Once DQS latches the DQ at the input to the part, the device must then synchronise the data to its local clock. If the clocks between say the NB and the DDR memory devices are to far out of alignment, it will not be possibble to compensate for this mis-alignment in the recieving device. This will mean that the data cannot be properly sampled in the local devices recieving clock domain. Best Regards John > -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Jack W.C. Lin > Sent: 17 October 2002 09:32 > To: si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] Constraints on DQ/DQS and CK/CK# in DDR > > > Hi All SI friends: > I met a question about DDR. We all know that DDR is a source > synchronus > structure, data transfer will not depend on bus clock any > more. In chip > design, there exist Delay mechanism for DQ and DQS (90 degree > phase shift) > which is controlled by bus clock. But we found that either in > Intel 845 chip > series or AMD K8 ¡K., they all have some layout constraint on > DQ/DQS and > command clock (CK&CK#). For example in Intel, if DQ/DQS > length is Y, CK/CK# > is X, then they should follow X-4.4<=Y. In AMD, Y=X+0.5 or > Y=X-0.5. Why? In > DDR mechanism, it seems no reason to put such constraints on > them. Maybe I > miss something, is there anyone could provide a reasonable > explanation? > Thanks > > Jack > > ------------------------------------------------------------------ > 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 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 > > > > __________________________________________________ > Virus checked by MessageLabs Virus Control Centre. > __________________________________________________ Virus checked by MessageLabs Virus Control Centre. ------------------------------------------------------------------ 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 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