[SI-LIST] Re: SSO and load capacitance
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Larry Smith <LSMITH@xxxxxxxxxx>
- Date: Tue, 02 Aug 2011 15:18:21 -0400
larry
The question was about SSO, not SSN. SSN, as you have defined it is
merely the inductive coupling component of the noise within the package.
However, SSO noise (by my definition) extends throughout the system, and
is influenced by all aspects of the system design. One thing that you
fail to take into account is the possibility that your driver design
might have an impact on the overall SSO noise. Even with the best end
of line termination, capacitive loads will still kick back to the driver
and interact with it's dynamic impedance - which is an aspect of the
silicon design. Simultaneously, there is the additional interaction of
the power system with the driver. This leads to some interesting
practical considerations.
* Some drivers have excessively low output impedance during
transient switching and can be problematic in this regard.
Clearly this can be a silicon issue, that I've seen way too many
times to count.
* Other drivers have dynamic slew rate control circuits (that can
generally only be modeled in Spice, and are often not modeled at
all) that can act unfavorably and turn off prematurely when these
reflected waves travel back to the driver. I've seen this on
numerous occasions across various memory interface types.
* These are not "just" system issues, but are instances of the
silicon interacting with the system in interesting ways.
Scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
On 8/2/2011 2:55 PM, Larry Smith wrote:
> Scott - You are correct about the reflection phenomenon with transmission
> lines that are not perfectly terminated. We will definitely get a bounce off
> the load capacitance and energy coming back towards the place where SSN
> originated. I consider that to be more of a bus termination problem than an
> SSN problem.
>
> I have a very narrow definition of SSN (simultaneous switch noise). It is
> the waveform launched into a victim (quiet-high or quiet-low) signal line
> when aggressor signals switch simultaneously. As mentioned before, there is
> an inductive coupling portion to the signal that is only launched during the
> aggressor rise (fall) time and there is a longer wavelength PDN component
> (that Steve referred to) that is initiated by the SSN event. The inductive
> coupling component tends to be about 200 pSec long (~ .35/.2 = 1.75 GHz
> content) and the PDN component tends to be at about 100MHz.
>
> I consider the reflections that you have discussed to be part of the "system
> response" to the SSN event. Yes, this is very important but I find it useful
> to divide things up into the initial noise stimulus and then the system
> response to that stimulus because you may choose to manage each of these
> independently. As a component house, we can only manage the initial
> stimulus. The system house needs to manage the
> board/reflection/termination/load issues. And of course, we need to provide
> the system houses with components that they can live with on their boards. :)
>
> Another point to be made is that the inductive coupling glitch (1.75MHz
> frequency content) fortunately suffers significant loss on its journey to the
> load and back to the source. This tends to damp out system resonances at
> least to some degree. When we go to measure inductive coupling SSN, we have
> to account for the loss in our measurement fixture (possibly 18 inches of
> board transmission line) and use de-embedding to find out the SSN amplitude
> that our customers are likely to see at some distance away from the SSN
> source. In this case, loss is our friend..
>
> Regards,
> Larry
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Scott McMorrow
> Sent: Tuesday, August 02, 2011 9:17 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: SSO and load capacitance
>
> Larry
>
> Be careful. Load capacitance looks like a momentary reflected short
> circuit to the driver. If the driver is not well matched (for example
> low output impedance drive on a DDR driver), we have effectively two low
> impedance discontinuities on either end of the line, setting up a
> half-wave resonant circuit. Now, you are quite right that the reflected
> discontinuity back from the load capacitance can never be larger than
> the initial SSO charge up of the line at the driver, however, it is
> possible to tune the delay such that the reflection reaches the driver
> at exactly the wrong time - when the driver switches during the next
> cycle. With the right bit rate, a standing wave can occur, causing SSO
> to peak.
>
> Scott
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
>
> http://www.teraspeed.com
>
> Teraspeed(r) is the registered service mark of
> Teraspeed Consulting Group LLC
>
>
> On 8/2/2011 11:18 AM, Larry Smith wrote:
>> Jason - the comment is correct: the worst case SSN waveforms will be found
>> with minimum load capacitance. But some explanation is required.
>>
>> First, SSN can be broken down into two components: inductive coupling and
>> PDN. Steve is referring to the PDN part in his response. But usually the
>> greatest SSN noise amplitude measured at the far end of a signal
>> transmission line comes from inductive coupling, not PDN.
>>
>> Inductive coupling is related to mutual inductance between aggressor signals
>> and the victim signal. It only happens during the rise (fall) time of the
>> driver because that is when the di/dt takes place. To a first
>> approximation, the voltage noise that gets launched into a victim
>> transmission line (under the BGA that makes the SSN) is proportional to
>> m*di/dt where m is the sum of the mutual inductance from all the aggressors
>> to the victim and i is the current in the aggressors. Mutual inductance
>> occurs in the wire bonds, package vias, balls and PCB vias and to a first
>> approximation is proportional to the length of these structures.
>>
>> These days, the aggressor rise time is on the order of 200pSec, which is the
>> time that it takes signals to travel about an inch down a transmission line.
