We find that that with the sub nano second edges on today's drivers,
almost every unterminated signal has an overshoot that exceeds the
maximum voltage rating on the input, if it is more than 1inch away from
the output.
As an example, I am driving with a 500ps edge into a SAMSUNG Memory
KM736V887. The SSRAM is 2 inch away from the driving chip, and my
simulation result shows an overshoot of 4.2V in a point-to-point
topology. Samsung specifies a 'absolute maximum voltage' for this input
of 3.6V.
Would you put a series termination on this net?
Those maximum voltage specs, we find in the data sheets consider a DC
voltage, but what would be the AC margin for this parameter. Of course
it is higher - but how high? Could I consider 2 times the DC maximum
voltage rating for transients as a safe and simple design rule?
I guess the question is, does anybody care about overshoot on
data/address busses before the actual sampling time? If yes, what would
you consider to be an acceptable overshoot?
Our products have to meet long term reliability requirements and I would
think latch up can not be the only failure mechanism to consider.
Any comments would be greatly appreciated
Joachim
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