[SI-LIST] Re: Non-monotonic

Oops!  I just realized you said that the non-monotonicity happens at the
SDRAM (apparently IC#3, already at the end of the trace).

The non-monotonic waveform there could be due to at least two causes.
One, if the package delay of the SDRAM is significant enough compared to
the risetime, and if you observe at its IC pin, it might be slightly
non-monotonic there while in fact it is monotonic at the die pad.
That's because the IC pin isn't the end of the transmission line.  There
is no way around this except by slowing down the driven edge rates.  On
the other hand, maybe the non-monotonicity doesn't really matter.  What
truly matters (to the IC) is the waveform at the die pad, which in this
case is clean.  Difficult to measure, but easy to see with a simulator.

The other cause is that the Flash IC and/or its stub (which IS a
transmission line, by the way, just not a terminated one) causes enough
of a reflection to make a glitch in the waveform.  You might try adding
series isolating resistors at the point where the Flash IC taps off the
main signal trace; it may or may not help.  Or, it might help to move
the Flash to the end of the trace.

Some ICs have a heavy enough electrical load that they cause
non-monotonic edges almost no matter what you do (short of slowing down
the edgerates a lot).

Andy

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