Lee, If the driver is really slow or the path very long that might be OK. For a clean driver on modest length links, 50ps skew can run 50% or more of Tr/Tf. I don't like that much mode conversion. Best Regards, Steve. Lee Ritchey wrote: > Actually, I was being conservative at 100 mils. A mismatch of 300 mils > total is acceptable throughout the total path. 300 mils is roughly 50 > pSec.out of a 320 pSec bit interval. > > > >> [Original Message] >> From: Peterson, James F (EHCOE) <james.f.peterson@xxxxxxxxxxxxx> >> To: <si-list@xxxxxxxxxxxxx> >> Date: 10/20/2009 6:09:07 AM >> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs >> >> One comment on length matching at 100 mils : a lot of times the board >> we're looking at is only one section of the interface. There is often >> two more boards involved (a backplane and endpoint). When we say 100 >> mils matched lengths, are we saying total matched length or at each >> board (so total could be 300 mils in that case)? The thread below says >> "matched at the receiver", which implies total, so the 100 mils should >> be budgeted through 3 boards. >> >> Regards, >> Jim Peterson >> Honeywell >> >> -----Original Message----- >> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] >> On Behalf Of steve weir >> Sent: Tuesday, October 20, 2009 1:13 AM >> To: icer world >> Cc: Lee Ritchey; Paul Hurst; si-list@xxxxxxxxxxxxx; chundi srikanth; >> Lambert Simonovich >> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs >> >> 1) Matching much closer than 1/4 Tr/Tf offers little value by itself. >> 100mils translates to about 16ps skew. That corresponds to a 64ns Tr/Tf >> >> for a 3.125G link which is a little to the outside, but it is not >> horrible. I would rephrase Lee's advice as: Don't take any >> extraordinary measures to match closer than 1/4 Tr/Tf as seen at the >> receiver. The longer the link, the slower Tr/Tf will be. There is a >> good treatment on this in Eric Bogatin's "Signal Integrity Simplified". >> >> If you get very tight matching truly for free, then fine. But obsessing >> >> about mechanical match to tight tolerances does not improve the design. >> >> It can in fact harm it if the matching is done with dense serpentines >> which introduce their own dispersion and timing skew that is not >> properly accounted in some tools. >> >> 2) Continuous return path is very important. Diff tolerates obscenities >> >> like crossing moats, but at undesirable costs. The best answer is don't >> >> interrupt the return path. >> >> 3) Surface ground guards are more often unintended resonators than of >> specific value. See if the cross-talk can be satisfied with spacing. >> If it can't, then consider alternatives. >> >> 4) I am not clear on what you are trying to recommend. Are you talking >> about termination at both ends of the link, or even and odd mode >> terminations? >> >> 5) An appropriate stitch density helps with EMC and signal integrity. >> Follow Bruce Archambeault's hierarchy on layer assignments and your life >> >> will be good: >> i. Route on one layer that faces a contiguous plane. >> ii. Switch between layers on either side of the same contiguous plane. >> iii. Switch between layers that reference planes on the same DC >> potential that are adequately stitched together. This is often >> misunderstood as needing to assign a return stitch via near each >> transition. The point is to raise the resonant frequency of the >> structure sufficiently so that it won't be a problem to the signal. >> iv. If you must switch between layers that are stitched with bypass caps >> >> and planar capacitance. Be wary of PDN resonances in the signal >> frequency range. These will tend to occur at much lower frequency than >> cavity resonances. >> >> Steve. >> >> icer world wrote: >> >>> 1) length matching must be consided seriously and mismatch should be >>> >> below 100mils; >> >>> 2) generally speaking,the differential pairs impedance must keep >>> >> 100ohm and the ground return path must not be choped ; >> >>> 3) the ground guarding trace should keep two times of differential >>> >> trace width away from the differantial pairs,which can not influences >> the differential pairs impedance and avoid crosstalk issues; >> >>> 4) using serial and parallel matching resistors simultaneously for >>> >> debug ; >> >>> 5) changing layers is not expected,but if necessary ,you should place >>> >> a groung via near the signal via ; >> >>> the above is just an advice ,and you'd better do a simulatiom to >>> >> decide the rules of matching resistors and crosstalk; >> >>> >>> ________________________________ >>> From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx> >>> To: Paul Hurst <paul@xxxxxxxxxxxxxxxxxxxxxx>; si-list@xxxxxxxxxxxxx >>> Cc: chundi srikanth <chundis@xxxxxxxxx>; Lambert Simonovich >>> >> <bertsimonovich@xxxxxxxxxx> >> >>> Sent: Mon, October 19, 2009 11:46:36 PM >>> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs >>> >>> Length matching does not need to be tighter than 100 mils. >>> >>> It is not necessary to back drill vias used to connect component pins >>> >> to >> >>> traces or to change layers. The 0.6 pF or so capacitance does not >>> adversely affect the signals at this data rate. >>> >>> It is not necessary to put "ground" vias next to routing vias as long >>> >> as >> >>> the PDS is well designed." >>> >>> "Guard" traces have no value. Proper spacing to avoid crosstalk >>> >> does. >> >>> The "rule of thumb" for spacing given, while crude, is not far off. >>> >>> Lee Ritchey >>> >>> >>> >>> >>>> [Original Message] >>>> From: Paul Hurst <paul@xxxxxxxxxxxxxxxxxxxxxx> >>>> To: <si-list@xxxxxxxxxxxxx> >>>> Cc: chundi srikanth <chundis@xxxxxxxxx>; Lambert Simonovich >>>> >>>> >>> <bertsimonovich@xxxxxxxxxx> >>> >>> >>>> Date: 10/18/2009 5:45:08 PM >>>> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs >>>> >>>> Srikanth, >>>> >>>> Additional to Bert's comments:- >>>> >>>> 1) Match the lengths of the pairs exactly >>>> 2) Use a solid ground return >>>> 3) Avoid stubs by design or by using stub-drilling >>>> 4) If you have vias in the path use a small drill and a large >>>> >> anti-pad and >> >>>> place a ground via next to each signal via >>>> >>>> Regards >>>> >>>> Paul >>>> >>>> -----Original Message----- >>>> From: si-list-bounce@xxxxxxxxxxxxx >>>> >> [mailto:si-list-bounce@xxxxxxxxxxxxx] >> >>>> >>>> >>> On >>> >>> >>>> Behalf Of Lambert Simonovich >>>> Sent: 16 October 2009 21:57 >>>> To: 'chundi srikanth'; si-list@xxxxxxxxxxxxx >>>> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs >>>> >>>> >>>> Srikanth, >>>> >>>> A rough rule of thumb is greater than 3 times trace width separation >>>> >>>> >>> between >>> >>> >>>> pairs. Using a 2D field solver will ultimately give you the crosstalk >>>> coupling factor for the exact geometry in your stackup, and dictate >>>> >> the >> >>>> routing rules you need to follow to satisfy your noise budget. >>>> >>>> Adding GND guarding will more often than not present more issues than >>>> >> it >> >>>> solves. You should stitch these guard traces at regular intervals >>>> approximately 1/10 of the wavelength of the highest frequency >>>> >> component of >> >>>> the aggressor signal to avoid it from resonating and coupling noise >>>> >> back >> >>>> onto other adjacent traces. This further reduces routing density of >>>> >> the >> >>>> board. By the time you factor in the additional space of one trace >>>> >> width >> >>>> between the guard trace and Diff pair, plus the additional via >>>> >> stitching, >> >>>> you will find you are already at 3 times separation between pairs and >>>> >> you >> >>>> would gain back more real estate. >>>> >>>> Regards, >>>> >>>> Bert Simonovich >>>> >>>> >>>> >>>> >>>> >>>> -----Original Message----- >>>> From: si-list-bounce@xxxxxxxxxxxxx >>>> >> [mailto:si-list-bounce@xxxxxxxxxxxxx] >> >>>> >>>> >>> On >>> >>> >>>> Behalf Of chundi srikanth >>>> Sent: October-16-09 12:40 PM >>>> To: si-list@xxxxxxxxxxxxx >>>> Subject: [SI-LIST] Routing guidelines for 3.12Gbps LVDS pairs >>>> >>>> Hi Techies, >>>> We have a 12-layer board in which we have lot of differential LVDS >>>> >> pairs >> >>>> operating at several hundreds MHz. And we have SERDES signals >>>> >>>> >>> (differential >>> >>> >>>> TX & RX pairs) operating at CPRI rate-4 i.e., 3Gbps. So can you just >>>> >> share >> >>>> me some inputs on exactly what are the guidelines to be followed >>>> >> while >> >>>> routing these signals. And is GND gaurding between the differential >>>> >> pairs >> >>>> improve the SI?Please share or refer me any documents in which i can >>>> >> get >> >>>> Good information on High-Speed design guidelines. >>>> >>>> Thanks >>>> >>>> With Best Regards >>>> Srikanth >>>> >>>> >>>> ------------------------------------------------------------------ >>>> 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 >>>> or at our remote archives: >>>> http://groups.yahoo.com/group/si-list/messages >>>> 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 >>>> or at our remote archives: >>>> http://groups.yahoo.com/group/si-list/messages >>>> 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 >>>> or at our remote archives: >>>> http://groups.yahoo.com/group/si-list/messages >>>> 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 >>> or at our remote archives: >>> http://groups.yahoo.com/group/si-list/messages >>> 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 >>> or at our remote archives: >>> http://groups.yahoo.com/group/si-list/messages >>> 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 >> (866) 675-4630 Business >> (707) 780-1951 Fax >> http://www.ipblox.com >> A Teraspeed Consulting Group LLC Partner >> >> Teraspeed Consulting Group >> 121 North River Drive >> Narragansett, RI 02882 >> (401) 284-1827 Business >> (401) 284-1840 Fax >> >> Teraspeed Labs >> 13610 SW Harness Lane >> Beaverton, OR 97008 >> (503) 430-1065 Business >> (503) 430-1285 Fax >> A Teraspeed Consulting Group LLC Partner >> >> http://www.teraspeed.com >> >> This e-mail contains proprietary and confidential intellectual property >> of IPBLOX LLC >> ------------------------------------------------------------------------ >> ------------------------------ >> Teraspeed(R) is the registered service mark of Teraspeed Consulting >> Group LLC >> >> ------------------------------------------------------------------ >> 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 >> or at our remote archives: >> http://groups.yahoo.com/group/si-list/messages >> 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 >> or at our remote archives: >> http://groups.yahoo.com/group/si-list/messages >> 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 > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > > > -- Steve Weir IPBLOX, LLC 150 N. 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