[SI-LIST] Re: matching within 1 mil
- From: Bill Owsley <wdowsley@xxxxxxxxx>
- To: jeff.loyer@xxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Fri, 1 Jun 2007 09:02:32 -0700 (PDT)
Jeff,
You said it much better than I was inclined to.
Thanks.
As for leading edge designs using an auto-router, the guys spend nearly as
much time working the constraint table as it would take to lay it out by hand
and then it can be done over and over. But they still get those annoying
leftover nets that auto-router chokes on, reducing them to a miniumum is their
objective.
"Loyer, Jeff" <jeff.loyer@xxxxxxxxx> wrote:
I think folks are missing Bill's point. I don't think he's saying that
the design is going to break at 2 mils routing difference, only that he
can easily attain a maximum 1 mil delta, so he uses that as his
constraint. If his CAD folks balked at that number, he'd probably
adjust accordingly and, if it got close to having significant impact,
would do a more detailed analysis.
This is very similar to what we've done for common-clock FSB signals:
tell our CAD folks to match them within 10 mils (or some number), even
though we know we can absorb more. But, 10 mils is easily attainable,
and guarantees essentially no skew due to length differences. Granted,
it's a little more work on CAD's part, but the tradeoff is I don't have
to do a grand analysis to figure out exactly how close they HAVE to be
matched, and then worry about whether my analysis was correct (and
require subsequent validation in the lab, etc.).=20
This is about compromises, and where you put your energy into in-depth
analysis. If you can easily do 1 mil spacing, no problem. It's a
little tighter than I'd generally specify, but it's not absurd, either.
If you're using an auto-router (that's surprising to me, I didn't think
leading-edge designs could use them), you probably have different
constraints.
Jeff Loyer
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