Here's a neat trick you can try. Most well designed modern processors have some kind of energy star, power saving power down mode that can literally go from deep sleep to full throttle in just a few clock cycles. At >1GHz or <1ns cycle time, the processor itself is one hack of a fancy function generator with a few ns timing resolution provided you can have the correct power testing program that can control the amount of computation (hence power) it executes. Couple that will a flexible PLL on die and a good tester, you can search and test your package power all day until you hit the resonance. It is just a matter of resources (product engineers and tester time). Most of the well designed high power Si chip should have secret pins that tap directly into the core power and gnd grids for measurements. In fact, there were proposals to directly use them to control the DC/DC regulator feedbacks. -----Original Message----- From: Larry Smith [mailto:Larry.Smith@xxxxxxx] Sent: Thursday, January 22, 2004 10:52 AM To: ctwardy@xxxxxxxxxxxxxxxxxx Cc: 'silist' Subject: [SI-LIST] Re: One last question for Chris & Larry re: Power/D ecoupling Craig - It is difficult to know the power supply voltage at the silicon with accuracy at clock frequencies. Sense probes can be wired into the chip but there are always questions about where on the chip to place them, how to route them out and how to measure them when you get out of the package. We have had real good results at the PCB level and at the top of the package by using a 50 Ohm coax probe soldered directly to power and ground. Measurements from an active probe are usually a little different. The coax probe has very high bandwidth and the 'unprotected' loop where it is attached is very small. You can certainly measure something on wires that probe into the silicon but I am not sure that you can believe the results. There is usually a relationship between the DC voltage and the maximum frequency that a processor will run. Alex Waizman from Intel has done some good work in this area. Most of us believe that noise on top of the DC voltage will decrease Fmax, but this relationship is probably frequency dependent and difficult to quantify. It may be possible to inject noise on low power systems, but for 100W processors, noise injection is difficult at best. This is a difficult area which is ripe for technical development. I'd be interested in comments from others on the list for this topic. regards, Larry Smith Sun Microsystems Craig Twardy wrote: ------------------------------------------------------------------ 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.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