Jeremy, This is a good point, and I'm sure there are different ways of handling it. One problem with taking your timing measurements at VREF, is that the component's timing specifications apply only when using some specified input edge (slew) rate. As long as your actual input edge rate (or the one you see in your simulations) happens to be the same as the one in the data sheet, and the component's timing was specified at VREF, then, theoretically, it is safe to do your system timing measurements at VREF. Provided, of course, that you have confidence in the vendor's data and in your work. If the input edge rate differs from the data sheet value, whether it is slower OR faster, then it may not be valid to measure your timings at VREF. Since the edge rate rarely equals the data sheet value, this is the norm. There are various reasons why a different edge rate causes the timings to change. One is that the input buffer has some unpredictable input offset voltage, so the actual "switch point" might be anywhere between Vil and Vih (an oversimplification, perhaps, but let's use it anyway). Another is gain bandwidth product. Even if the input buffer had no offset voltage, its response time still depends on overdrive. One approach to handle edge rate factors, is to take your measurements at Vil and Vih (or at Vil-X and at Vih+X, where X is some amount of padding you wish to include to account for noise and crosstalk), and then correlate them back to Vref using the data-sheet-specified edge rate. Something similar to this is done in the PCI specification. As for the question about how to trust the chip specification in the first place, that is a good question... Regards, Andy ------------------------------------------------------------------ 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 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