Steph, Since the main benefit of this configuration is reduction of switching current drawn from the PWR/GND structure, the increased loop inductance is not a concern. This is assuming of course PWR is dedicated I/O power. Thanks, Vinu Stephane Tremblay wrote: > Vinu, > > I like your idea of symmetrically reference the signal to a ground and a > vddio plane. I do understand the benefits of doing so. On a typical package > stackup, it also means that instead of having (representing only 3 layers of > interest) SIG-GND-PWR, you would change to GND-SIG-PWR. Doing so will > realize your plan but it will increase the loop inductance from PWR to GND. > > Can anyone comment on this practice? Anyone ever tried that? > > Thanks for the help, > Steph. > > -----Original Message----- > From: Vinu Arumugham [mailto:vinu@xxxxxxxxx] > Sent: Wednesday, April 09, 2008 12:26 PM > To: Stephane Tremblay > Subject: Re: [SI-LIST] On-die caps for IO supply > > If you can afford to symmetrically reference the signal to a ground and > vddio plane along the entire length of the interconnect and use a > thevenin termination to vddio and ground, the die capacitance required > for signal switching is close to zero. You will only need enough I/O die > capacitance to handle the parasitic 3.5pF, any crowbar current and > pullup/pulldown asymmetry. > > Thanks, > Vinu > > Stephane Tremblay wrote: > >> Hi SI-Listers, >> I am looking for a rule of thumb on the required on-die capacitors >> > needed for proper operation of a given IO (a DDR mem IO in my case). I am > looking for a number of "x" capacitance per IO. > >> My IO can be programmed to 18 Ohms of drive and the measured slew-rate is >> > slightly higher than 6 V/ns for both rise and fall. The parasitic > capacitance of this IO is about 3.5 pF. The toggle rate could be as high as > 2 Gbps so I expect the IO rail to recover within reasonable limits within > half a period. > >> I could always start by assuming the current I need just for the >> > transmission line (being a 50 Ohms one): > >> dI/dt = 6 V/ns / 50 Ohms = 120 mA / ns (per IO) >> >> but this basic current demand calculation neglected the parasitic >> > capacitance of 3.5 pF. > >> Some will say it is also greatly dependant of my power-gnd loop inductance >> > from IO to on-package decaps. On the other end, if my noise spectrum is in > the hundreds of MHz, I can't do much at the package level to clean my die > supply. > >> So from your experience and knowledge, what would be a good start as a >> > required on-die capacitance required. I want to avoid my on-die IO rail > collapsing at a frequency that my package could not keep up. > >> Thanks for the answers, >> Steph. >> >> >> >> ------------------------------------------------------------------ >> 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