RE: [SI-LIST] : Parallel Termination in Theory and Practice

About this list Date view Thread view Subject view Author view

From: lborbely (lborbely@micron.com)
Date: Tue Aug 01 2000 - 14:03:17 PDT


Scott,

How do they show that programmability in their IBIS models?

Lesley Borbely
Micron Technology

> -----Original Message-----
> From: Scott McMorrow [mailto:scott@vasthorizons.com]
> Sent: Tuesday, August 01, 2000 1:12 PM
> To: abe riazi
> Cc: 'si-list@silab.eng.sun.com'
> Subject: Re: [SI-LIST] : Parallel Termination in Theory and Practice
>
>
> Abe,
>
> There are some very nice buffers now available in some ASIC vendor's
> families which have programmable parallel termination. You can
> program the termination to have either a 50 ohm terminator or
> not on inputs
> and program drivers to have either a 50 ohm or 25 ohm source
> impedance.
> Both allow you to tailor the driver to end or middle of line
> conditions.
>
> regards,
>
> scott
>
>
> abe riazi wrote:
>
> > Dear Chuck and Scott:
> >
> > Thank you very much for your interesting replies.
> >
> > Another important point related to topic of termination is
> appraisal of internal vs. external termination. The internal
> termination appears to offer advantages of eliminating the
> need for external stubs and components. But this is achieved
> at the price of sacrificing flexibility. For instance an IC
> having internal series termination eliminates the need for an
> external series resistor and the associated stub. But the
> disadvantage is that series termination is suitable only for
> certain routing topologies (such as point to point routing)
> and not the right for choice for other types of topologies
> (such as daisy-chain ). Similar reasoning applies to
> internal parallel termination. Variations in the
> characteristic impedance of the trace can be also better
> dealt with by use of external terminators.
> >
> > What are your thoughts on this subject ?
> >
> > Thanks again.
> > Abe
> >
> > -----Original Message-----
> > From: Chuck Hill [SMTP:chuckh@altaeng.com]
> > Sent: Tuesday, August 01, 2000 6:19 AM
> > To: Scott McMorrow; abe riazi
> > Cc: 'si-list@silab.eng.sun.com'
> > Subject: Re: [SI-LIST] : Parallel Termination in
> Theory and Practice
> >
> > All,
> >
> > Very good points from Scott. I'd like to add in regard to
> point #4 that
> > the stubs are also capacitively and inductively loaded. So
> the stubs can
> > resonate at frequencies far lower than the 1/4 or 1/2
> wavelength value.
> > When these resonances are lower in frequency than the bit rate, they
> > contribute to pattern dependent jitter (as described in point #1).
> > Remember that two capacitively (or inductively) loaded
> stubs can resonate
> > together as the transmission line performs an impedance
> transformation.
> >
> > "Critical length" is used as a decision criterion to
> separate a lumped
> > parameter circuit from a distributed parameter circuit
> model. Scott's
> > comments address the limitations due to dispersion, loss in the
> > transmission line. My comments raise the issue of
> distributed circuit
> > effects appearing much lower in frequency than would be
> expected with
> > oversimplified modeling.
> >
> > Since there are many other considerations, I apply the
> critical length
> > approximation with great care.
> >
> > Chuck
> >
> > At 01:32 AM 8/1/00 -0700, Scott McMorrow wrote:
> > >abe riazi wrote:
> > >
> > >> Scott:
> > >>
> > >> The critical length Lc (rather than a specific
> frequency or rise
> > time) is often used as a yardstick for distinguishing lumped and
> > distributed circuit elements and for setting acceptable
> limits on stub
> > lengths.
> > >>
> > >> Does concept of critical length break down at
> frequencies above 1GHz?
> > >
> > >Abe,
> > >
> > >
> > >Critical length is an interesting approximation. The
> evaluation of what
> > >is "critical" depends upon how much error can be tolerated in the
> > >result when one is using a lumped circuit approach over a
> distributed
> > >one. There is always error when using lumped circuits to model
> > >any sort of waveguide. For example, according to Christopoulous
> > >in "The Transmission-Line Modeling Method TLM", page 24, if
> > >lambda/10 is used as the size for lumped elements, there is
> > >still almost a 2% error in the propagation delay of the
> circuit over
> > >the true distributed circuit. The breakdown comes in the size of
> > >errors we can tolerate at high frequencies and the effects which
> > >are masked by oversimplified modeling.
> > >
> > >Several things happen with high frequency signaling:
> > >
> > >1) the period is reduced, increasing the chances of intersymbol
> > >interference occurring because of discontinuities in the line.
> > >(i.e. - ringing and jitter spill over into the next bit
> period.) This
> > >translates into less overall margin.
> > >
> > >2) the edge rate is increased to support the higher signaling rate
> > >which increases the bandwidth of the signals.
> > >
> > >3) the increased bandwidth of the signals causes a subsequent
> > >increase in sensitivity of the circuit to discontinuities.
> > >
> > >4) the increased bandwidth of the signals can excite stubs into
> > >operation at quarter wave resonances. ( large packages
> > >like BGA's make for very nice stubs with a large discontinuity
> > >at both ends. A capacitive discontinuity at the die and a
