[SI-LIST] Re: delay vs. transmission line length
- From: "Tabatchnick, Justin" <justin.tabatchnick@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 11 Jun 2003 19:17:29 -0700
Scott;
I don't understand how ground/power bounce will affect delay or maybe I
got it wrong. Could you elaborate ?=20
Thanks
Justin@Intel
-----Original Message-----
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]=20
Sent: Wednesday, June 11, 2003 7:02 PM
To: Raymond.Leung@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: delay vs. transmission line length
All,
Okay, I'm tired of hearing about Report generation software now, so=20
instead of discuss something that I truly know nothing about I'll=20
respond to this thread.
Hmmm, if an ideal transmission line is perfectly terminated in it's=20
characteristic impedance, the length of the transmission line does not=20
"matter" to the driver. As Art points out, the transmission line looks=20
resistive to the driver. What the original poster did not state was=20
whether the transmission line was terminated or not, and whether it was=20
a lossless or lossy transmission line. In addition, he did not state=20
whether the driver was simulated by itself, or with the associated=20
device package, and if simulated with the package, whether package=20
coupling was included. =20
Now, if the transmission line is open ended, I would expect that the=20
measured Tpd would be reduced as the transmission line becomes shorter=20
and shorter, since the reflection will add to the output voltage of the=20
driver, causing a seemingly faster transition through the threshold=20
w.r.t. the driver input stimulus. If the driver is driving an ideal=20
transmission line with perfect termination, I would expect absolutely no
change in Tpd at all, and, in fact, delay can then be measured into a=20
pure resistive termination. The only times I've ever seen the sort of=20
Tpd relationship that Yoni described are in two cases. First, where a=20
power delivery SSO problem is this issue, with some sort of power/ground
bounce causing delay changes. Or second, where there is a package and=20
the package has high reverse crosstalk, and this reflected reverse=20
crosstalk causes an apparent increase in Tpd when the device drives and=20
unterminated load. But, I am only guessing.
Seems to me that in order to answer the original question and not=20
speculate, one needs to know the complete story.
regards,
scott
--=20
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
Come visit us see our new=20
Teraspeed IBIS Model Library=20
and IBIS Reference Page
Raymond.Leung@xxxxxxxxxxx wrote:
>
>Art,
>
>No one would object to your analysis about T-line, after the popularity
>of the "black magic handbook". However, the issue is talking about the
>loading effect to the driver that the delay within the driver will be
different
>when the T-line length is changing. In view of this theory you can say
>there is no added "C load" in short T-line, yet the EFFECT of cap
loading
>is apparent to the driver, as analised both by you and Jon.
>
>Raymond
>
>
>
>Art Porter wrote:
>
>Well, that's the definitive behavior of a transmission line. A properly
>terminated transmission line "looks" resistive (i.e. voltage and
current are in
>phase at all frequencies). If it isn't properly terminated, it still
"looks"
>resistive at the time of the incident wave. The driver doesn't "see"
the
>reflection of whatever is at the end of the transmission line until a
time
>equal to twice the delay of the transmission line. A "short"
transmission line
>doesn't have any inherent added "C load." With a short transmission
line, there
>is less time between the incident edge and the reflection from the C
(or
>whatever else) at the end of the transmission line. If the length of
the
>transmission line is short compared to the transition time of the edge,
then
>it's difficult to distinguish the incident edge from the reflection.
That is
>the source of the "rule of thumb" that you can model transmission lines
as
>lumped elements if their length is <1/5 (or 1/3, or 1/10, depending on
which "a
> uthority" you prefer) of the transition time.
>
>Art
>
>-----Original Message-----
>From: Raymond.Leung@xxxxxxxxxxx [mailto:Raymond.Leung@xxxxxxxxxxx]
>Sent: Tuesday, June 10, 2003 6:20 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: delay vs. transmission line length
>
>
>
>
>
>I think it is more or less like a resistive load seen by the driver
>when the T-line is long enough. As what Jon has described
>below, the C load of a short line would cause longer Tpd.
>
>Raymond
>
>
>
>
>"Jon Powell" <jonpowell@xxxxxxxxxxxx> on 11/06/2003 01:07:31
>
>Please respond to jonpowell@xxxxxxxxxxxx
>
>To: yonitz@xxxxxxxxxxxxx, "Si-List (E-mail)" <si-list@xxxxxxxxxxxxx>
>cc: (bcc: Raymond Leung/sdc)
>
>Subject: [SI-LIST] Re: delay vs. transmission line length
>
>In a lot of ways this relates to a previous question on how to measure
>time-of-flight and relate that properly to the CLK->Q data in a static
>timing program. Since the actual CLk->Q (Tpd or whatever) of the device
is
>dependent on load, the datasheets will spec it into a specific load and
then
>it becomes the job of the SI and Timing tools to figure out how to
properly
>change that data for accurate total path timing. It used to be that
most
>drivers were spec'd into 50pf loads because that happened to be the
loads on
>the chip testers. Now days 50pf is so far away from the real load that
that
>spec isn't good enough for many purposes.
>As other people have pointed out, if the load is closer to the driver
than a
>round-trip, the C of the load is seen by the driver and can change the
>transition rate of the driver, which effectively changes Tpd. If the
load is
>farther away than a round-trip it is effectively non-existent to the
driver
>(at least starting from steady-state) and can have no effect on Tpd.
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On
>Behalf Of Yoni Tzafrir
>Sent: Tuesday, June 10, 2003 5:45 AM
>To: Si-List (E-mail)
>Subject: [SI-LIST] delay vs. transmission line length
>
>
>hi,
>i run some simulations, for measuring Tpd from input of the of the =3D
>driver to the output of the driver.=3D20
>
> |\
> | \driver transmission line
> input------------| =3D
>\---------------------------------------------------LOAD
> | /out
> |/
>
>I have noticed that the longer my transmission line, the Tpd becomes =
=3D
>shorter.
>does it make sense? as i understand it a longer transmission line means
=3D
>more capacitance (and resistance) so the Tpd should be longer, doesn't
=3D
>it?
>
>Yonatan (Yoni) Tzafrir
>*Tel: (972) - 3 - 7552300 (T/L: 351)
>*Fax: (972) - 3 - 6177130
> *Mobile: (972) -54- 459469
>*E-mail: <mailto:yonitz@xxxxxxxxxxxxx>
>
>
>
>
>
>
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