[SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs

  • From: "Peterson, James F (EHCOE)" <james.f.peterson@xxxxxxxxxxxxx>
  • To: <leeritchey@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 20 Oct 2009 11:08:40 -0400

Wow, the crossing point isn't too critical...that's news to me...

Also, actually I was mistaken on the impact, the diff-pair crossing
point would be even worse than 20% with a 50ps skew with a Tr/Tf of
64ps...probably more like 0% or worse.

I am looking for a consensus from this group on this (it could mean
changing my assumptions and relaxing some of my guides in this area).

Best regards,
Jim

-----Original Message-----
From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx] 
Sent: Tuesday, October 20, 2009 11:02 AM
To: Peterson, James F (EHCOE); si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs

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
> > >>
> > >>
> > >>
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> > -- 
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