[SI-LIST] Re: Matching within 1 mil is just plain sillyness
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
- To: <jseeger@xxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 1 Jun 2007 13:46:10 -0600
Jeff et al -
Anytime I go under a 100 mils, my CAD guys want an explanation. And they
don't want theory. I match lengths according to degrees of phase at the
frequency of operation. The max I go is 4 to 5 degrees of phase. At 1GBs,
one UI(bit width)is 1 nS. For FR4, Er = 4.0, 1 deg = 33 mils. 3 deg ~ 100
mils. The maximum frequency for 100 mils matching would be ~ 875MHz.
Anthing faster than that I would match at less than 100 mils. I think this
whole area is fertile ground for research, since it seems to be pretty
arbitrary in the literature.
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Jeff Seeger
Sent: Friday, June 01, 2007 12:38 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Matching within 1 mil is just plain sillyness
Thank you, Scott and Lee, for defending the physical designer.
For decades now, we've been asked to meet rules like 1 mil, 10 mils,
100 mils. I often dig further and find we're working with 1 nS edges
or skew budgets in 100s of pS.
Unlike the machine shop, we can't laugh as the next guy will simply
put the take-up wire length under the pad.
And FWIW when arranging skew on a complex topology, what sounds like
"free" on one path increases difficulty, layers, or both by orders
of magnitude on wide busses or between large, densely arrayed parts.
I find this "free" effort is usually coming out of my hide. Maybe
there are in-house groups where this effort is intentionally funded?
--
Jeff Seeger Applied CAD Knowledge Inc
Chief Technical Officer Tyngsboro, MA 01879
jseeger "at" appliedcad "dot" com 978 649 9800
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