[SI-LIST] Re: Reducing SSO noise in an FPGA

I cannot reduce the drive strength, the drivers are SSTL2.  We reduced
the SSO noise significantly by turning off half the data bits.

-----Original Message-----
From: Chris Cheng [mailto:chris.cheng@xxxxxxxxxxxx]=20
Sent: Tuesday, July 15, 2003 2:27 PM
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Reducing SSO noise in an FPGA

What's wrong with reducing the driving strength of the drivers in the
FPGA ?
Are you at the limit of the lowest possible drive strength already ?

-----Original Message-----
From: Fabrizio Zanella [mailto:fzanella@xxxxxxxxxxxx]
Sent: Tuesday, July 15, 2003 5:54 AM
To: bhenson@xxxxxxxxxxxx; Ken.Cantrell@xxxxxxxxxxx
Cc: bill.panos@xxxxxxxxxxxxxxx; Chris Cheng; scott@xxxxxxxxxxxxx;
si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Reducing SSO noise in an FPGA


This discussion has turned quite interesting.  There have been several
comments which imply that the only way to reduce SSO noise in an
FPGA/ASIC is to add decoupling at the die or inside the package.  These
are fixes which only the device manufacturers can make. =20
Does anyone have measurement/simulation data on what effect adding many
decoupling capacitors under the BGA package, between VCC and ground
balls, will have on the SSO noise?

Thanks and regards,=20
Fabrizio

-----Original Message-----
From: Bradley S Henson [mailto:bhenson@xxxxxxxxxxxx]=20
Sent: Monday, July 14, 2003 12:23 PM
To: Ken.Cantrell@xxxxxxxxxxx
Cc: bill.panos@xxxxxxxxxxxxxxx; Chris Cheng; scott@xxxxxxxxxxxxx;
si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Reducing SSO noise in an FPGA






I think Ken provides a good summary that I generally agree with also.
There
is an interesting side note to the on-package decoupling that is not
always
clear.

On-chip/package decoupling helps the SSO noise problem by paralleling
the
local I/O power and ground inductance during the switching event. This
lowers the effective inductance. However, without decoupling there is
often
a degenerative feedback that takes place, essentially turning off the
output driver a tad as a result of the SSO noise. Sometimes we see this
covered in specs as SSO timing pushout. The subtle point about on
package
(or on chip) decoupling is that it not only parallels the I/O power and
ground pins in the area, but it also stiffens the local rails to the
offending driver. Less rail lift or droop means less degenerative
feedback
and more noise generation. Because of this, you will not always see the
expected improvement in SSO noise with the application of on-package I/O
power decoupling. I'm not advocating neglecting this important design
detail, but rather explaining a subtle 2nd order impact.

Regards,
Brad Henson
Raytheon


=20

                      "Ken Cantrell"

                      <Ken.Cantrell@src         To:
<scott@xxxxxxxxxxxxx>, <bill.panos@xxxxxxxxxxxxxxx>  =20
                      comp.com>                 cc:      "Chris Cheng"
<chris.cheng@xxxxxxxxxxxx>,            =20
                      Sent by:                  <si-list@xxxxxxxxxxxxx>

                      si-list-bounce@fr         Subject: [SI-LIST] Re:
Reducing SSO noise in an FPGA          =20
                      eelists.org

=20

=20

                      07/14/2003 08:13

                      AM

                      Please respond to

                      Ken.Cantrell

=20

=20





Scott,
My lab measurements(anything else is speculation)are in agreement with
your
statements.  It is a package problem, and all the BC in the world won't
change it.  I have varied the noise levels going into the chip from 2%
to
>10% with no significant measured change in SSO, with Zo vs Frequency
running 2x to 3x the calculated target value.
I am increasing the BC via diameters and break out trace widths from
ball
to
via as general improvements, but don't expect that it will make any
difference.  Layering and loop size are already optimized.  There's a
joke
for you, minimizing loop size in dense, thick boards.  Do you have data
on
any board geometry changes that produce a positive effect?
Ken

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Scott McMorrow
Sent: Friday, July 11, 2003 4:45 PM
To: bill.panos@xxxxxxxxxxxxxxx
Cc: Chris Cheng; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Reducing SSO noise in an FPGA


Bill,
I respectfully disagree with you and agree with Chris.  The SSO problem
wrt. to a device driver is a chip and package problem.  The round trip
time from the driver, through the package and out to a BC layer is
longer than the risetime for almost all packages and devices where this
is a problem.  SSO is generally a loop inductance issue.  No amount of
capacitance at the end of a large loop inductance in a power delivery
system can effect the problem. For the capacitance to be effect, it has
to be in the package or on the die. As a result, the PCB has no effect
on instantaneous SSO problems.  You can make the problem worse, but if
the package and silicon are already hosed, their ain't much you can do
to make it better.  SSO problems are almost always I/O power issues.
This is a region of silicon and package design where there are many
tradeoffs, quite a few of them wrong.  If this portion of the design is
already broken (and it is in quite a few FPGA, ASICs and even in custom
devices) then all the BC in the world will not fix it.

