RE: [SI-LIST] : Diff clocks length matching

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From: Bill Owsley (owsley@cisco.com)
Date: Thu Apr 12 2001 - 07:26:49 PDT


The purpose of the end result may have an effect on the requirement for
matched length.
For SI, +/- 5% may be close enough for a decent eye pattern.
For EMI, that may still be an emission problem.
ps. Can you get your simulator to show the effects on the sum of the diff
signals for mis-matched pairs ?
- Bill

At 07:50 PM 04/11/2001 -0700, Larry Miller wrote:
>I do not disagree with what you said. However, there are those who say
>that we see things the way we do because that is what we are looking for
>in our methodology (rational empiricism or The Scientific Method).
>
>And there are many who say that "unreasonable people are those who change
>the world; change is not brought about by reasonable people" (i.e., those
>who think "outside the box"). This is probably true, but I have heard this
>argument used to justify an awful lot of bull. (This particular length
>match requirement seems to fit into that category IMHO.)
>
>Since we had digressed to philosophy, I waxed philosophical <grin>.
>
>Larry
>>-----Original Message-----
>>From: Sainath Nimmagadda [mailto:sainath@lsil.com]
>>Sent: Wednesday, April 11, 2001 6:32 PM
>>To: Signal Integrity
>>Subject: Re: [SI-LIST] : Diff clocks length matching
>>
>>It appears that I was mistaken by some. The message was about the laws
>>of physics. No reference to a particular situation. I did not imply
>>that physical laws change. All I meant was the possibility of
>>new/additional laws (which do not violate existing laws) as we understand
>>science.
>>
>>Sainath
>>
>>Larry Miller wrote:
>>>I think that pushing the envelope is OK if appropriate. I certainly
>>>never expected to be running digital logic signals around at microwave
>>>frequencies-- cheaply at that.The operative word is "appropriate".Most
>>>physical laws have not changed within my memory. Certainly not these,
>>>anyway.As a physicist friend of mine at Cambridge once said, "the more
>>>we measure the speed of light the more constant it gets".I do think
>>>there is a lot of BS physics in the last 20 years. I don't believe in
>>>any of these so-called demonstrations of hyper-light signal information
>>>propagation (every test setup looks "rigged"). And I think that most of
>>>the popular explanations of chaos theory are bunk written by people who
>>>don't seem to realize that nonlinear finite-state systems naturally have
>>>limit cycles, which is NOT new and certainly nothing like the "magic"
>>>ascribed to it. Anyone who has had to deal with digital servo systems
>>>has contemplated this phenomenon for many hours. (BTW, I was an alpha
>>>tester along with Mr. James Gleick on Word For Windows <1.0.)See
>>><http://www.badscience.com/>http://www.badscience.com/.Larry
>>>-----Original Message-----
>>>From: Sainath Nimmagadda
>>>[<mailto:sainath@lsil.com>mailto:sainath@lsil.com]
>>>Sent: Wednesday, April 11, 2001 12:19 PM
>>>To: Signal Integrity
>>>Subject: Re: [SI-LIST] : Diff clocks length matching
>>>Hey Larry,
>>>
>>>Don't you think we keep adding new laws to physics as we push the
>>>envelope. I believe that pushing the envelope happens when somebody
>>>*insists* on something. Don't you agree with me?
>>>
>>>Sainath
>>>
>>>Larry Miller wrote:
>>>>...and when I was in the professional audio business (Ampex in its Days of
>>>>Glory) I had guys tell me they could hear 30 kHz.
>>>>
>>>>0.002" = 0.3 picoseconds with your own calculator (which I use frequently,
>>>>thanks!). I don't believe you can measure that in a video system, much
>>>>less
>>>>see any effects from it. How much of a pixel is that in any analog system,
>>>>let alone a digital one that is clocked?
>>>>Rich Hollywood nuts may demand it and even pay for it, but it doesn't mean
>>>>it is necessary. Personally, I stand by the laws of physics, not psychics.