> > >Z to Z/2 mismatch at the pin breakout when the device is
> > >placed on a line terminated at the far end. This structure
> > >forms a very nice resonator if excited with a high edge rate
> > >source.) Hmmm ... I wonder what might happen at say ...
> > >400 MHz with 800 Mb/s signaling on a bus with a single
> > >parallel end terminator and one BGA driving another? This
> > >might form two resonant circuits ... one from device to
> > >device and the other from trace to package. Like this:
> > >
> > >
> > >BGA ----------------------------------terminator
> > > | (package resonance)
> > > BGA
> > >
> > >|<- resonant circuit ->|
> > >
> > >
> > >5) these quarter wave resonant stubs can perturb signals
> > >causing excessive jitter.
> > >
> > >6) Multiple stubs on a single line with nearly similar resonant
> > >frequencies can form high frequency bandpass filters which
> > >actually amplify the resonances. This will greatly increase
> > >signal jitter and can cause high bit error rates which are
> > >pattern sensitive. (If multiple devices of the same type and
> > >package are daisy chained on a parallel terminated line
> > >then it is most likely that the package interconnects have
> > >nearly the same resonant frequency. This greatly increases
> > >the chances of something bad happening.)
> > >
> > >7) Resonant points of all circuits involved can change
> > >due to even and odd mode coupling to neighboring circuits.
> > >This makes it even more interesting to diagnose and track in
> > >operating systems.
> > >
> > >8) Unbalanced data coding as used in most computer systems
> > >will cause large average DC level variations dependent upon the
> > >data pattern being transmitted. These DC level variations
> > >translate into decreased eye margin for differential signals
> > >and increased timing jitter (skew) for non differential signals.
> > >
> > >9) A capacitor is not just a capacitor any more ... and this
> > >includes die capacitance. Since all include some physical
> > >length of interconnect to get to the capacitance there is a
> > >delay and an associated inductance. Ignore the inductance
> > >and the nifty little trace width impedance compensation circuit
> > >that you might design will not work so nicely.
> > >
> > >10) There are little capacitors everywhere ... especially in
> > >device pads and pad stacks. These little capacitors reflect
> > >quite a bit of "stuff" when hit with fast edges. Removing
> > >excessive capacitance in layouts removes a lot of excessive
> > >jitter ... which is just a by product of "stuff" reflecting.
> > >
> > >These are some of the interesting effects at high frequencies that
> > >can be easily ignored when moving up from SI engineering at lower
> > >frequencies. The guys who have experience doing RF and Microwave
> > >work have been used to these effects for years.
> > >
> > >A frequency domain sweep will uncover unwanted resonances
> > >quite nicely ... and help to better understand what is happening
> > >in the time domain. This is where transmission line simulators
> > >like XTK, SpectraQuest, ICX and Hyperlynx fall short. These can
> > >all do a good job of simulating in the time domain at high
> frequencies
> > >but can't perform simple AC sweeps. An AC sweep will often
> > >explain why a circuit won't perform beyond a particular frequency
> > >or why jitter can rise to unacceptable levels.
> > >
> > >This was a long winded answer to a simple question.
> > >I hope it helps.
> > >
> > >best regards,
> > >
> > >scott
> > >
> > >
> > >--
> > >Scott McMorrow
> > >Principal Engineer
> > >SiQual, Signal Quality Engineering
> > >18735 SW Boones Ferry Road
> > >Tualatin, OR 97062-3090
> > >(503) 885-1231
> > >http://www.siqual.com
> > >
> > >
> > >
> > >
> > >**** To unsubscribe from si-list or si-list-digest: send e-mail to
> > >majordomo@silab.eng.sun.com. In the BODY of message put:
> UNSUBSCRIBE
> > >si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
> > >si-list archives are accessible at http://www.qsl.net/wb6tpu
> > >****
> > >
> >
> > **** To unsubscribe from si-list or si-list-digest: send e-mail to
> > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
> > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
> > si-list archives are accessible at http://www.qsl.net/wb6tpu
> > ****
> >
> > **** To unsubscribe from si-list or si-list-digest: send e-mail to
> > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
> > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
> > si-list archives are accessible at http://www.qsl.net/wb6tpu
> > ****
>
> --
> Scott McMorrow
> Principal Engineer
> SiQual, Signal Quality Engineering
> 18735 SW Boones Ferry Road
> Tualatin, OR 97062-3090
> (503) 885-1231
> http://www.siqual.com
>
>
>
> **** To unsubscribe from si-list or si-list-digest: send e-mail to
> majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
> si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
> si-list archives are accessible at http://www.qsl.net/wb6tpu
> ****
>

**** To unsubscribe from si-list or si-list-digest: send e-mail to
majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
si-list archives are accessible at http://www.qsl.net/wb6tpu
****


About this list Date view Thread view Subject view Author view

This archive was generated by hypermail 2b29 : Wed Nov 22 2000 - 10:50:56 PST