By the time the signal arrives at the receiver, even with a short path, the rise time is far slower than 64 pSec. In long paths the rise time approaches half the UI. 50 pSec is 1/6 of a UI. Crossing point isn't too critical as long as it doesn't move around from bit to bit (jitter). > [Original Message] > From: Peterson, James F (EHCOE) <james.f.peterson@xxxxxxxxxxxxx> > To: <leeritchey@xxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx> > Date: 10/20/2009 7:54:14 AM > Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs > > A 50ps skew with a 64ps Tr/Tf?... > > So instead of a diff-pair crossing point at 50%, I get a crossing point > at 20%? > > Are we ready to say that's ok? > > -----Original Message----- > From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx] > Sent: Tuesday, October 20, 2009 10:30 AM > To: Peterson, James F (EHCOE); si-list@xxxxxxxxxxxxx > Subject: RE: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs > > 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. 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