[SI-LIST] Re: [SI-LIST]: Distribution/Filtering/Decoupling Gu ideThe 100Mhz clean cut off.
- From: Mark Alexander <mark.alexander@xxxxxxxxxx>
- To: Chris.Cheng@xxxxxxxxxxxx
- Date: Fri, 16 Jan 2004 15:27:17 -0700
Chris,
I doubt that anyone on the list strictly follows every aspect of EIA or
JEDEC specified IO standards. We all mix and match to get what we want,
as you say. If we didn't, it wouldn't be engineering.
Still, there's a lot of value in standards -- they get people into the
same ballpark and provide an anchor. It's pretty nice that most designs
have standardized in the vicinity of 50 ohm trace impedance for
single-ended interfaces. This has allowed for advances in tools and
techniques that might not have been possible if everyone was haphazardly
choosing t-line impedances at random.
Right now we're in that haphazard area with PDS design. There's a few
camps with favorite techniques, but even those aren't well defined.
Coming up with some relevant standards for PDS's based on PCB types (ala
Steve W's four PCB categories), would draw some useful lines in the
sand. For one thing, it gives silicon vendors some information on what
environments they should have to work in.
No one is intending to put designers in a box by defining some PDS
standard references or benchmarks. The idea is to agree on some common
structures. Based on that, we can intelligently evaluate our own
deviations from the standards.
-mark
Chris Cheng wrote:
>First, let me remind you of what I said below :
>
>"As a side note for those who want to built benchmark system design to
>evaluate system to package distribution, good luck ! If you think you can
>pick up something as subtle as the above, my hats off to you. I have enough
>trouble doing a unique case where I know everything ahead of time."
>
>I don't believe in standards or references or papers or textbooks telling me
>what I should do and what not. You come up with your own analysis based on
>your own unique experiences and challenges. I suppose you have your own
>experiences with >100W >GHz Si core power design and they may very well be
>different from mine or Larry's situation. Most FPGA,ASIC or ASSP have lower
>power and with lower core speed. But that doesn't mean they can get away
>easy.
>
>If there is anything I said below I want you to take, it will be "do you own
>analysis before drawing your conclusion". It is just an example of how over
>design can cost you money but at the end of the day, a bad engineer under
>design and his part fails. A good engineer over design and his part works. I
>think I understand enough of Larry's company design methodologies (afterall,
>I developed a lot of them) to say they don't throw pins in without reasons
>but for every action you take, there is a consequence you pay.
>
>I don't understand what do you mean by "Increasing the inductance to the
>board has a good chance of
>increasing the coupling between different parts of the chips." Are you
>suggesting someone is deliberately adding inductance to the board ? I am
>not, neither is Larry.
>
>-----Original Message-----
>From: Ed Priest [mailto:epriest@xxxxxxxxxxxxxxxxxxx]
>Sent: Friday, January 16, 2004 10:07 AM
>To: silist
>Subject: [SI-LIST] Re: [SI-LIST]: Distribution/Filtering/Decoupling
>Guide The 100Mhz clean cut off.
>
>
>This is getting scary. Please do not globally take the number of
>required power and grounds on a specific project and apply it to all
>applications. Microprocessors, FPGA's, ASSP, ASICs all have a different
>on chip environment. The amount of on chip capacitance, the closeness of
>the di source to the chip capacitance, the presence of on chip mixed
>signal IP, all could make reducing the number of power and grounds a bad
>idea. Increasing the inductance to the board has a good chance of
>increasing the coupling between different parts of the chips. Again,
>microprocessors where both the circuits and power distribution are
>custom designed can have a lot more friendly environment then other
>chips you deal with.=20
>
>This discussion and work can be very beneficial but applied incorrectly
>can have disastrous effects.
>
>Ed
>
>-----Original Message-----
>From: Ravinder.Ajmani@xxxxxxxx [mailto:Ravinder.Ajmani@xxxxxxxx]=20
>Sent: Friday, January 16, 2004 8:35 AM
>To: Chris.Cheng@xxxxxxxxxxxx
>Cc: silist
>Subject: [SI-LIST] Re: [SI-LIST]: Distribution/Filtering/Decoupling
>Guide The 100Mhz clean cut off.
>
>Chris,
>Thanks a lot for the excellent explanation. Will you please tell me
>what=20
>is the adequate number of power and ground pins for a 324 pin BGA
>package.=20
> We use this package in two versions, C4 attach and wire bonded.
>Regards, Ravinder
>Server PCB Development
>Hitachi Global Storage Technologies
>
>Email: Ravinder.Ajmani@xxxxxxxx
>
>
>
>
>Chris Cheng <Chris.Cheng@xxxxxxxxxxxx>
>Sent by: si-list-bounce@xxxxxxxxxxxxx
>01/15/2004 06:56 PM
>Please respond to Chris.Cheng
>
>=20
> To: "'Charles Grasso'" <cgrassosprint1@xxxxxxxxxxxxx>, Chris
>Cheng=20
><Chris.Cheng@xxxxxxxxxxxx>, "'Larry Smith'" <Larry.Smith@xxxxxxx>
> cc: scott@xxxxxxxxxxxxx, silist <si-list@xxxxxxxxxxxxx>
> From: si-list-bounce@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Distribution/Filtering/Decoupling
>Guide The 100Mhz clean=20
>cutt off.
