[SI-LIST] Re: PDS analysis?

I've got some ICs with core frequency transients well into the hundreds of
MHz.  Looks like the on die and on package capacitance was not done well. 
Fixed with a little more plane capacitance.


> [Original Message]
> From: steve weir <weirsi@xxxxxxxxxx>
> To: Chris Cheng <Chris.Cheng@xxxxxxxx>
> Cc: Bowden, Ivor <ibowden@xxxxxxxxxxxxxxxxx>; Gil Simsic [IEEE]
<gilsimsic@xxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Date: 3/29/2008 10:23:32 PM
> Subject: [SI-LIST] Re: PDS analysis?
>
> Chris, I appreciate your experience w/ changing out caps.  Sprinkling 
> caps of some arbitrary values to cure EMI ills is a bit akin to hunting 
> rabbits with a revolver on a dark moonless night in the desert.
>
> 100MHz as a high cut-off for IC core currents is often a reasonable 
> number due to limitations of both planes and IC packages.  It is not 
> however a reasonable number for many I/O rails.  Nor does it account for 
> the evils of packaged ICs that include significant problematic 
> resonances.  I've got examples that occur way above 100MHz.  We just 
> worked one recently where the resonance was about 600MHz.  We handily 
> corrected it with a single X2Y capacitor.
>
> The IC spec deficiencies are well known evils.  Getting decent specs is 
> the starting point for a competent design.
>
> For the two rules, personally I like coefficients for any problem.  Over 
> design costs too much money.  Under design invites disasters.  Ask 
> Microsoft about their ever ballooning liabilities w/ the Xbox 360.  I 
> haven't seen a single problem with that fiasco yet that doesn't trace 
> back to engineering deficiencies.
>
> The idea of using bypass caps to cover to 100MHz only is a demonstrably 
> dubious truism.  There are a number of chips out there that I can 
> demonstrate misbehave when the bypass network doesn't deal with 
> frequencies at 300MHz or higher.  So, this is one time that I have to 
> disagree with your advice. 
>
> We do agree on the value and wisdom of managing the I/O return path.  
> "Ask not what your PDN can do for signal returns.  Ask what your 
> physical design can do to avoid injecting unnecessary noise into the PDN."
>
> A parting comment on using a ton of 100nF caps.  It used to be that this 
> could work rather well when the overall density of the capacitors pushed 
> the PRF between the bypass network and the planes out beyond the spectra 
> of both the bit and edge rates.  Nowadays that is just a very expensive 
> if not just impossible approach. 
>
> Best Regards,
>
>
> Steve.
>
> Chris Cheng wrote:
> > My first disclaimer will be :
> > YMMV
> > my second will be :
> > I still don't have the balls to do single value highspeed ceramic caps 
> > but I do think .1u is a good starting point to go up to 1u and down to 
> > .01u or even 1nf if you have a well designed flip chip package. For a 
> > el-cheapo wire bond package .1u might even be good enough as "external 
> > highspeed cap" to begin with.
> >  
> > That said, I have at least two experiences that make me have second 
> > thoughts :
> > 1) In one of my large product platforms (big server type machine), 
> > during the first proto stage on the first EMI scan, my board design 
> > guy accidentally generated the wrong BOM and stuff all the highspeed 
> > caps that were supposed to be 1u,0.1uf and 0.01uf spread to a single 
> > value 0.1uf. We actually send it through the EMI scan and it actually 
> > pass FCC scan with good margin. And the system ran through all PVT 
> > margin tests. Is it our luck or good enough to begin with ? I'll let 
> > you speculate.
> > 2) In the workstation/server company we came from, on an occasion 
> > after we had a big screaming session with the EMI engineer on our 
> > designs, we secretly rework out all the 10pf,100pf and 100pf caps that 
> > she sprinkled on our board with 1u caps and send it to her to scan. 
> > And as expected it passed FCC with good margin. We lost all our 
> > respect of her and probably EMI engineers in general after that.
> >  
> > Typical IC spec calls out the peak current demand. A more 
> > sophisticated one calls out the peak di/dt. A even more sophisticated 
> > one calls out the voltage tolerance between different frequency bands.
> > No where does it tell you how it can ramp from zero current to peak 
> > during normal operation.
