[SI-LIST] Re: matching within 1 mil
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- Date: Mon, 04 Jun 2007 16:01:01 -0400
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® 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® 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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