Peter, From the discussion, I believe that you have the following configuration SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS Layer 1 (Signal) P1PP1P1P1P1P1P1 P2P2P2P2P2P2P2P2P2P2 Layer 2 (Split Power) GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG Layer 3 (GND) SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS Layer 4 (Signal) For the signals on layer 1, the issue is that the return current will be on the power planes not the ground. The Power to Ground natural capacitance will allow the return current to cross the split, but the effectiveness will depend on the dielectric thickness (Thinner is better). Other options to manage the return issue are: 1) You can put coupling capacitors between the 2 power regions (But this will couple supply noise between the supplies. This ill also have multiple signals sharing the return path, increasing crosstalk) 2) You can place decoupling capacitors on either side of the split, thus providing the return path, but again you may have multiple signals sharing the same capacitor, increasing signal crosstalk The split will also be a region of emissions due to the discontinuity There are no elegant solutions with 4 layers. Doug -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Pang Ning Sent: Tuesday, April 19, 2005 10:47 AM To: Pang Ning; weirsi@xxxxxxxxxx; Shikha soni Cc: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: DDR traces across different power plane Steve/Shikha, Sorry, DQS is still 250MHz, but data rate is 500Mt/s. Best Regards, Peter Pang ----- Original Message ----- From: Pang Ning To: weirsi@xxxxxxxxxx ; Shikha soni Cc: si-list@xxxxxxxxxxxxx Sent: Tuesday, April 19, 2005 10:41 PM Subject: Re: [SI-LIST] Re: DDR traces across different power plane Steve/Shikha, Thanks for your reply. It may cause problems as you mentioned. But I'm still confused. Even if the high speed traces (on layer 1) cross different VCC planes on layer 2, the ground return current of these traces will still flow on the GND plane which is intact on layer 3. The problem that I'm worried is the impedance discontinuous for these traces. Could you help to explain why you think this kind of routing will cause SI issue except impedance broken? Thanks a lot. I tried to run some simulation with SPEED2000 and didn't found serious problems. I thought of another routing scheme, parts of address & control traces of DDR are routed on VCC plane. In this way, most of traces won't cross power plane. However, there are some address trace segments which will be located above the routing area on VCC plane. I don't know whether this kind of routing will be better than previous one. The bottom layer is full of DDR traces as well so it's useless to exchange VCC and GND layers. It's a challenge that MCLK frequency need to achieve 250MHz (DQS 500MHz). Thanks for your help! Best Regards, Peter Pang ----- Original Message ----- From: steve weir To: pangning2000@xxxxxxxxx ; si-list@xxxxxxxxxxxxx Sent: Tuesday, April 19, 2005 8:34 PM Subject: [SI-LIST] Re: DDR traces across different power plane Peter, if you mean a 250MHz clock and 500Mt/s data rate, then you likely have some serious problems. If you mean a 125MHz clock and 250Mt/s, then with some care, you should be able to manage getting the address and control lines across a split. Steve At 06:37 PM 4/19/2005 +0800, Pang Ning wrote: >Dear experts, >I am currently working on a 4-layer PCB layout. Due to the complicated >power scheme, some of the DDR traces have to be routed across different >power planes. In details, DDR traces are mainly on the layer 1, and VCC is >layer 2. Many address & control traces must be routed above the gap >between different VCC planes. I understood the impedance will be >discontinuous for these traces, but it's unavoidable in order to reduce >cost. I want to know whether there is any other problems for this kind of >routing? The GND plane is intact on layer 3. The return path of these >traces should be good since GND plane is good, right? > >The reflection may not be serious because these signal has relatively slow >switching rate. Did anybody try this kind of routing experience for DDR >before? Our target frequency will be 250MHz. Thanks for your comments. > >Best Regards, >Peter Pang >04/19/2005 >------------------------------------------------------------------ >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 ------------------------------------------------------------------ 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