> > No where can it tell you if your system architecture can actually pin 
> > the I/O buses or subsystem to maximum activities during normal operation
> > No where does it tell you your super multi-processors backplane can 
> > actually maintain peak bandwidth on all cluster ports when the per 
> > node bandwidth is choked by say your on board memory bus or real I/O 
> > sustain bandwidth on board
> > No where does it tell you in a typical multi-giga bit SERDES, even if 
> > the I/O is idle, the idle pattern actually draws zero power vs. peak 
> > activity
> > No where does it tell you the bandwidth of your memory refresh cycle 
> > and access activity factor variations
> >  
> > Once again, I won't say with only .1uf will work. But with sensible up 
> > and down values around it, it is a good starting point.
> >  
> > And remember the two Chris Cheng rules of SI
> > a) Manage your core power distribution with stages from die to package 
> > to PCB with highspeed at die/package and medium/low speed on PCB
> > b) Manage your I/O return current properly
> >  
> > If you buy the above, PCB decoupling is a matter of 100MHz or below 
> > PROVIDED you manage your return current properly.
> >  
> > Another important point is also related to rule a) is if you believe 
> > your critical PCB distribution responsibility is around a few MHz to 
> > 100MHz, can you afford to waste your limited space on your PCB to 
> > populate those junk 10pf or 100pf other people love to sprinkle ? Back 
> > in the days when boards are big and performance and financial 
> > budget are loose, we chose to let whoever speculate whatever is needed 
> > provided he/she sign the design off in a bureaucratic company. Can we 
> > afford to do that anymore ? Not in a small company like mine and we 
> > make a conscious decision to hire one type of engineer responsible for 
> > both SI and EMI. And it worked out great.
> >  
> > And if you have time/resources or stupid enough like me, why not try 
> > experiment 1) or 2) :-D.
> >  
> >
> > ------------------------------------------------------------------------
> > *From:* si-list-bounce@xxxxxxxxxxxxx on behalf of Bowden, Ivor
> > *Sent:* Sat 3/29/2008 1:31 PM
> > *To:* steve weir
> > *Cc:* Gil Simsic [IEEE]; si-list@xxxxxxxxxxxxx
> > *Subject:* [SI-LIST] Re: PDS analysis?
> >
> > Hi Steve,
> >
> >
> > Thank you for your reply! This is the type of answer I was looking 
> > for. I'm not "going anywhere with this", not advocating any particular 
> > position, just looking for information about ramifications of typical 
> > design practices.
> >
> >
> >
> > I know that PDS analysis tools are not as ubiquitous as simulation / 
> > crosstalk tools, and many companies, especially smaller ones, tend to 
> > skip this step, instead relying on experience with past designs. I was 
> > wondering how frequently these designs have problems due to 
> > sub-optimal PDS, the nature of the problems, and the resolutions. For 
> > instance, have you seen many cases where going to smaller value bypass 
> > caps at specific points improved a broken circuit?
> >
> >
> >
> > I apologize for not doing more research before posting the question. 
> > I'm sure I can find a variety of examples with enough searching. 
> > Mainly I was looking for opinion, preferably backed by knowledge.
> >
> >
> >
> > As sort of an informal survey, it was interesting to note that the off 
> > list replies tended more towards "for typical boards PDS isn't needed 
> > if proper rules are followed", generally backed up with statistics of 
> > successful boards not PDS analyzed, while the on list replies tend to 
> > be more of the "always do PDS analysis" flavor.
> >
> >
> >
> > Again, not going anywhere with this. I agree that the safest design 
> > practice is to fully simulate the design, including PDS.
> >
> >
> >
> > Ivor
> >
> >
> >
> >
> >
> > -----Original Message-----
> > From: steve weir [mailto:weirsi@xxxxxxxxxx]
> > Sent: Saturday, March 29, 2008 3:00 AM
> > To: Bowden, Ivor
> > Cc: Gil Simsic [IEEE]; si-list@xxxxxxxxxxxxx
> > Subject: Re: [SI-LIST] Re: PDS analysis?
