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