>>>>
>>>>Regards,
>>>>
>>>>Larry Miller
>>>>
>>>>-----Original Message-----
>>>>From: Doug Brooks [<mailto:doug@eskimo.com>mailto:doug@eskimo.com]
>>>>Sent: Wednesday, April 11, 2001 8:55 AM
>>>>To: Larry Miller; si-list@silab.eng.sun.com
>>>>Subject: RE: [SI-LIST] : Diff clocks length matching
>>>>
>>>>Actually we had a customer once that wanted a large number of bus's ALL
>>>>matched to within a spec like that. The bus's were for a matrix of video
>>>>signals. The argument was that you couldn't measure the differences in
>>>>time, but you could SEE it on the display matrix. It cost them a lot in
>>>>time and in layers to get that, but they insisted it was important.
>>>>
>>>>Doug Brooks
>>>>
>>>>At 07:06 PM 4/10/01 -0700, you wrote:
>>>> >2 mils (0.002") is a ridiculous spec unless you are operating at 200
>>>> GHz.
>>>> >
>>>> >0.1" is close enough for 2 GHz at anything like normal propagation
>>>> >velocities (in the vicinity of 5.6" per ns).
>>>> >
>>>> >Search on polar.com to get tutorials on differential pairs or search on
>>>>Eric
>>>> >Bogatin or just search on differential pairs. National Semi has quite a
>>>> >tutorial in their LVDS literature, You have the right idea, but you
>>>> are not
>>>> >looking at the correct parameters. When you find it, show it to whomever
>>>> >asked for 0.002" phase matching!
>>>> >
>>>> >Larry Miller
>>>> >
>>>> >
>>>> >-----Original Message-----
>>>> >From: AA [<mailto:alokbya@yahoo.com>mailto:alokbya@yahoo.com]
>>>> >Sent: Tuesday, April 10, 2001 6:34 PM
>>>> >To: Todd Westerhoff; Kim Helliwell; Anthony Davidson
>>>> >Cc: 'Dunbar, Tony'; si-list@silab.eng.sun.com
>>>> >Subject: [SI-LIST] : Diff clocks length matching
>>>> >
>>>> >
>>>> >
>>>> >This questions concern routing fast differential
>>>> >clocks pairs from a clock driver to chipset. The pair
>>>> >is series terminated and the termination resistor set
>>>> >near the clock driver. I learned that the pair needs
>>>> >to be length matched to within 2 mills.
>>>> >- First questions is which trace do we need to length
>>>> >match is the one between the chipset and the
>>>> >termination resistor or is the entire trace length
>>>> >(between clock drive and chipset) and why?
>>>> >- Second it was suggested that the spacing between
>>>> >the pair should be set to 12 mills ( a multiplier of
>>>> >the distance to the GND plane). This was based on
>>>> >simulation. How does one come up with the ideal
>>>> >spacing and what factors impact this? I thought
>>>> >separating the differential pair to far would be
>>>> >counter intuitive since it impacts their common mode
>>>> >noise rejections? But then getting them to close may
>>>> >present cross talk issues!!
>>>> >
>>>> >Your input is very much appreciating it.
>>>> >
>>>> >Adan
>>>> >
>>>> >--- Todd Westerhoff <twester@hhnetwk.com> wrote:
>>>> > > "So I think the accuracy issue is illusory."
>>>> > >
>>>> > > I like that. Truer words were never typed ;-).
>>>> > >
>>>> > > The argument of IBIS vs. HSPICE is a recurring one.
>>>> > > While I think there is
>>>> > > a lot of substance to it, I also think the whole
>>>> > > issue is incredibly
>>>> > > over-hyped. We're talking about analog analysis
>>>> > > after all; error is
>>>> > > inherent. It *cannot* be avoided, and therefore,
>>>> > > the real issue is keeping
>>>> > > the "accuracy" of the analysis in perspective. It
>>>> > > doesn't do you much good
>>>> > > to go after the last 1% of accuracy with your
>>>> > > simulator algorithms when your
>>>> > > models are only +/- 5% to start with. However,
>>>> > > understanding how models
>>>> > > correlate back to reality is difficult, at best.
>>>> > >
>>>> > > The real problem, I suspect, is that "HSPICE is more
>>>> > > accurate than IBIS"
>>>> > > makes for a good sound bite, and "you really have to
>>>> > > understand what you're
>>>> > > modeling, and how" doesn't. After all, modeling
>>>> > > isn't fun. Right?
