FreeLists / si-list / [SI-LIST] Re: overshoot and undershoot different swing

["Tom Dagostino" <tom@xxxxxxxxxxxxx>]

I think Rajeev's question has to do with the amount of overshoot seen =
and
whether or not the falling overshoot is supposed to be the same as the
rising overshoot.  Rajeev is seeing on his scope display different =
amounts
from the rising and falling waveforms. =20

Overshoot is caused by energy in the input waveform stimulating a tuned
circuit.  If there is more energy in the falling waveform than the =
rising
then the falling edge will display more overshoot.  The faster the
transition time the more energy in the higher frequencies. =20

The clamp devices in the net will have the effect of limiting the =
overshoot.
For many inputs there are both power clamps and ground clamps.  These
usually have similar characteristics so the input will limit the =
overshoot
sort of symmetrical.  In many cases there is a ground clamp but no power
clamp. In these cases the falling overshoot will be clamped if it is =
large
enough and the rising overshoot will not be clamped.

Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx=20
www.teraspeed.com=20

Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On
Behalf Of Jim (James) Antonellis
Sent: Tuesday, April 03, 2007 10:44 AM
To: rajeev.kommera@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: overshoot and undershoot different swing


Rajeev,

I see a few situations to consider:

1) ESD
2) Timing
3) Fidelity

The ESD structures typically dictate what is allowed for
over and under shoot and they are not necessarily identical=3D20 even =
after
polarity has been taken into account. Some of the=3D20
(better?) specifications will provide an area_under_the_curve=3D20
specification for the over/under shoots.

In regards to timing and fidelity, these will be application dependent. =
For
example, one example bus may allow for a lot of settling time before
sampling and as such will tolerate the time that the over/under shoot =
does
not meet the logic=3D20 threshold. Kind of the same thing with fidelity =
except
you=3D20 need to be careful of false transitions that may occur on=3D20
clocks/strobes if a sequence of underdampened over/under- shoots occur =
(may
be referred to as underdamped ringing in=3D20 some circles).=3D20

Jim


-

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janton@xxxxxxxxxxxx

Broadcom Corp www.broadcom.com

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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of rajeev.kommera@xxxxxxxxx
Sent: Tuesday, April 03, 2007 1:36 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] overshoot and undershoot different swing


Hi all,

We are having 76 MHZ clock output from FPGA in our design.
I am seeing on oscilloscope that the overshoot and undershoot is not =
same.

I want to know whether it is necessary that overshoot and undershoot =
values
needed to be same? If yes then what factors effects the values to be
different?


Regards,
Rajeev.
Failing to Plan is Planning to Fail



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