>
>
>
>
>Well, I will give it a try based on what I now see as a complete picture
>
>of
>what's going on with Larry's example.
>
>Disclaimer first
>Since I was responsible for a lot of packaging and module designs in=20
>Larry's
>company before I departed many years ago, I have to say I have never
>seen=20
>or
>reviewed or involved in any of the current or whatever future generation
>package or module he is referring to. Everything I am saying is simply=20
>based
>on what Larry has disclosed in this public forum. If my guess is wrong,
>please accept my apology.
>
>To begin the discussion, I have to requote Zhiping's question below on
>how
>to "shift the power integrity problem to the PCB level". I claim there
>is=20
>a
>hard limit of 100MHz on the package power distribution, by that I mean=20
>with
>the existing number of pins/sockets and package caps and on die=20
>decoupling,
>all the available decoupling charge on the package and die will be=20
>consumed
>and core voltage will start to drop after 10ns of continues current
>drain=20
>on
>die. Something on the PCB and external DC/DC regulator has to start
>providing the charge to maintain the voltage level on die. By the same
>token, due to the exiting impedance at the pins/socket and package, the
>system (PCB or regulator) cannot reach the Si core power faster than=20
>100MHz.
>This is based on an optimized pin analysis where I want to provide the=20
>least
>amount of power and ground pins (if you ship a huge volume, like a 100M
>of
>them, @ a few cents per pin, you save a LOT of money) that can maintain=20
>the
>minimum core noise level on die. The added benefit of it is you don't
>need
>to demand exotic >100MHz decoupling outside the package (since it won't=20
>help
>anyways) and you also don't need to over kill your decoupling on package
>
>by
>providing <100MHz decoupling bulk caps, just pass that responsibility to
>
>the
>system folks.
>
>Now let's say you throw in too many power and ground pins on your
>package.
>Not only does it cost you money (@ a few cents per pin), it now also=20
>starts
>to perforated your PCB power and gnd planes to the extend that the PCB
>to
>package impedance starts to pick up (shifting below 100MHz). Now you=20
>really
>need to have a thin core PCB to lower the impedance back to equal the
>case
>when you have less power/ground pins with a smaller package. But in a=20
>sense,
>this is exactly what Zhiping was asking for, shifting some of the=20
>integrity
>problem to the PCB level. You can over design to an extend that what=20
>happen
>in the PCB really start to impact the package performance.=20
>
>It's your choice, too many pins and you will need BC and cost you
>$$$$$$,
>just enough pins and you don't need BC and save you some $ also. But if=20
>you
>don't have enough pins, you are dead and no system or PCB design can
>save
>you.
>
>As a side note for those who want to built benchmark system design to
>evaluate system to package distribution, good luck ! If you think you
>can
>pick up something as subtle as the above, my hats off to you. I have=20
>enough
>trouble doing a unique case where I know everything ahead of time.
>
>Same old song here, there is no need for BC. Hey, that may be in my
>second
>broken record.
>
>-----Original Message-----
>From: Charles Grasso [mailto:cgrassosprint1@xxxxxxxxxxxxx]
>Sent: Thursday, January 15, 2004 5:34 PM
>To: Chris.Cheng@xxxxxxxxxxxx; 'Larry Smith'
>Cc: scott@xxxxxxxxxxxxx; silist
>Subject: Distribution/Filtering/Decoupling Guide The 100Mhz clean cutt
>off.
>
>
>Chris, Larry et al...
>
>Can you please explain the 100MHz clean cut so often mentioned in ths
>thread? Is it bandwidth (i.e related to rise time?) is it the crystal=20
>or...
>
>Puzzled.
>Chas in Colorado..
>
>
>-----Original Message-----
>From: zhiping yang [mailto:zhiping@xxxxxxxxx]
>Sent: Tuesday, January 13, 2004 5:55 PM
>To: Chris.Cheng@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: Re: [SI-LIST] Re: Power Supply
>Distribution/Filtering/Decoupling Guide]
>
>
>Hi Chris,
>
>Thank you for sharing your thoughts and Very good points about the power
>integrity.
>
>One comment about your "total system approach". Technically speaking,
>it=20
>is
>best to put the equal amount of efforts on the die, package and PCB
>power
>distribution since they are equally important in the complete power=20
>delivery
>system. More important, the PCB COULD NOT fix the problems with the die
>
>and
>package power delivery systems in some cases.
>
>In the real word, when cost and $$ is involved, the trade-offes must be
>made, so the design may not be optimized in technical side. For
>example,
>due to the high cost of die size increase and package limitations, it
>may=20
>be
>more cost effective by shifting the power integrity problem to the PCB=20
>level
>at a certain degree. This may require using BC or more decoupling caps
>on
>the board, but it could still be cost effective from the "total system"
>point view.
>
>This is my $0.02 input. Thanks.
>
>Zhiping
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