Sean Breheny wrote: > On Fri, Jul 1, 2011 at 4:04 PM, Dave Tweed wrote: > > Well, that idea (the actual quote is, "No choice between near-simultane= ous > > events can be made unambiguously within a preset deadline.") comes from= the > > same paper and the reference goes to an enormous website in which it wo= uld > > take some time to track down the context in which it was made. >=20 > I was actually only presenting that article for the fact that it > mentions m-stability persisting after additional clock pulses. I got > my statement of this fundamental property from: >=20 > http://research.microsoft.com/en-us/um/people/lamport/pubs/buridan.pdf >=20 > Although, I too will admit that I am not completely convinced by his > arguments. There's nothing wrong with the argument; but the entire discussion is about a single discrete decision made starting at a definite time. In a FF, the clock edge represents the start time of the decision. If there's a second clock edge, then you're simply throwing the first decision away and starting a separate one. And also in the context of FFs, the input variable (voltage) is only "continuous" for the duration of any actual transition through the metastability window (bounded in both time and voltage) for the FF in question. At all other times, the system functions in a discrete way. In analogy with the "ass between two bales of hay" scenario, this window is based on the ass's eyesight -- if he can perceive that he's closer to one b= ale than the other, he'll simply walk to it and eat it. There's only a small ra= nge of initial positions within which he can't tell the difference and will dit= her indefinitely. > > My understanding is that the "choice" we're talking about here is which > > came first, the data edge or the (first) clock edge? The fact that the > > metastability is gone after the second clock edge does not answer that > > particular question; in fact, it erases all of the available informatio= n > > about that question. You now have the answer to a different question: > > Did the data edge occur before the second clock edge? (Yes!) >=20 > I interpret the question a little differently: what was the logic > state of the signal at the first clock edge? It's the same question, really. -- Dave --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .