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

[Scott McMorrow <scott@xxxxxxxxxxxxx>]

Jack
I probably was not as clear as I could have been.

We're all Bozo's on this bus!

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® is the registered service mark of
Teraspeed Consulting Group LLC



Jack Olson wrote:
> 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/>         /
>>>>>       ************************************
>>>>>
>>>>>
>>>>>           
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