[SI-LIST] Re: Current Return Vias
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: perry.qu@xxxxxxxxxxx
- Date: Wed, 26 Jul 2006 13:30:42 -0400
Ding, ding, ding ... this is a good example of a design with a poor
return path due to signal via crowding. Adding ground vias within the
signal via field, for a gnd referenced design, will undoubtedly help, as
long as the ground vias are interspersed within the signal via field.
The PDS will not help you when signal edge rates are in the several
hundred picosecond range.
Those who believe that "good" PDS design will solve all signal via
related crosstalk problems are woefully mistaken.
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
Perry Qu wrote:
> Interesting topic. We recently had a design with a SDRAM interface consists
> of 5 SDRAMs, 2 pairs clamshelled and one standalone. It's running on 155 MHz
> clock single data rate so not a fast interface by today's standard. For the
> clamshelled SDRAMs, we measured the address signals and the waveform on the
> top and bottom sometimes are quite different, specifically, one of the SDRAM
> (could be either top or bottom depending on which address pin we probe)
> shows very visible crosstalk and worse shelf on edge. The result is less
> margin for timing.
>
> This can't be caused by topology as we length-matched the trace from SDRAM
> pin to the branch via in a mixture of star/far-end cluster topology. The
> only thing I can think of is return current as most of the signals vias are
> under the middle of the SDRAM packaging with no ground/power via really
> closeby. The SDRAM packaging is not well done in my opinion as they have
> most of the power/ground pins on one half of the chip, and very few
> reference pins on the other half where all address pins locate. To make it
> worse, the routing layer is all over the stackup and it can jump from a
> layer close to top to a layer close to bottom and then jump back, with no
> return vias close to signal vias, although they all reference to GND (we
> have multiple GND planes throughout the stackup). The worse waveform seems
> always occurs at the SDRAM with bigger jump in terms of reference planes
> thus larger return loop area, which is the cause for crosstalk and higher
> inductance on return and shelf on edge. I decide to add some ground vias
> close to the signal vias for our next design which uses same chips.
>
> Regards
>
> Perry
>
>
> =======================================
>
> Perry Qu
>
> Design & Qualification, Alcatel Canada
>
> 600 March Road, Ottawa ON, K2K 2E6
>
> DID: 613-7846720 Fax: 613-5993642
>
> Email: perry.qu@xxxxxxxxxxx
>
> =======================================
>
>
> ------------------------------------------------------------------
> 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 FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
> http://www.si-list.org
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> 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 FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
