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Subject: Is angle routing recommended for XAUI??
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jithutharoor
Posts: 2
Online: User is Offline
5/27/2008 12:14 AM  
Hai

   Is there any specific recommendation for XAUI rouitng. If so can anyone say what is the recommendation.


Jithu Tharoor
vealmic@uk.ibm.com
Posts: 23
Online: User is Offline
6/02/2008 9:16 AM  
I've never dealt with XAUI, but I have used plenty of fibre channel and PCIe. These are pretty similar.

1/ They're diff pairs. Look up the required differential impedance in the spec.
2/ Look up the allowed intra lane skew.
3/ Look up the lane to lane skew.

That should give you a good basis for some simple constraints.

Pay special attention to any point that gives an impedance discontinuity, layer changes, BGA breakouts and passives in your diffpair.

Make sure your board is properly decoupled.

If you're new to high speed signal integrity, I suggest a copy of "High Speed digital design (black magic)" and "High speed signal propagation (advanced balck magic)".

If you're looking for someone to give you spacing rules, I doubt you'll get that here - tell your boss he needs to hire a SI engineer.

Hope that helps.
Mike V
jithutharoor
Posts: 2
Online: User is Offline
6/02/2008 8:38 PM  
Thanks Mike,

what I really want is , here we usually follow 45 degree routing.In some PHY specification it is said like XAUI needs ARC routing(Routing with small angles).I need to confirm like is that roting specific to that PHY or XAUI need ARC routing.


Jithu Tharoor
vealmic@uk.ibm.com
Posts: 23
Online: User is Offline
6/05/2008 9:02 AM  
Do you mean 45% champher on bends?

Nobody can answer that question, there are too many other variables, like how long is your trace, how good is your impedance control, what are your geometeries within each pair? Even how many bends you're introducing.

IF your impedance control is excellent, your traces very short, and you have little crosstalk in your nets, then you will have room for a bit of noise introduced by 45deg or 90 degree bends.


Angled corners radiate more than curved ones (which means impedance discontinuities and SI issues too). There is also more skew introduced.

Whether you can live with this or not is up to you. Lay it out, run a sim and see.
If you're not sure, curvy traces are better for SI. Whilst it may tke you longer, you know that this particular aspect of layout can't be improved.

I guess what I'm saying is do the SI work. If you don't have the resources, then you need to balance the cost to your schedule (to add rounded corners) with the risk of getting a card back with "issues". I know nothing about XAUI, will it autonegotiate to a lower speed? Is the card a development prototype, or will this ship to a customer?

I don't think there are hard and fast rules. You need to decide based on your application.
dschaefer
Posts: 42
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6/05/2008 10:08 AM  
Check out the following paper on the subject:

http://www.ultracad.com/articles/90deg.pdf

I also attended a presentation by Dr.Hubing presenting the results sumamrized in the document which debunk many of the myths of route angles. UMR, Dr.Hubing, or Dr.Van Doren may have other published papers on the subject.
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Forums > Silicon-package-board > PCB Design > Is angle routing recommended for XAUI??


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