Data signals are supposed to run against GND. A/C are supposed to run against VDD. Steve On 9/8/2012 7:19 AM, Stefan Milnor wrote: > But they are not against VDD, they are against GND. Hence my question; are we > getting conflicting advice from the vendor (route signals against GND but > bypass VTT to VDD) > > On Sep 7, 2012, at 10:55 PM, "steve weir" <weirsi@xxxxxxxxxx> wrote: > >> d) makes sense if you are also routing according to the recommendations, >> which runs A/C against VDD in the PCB. >> >> Steve >> On 9/7/2012 10:17 PM, Stefan Milnor wrote: >>> Experts - >>> >>> >>> For a DDR3 interface, for an SOC from a vendor that you all have heard >>> of, we are being advised to: >>> >>> >>> >>> a) Reference all DDR3 signals to GND >>> >>> b) Terminate certain signals at the far end, to VTT (0.75V) >>> >>> c) Make sure the VTT supply can sink and source current >>> >>> d) Bypass the VTT supply with caps from VTT to VDD (1.5V) (and not >>> to GND) >>> >>> >>> >>> Does item d) make sense, if the signals are GND referenced? Wouldn't it >>> make more sense if those signals we referenced against VDD? >>> >>> >>> >>> The vendor's rationale for bypassing with caps from VTT to VDD is quoted >>> as follows: >>> >>> >>> >>> "The reason for bypassing to 1.5V instead of GND is that VTT is both >>> sinking and sourcing current. When high side mosfet of the buffer is >>> turned ON, for logic HI, the current flow is from 1.5V to MOSFET to >>> termination resistor to VTT power supply. Signal is switched referenced >>> to VTT, so we need to decouple the buffer's 1.5V pin to VTT, not to GND. >>> >>> No need for the decap between buffer's GND to VTT, because GND is much >>> more robust than 1.5V rail (usually a massive plain) and can supply >>> necessary instantaneous current for logic LOW." >>> >>> >>> >>> What do you think? >>> >>> >>> >>> Thanks, Stefan M. >>> >>> >>> ------------------------------------------------------------------ >>> 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 forum is accessible at: >>> http://tech.groups.yahoo.com/group/si-list >>> >>> 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 >> >> All contents Copyright (c)2012 IPBLOX, LLC. All Rights Reserved. >> This e-mail may contain confidential material. >> If you are not the intended recipient, please destroy all records >> and notify the sender. >> >> ------------------------------------------------------------------ >> 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 forum is accessible at: >> http://tech.groups.yahoo.com/group/si-list >> >> 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 All contents Copyright (c)2012 IPBLOX, LLC. All Rights Reserved. This e-mail may contain confidential material. If you are not the intended recipient, please destroy all records and notify the sender. ------------------------------------------------------------------ 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list 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