> >
> >
> >
> > Ivor, you can find examples in papers by people like Larry, Istvan, and
> >
> > books that are out there.  But, I am not sure where you are going with
> >
> > this.  Since terms like "well fed" are ambiguous, I don't know what
> >
> > example you are looking for or to what purpose.  My sense is that you
> >
> > have a belief that if one follows some ambiguous "industry practices"
> >
> > that the PDN will usually "work fine".  Undefined engineering leads to
> >
> > undefined results.  If you want predictable results then:  Define the
> >
> > requirements and engineer to them.
> >
> >
> >
> > We can characterize components to determine their actual power delivery
> >
> > requirements.  Once known we can easily demonstrate failures that result
> >
> > when we don't meet those requirements.  It is also easy to demonstrate
> >
> > what different PDN practices do to the impedance profile under any
> >
> > particular set of controlled circumstances we wish to create.
> >
> >
> >
> > If you want to see resonance effects for yourself then:  Design some
> >
> > board following one of the popular ad-hoc rules.  If for example you
> >
> > design a board with one 0402 capacitor every two square inches and the
> >
> > plane cavity is 4mil FR406 on layers 2/3, the bypass to cavity PRF will
> >
> > land around 330MHz depending on how you do your vias. If the planes are
> >
> > deeper in the board that frequency will just go down.  Then get yourself
> >
> > a programmable clock generator and use it to drive some device like a
> >
> > PLD or FPGA with a bunch of outputs simultaneously in a simple 1-0-1-0
> >
> > sequence while you monitor the power supply planes with a decent scope. 
> >
> > Just step frequency until the bit rate / 2 excites the lowest parallel
> >
> > resonance in the power system.  After that experiment see if your
> >
> > feelings about what works fine are still the same.
> >
> >
> >
> > Steve
> >
> >
> >
> > Bowden, Ivor wrote:
> >
> > > Hi Gil,
> >
> > > 
> >
> > >
> >
> > > Thanks for sharing.
> >
> > >
> >
> > > 
> >
> > >
> >
> > > By "real world" I do not mean rules of thumb and shortcuts, I mean 
> > examples. I expect that most folks on this list, especially those for 
> > whom SI is primary activity, can demonstrate examples of operational 
> > failures due to bad board layout. I also know that a large number of 
> > designs never get PDS analysis, yet work fine. I'm interested in 
> > typical modern technology designs that are laid out in standard 
> > industry practice: contiguous ground planes, tandem heavy split power 
> > planes not used as return reference, plenty of properly located bypass 
> > caps, etc that have operational failures which could have been 
> > pre-determined by PDS analysis, and if so, how did that operational 
> > failure manifest? For example, it would be interesting to see a scope 
> > (or simulation) snap shot of the voltage measured directly across vias 
> > to contiguous well fed planes as a device current requirement 
> > approaches the planes resonant frequency. And I mean a real device, 
> > not a hypothetical entity tuned to instig
> >  at
> >
> > >  e the problem.
> >
> > >
> >
> > > 
> >
> > >
> >
> > > -Ivor
> >
> > >
> >
> > > 
> >
> > >
> >
> > > -----Original Message-----
> >
> > > From: Gil Simsic [IEEE] [mailto:gsimsic.ieee@xxxxxxxxxxxxx]
> >
> > > Sent: Friday, March 28, 2008 1:08 PM
> >
> > > To: Bowden, Ivor; si-list@xxxxxxxxxxxxx
> >
> > > Subject: Re: [SI-LIST] Re: PDS analysis?
> >
> > >
> >
> > > 
> >
> > >
> >
> > > Ivor
> >
> > >
> >
> > > 
> >
> > >
> >
> > > I am involved in design since 1993, and have numerous of successful 
> > designs
> >
> > >
> >
> > > under my belt. (I know - not very humble...)
> >
> > >
> >
> > > 
> >
> > >
> >
> > > In one way or another I worked and learned from SI leaders (aka -
gurus)
> >
> > >
> >
> > > like Lee Ritchey, Istvan Novak, Eric Bogatin, Scott McMorrow and
Howard
> >
> > >
> >
> > > Johnson (and others that due to a senior moment I did not mention
here -
> >
> > >
> >
> > > sorry) I gratefully owe my knowledge to them.
> >
> > >
> >
> > > 
> >
> > >
> >
> > > by the way - all I tried to do is to learn about "real world picture".