>> The capacitance load in question is down at the far end of the transmission
>> line, let's assume 6 inches. The 200pSec rise time aggressors launch an SSN
>> noise pulse into the victim signal net that is approximately 200pSec wide
>> and it arrives at the capacitance load about 1000pSec later. The load
>> capacitance at the far end will have no effect on the SSN event that
>> launches the SSN glitch into the victim transmission line.
>>
>> When the SSN glitch arrives at the far end of the transmission line, it
>> often finds a 50 ohm termination. The noise measured at the far end is
>> identical to the glitch launched into the near end, assuming lossless lines.
>> Now if there is any capacitance load at the far end, glitch energy goes
>> into charging up the load. The measured SSN glitch voltage amplitude will
>> be less with more load capacitance.
>>
>> Regards,
>> Larry Smith
>>
>> -----Original Message-----
>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
>> Behalf Of steve weir
>> Sent: Tuesday, August 02, 2011 4:41 AM
>> To: si-list@xxxxxxxxxxxxx
>> Subject: [SI-LIST] Re: SSO and load capacitance
>>
>> Oops the second formula was energy not charge. It should have read
>> Qload comes from Qbypass = Vdroop*Cbypass.
>>
>> Steve
>> On 8/2/2011 4:24 AM, steve weir wrote:
>>> Jason, there are two possible sources of confusion. The first is
>>> possible confusion between output load capacitance with die capacitance
>>> per output driver. Your intuition is correct: If we simplify the PDN /
>>> driver network to a switched capacitor representation, then we deposit
>>> Qload = Vdd*Cload on each output line that switches from low to high,
>>> and remove Qload from each output that switches from high to low. For
>>> the low to high switching outputs: Qload comes from Qbypass = (Vdd -
>>> Vdroop)^2/2*Cbypass.
>>>
>>> The second source of confusion comes from the fact that any loads that
>>> remain statically high can draw current from any load capacitance that
>>> connects to the driver outputs, supporting other outputs that switch
>>> from low to high.
>>>
>>> Steve.
>>>
>>>
>>> On 8/2/2011 3:01 AM, Jason Young wrote:
>>>> Dear Experts,
>>>> I have read a couple of documents are from silicon IP vendors discussing
>>>> the number of power/ground pads needed to meet SSO requirements for a
>>>> given number of output drivers. These documents mention that worse case
>>>> conditions for SSO are with the smallest output load capacitance. At first
>>>> this seems counter intuitive. My initial reasoning would be that a larger
>>>> capacitance would present a lower impedance load and hence greater dI/dt,
>>>> greater IR drop and greater supply rail bounce. Could you please help me
>>>> understand?
>>>> Regards,
>>>> Jason
>>>>
>>>>
>>>>
>>>> ------------------------------------------------------------------
>>>> To unsubscribe from si-list:
>>>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>>>
>>>> or to administer your membership from a web page, go to:
>>>> //www.freelists.org/webpage/si-list
>>>>
>>>> For help:
>>>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>>>
>>>>
>>>> List technical documents are available at:
>>>> http://www.si-list.net
>>>>
>>>> List archives are viewable at:
>>>> //www.freelists.org/archives/si-list
>>>>
>>>> Old (prior to June 6, 2001) list archives are viewable at:
>>>> http://www.qsl.net/wb6tpu
>>>>
>>>>
>>>>
>> --
>> Steve Weir
>> IPBLOX, LLC
>> 150 N. Center St. #211
>> Reno, NV 89501
>> www.ipblox.com
>>
>> (775) 299-4236 Business
>> (866) 675-4630 Toll-free
>> (707) 780-1951 Fax
>>
>>
>> ------------------------------------------------------------------
>> To unsubscribe from si-list:
>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>
>> or to administer your membership from a web page, go to:
>> //www.freelists.org/webpage/si-list
>>
>> For help:
>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>
>>
>> List technical documents are available at:
>> http://www.si-list.net
>>
>> List archives are viewable at:
>> //www.freelists.org/archives/si-list
>>
>> Old (prior to June 6, 2001) list archives are viewable at:
>> http://www.qsl.net/wb6tpu
>>
>>
>>
>>
>> Confidentiality Notice.
>> This message may contain information that is confidential or otherwise
>> protected from disclosure. If you are not the intended recipient, you are
>> hereby notified that any use, disclosure, dissemination, distribution, or
>> copying of this message, or any attachments, is strictly prohibited. If
>> you have received this message in error, please advise the sender by reply
>> e-mail, and delete the message and any attachments. Thank you.
>>
>> ------------------------------------------------------------------
>> To unsubscribe from si-list:
>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>
>> or to administer your membership from a web page, go to:
>> //www.freelists.org/webpage/si-list
>>
>> For help:
>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>
>>
>> List technical documents are available at:
>> http://www.si-list.net
>>
>> List archives are viewable at:
>> //www.freelists.org/archives/si-list
>>
>> Old (prior to June 6, 2001) list archives are viewable at:
>> http://www.qsl.net/wb6tpu
>>
>>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
> Confidentiality Notice.
> This message may contain information that is confidential or otherwise
> protected from disclosure. If you are not the intended recipient, you are
> hereby notified that any use, disclosure, dissemination, distribution, or
> copying of this message, or any attachments, is strictly prohibited. If you
> have received this message in error, please advise the sender by reply
> e-mail, and delete the message and any attachments. Thank you.
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
