[SI-LIST] Re: DDR-Length matching
- From: "Aubrey Sparkman" <asparky@xxxxxxxxxxxxxxxxxxxxxxxx>
- To: "'Moran, Brian P'" <brian.p.moran@xxxxxxxxx>, "'Loyer, Jeff'" <jeff.loyer@xxxxxxxxx>, "'steve weir'" <weirsi@xxxxxxxxxx>
- Date: Sat, 30 Apr 2011 16:41:27 -0500
Brian,
If you are doing a design where you have real estate to spare, I don't
disagree with what you have said.
On the other hand, you asked why I might leave margin on the table. I would
ask you to consider the scenario where you working on a very small board and
are cramped for space. Would you still recommend the extra serpenting if it
would cause you to add a pair of (you can't add just one) layers?
Thanks,
Aubrey Sparkman
512-461-6165
asparky@xxxxxxxxxxxxxxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Moran, Brian P
Sent: Thursday, April 28, 2011 12:09 PM
To: Loyer, Jeff; steve weir
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: DDR-Length matching
Hi All,
The tight +/-10 mil spec usually refers to matching within the DQS pairs and
in some cases between DQ and their respective DQS strobes. +/-10 mils is
reasonable for the diff pair matching, but perhaps a bit tight for DQ to
DQS. The tighter you match the better your margins.
Most designs have some margin to give, but on the other hand, once you start
matching traces you might as well match to the tightest reasonable
guideline. Why leave margin on the table. But you can do the math as far as
how much margin you give up by loosening the matching. I agree its not a
matter of pass/fail. Its more a matter of optimization.
Note that most controllers provide timing control per byte lane, so there is
no need to length match across byte lanes. Individual byte lanes themselves
are usually matched to some relatively large window around CLK length,
similar to how CTRL, and CMD/ADR groups are length matched. In Intel
guidelines we generally recommend matching CTRL groups and CMD/ADR groups
for a given channel within the group to a fairly tight guideline, but then
allow the group length as a whole to be matched to CLK using a more generous
guideline. This then allows
the CTRL or CMD/ADR groups to be alighned to CLK using internal timing
circuits.
Brian Moran
Signaling Development Group
Client Platforms
Intel Corporation
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Loyer, Jeff
Sent: Thursday, April 28, 2011 9:16 AM
To: steve weir
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: DDR-Length matching
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
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