> >
> > >
> >
> > > 
> >
> > >
> >
> > > My greatest lesson is - there are no short cuts and no rule of
thumbs. I
> >
> > >
> >
> > > read daily emails on this list that prove this point over and over 
> > again.
> >
> > >
> >
> > > 
> >
> > >
> >
> > > And than again I might misunderstand you...
> >
> > >
> >
> > > If the by 'real world' you refer to 'rules of thumbs' and 'short 
> > cuts', I
> >
> > >
> >
> > > will strongly recommend you to shy away from that 'real world'. I
really
> >
> > >
> >
> > > think that any design as hypothetic or rhetorical as it is, needs 
> > analysis
> >
> > >
> >
> > > (the subject of your email is PDS analysis...).
> >
> > >
> >
> > > 
> >
> > >
> >
> > > Most of the SI phenomena are frequency depended.
> >
> > >
> >
> > > 
> >
> > >
> >
> > > You said - * I'm more interested in answers like "you might see a 
> > waveform
> >
> > >
> >
> > > of xxx characteristics across the bypass capacitor". *
> >
> > >
> >
> > > My answer - "you might see a waveform of xxx characteristics across
the
> >
> > >
> >
> > > bypass capacitor". ;-)
> >
> > >
> >
> > > How can one attempt giving you any 'ball-park' numbers for a frequency
> >
> > >
> >
> > > dependent component with out knowing what the frequency parts are?
> >
> > >
> >
> > > 
> >
> > >
> >
> > > Good luck!
> >
> > >
> >
> > > 
> >
> > >
> >
> > > Gil
> >
> > >
> >
> > > ----- Original Message -----
> >
> > >
> >
> > > From: "Bowden, Ivor" <ibowden@xxxxxxxxxxxxxxxxx>
> >
> > >
> >
> > > To: <si-list@xxxxxxxxxxxxx>
> >
> > >
> >
> > > Sent: Friday, March 28, 2008 12:11 PM
> >
> > >
> >
> > > Subject: [SI-LIST] Re: PDS analysis?
> >
> > >
> >
> > > 
> >
> > >
> >
> > > 
> >
> > >
> >
> > >  
> >
> > >> Hi,
> >
> > >>    
> >
> > >
> >
> > >  
> >
> > >
> >
> > >  
> >
> > >
> >
> > >  
> >
> > >> I'd like to thank everyone who replied so far, off and on the list. I
> >
> > >>    
> >
> > >
> >
> > >  
> >
> > >> emphasize that this is a rhetorical question, it doesn't represent
any
> >
> > >>    
> >
> > >
> >
> > >  
> >
> > >> specific product. I also emphasize that I'm interested more in the 
> > "real
> >
> > >>    
> >
> > >
> >
> > >  
> >
> > >> world observation" part. Instead of answers like "your power plane
may
> >
> > >>    
> >
> > >
> >
> > >  
> >
> > >> have a resonance at xxx frequency", I'm more interested in answers
like
> >
> > >>    
> >
> > >
> >
> > >  
> >
> > >> "you might see a waveform of xxx characteristics across the bypass
> >
> > >>    
> >
> > >
> >
> > >  
> >
> > >> capacitor". Also, this question is more about power distribution than
> >
> > >>    
> >
> > >
> >
> > >  
> >
> > >> signal return path. All answers appreciated.
> >
> > >>    
> >
> > >
> >
> > >  
> >
> > >
> >
> > >  
> >
> > >
> >
> > >  
> >
> > >
> >
> > >  
> >
> > >> -Ivor
> >
> > >>    
> >
> > >
> >
> > >  
> >
> > >
> >
> > >  
> >
> > >
> >
> > >  
> >
> > >
> >
> > >  
> >
> > >> ________________________________
> >
> > >>    
> >
> > >
> >
> > >  
> >
> > >
> >
> > >  
> >
> > >> From: Bowden, Ivor
> >
> > >>    
> >
> > >
> >
> > >  
> >
> >
> >
>
>
> -- 
> Steve Weir
> Teraspeed Consulting Group LLC 
> 121 North River Drive 
> Narragansett, RI 02882 
>
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>
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>
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>
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