[SI-LIST] Re: SSO pushout, ground bounce definition

  • From: Ege Engin <engin@xxxxxxxxxx>
  • To: chen@xxxxxxxxxxx
  • Date: Tue, 16 Apr 2002 09:44:53 +0200

Raymond,

The first thing that comes up to my mind is the concept of partial inductance. 
Some
experts work with a unique defined partial inductance (e.g., C.R.Paul, 
A.E.Ruehli),
whereas some are against such a unique definition (e.g., B.Young). I suppose you
fall into the second category. Indeed SI issues can be investigated using solely
terminal voltages (your 2.point). But voltage drop *along the conductor itself* 
can
also give valuable information regarding the EMI behavior, because it can model 
some
unintended dipole antennas (according to the experts falling into the first
category). The question is, how can one resolve this contradiction, that an
ill-defined voltage in terms of SI becomes a useful tool in terms of EMI?

Regards
Ege

"Raymond Y. Chen" schrieb:

> Kevin,
>
> You raised a good question, and I?ve been asked many times on this issue, so
> please see my comments after your post:
>
> > Raymond,
> > Thanks for the link to the doc. I read
> > "http://www.sigrity.com/papers/ECTC2001/ECTC_LI1.pdf";.
> > In there, the authors show the effect of on-die decoupling on SSO
> > power/gnd
> > noise. The tables and conclusion say
> > on-die decoupling cap can reduce power/ground noise. I can see how it
> > reduces the power line noise.
> > However, I can not understand how the on-die decoupling cap which
> > is placed
> > between power and
> > gound (on die, vss or substrait) can reduce the gound bounce. None of the
> > waveforms in this paper shows
> > the ground line. Can you please comment on that? Thanks.
> > Regards,
> > Kevin Hui
> > LSI Logic
>
> First of all, voltage is defined between 2 nodes. So power bounce is not
> something happening on power rail alone, it is the voltage fluctuation
> between power and ground.
>
> So where is the ground bounce voltage? It must be defined between 2 nodes.
> And usually people will show such circuit diagram to define ground bounce (I
> borrowed the drawing from ADEEL AHMAD?s 4/10 post on this topic):
>
>                         -------PAD
>                         |
>                         |
>                       NMOS
>                         |
>                         |<<internal GND
>                      INDUCTOR
>                         |
>                     GROUND PIN
>
> Here the ground bounce is defined as the voltage drop crossed the inductor,
> which models the interconnect parasitics (ground via, trace, plane)
>
> However, this definition derived from circuit-theory point of view often is
> not valid and causes misleading concepts, especially in the high-speed
> design arena where Electromagnetic (EM) phenomena is the basics for all
> circuit or SI issues. Because:
>
> 1.      measuring voltage across a big distance (compare to wavelength) is not
> well defined. For example, defining the voltage between die-pad and the
> package-pin; or defining the voltage between a point in the middle of a PCB
> and a point at the edge of the PCB, are not good if you are working in the
> hundreds of MHz range and beyond.
> 2.      measuring AC voltage drop along the conductor itself is not defined. 
> For
> example, we often measure the voltage at the ends of a transmission line
> (the 2 ports); we don?t measure the voltage drop across the individual
> transmission line conductor. Voltage drop along the ground conductor (except
> DC) is not well defined based on EM theory. Think about this, in the above
> drawing, the inductor can not associate with just the conductor, the
> inductor has to associate with a loop, where is the loop?
>
> After all, the terminology of ground bounce itself can be misleading,
> because lot of people think that ground bounce can be viewed ON the ground
> conductor, whereas actually ground bounce happens BETWEEN power and ground.
> The only time you may and you can well define the voltage between 2 ground
> points is if these 2 ground points are very close (local port). For example,
> between 2 ground C4 bumps. At that time, most flux in this loop is well
> captured between these 2 points.
>
> Therefore it may be better to use the term Power/ground fluctuation instead
> of ground bounce.
>
> Lastly, please take a look at the commonly used one-dimension power delivery
> model (illustrative):
>
>           L     R         L     R         L     R         1
>      |---ooo---^^^---|---ooo---^^^---|---ooo---^^^---|----|
>      |               |               |               |    |
> VRM ___              |               |               |    |
>      -               |               |               |    |
>      |        decap ===       decap ===        Cdie ===  |> buffer
>      |               |               |               |    |
>      |     PCB       |    Pkg        |    chip       |    |
>      |               |               |               |    |
>      |---ooo---^^^---|---ooo---^^^---|---ooo---^^^---|----|
>      2                                                    0
>
> To a driver, what is important is the local supply voltage between node 1
> and 0 (power and ground). And only V(1,0) can be well defined in the correct
> EM sense. The so-called ?ground bounce? between node 2 and 0 will be ill
> defined and meaningless to the driver.
>
> Regards,
>
> Raymond Y. Chen
> Sigrity, Inc.
>
> > ----- Original Message -----
> > From: "Raymond Y. Chen" <chen@xxxxxxxxxxx>
> > To: <si-list@xxxxxxxxxxxxx>
> > Cc: <linwee70@xxxxxxxxx>; <Andrew.Ingraham@xxxxxxxxxx>
> > Sent: Thursday, April 11, 2002 2:57 PM
> > Subject: [SI-LIST] Re: SSO pushout, ground bounce, IO and core switching
> >
> >
> > >
> > > During Simultaneous Switching Output (SSO) stage of I/O
> > buffers, the basic
> > > Electromagnetic (EM) phenomena affect the timing and waveform of the
> > > signals. Trace coupling (even/odd mode for example) affect timing and
