RE: [SI-LIST] : What first-routing/plane splitting?

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From: Ken Cantrell (Ken.Cantrell@srccomp.com)
Date: Wed Dec 20 2000 - 07:34:26 PST


Reality.

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Michael Nudelman
Sent: Tuesday, December 19, 2000 8:10 AM
To: Aloke Bhattacharya
Cc: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : What first-routing/plane splitting?

Aloke,

I ususlly try to do this:

1. I define "critical" groups of signals.
2. I wait 'till the placement is done.
3. Based on placement the planes shapes are defined.
4. Whatever can be nicely routed, gets routed. One can use keepouts, rules,
manual routing etc.
5. Once evrything is routed, I highlight criticals one by one and see if and
where and how they cross gaps (if any). Everything (layer location,
de-coupling etc) is evaluated and change according to my own understanding
of how a crossing to be handled.
6. You say a prayer, look in the PCB guy's eyes in such a way that he shares
your anxiety, and release the board to DFM.
7. Go to #1 and spin the sucker.

Mike.

Aloke Bhattacharya wrote:

> Hello all,
> I would like to know which one among the following two is the preferred
> way of making a plane split so that we can avoid signals crossing plane
> splits:
>
> Option1: Route all signals first, and then perform the plane split.
> My opinion is that this may not be a good choice because as the routing
> has been done prior to plane splitting, generally signals may go left to
> right/top to bottom(considering the worst case) and later on it may be
> very difficult to find the optimum plane split which will avoid any
> signal crossing the splits. Particularly for an autorouted board, this
> may not be a good choice. Avoiding all the plane splits may require lots
> of changes in the routing which may take considerable amount of time.
>
> Option2: Do the plane splitting before starting the routing.
> This looks like a good choice as we already know the location of the
> splits and we can route accordingly to avoid the splits. For
> autorouting, probably we can define a route keepout in the adjacent
> layers wherever a split is there in the power plane( I have not tried
> this keepout method yet, and I would like to get your comments on this).
>
> I still see one more practical problem in this- when we start the
> routing, generally it happens that placement is not 100% finished- and
> also there are the last minuite schematic changes which adds , other
> than resistors and capacitors, a few ICs also(like inverters/buffers
> etc), and the earlier shape of the plane split may not be sufficient for
> these new components. Now comes the problem of changing some routed nets
> to avoid this new split.
>
> So I am not able to bring a final conclusion on which one will be a
> better choice.As you must have faced this problem several times in the
> design phase, I would like to get your opinion on which one will be
> better- routing first or splitting first or some other solution which is
> better then both of the above methods.
>
> Thanks and regards,
> Aloke
>
> --
> **********************************************************************
> * Aloke Bhattacharya, *
> * Senior Engineer-VLSI/System Design,
> * *
> * Wipro Infotech, *
> * Global R&D, *
> * 88, M.G. Road, 5th Floor, *
> * S.B. Towers, *
> * Bangalore- 560 001 , *
> * INDIA *
> * Tel : 91-80-5588422(Ext. 520) *
> * email: aloke.bhattacharya@wipro.com
> * *
> **********************************************************************
>
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