All, A few clarifying additions on the appnote Steve referenced: XAPP623 is the PDN design guideline for older Virtex products like Virtex-2 Pro. The china.xilinx.com link below should pull up the correct document, but here's the link through our main website: http://www.xilinx.com/support/documentation/application_notes/xapp623.pd f For current FPGAs like Virtex-4 and Virtex-5, we made the guidelines specific to each product family. This was necessary because the device power systems have been getting more unique. Consult the respective PCB Design Guide for PDN design guidelines: Virtex-4: http://www.xilinx.com/support/documentation/user_guides/ug072.pdf Virtex-5: http://www.xilinx.com/support/documentation/user_guides/ug203.pdf = Virtex-5 is the first family where we've made the decoupling guidelines completely prescriptive: it's a fixed network per-device. -mark = -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Salkow, Steven Sent: Monday, June 23, 2008 11:45 AM To: Dan Bostan; Ambr Amit Cc: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Number of layers Picture is worth a 1000 words. (See my website: http://www.bychoice.com/XC2VP70_6FF1704C.jpg This is a Xilinx part from one of our designs with the GND pins highlighted. You can see what they have done. The layout of an IC and the ability to PIN it out is always a compromise. Xilinx, however, has done something rather smart as far as decoupling the IC has they have included capacitance in their packaging that minimizes the effect of ground bounce. The complex nature of electronic packaging has kept many fine electronic engineers employed for life and that cannot be conveyed in an email. Many designs, like PC are executed in 4 layers as it's real cheap. To make that even practical, SI engineers at Intel and other companies often iteratively redesign the board chip sets and even the processor until they have something that works. Are these OPTIMUM solutions as far as signal integrity? -- Absolutely not. = The number of pins, the amount of signals switching at the same time, the pin inductance, differential coupling, amount of power needed, etc. . . This app note from Xilinx may help a lot: http://china.xilinx.com/support/documentation/application_notes/xapp623. pdf Good Luck Steve Salkow -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Dan Bostan Sent: Monday, June 23, 2008 10:48 AM To: Ambr Amit Cc: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Number of layers Why use only four layers for the board? In order to break the signals properly under the chip you (probably) need six layers for the board. Going that way you would improve the SI. /dan ----- Original Message ---- From: Ambr Amit <ambr_amit@xxxxxxxxx> To: si-list@xxxxxxxxxxxxx Sent: Monday, June 23, 2008 7:01:10 AM Subject: (no subject) Hi, I'm working on a 6-layer flipchip package design with following stackup: VSS signal VDD VSS VDD Balls This package gets used on a 4-layer board with stackup of signal VSS VDD signal The design supports high speed interfaces like DDR3-1600 & multiple Serdes (upto 5Gbps). While doing ball assignment I'm proposing to place power/ground (VDD/VSS) balls in the perimeter of package (to get better return paths for high speed signals). But based on the board routing this is turning out to be bad idea because it blocks lot of signal routes and proposal is to move all the power/ground (VDD/VSS) inside towards the centre of the package. I'm not comfortable with this because = - this will affect the return path = - it results in current crowding in the central area where lot of VDD/VSS balls are places - and ineffective use of package internal planes This can result in SI issues. But sadly I do not have any data which can support that moving all the VDD/VSS towards centre will result in SI issues. One part is doing simulation to see the impact of bad return path, which I'm working on. But if someone in SI-list has already done study on this scenario, I would appreciate their opinions and data on this subject. Thanks and regards Amit = ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net 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 technical documents are available at: http://www.si-list.net 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 technical documents are available at: http://www.si-list.net 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 = This email and any attachments are intended for the sole use of the named r= ecipient(s) and contain(s) confidential information that may be proprietary= , privileged or copyrighted under applicable law. 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