FreeLists / si-list / [SI-LIST] Re: Matching within 1 mil is just plain sillyness

["Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>]

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

------------------------------------------------------------------
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