[SI-LIST] Re: matching within 1 mil
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: silist <si-list@xxxxxxxxxxxxx>
- Date: Mon, 04 Jun 2007 19:37:43 -0400
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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