>>>> > >
>>>> > > I think there are lots of places where the "SPICE or
>>>> > > not to SPICE" arguments
>>>> > > have merit. But I've also spent enough time
>>>> > > trudging through models and
>>>> > > data where the most basic things were wrong, to know
>>>> > > that until we have a
>>>> > > firm, common foundation, arguing about details
>>>> > > doesn't make much sense. And
>>>> > > guess what - we're not there yet!
>>>> > >
>>>> > > The disguised blessing with IBIS is that by
>>>> > > standardizing the model format,
>>>> > > it made it easier for users to find problems with
>>>> > > the models they use, and
>>>> > > to correlate those models against test load
>>>> > > conditions and datasheets.
>>>> > > HSPICE models, in contrast, are often encrypted and
>>>> > > have unique interface
>>>> > > requirements (control pins, voltages and slew
>>>> > > rates). Bottom line, an IBIS
>>>> > > model is a lot easier to check and use than a
>>>> > > corresponding HSPICE model.
>>>> > >
>>>> > > If you're analyzing phenomena that only a SPICE
>>>> > > model can represent, then
>>>> > > there's no choice. But I'd use IBIS as the "first
>>>> > > line of defense" in any
>>>> > > situation where I could, and only back that analysis
>>>> > > up with SPICE when
>>>> > > needed.
>>>> > >
>>>> > > My $0.01 (only worth half of a typical opinion).
>>>> > >
>>>> > > Todd.
>>>> > >
>>>> > >
>>>> > >
>>>> > > -----Original Message-----
>>>> > > From: owner-si-list@silab.eng.sun.com
>>>> > >
>>>> [<mailto:owner-si-list@silab.eng.sun.com>mailto:owner-si-list@silab.eng
>>>> .sun.com]On Behalf Of
>>>> > > Kim Helliwell
>>>> > > Sent: Tuesday, April 10, 2001 4:19 PM
>>>> > > To: Anthony Davidson
>>>> > > Cc: 'Dunbar, Tony'; 'si-list@silab.eng.sun.com'
>>>> > > Subject: Re: [SI-LIST] : Hspice: Windows vs Unix
>>>> > >
>>>> > >
>>>> > > Actually, accuracy isn't really the issue, or at
>>>> > > least not in the
>>>> > > sense your management probably means it, Anthony.
>>>> > >
>>>> > > SpecctraQuest uses a spice-like simulator, TLSIM, to
>>>> > > do the work.
>>>> > > This simulator is a derivative of SPICE (I don't
>>>> > > know which flavor),
>>>> > > and therefore has all the usual accuracy plusses and
>>>> > > minuses of
>>>> > > any SPICE, including HSPICE.
>>>> > >
>>>> > > In addition, TLSIM has a coupled transmission line
>>>> > > model, and the
>>>> > > diode and transistor models have been removed, and
>>>> > > the whole simulator
>>>> > > has been optimized for the problem it's intended to
>>>> > > solve.
>>>> > >
>>>> > > The question of whether TLSIM's coupled transmission
>>>> > > line is more
>>>> > > or less accurate than HSPICE's W-element is one of
>>>> > > the issues, and
>>>> > > I cannot quantify it, except that I've never seen
>>>> > > any reason to
>>>> > > distrust either one. From that I conclude that they
>>>> > > are probably
>>>> > > equally good.
>>>> > >
>>>> > > The real issue you face is the classical conundrum
>>>> > > of SPICE: that accuracy
>>>> > > of results depends on accuracy of the models. So
>>>> > > the issue is: what's
>>>> > > more accurate: the manufacturer's original BSIM
>>>> > > models of their buffers,
>>>> > > or their IBIS models? The answer is obvious, since
>>>> > > presumably the IBIS
>>>> > > models derive from the SPICE buffer models (almost
>>>> > > no one creates IBIS
>>>> > > models from lab measurements, you see). But IBIS
>>>> > > models can be very
>>>> > > close to the buffer models they derive from, and
>>>> > > it's possible to lose
>>>> > > very little accuracy in using them. Whereas you
>>>> > > might not be able to even
>>>> > > get the buffer model. And then you are forced to
>>>> > > use HSPICE's IBIS buffer
>>>> > > model, at which point the accuracy of the two is on
>>>> > > an even footing, and
>>>> > > it's *MUCH* harder to use HSPICE in this way than to
>>>> > > use SpecctraQuest.
