[SI-LIST] Re: DDR-Length matching
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>
- Date: Thu, 28 Apr 2011 09:15:51 -0700
My apologies - I should not have used the word 'spec'. I was referring to what
are often called 'Design Guidelines'.
For most accurate reading, replace 'spec.' w/ 'guideline'. I think the terms
'specify' and 'specified' are ok.
Jeff Loyer
-----Original Message-----
From: steve weir [mailto:weirsi@xxxxxxxxxx]
Sent: Thursday, April 28, 2011 9:00 AM
To: Loyer, Jeff
Cc: karthi keyan; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: DDR-Length matching
For all you guys that sit on these committees, I recommend calling these
unnecessarily tight matches "Guidelines" and "Best Practices", and
restricting specifications to actual performance requirements. This
avoids building assumptions about tools and practices into
specifications, without losing the benefit of practical experience in
the guidelines.
Steve.
n 4/28/2011 8:25 AM, Loyer, Jeff wrote:
> +/-10 mils tolerance means that all signals in that group must be within 20
> mils of each other.
> If your longest trace is 6.253", and your shortest is 6.234", you have met
> the spec.
> If your longest trace is 6.253", and your shortest is 6.232", you fail the
> spec.
>
> The spec. could call out '+/-10 mils' or 'within 20 mils', with the same
> meaning.
>
> The '+/- 10 mils' verbiage is usually used to align with popular layout
> tools' conventions, where you specify +/- xx mils of a defined target.
> Finding that target is part of the process for your particular design.
>
> When they specify this kind of tolerance, they usually also insist on routing
> on the same layer, so propagation velocity differences don't come into play.
>
> This tight a tolerance (within 20 mils, or about 3-4ps) is usually specified
> because experience has proven that it doesn't take CAD folks much longer to
> meet a +/- 10 mil spec. than a +/- 100 mil spec., and we can reduce the skew
> from routing to essentially zero.
>
> Jeff Loyer
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of karthi keyan
> Sent: Thursday, April 28, 2011 2:42 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] DDR-Length matching
>
> Hi experts,
> I am working on boards having DDR interface. on layout we are
> following the below groupings& length matching
>
>
> Group1- Data signals,strobe,Mask with in group +/-10 mils tolerance
> Group2- Add/Ctrl/Cmd/Clk--with in group +/-10mils tolerance
>
> i am clear on groupings but on length matching i want to know how
> to calculate the exact Min& max length matching tolerance .
>
> can you please let me clear on DDR length matching?
>
>
> Thanks,
> Karthikeyan
>
>
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Steve Weir
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