltages Greater Than
Vih(min) As Being High. (It doesn't say that you can't also recognize
some voltages below Vih(min) as being High.)
For 5V PCI, Vih(min) is 2.0V. If you designed an IC that recognizes
everything greater than 1.5V as High, it satisfies the 5V PCI spec too,
because input voltages greater than 2.0V are also greater than 1.5V. So
it is actually better than the spec for 5V PCI. Nothing wrong with
being better.
It's also good enough for 3.3V PCI.
Taking this a step further, if your IC also had an actual Vil(max) of,
say, 1.1V, then it would satisfy both the 5V and 3.3V PCI specs
simultaneously.
Therefore, you can use a single input cell for both 3.3V and 5V PCI, and
you don't even need to make the threshold or switching point
programmable.
If the threshold levels didn't overlap this much, it would still be
possible to do it with a single input cell, by making use of the +Vi/o
voltage to set the input buffer's switching point.
Regards,
Andy
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