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

  • From: Vinu Arumugham <vinu@xxxxxxxxx>
  • To: "Nash, Timothy J" <timothy.j.nash@xxxxxxxx>
  • Date: Tue, 20 Oct 2009 09:41:58 -0700

The 3.125Gbps XAUI spec. allocates 60ps for interconnect skew. So as Lee 
wrote, in theory 300 mil skew is acceptable. However, I agree with Jeff 
Loyer's approach. If it is easy to do +/-2.5 mil matching, the rest is 
margin for other impairments that are more difficult to control.
Thanks,
Vinu


Nash, Timothy J wrote:
> I agree with that rule of thumb, except...
>
> Many layout folks complain that we sometimes over-constrain our designs.  
> That is to say, whether hand-routed or auto-routed, the DRC constraints can 
> be prohibitive - especially when dealing with wide matched-length 
> differential busses on minimal layer counts.  I think it is important to keep 
> in mind that there is no magic number, rather the number is a function of the 
> characteristics of the interface.  I would agree that if you are only trying 
> to match a few diff pairs or you have more than enough routing layers then 
> matching to +/- 2.5 mils is a non-issue.  But, why make it difficult matching 
> to +/- 2.5 mils when 100 mils would suffice?  The bottom line is, rules of 
> thumb can be dangerous if we forget the caveats associated with them.
>
> Tim
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
> Behalf Of Joel Brown
> Sent: Tuesday, October 20, 2009 9:54 AM
> To: 'Loyer, Jeff'
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs
>
> We use a similar approach but use 5 mils. It is a number I have seen in
> various design guidelines including PCI express published by Intel.
> I think Jeff is right it's easier and faster to have a one size fits all
> rule even if it is overly conservative than to have to scrutinize each route
> to see if the length matching is below some number related to rise time.
> Our layout guy never has complained.
>
> Joel
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Loyer, Jeff
> Sent: Tuesday, October 20, 2009 8:21 AM
> To: steve weir; Lee Ritchey
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs
>
> After fighting this (how tightly must my diff pair be aligned?) forever,
> I've taken a more pragmatic approach.  I have my CAD folks use +/-2.5 mils
> for ALL their differential pairs.  This seems to be the quickest for all
> involved.
>
> 1) CAD folks tell me that if they're going to match within 100 mils,
> matching within 2.5 mils isn't much different.  They don't care.
> 2) They use the same constraints for ALL diff pairs - less chance for
> messing that up, much quicker.  They like that, A LOT!!!
> 3) I don't spend any time worrying that they didn't match some diff pair
> correctly, including accounting for multiple boards, etc.
>
> Of course, it does have the detrimental effect of lessening my workload and
> hence my innate value (providing expert guidance on exactly how much
> matching is required for each bus), but I'll take it.  I have more job
> security when the design is done more reliably and quickly.
>
> Does it make anyone else giggle that someone is hoping we'll provide a
> complete Design Guide via e-mail?  I would suggest to the original poster
> that you'll need some other means to supply all the guidelines you'll need
> to properly design a high-speed interface.  Asking for specific
> clarification is one thing, to ask for all the guidelines necessary for a
> complete design is another.
>
> My $0.02...
>
> Jeff Loyer
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of steve weir
> Sent: Tuesday, October 20, 2009 7:43 AM
> To: Lee Ritchey
> Cc: Peterson, James F (EHCOE); si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs
>
> 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
>>>>>
>>>>>
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>>> --
>>> Steve Weir
>>> IPBLOX, LLC
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> --
> Steve Weir
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>
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