BC can deal with lower frequency noise components and lower the overall
noise floor for a PCB power delivery system.  BC can reduce the power
impedance profile. BC can even be used to change the resonant frequency
of a board, if the dielectric constant is higher than FR4.  But BC
cannot help the classical SSO problem.  If you have real engineering
data that shows differently, I would enjoy seeing it.

best regards,

scott

--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com



bpanos wrote:

>Chris
>I think it's "guest" not "guess", (might want to check your syntax)
>I disagree with your notion that BC is useless in an SSO applications,
Even
with
>as you say with good reference pane placement. SSO problems may be
manifested by
>supply bounce or ground. Concerning supply bounce in the presence of
bypass
caps
>and issues with ESL (low ESL caps are not always provided or an
option),
BC
>could be entertained. To what degree BC would be of a benefit is up to
debate.
>Bill
>
>
>
>
>Chris Cheng wrote:
>
>
>
>>BC is useless to solve packaging SSO or any SSO problem for that
matter
if
>>you manage your signal reference plane correctly on the PCB. If you
choose
>>to use the wrong reference plane for your I/O and then decided to
throw
>>money to fix it with BC, be my guess.
>>
>>Chris
>>
>>-----Original Message-----
>>From: bpanos [mailto:bill.panos@xxxxxxxxxxxxxxx]
>>Sent: Friday, June 27, 2003 4:05 PM
>>To: perry.qu@xxxxxxxxxxx
>>Cc: zhang_kun@xxxxxxxxxx; fzanella@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
>>Subject: [SI-LIST] Re: Reducing SSO noise in an FPGA
>>
>>I personally have not tried implementing this particular application,
in
>>this fashion, but I wonder how well using BC (buried capacitance)
material
>>would work with SSO issues with an FPGA.
>>Granted, there may not be enough capacitance / sq, though with 0.5mil
BC
it
>>might be an option..
>>Regards,
>>Bill
>>
>>Perry Qu wrote:
>>
>>
>>
>>>Hi, Zhangkun:
>>>
>>>We have been using 100nF cap extensively in our high speed design (>
>>>
>>>
>>100MHz) and so far so good. In my understanding, small ESL is the key
to
>>make a flat impedance curve in frequency
>>
>>
>>>range, as explained here many times by Larry Smith and Istvan Novak.
Small
>>>
>>>
>>caps such as NPO 1nF will normally give us  high Q since resonance
factor
>>increase when ESL increase, and when
>>
>>
>>>ESR and C decrease. For NPO cap, it has very small ESR and small C,
so
if
>>>
>>>
>>we do not control ESL to very small value, we will end up with high Q.
Such
>>high Q cap may cause strong
>>
>>
>>>anti-resonance when mixed with other cap and/or plane pairs. In our
>>>
>>>
>>designs, since we can't control ESL very well with special
stackup/routing,
>>we use 1nF very carefully.
>>
>>
>>>Regards
>>>
>>>Perry
>>>
>>>Zhangkun wrote:
>>>
>>>
>>>
>>>>Dear Zanella
>>>>
>>>>I think 0.1uF and 0.22uF is too large for decoupling. When the
frequency
>>>>
>>>>
>>goes up to 100MHz, these two kind of caps will be of no use. Istvan
has
>>writen one paper about measuring caps.
>>
>>
>>>>Best Regards
>>>>
>>>>Zhangkun
>>>>2003.06.26
>>>>----- Original Message -----
>>>>From: "Fabrizio Zanella" <fzanella@xxxxxxxxxxxx>
>>>>To: <si-list@xxxxxxxxxxxxx>
>>>>Sent: Thursday, June 26, 2003 12:01 AM
>>>>Subject: [SI-LIST] Reducing SSO noise in an FPGA
>>>>
>>>>
>>>>
>>>>>I would like to hear about experiences regarding methods of
reducing
>>>>>simultaneous switching noise in a large FPGA, BGA package.  Let's
>>>>>
>>>>>
>>assume
>>
>>
>>>>>a 128bit bus, with a signal frequency of 100MHz.
>>>>>How effective is adding ground planes 2 mils from the VCC planes in
>>>>>reducing SSN? If one uses BC, does every VCC pin in the FPGA
require
>>>>>decoupling? And should the caps be tied to the BGA pins with blind
>>>>>
>>>>>
>>vias
>>
>>
>>>>>so they can be placed directly under the BGA?  What are the optimal
>>>>>values for the decoupling capacitors, 0.1uf, 0.22uF?
>>>>>
>>>>>Thanks very much and regards,
>>>>>
>>>>>
>>>>>Fabrizio Zanella
>>>>>Principal Hardware Design Engineer
>>>>>Broadbus Technologies
>>>>>fzanella@xxxxxxxxxxxx
>>>>>=3D20
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>>>--
>>>Perry Qu
>>>
>>>Design & Qualification       |      600 March Road
>>>Alcatel Canada               |      Ottawa, ON K2K 2E6, Canada
>>>
>>>DID: (613) 7846720           |      FAX: (613) 5993642
>>>
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