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

[steve weir <weirsi@xxxxxxxxxx>]

Jack, while it is impossible to out calculate a computer, expertise is 
what we use to outperform results one gets from push button reliance on 
simplified assumptions built into most tools.  The first thing to know 
when deciding where to put resources is how much different trade-offs 
matter.  We are hardly alone in this view.  Two things that you will 
repeatedly hear from people like Eric Bogatin and Lee Ritchey is the 
importance of understanding how much changing a given parameter 
matters.  Eric spends a great deal of time in his book and in a number 
of his on-line seminars demonstrating this with real numbers.  When it 
comes to the differential trace matching, 1 mil is massive overkill.  If 
it really comes for free, great.  For reasons that have kind of gotten 
beat to death here, there are any number of cases where it isn't.  I 
think it is very wise of Bill to understand the metrics of this and 
other parameters in his designs so that he can make intelligent 
decisions when and if he needs to make trade-offs.

Regards,


Steve.

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         /
>>   ************************************
>>
>>     
>
>
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