Since this is a DDR interface, the +/- 10 mils probably refers to the DQ length relative to the DQS. This means the DQ can be routed 10 mils shorter than the DQS or 10 mils longer. This seems to be a tight spec, more like what I'd see on a package and not a board. John Ellis Sr. Staff R&D Engineer Mixed Signals & I/O Libraries Group Synopsys, Inc. email: jellis@xxxxxxxxxxxx Phone: 508.263.8194 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Loyer, Jeff Sent: Thursday, April 28, 2011 11:25 AM To: karthi keyan; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: DDR-Length matching +/-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 ------------------------------------------------------------------ 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