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

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From: D. C. Sessions (dc.sessions@vlsi.com)
Date: Thu Jan 06 2000 - 12:23:28 PST


"Matt (boomer) Russell" wrote:
>
> Doesn't the 5V spec also have a clamp requirement?
> It is just a clamp to 5V.
>
> Just curious about how many preople have seen high side
> ringing problems with 5V tolerant universal buffers in
> a 5V system becuase it drives to 3.3 but the bus clamps at 5V.

That's why 5v tolerant PCI drivers have to use 3.3v pullup specs.
It keeps the ringing managable. Honking pulldowns aren't nearly
the problem because all of the devices have lossy ground clamps.

Unfortunately, Intel put most of their effort into 3.3v PCI, which
is almost nonexistent, then into 5v PCI, which is extinct, and
least of all into 5v tolerant PCI, which is almost 100% of the
market. Their own products take whatever liberties with the spec
they need in order to work, and are thus pretty well-suited to their
applications (it's not like anyone is going to refuse to buy Intel
products for lack of standards compliance.) The rest of us have to
skate a pretty fine line between complying with the letter of the
spec (which our customers insist on) and making the products work
reliably (which they unreasonably expect.) Conflicts between these
two requirements are not uncommon.

Sorry for the attitude today -- I just saw the weather forcast and
it's warm and dry. Bummer. Why can't we get some freaking SNOW?????

> >
> > 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).

-- 
D. C. Sessions
dc.sessions@vlsi.com

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