I think Bob Davis has done a great job of clearly articulating the
reasons why IC I/O protection diodes should not be relied upon as
terminations.
The recent advent of IBIS models is a wonderful thing, but there's at
least one drawback: many of these models contain I/V curves for
clamp diodes. It's all too easy to throw together a simulation, see
an acceptable result, and forego resistive termination, never
realizing that it's the (poorly-characterized, vendor-dependent)
clamp diodes that have cleaned up the signal. At any rate, it's
something to watch out for.
Bob Perlman
Cambrian Design Works
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