RE: [SI-LIST] : non-monotonic

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Fri Mar 30 2001 - 08:14:59 PST


> Can someone explain the cause of non-monotonicity. Is it caused by the
> change of impedance in the transmission medium? At times, I have also
> seen
> non-monotonic signals at the output pins of driver ICs.
 
Non-monotonic signals often happen at output pins.

In the ideal case, if a driver's output impedance roughly equals the trace
characteristic impedance and there are no inductive effects, and there is
only one trace and it is point-to-point, ideally one would expect to see a
plateau in the middle of each edge. That is, it rises (or falls) only half
way, then sits there at Vdd/2 for twice the propagation delay of the trace.
Then it goes the rest of the way. This is the ideal source terminated case.
The waveform at the far end has no such plateau.

Add some inductance to the picture, and the waveform at the driving IC might
bounce a little around the plateau.

Many other things can also cause the otherwise flat plateau to move in the
wrong direction.

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