[SI-LIST] Re: Matching within 1 mil is just plain sillyness
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Jeff Seeger" <jseeger@xxxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Sat, 2 Jun 2007 11:26:05 -0700
Jeff,
This is why I responded. I, too, have seen engineers impose constraints on
layout that make life very difficult without any detectable performance
payoff. As I'm sure you have seen, often it comes down to "I'm the
engineer and I said so" rather than showing that the constraint is founded
in good engineering. Imagine using this approach to design a mile long
suspension bridge! That's the reason there is a photo of the Golden Gate
Bridge on the cover of both my books. We electrical engineers ought to be
at least as disciiplined as the structural engineers who design bridges.
Sadly, that is too often not the case. Just visit some of the absurd rules
of thumb that keep being circulated about EMI control. 20H rule, etc.
Lee Ritchey
> [Original Message]
> From: Jeff Seeger <jseeger@xxxxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 6/1/2007 12:12:04 PM
> 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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