>>>> > >
>>>> > > So I think the accuracy issue is illusory. If your
>>>> > > management has enough
>>>> > > confidence in you, you have a chance to educate
>>>> > > him/her/them as to the
>>>> > > realities of the situation.
>>>> > >
>>>> > > Personally, I've used SpecctraQuest a lot in the
>>>> > > last 2 years, and HSPICE
>>>> > > only
>>>> > > occasionally. I use it for 2 things: as a field
>>>> > > solver when the problem is
>>>> > > not easily expressible in terms that SQ understands,
>>>> > > and perhaps to create
>>>> > > an IBIS model when the vendor provides an HSPICE
>>>> > > buffer model but no IBIS
>>>> > > model.
>>>> > > A third possibility is when IBIS doesn't accommodate
>>>> > > a particular type of
>>>> > > buffer.
>>>> > >
>>>> > > So I think Tony has a good question, and it's also
>>>> > > not clear to me what
>>>> > > value-added HSPICE provides in your management's
>>>> > > view. There is some, but
>>>> > > perhaps not where they are looking for it.
>>>> > >
>>>> > > Kim
>>>> > >
>>>> > > Anthony Davidson wrote:
>>>> > > >
>>>> > > > Perhaps that's where I have seen your name.
>>>> > > >
>>>> > > > I am a new user to Hspice, and SpecctraQuest for
>>>> > > that matter. But the
>>>> > > > opinions of my team leaders is that the tools that
>>>> > > are able to do analysis
>>>> > > > on complete boards and board-to-board
>>>> > > interconnects are not as accurate as
>>>> > > > Hspice. And Hspice is more accurate, however, the
>>>> > > analysis of complex
>>>> > > (many
>>>> > > > connections) boards is very difficult.
>>>> > > >
>>>> > > > Note that the "less accurate" and "more accurate"
>>>> > > statements are the
>>>> > > > opinions of others and not necessarily
>>>> > > quantifiable.
>>>> > > >
>>>> > > > Anthony Davidson
>>>> > > >
>>>> > > > -----Original Message-----
>>>> > > > From: Dunbar, Tony
>>>> > > [<mailto:tony_dunbar@mentorg.com>mailto:tony_dunbar@mentorg.com]
>>>> > > > Sent: Tuesday, April 10, 2001 11:18 AM
>>>> > > > To: 'Anthony Davidson'
>>>> > > > Subject: RE: [SI-LIST] : Hspice: Windows vs Unix
>>>> > > >
>>>> > > > Hi Anthony,
>>>> > > >
>>>> > > > No, neither Nortel, nor Univ of Western Ontario.
>>>> > > Maybe you've seen my name
>>>> > > > on the list or something.
>>>> > > >
>>>> > > > The reasoning behind my question is that the
>>>> > > platform on which you're
>>>> > > > running the other tools might give some pointers
>>>> > > to the H-SPICE platform
>>>> > > > choice. Actually, since you're going with
>>>> > > SPECCTRAQuest, I don't really
>>>> > > see
>>>> > > > the need for H-SPICE for the so-called "fewer,
>>>> > > critical nets". On these
>>>> > > > nets, what is it you're looking for SPICE to
>>>> > > provide that SPECCTRAQuest
>>>> > > > can't? I'm not saying there is never room for
>>>> > > co-existance,
>>>> >=== message truncated ===
>>>> >
>>>> >
>>>> >__________________________________________________
>>>> >Do You Yahoo!?
>>>> >Get email at your own domain with Yahoo! Mail.
>>>> ><http://personal.mail.yahoo.com/>http://personal.mail.yahoo.com/
>>>> >
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>>>>.
>>>>************************************************************
>>>>Doug Brooks' book "Electrical Engineering for the Non-Degreed
>>>>Engineer" is now available. See our web site for details.
>>>>.
>>>>Doug Brooks, President doug@eskimo.com
>>>>UltraCAD Design,
>>>>Inc. <http://www.ultracad.com>http://www.ultracad.com
>>>>
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>>
>
>

----------------------------
Bill Owsley, owsley@cisco.com
919) 392-8341

Compliance Engineer
Cisco Systems
7025 Kit Creek Road
POB 14987
RTP. NC. 27709

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