> > > waveform during SSO. Power/Ground bounce (also referred as Simultaneous
> > > Switching Noise - SSN) is another major mechanism that affects Signal
> > > Integrity, since power/gound is the signal return path. Any voltage
> > > fluctuations on the power/gound affect the driver switching
> > characteristics,
> > > driver/receiver end waveforms, and signal waveforms along the path where
> > > power/gound noise can reach in the form of EM wave propagation. You can
> > > think as moving charge, flux, inductance, and displacement current as
> > well.
> > > In essence, Maxwell equations rule.
> > >
> > > To illustrate this point and to see the mechanism that
> > contribute to "SSO
> > > Pushout", I just put up an animated slides on our web site to show the
> > > relation between SSO, power/gound bounce, signal Return Path
> > Discontinuity
> > > (RPD), and signal timing and waveform degradation.
> > > http://www.sigrity.com/papers/reply20020411/speedxp_ssn.ppt
> > >
> > > And also here is a functional demo software to simulate this topic.
> > > http://www.sigrity.com/prod01_demodl.htm
> > > An application example is included (application notes, example 7) to see
> > the
> > > SSO of 17 nets with gound bounce and RPD, with and without decoupling
> > > capacitors. You can change many parameters, such as with/without trace
> > > coupling, edge rates, power/ground structures, various decoupling
> > capacitors
> > > (models and locations), different switching bit patterns,
> > even/odd mode. A
> > > good technical paper to reference on this kind SSO simulation is "A
> > > Simulation Study of Simultaneous Switching Noise" at:
> > > http://www.sigrity.com/papers/ECTC2001/ECTC_LI1.pdf
> > >
> > > In the case of core logic SSO, the strong power/gound transient currents
> > > will generate dynamic EM noise and couple into I/O. This phenomenon is
> > > pointed out on another thread these two days - by Gil Gafni on topic of
> > > "PCI Buffer Slew Rate", and you can also refer to this paper "Integrated
> > > Modeling Methodology for Core and I/O Power Delivery" for some
> > discussions:
> > > http://www.sigrity.com/papers/ECTC2001/ECTC_LI2.pdf
> > >
> > > Finally I want to point out, just as Andy said, SSO noise
> > phenomena could
> > > either increase or reduce the signal delay. You can try to build an
> > example
> > > with the demo software to show this, even though this time I
> > only provided
> > > the slides for "SSO Pushout".
> > >
> > > Raymond Y. Chen
> > > Vice President, Products and Services
> > > Sigrity, Inc.
> > > ********************************************
> > > 4675 Stevens Creek Blvd. Suite 130
> > > Santa Clara, CA 95051
> > > 408.260.9344 Ext 102
> > > ********************************************
> > > www.sigrity.com
> > >
> > >
> > > > -----Original Message-----
> > > > From: si-list-bounce@xxxxxxxxxxxxx
> > > > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ingraham, Andrew
> > > > Sent: Thursday, April 11, 2002 6:34 AM
> > > > To: linwee70@xxxxxxxxx
> > > > Cc: si-list@xxxxxxxxxxxxx
> > > > Subject: [SI-LIST] Re: SSO pushout
> > > >
> > > >
> > > > > " SSO pushout is a result of multiple drivers
> > > > > switching simultaneously. It impacts signal integrity
> > > > > through adding extra delay to the propagating signal"
> > > > >
> > > > > I understand that we'll have more current in the
> > > > > return path due to simultaneous switching that may
> > > > > cause ground/plane bounce, but couldn't relate that to
> > > > > delay.
> > > > >
> > > > > Can someone explain this  delay, as the text doesn't
> > > > > explain that ?
> > > >
> > > > There are a few mechanisms that result in extra delay.  (It isn't
> > > > really a delay in the propagating signal, i.e. wires, but rather in
> > > > the driven signal coming out of the IC outputs.)
> > > >
> > > > One of them is simply this.  When several outputs switch from high to
> > > > low, switching current flows in through the signal pins and N times
> > > > as much current goes out through the "ground" pins.
> > > >
> > > > The ground system impedance turns this current into ground bounce,
> > > > where on-die "ground" momentarily lifts above board "ground".
> > > >
> > > > For the outputs switching low, the pull-down transistor is on, so
> > > > this ground bounce gets added to their outputs.  Imagine simply
> > > > adding a small positive pulse to the outputs, while they switch.
> > > > Their falling edges are lifted up slightly, which also has the
> > > > appearance of moving them a little to the right --> greater delay.
> > > >
> > > > So it is really just crosstalk by way of the on-die ground bounce.
> > > >
> > > > The same thing happens for rising edges, except it is the VDD or VCC
> > > > that bounces or sags.
> > > >
> > > > I'm sure there are other mechanisms at work too.  Note that SSO
> > > > can both increase and decrease delays, at least in principle.
> > > >
> > > > Regards,
> > > > Andy
> > > >
> > > > -----Original Message-----
> > > > From: si-list-bounce@xxxxxxxxxxxxx
> > > > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Gil Gafni
> > > > Sent: Wednesday, April 10, 2002 12:21 PM
> > > > To: Si-List
> > > > Subject: [SI-LIST] PCI Buffer Slew Rate
> > > >
> > > >
> > > > Hello Experts,
> > > >
> > > > As part of an SSO analysis that we are conducting, we noticed an odd
> > > > behavior of the PCI buffer -
> > > > 1. It looks like it is influenced by the noise on the core Vdd
> > > > (logic side of it) and NOT to the IO Vdd.
> > > > 2. The sensitivity to Vdd core changes is very high; any added
> > > > 100mV of noise can add 1nSec to the pushout.
> > > >
> > > > The bottom line is that with a moderate noise on Vdd of 375 mV we have
> > > > 1nS of push out, and find it very hard to pass (no or
> > negative margins)
> > > > Any recommendations?
> > > >
> > > > Regards,
> > > >
> > > > Gil Gafni
> > > >
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