Hi Aubrey, Point well taken. A well optimized design would length match to the point where additional margin is no longer free for the taking. At that point, if margins are adequate for the design objectives you are done. I certainly would not advocate adding layers if the deisgn does not require. Brian Moran Signaling Development Group Client Platforms Intel Corporation -----Original Message----- From: Aubrey Sparkman [mailto:asparky@xxxxxxxxxxxxxxxxxxxxxxxx] Sent: Saturday, April 30, 2011 2:41 PM To: Moran, Brian P; Loyer, Jeff; 'steve weir' Cc: si-list@xxxxxxxxxxxxx Subject: RE: [SI-LIST] Re: DDR-Length matching 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