I refer to the 'F12x because that's the one I have in my desk drawer, but you're certainly right. I don't see the advantage of using USB, as async serial, though slower, would work just as well. There's no reason not to use USB, of course.
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I was thinking about the 'F120 because it can be operated at 100 MIPS, and therefore perhaps do the translation on-the-fly. With that board DLP device that Michael recommended, perhaps it could all be done within the PC. My thought was that, in reality, any MCU that has an adequate, and adequately fast, ADC would work, once the data transfer to the PC was provided in some form. Of course, the sample depth has to be very large, since the time interval over which the desired observation, namely the rise and fall of Vcc could be observed. With as much capacitance as some people put on their boards, it could take minutes ... many minutes ... to decay to a point at which the MCU and other circuits actually stop operating.