FreeLists / si-list / [SI-LIST] Re: matching within 1 mil

["Jack Olson" <pcbjack@xxxxxxxxx>]

oh geez, I really feel like a bozo (again)
I see what you mean now,
and thank you for your patience.

(this is probably the third time I've made a
fool of myself on this list. maybe its time
to get a fake name? ha....)

Thanks again,
bozo


On 6/4/07, Scott McMorrow <scott@xxxxxxxxxxxxx> wrote:
>
> Jack
> There is a considerable difference between the measured length of two
> traces and the electrical delay of two traces.  This is what I was
> getting at.  When you force an autorouter to try and meet a tight
> matching constraint, it is looking at the problem in the physical
> domain, and not the electrical.  If the router places a serpentine, or a
> jog, or "blip" in the line to make the matching constraint correct in
> the physical domain, it will almost always not be correct in the
> electrical domain, because of coupling terms that were not incorporated
> into the router's delay algorithms.  For example, autorouters
> occasionally use structures like this, to add delay:
>
>                        ___
>                        | |
> ________________________| |_____________________
>
>
> Please excuse my lousy email art.  That's a straight trace with a little
> loop "blip" that is often used to add delay.  The router would then say
> that this trace met it's specified delay exactly.  However, the
> electrical delay error inherent in this "blip" could be anywhere from
> 0.5 to 2 ps, depending on how close the blip traces were routed.  This
> is equivalent to anywhere from 5  to 10 mils of error.  Thus if the
> router were to have placed 10 of these structures into the trace to
> match it to other traces, there could easily be a 5 to 20 ps delay
> error.  However, if the router were to have been given 50 mils latitude,
> it might very well have been able to route that trace without any
> serpentines at all, and actually been better matched.
>
>
> Scott
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
>
> http://www.teraspeed.com
>
> Teraspeed(r) is the registered service mark of
> Teraspeed Consulting Group LLC
>
>
>
> Jack Olson wrote:
> > Yes, I agree.
> >
> > I just thought two things about this discussion were humorous:
> >
> > 1) complaining that a designer is giving better performance than you
> > asked for,
> > not believing him when he says it is no extra trouble, and wanting to
> > "educate" him because of it
> >
> > 2) complaining that a constraint should be set looser because the
> > inherent error is looser.
> > If the router says they match EXACTLY and you take the time to measure
> > a pad width and discover it is really off by 2mils, setting a rule for
> > 500mils is still going to have a range of error about the same. So
> > what is the harm in letting the router shoot for perfection? All you
> > will get is the inherent error and no slop.
> > All I'm saying is, if it is important enough to mention that you want
> > a match within 500mils, you shouldn't complain if the router gives you
> > less than 100. silly.
> > but on a tight design where the router is struggling to give you what
> > you asked for (which is USUALLY the case),
> > your points are right on. (the "if its no trouble" phrase is the key)
> >
> > ok, back to "lurking" mode,
> > Jack
> >
> >
> > On 6/4/07, *Scott McMorrow* <scott@xxxxxxxxxxxxx
> > <mailto:scott@xxxxxxxxxxxxx>> wrote:
> >
> >     Jack
> >
> >     Unfortunately, the things that an autorouter does to "easily"
> >     route to 1 mil trace matching can sometimes end up with a solution
> >     that has more real delay than a larger constraint might.  I have
> >     no problem letting the autorouter do it's absolute best.  But I
> >     find, more often than not, after looking at 100's of designs, that
> >     autorouters have a tendency to do some really wacky things that
> >     frak the electromagnetics around pins and pads.
> >
> >     One of the things that autorouters do not innately know, is that
> >     for differential pairs, matching should be accomplished
> >     incrementally across the entire pair length, not as something that
> >     occurs only at one of the ends.  Invariably, if the router
> >     performs matching at the ends, where it is easier, 1/2 of the
> >     time, it will be on the receiving end, which is the worst place to
> >     perform matching, since it guarantees that a common mode signal
> >     will propagate across the entire path length.
> >
> >     When it comes to wide bus matching, I've seen Mentor's
> >     lost-Expedition router do some wonderfully "clever" serpentines
> >     and jogs to achieve matching, that are absolutely guaranteed to
> >     create 10's of ps of delay.
> >
> >     Scott
> >
> >     Scott McMorrow
> >     Teraspeed Consulting Group LLC
> >     121 North River Drive
> >     Narragansett, RI 02882
> >     (401) 284-1827 Business
> >     (401) 284-1840 Fax
> >
> >     http://www.teraspeed.com <http://www.teraspeed.com/>
> >
> >     Teraspeed(r) is the registered service mark of
> >     Teraspeed Consulting Group LLC
> >
> >
> >
> >
> >     Jack Olson wrote:
> >>     I don't know what software you use, but
> >>     in Mentor's AutoActiveRE constraint editor,
> >>     its easier to type "1" than it is to type "100"
> >>     That's two less keystrokes, bub!
> >>
> >>     Do all the math you want, but if the router
> >>     can EASILY route to a tighter constraint,
> >>     LET IT.
> >>     If it has trouble, THEN you can loosen the belt.
> >>
> >>     It doesn't really seem worth "venting" about.
> >>
> >>     If anyone has constructive ways of educating
> >>     engineers not to out-think a computer, I'd like
> >>     to hear them
> >>
> >>     regards,
> >>     Jack (a layout guy)
> >>
> >>
> >>     On 6/3/07, Bill Wurst <billw@xxxxxxxxxxx> <mailto:billw@xxxxxxxxxxx>
> wrote:
> >>
> >>>     <snip>
> >>>
> >>>     Today, we now have some interfaces where 100mils is no longer
> adequate.
> >>>     I know that because I take the time to go through the math, and
> will
> >>>     specify what I believe is appropriate given all of the other
> variations
> >>>     that can affect skew.  Here in lies the frustration:  more often
> than
> >>>     not, the layout designer will come back and say to me, "I know you
> only
> >>>     needed this matched to XXmils, but it was just as easy for me to
> match
> >>>     it to 1mil, so that's what I did."  Now, I have a hard time
> believing
> >>>     that it didn't involve a lot of extra work to get down to 1mil,
> but I'm
> >>>     not about to do his job for him nor do I wish to micro-manage
> him.  In
> >>>     all other respects, these folks are excellent at what they do, but
> this
> >>>     typical response makes me wonder why I went through the trouble of
> >>>     figuring out a more practical number in the first place.  Judging
> from
> >>>     the various responses, I'm not alone.  And I know that while the
> tool
> >>>     reports the lengths as matching to within 1mil, there may be as
> much as
> >>>     a few mils difference within the pad itself.  I know because I've
> sat
> >>>     down with designers and together we've discovered this.  (As an
> aside,
> >>>     it would be nice if CAD tools could report trace length minus the
> trace
> >>>     segments (or portions thereof) buried in pads.)
> >>>
> >>>     Mainly I'm venting and not looking for a response, but if anyone
> has had
> >>>     similar experiences and can think of constructive ways of
> "educating"
> >>>     layout designers, I'd like their opinions.
> >>>
> >>>     Regards,
> >>>         -Bill
> >>>
> >>>
> >>>           /************************************
> >>>          /         billw@xxxxxxxxxxx <mailto:billw@xxxxxxxxxxx>
> /
> >>>         /                                   /
> >>>        / Advanced Electronic Concepts, LLC /
> >>>       /           www.aec-lab.com <http://www.aec-lab.com/>         /
> >>>       ************************************
> >>>
> >>>
> >>     ------------------------------------------------------------------
> >>     To unsubscribe from si-list:
> >>     si-list-request@xxxxxxxxxxxxx <mailto:si-list-request@xxxxxxxxxxxxx>
> with 'unsubscribe' in the Subject field
> >>
> >>     or to administer your membership from a web page, go to:
> >>     http://www.freelists.org/webpage/si-list
> >>
> >>     For help:
> >>     si-list-request@xxxxxxxxxxxxx <mailto:si-list-request@xxxxxxxxxxxxx>
> with 'help' in the Subject field
> >>
> >>
> >>     List technical documents are available at:
> >>                     http://www.si-list.net <http://www.si-list.net/>
> >>
> >>     List archives are viewable at:
> >>              http://www.freelists.org/archives/si-list
> >>     or at our remote archives:
> >>              http://groups.yahoo.com/group/si-list/messages
> >>     Old (prior to June 6, 2001) list archives are viewable at:
> >>                      http://www.qsl.net/wb6tpu
> >>
> >>
> >>
> >
> >
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
>                http://www.si-list.net
>
> List archives are viewable at:
>                http://www.freelists.org/archives/si-list
> or at our remote archives:
>                http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>                http://www.qsl.net/wb6tpu
>
>
>


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu