[SI-LIST] Re: No need of termination within critical length??
- From: Knut Georg Wiljugrein <kwiljugrein@xxxxxxxxxxxxxxxxx>
- To: "Sonu Kurian N.D" <sonukurian@xxxxxxxxxxxxxxx>
- Date: Thu, 17 Sep 2009 09:47:45 +0200
Hi Sonu,
The reflections are still there. However, the critical length is defined
from the rise/fall time of the driver, such that the round trip time for
the reflection becomes short compared to the rise/fall time. This means
the driver only changes its output voltage a fraction of the full swing
during the time of a reflection round trip. The short round trip time
means reflections die out faster than the signal rises/falls, and the
small change in output voltage during the round trip time means the
initial reflection is small (compared to the full signal swing). Thus
for traces shorter than the critical length the reflections are small
enough and die out fast enough to be ignored.
In theory, the reflections would make a small "ripple" on the
rising/falling edge of a signal. This would not be good for a edge
sensitive receiver, like a clock input, but I think with real
transmitters/receivers the parasitics of the components will smooth this
out to produce a monotonically rising/falling edge. Could someone else
comment on this last point?
Regards,
Knut Georg Wiljugrein
Sonu Kurian N.D wrote:
> Hi All,
>
> I am puzzled about why termination is not required if trace length is
> less than critical length.
> I read like if a driver encounters a reflected signal when it is
> rising(driving) , its able to absorb reflection.Is it so?
> Or is the situation like ..the reflections will be there..but the Setup
> condition can be met..so we need not worry..??
> Can anyone please throw some light on this..
>
> Many thanks,
> Sonu
>
>
>
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