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

[Scott McMorrow <scott@xxxxxxxxxxxxx>]

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

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> 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         /
>>     /                                   /
>>    / Advanced Electronic Concepts, LLC /
>>   /           www.aec-lab.com         /
>>   ************************************
>>
>>     
>
>
> ------------------------------------------------------------------
> 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