[SI-LIST] Re: 22.1 Ohm vs 50 Ohm Termination
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: <ikhan@xxxxxxxxxxxxxxxxxxx>
- Date: Fri, 30 Aug 2002 15:27:59 -0400
Ibrahim,
Overshoot can be an electrical/reliability problem if it is extreme, but
rarely causes the wrong signal state to be received.
Undershoot (not reaching the full swing, whether it is on a high-going
or low-going edge) can cause actual signaling problems on clocks if the
undershoot knee or plateau is anywhere in the input "threshold" region.
If a clock signal plateaus within the threshold region, it could make
the clock input receiver switch more than once, the result being double
clocking. Or, it might just make the clock input receiver not switch
until after the plateau is done, which means more delay (more clock
skew) than you may have counted on.
Of course if you have overshoot, you also want to watch out for
ringback. A little overshoot without much ringback, may help keep your
edges clean.
Note that I am defining overshoot to be going beyond the final or
steady-state voltage, in either direction. Some people (Intel, for
example) use "undershoot" to mean the same thing as others (including, I
believe, IEEE) would call "overshoot" on falling edges.
Regards,
Andy
> ----------
> What is the reason for the clock to be able to tolerate the overshoot
> but not the undershoot.
> Thanks
> Ibrahim
>
>
>
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