Hi all,
I agree with Remco, you always want to change something after the
autorouter has finished. The "DWIT" command (do what I'm thinking) still
has not been implemented .... :-)
But I want to speak in favour of Swiss cheese, not only because of its
excellent taste.
It is true that an autorouter like e.g. SPECCTRA will not generate these
parallel, evenly spaced bus structures on dense boards that you find on
many hand-routed boards (especially PC motherboards, where you have all the
space of the world for routing), which results in far less vias than what
an autorouter will produce. But is this really desirable? There are pros
and cons for both approaches. In very high speed design, every via creates
a tiny reflection on the signal that may be intolerable with signal
frequencies of several 100 MHz and above, whereas with frequencies below
100 MHz this will probably not be a critical issue. On the other hand,
parallel bus structures can cause crosstalk problems hard to debug, which
will simply not be there if the board was autorouted "like Swiss cheese".
Critical signals like fast clock traces or fast line interfaces I always
route manually before invoking the autorouter. Of course also these signals
can be routed satisfactory by a good autorouter, but it is necessary to
define very precise rules for every critical signal and having purchased
the router option needed to enable it to follow these rules (FST option
with SPECCTRA, this option being about the same price as the complete
router package including all other options).
Gisbert Auge
N.A.T. GmbH
"Remco v/d Heuvel"
<R.vd.Heuvel@fusion-elect An: "Protel EDA Forum"
<[EMAIL PROTECTED]>
ronics.nl> Kopie:
Thema: Re: [PEDA] Autorouting or
manual routing, or both?
22.01.2002 08:20
Bitte antworten an
"Protel EDA Forum"
Dear Matt,
Normally I only route the boards by hand (4 & 6 layers) , my expierence
with
autorouting utillities (not only Protel) is that you still want to change
tracks which you don't like so you will wind up re routing the whole board
:)
The only thing i sometimes use the autorouter for is routing the databus on
the pcb, only if there aren't any other tracks routed or else you get your
swiss cheese...
Remco van den Heuvel.
Hardware Engineer.
Please feel welcome to visit our website at:
http://www.fusionelectronics.org
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----- Original Message -----
From: Matt Polak <[EMAIL PROTECTED]>
To: Protel EDA Forum <[EMAIL PROTECTED]>
Sent: Monday, January 21, 2002 4:51 PM
Subject: [PEDA] Autorouting or manual routing, or both?
>
> Hey folks,
>
> It seems that a majority of you are doing some very dense, high-speed
> layouts with 4-6 layers being quite a common occurrence. I'm just
wondering
> how much you typically route by hand, and how much you let the
auto-router
> whack away at.
>
> Being primarily self-taught in the ways of Protel, and with the help of a
> few 'older school' engineer friends here and there, I've done a number of
> successful design layouts thus far, but these have been relatively simple
2
> and 4 layer designs without many small-pitch/high pin-count devices. I'm
> moving more towards laying out more high-speed designs in the near future
> where a lot of stuff needs to be fit into a small place, and all connect
> together without traces and vias meandering all over.
>
> When I look at sample six layer boards (such as the 5407 EVM reference
> design Motorola has released) the bussing and interconnects are extremely
> elegant and efficient in appearance. For fun, I unrouted the 5407 board
and
> then let the autorouter chew on it. It immediately made 'via
swiss-cheese'
> out of the board and created little more than a large mess. I'm GUESSING
> quite a bit of these sort of designs are laid out by hand, or at least
> pre-routed to give the auto-router a sense of direction?
>
> Can anyone offer some basic pointers to getting started into planning and
> laying out PCBs for multi-layer, high-speed designs such as these? I have
> no idea where one would really even start with something like this. It
> seems most of the important knowledge gets passed directly from engineer
to
> engineer; there are certainly no university classes (that I know of,
> anyway) that teach you how to lay out a dense, 6 layer board. :/ I
> appreciate the patience and wisdom of those who remember once being where
I
> am now, who are willing to take the time to pass some of the tips and
> tricks down the engineering family tree.
>
> Thanks again for any pointers anyone can provide (either through the
list,
> or privately.)
>
> Best regards,
> -- Matt
>
>
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