FreeLists / si-list / [SI-LIST] Re: delay vs. transmission line length

["Jon Powell" <jonpowell@xxxxxxxxxxxx>]

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 =
driver to the output of the driver.=20

                |\
                | \driver               transmission line
     input------------|  =
\---------------------------------------------------LOAD
                |  /out
                |/
I have noticed that the longer my transmission line, the Tpd becomes =
shorter.
does it make sense? as i understand it a longer transmission line means =
more capacitance (and resistance) so the Tpd should be longer, doesn't =
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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