[SI-LIST] Re: DDR2 2-slot design preference...
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: leeritchey@xxxxxxxxxxxxx
- Date: Mon, 29 Oct 2007 19:18:19 -0400
Lee
Actually that quote should either be attributed to Mark Twain, or
Benjamin Disraeli, not Truman. Twain claimed he'd heard it from Disraeli.
As for whether cavity resonances occur, that's an indisputable fact.
They do occur, and they can be very easily measured and correlated to
simulation. Whether they cause an issue is a matter of magnitude,
whether it be EMI, PI, or SI. I can assure you that many a memory
system has failed in the past, and will fail in the future, because of
cavity resonance in the power/ground cavity, when signals transition.
This was one of the root causes (there were several) of early Rambus
system failures, and is the reason why all DDR-II double data-rate
signals are specified to be ground referenced from controller to memory
module.
scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
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Lee Ritchey wrote:
> Steve,
>
> I have that paper and have read it. It does not corellate from the tests
> made to any real EMI failures. It just shows that ground stitching
> results in a difference. There is a claim that this helpes solve a problem
> with a real product, but no data was presented. (There are many such
> claims by EMI "Gurus" about their rules of thumb that turn out to be just
> that and this paper shows one, the 20H rule for one.)
>
> Again, show me a real failure from cavity resonances, either functional or
> EMI not a simulation.
>
> Truman used to say there are lies, damn lies and statistics. In our
> profession there are lies, damn lies and unvalidated simulations, and, no,
> that paper does not demonstrate a direct correlation between the things
> simulated and real failures, just before and after differences.
>
> Lee
>
>
>
>> [Original Message]
>> From: steve weir <weirsi@xxxxxxxxxx>
>> To: <leeritchey@xxxxxxxxxxxxx>
>> Cc: <Monji.Jabori@xxxxxx>; <si-list@xxxxxxxxxxxxx>
>> Date: 10/29/2007 10:27:21 AM
>> Subject: [SI-LIST] Re: DDR2 2-slot design preference...
>>
>> Lee, the case at issue was a GND-GND cavity, but applies equally well
>> for mixed voltages. The physics are indisputable. The issue is how bad
>> does the problem have to be before we get: a) EMI radiation, and EFT /
>> ESD susceptibility, or b) signal integrity issues.
>>
>> The good news is that until we get to ultrafast edge rates the simple
>> remedy for the GND-GND case is simply stitch with sufficient density.
>> That is becoming tougher for GND-VCC cavities, but can still be done in
>> many cases. It depends on just how much energy one wants injects into
>> the cavity and what the spectrum of that energy looks like.
>>
>> For an example of a real-life case where adding stitching fixed a real
>> problem get a hold of EMC2 / Ansoft paper delivered in Santa Clara just
>> last week. There you will see good correlation between both the models
>> and the measurements demonstrating the phenomenon. In their case the
>> stitch was good enough for signaling but was generating an EMI issue.
>>
>> In the past few years the speed and accuracy of various simulation
>> methods has improved immensely. This permits designers to play what-if
>> and get excellent performance without becoming slaves to ad-hoc rules.
>>
>> Best Regards,
>>
>>
>> Steve.
>>
>> Lee Ritchey wrote:
>>
>>> I have heard many times about cavity resonances between power and ground
>>> planes being the source of problems. However, I have yet to see any
>>>
> clear
>
>>> proof that such things happen and I have never seen a product fail from
>>> this in the design of 3000+ PCBs. Could those who claim this happens
>>> please supply some evidence of this? Not simulations, but actual
>>>
> measured
>
>>> results and not a statement that the last place it happened was at a
>>>
> client
>
>>> who has an NDA in place that prevents revealing the data.
>>>
>>> Lee Ritchey
>>>
>>>
>>>
>>>
>>>> [Original Message]
>>>> From: steve weir <weirsi@xxxxxxxxxx>
>>>> To: <Monji.Jabori@xxxxxx>
>>>> Cc: <si-list@xxxxxxxxxxxxx>
>>>> Date: 10/27/2007 2:09:16 PM
>>>> Subject: [SI-LIST] Re: DDR2 2-slot design preference...
>>>>
>>>> Jabori, the issue in each case will be the resonant frequency of the
>>>>
> GND
>
>>>> - GND cavity. Stitch the cavity with an adequate via density to get
>>>>
> the
>
>>>> resonance well above your signal energy and either topology can be
>>>>
> made
>
>>>> to work well.
>>>>
>>>> Steve.
>>>> Jabori, Monji wrote:
>>>>
>>>>
>>>>> Hi Experts,
>>>>>
>>>>>
>>>>> I am looking at two designs for a 2-slot DDR2-800 memory system. One
>>>>>
> is
>
>>>>> a butterfly design and the other has the slot on opposite sides of the
>>>>> motherboard (top and bottom).
>>>>>
>>>>> I am looking at the GND reference for both designs and have the
>>>>> following question knowing that I have to use GND referenced layers
>>>>>
> for
>
>>>>> each DDR2 channel.
>>>>>
>>>>>
>>>>>
>>>>> For a butterfly design, my traces on an 8-layer design will flow from
>>>>> the memory controller via 2 internal layers with 2 different GND
>>>>>
> layers,
>
>>>>> i.e., one slot will have to change GND reference while the other will
>>>>> not.
>>>>>
>>>>>
>>>>>
>>>>> In an opposite 2-slot design, however, we can make the traces that go
>>>>>
> to
>
>>>>> each internal layer have the same GND reference as the DIMM that is
>>>>> closer to it. For example, the Top DIMM will have the same GND
>>>>>
> reference
>
>>>>> (L2) as its channel traces coming on L3 while the Bottom DIMM will
>>>>>
> have
>
>>>>> its GND reference (L7) as its channel traces coming on L6.
>>>>>
>>>>>
>>>>>
>>>>> Having said the above, would you guys prefer one design over another
>>>>> from an Signal Integrity point of view??
>>>>>
>>>>>
>>>>>
>>>>> Thanks in advance.
>>>>>
>>>>>
>>>>>
>>>>> Monji
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
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>>>>>
>>>> --
>>>> Steve Weir
>>>> Teraspeed Consulting Group LLC
>>>> 121 North River Drive
>>>> Narragansett, RI 02882
>>>>
>>>> California office
>>>> (408) 884-3985 Business
>>>> (707) 780-1951 Fax
>>>>
>>>> Main office
>>>> (401) 284-1827 Business
>>>> (401) 284-1840 Fax
>>>>
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>>>>
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>>>
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>>>
>>>
>> --
>> Steve Weir
>> Teraspeed Consulting Group LLC
>> 121 North River Drive
>> Narragansett, RI 02882
>>
>> California office
>> (408) 884-3985 Business
>> (707) 780-1951 Fax
>>
>> Main office
>> (401) 284-1827 Business
>> (401) 284-1840 Fax
>>
>> Oregon office
>> (503) 430-1065 Business
>> (503) 430-1285 Fax
>>
>> http://www.teraspeed.com
>> This e-mail contains proprietary and confidential intellectual property
>>
> of Teraspeed Consulting Group LLC
>
> ----------------------------------------------------------------------------
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>
>> Teraspeed(R) is the registered service mark of Teraspeed Consulting Group
>>
> LLC
>
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