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From: Brian Drummond on 25 May 2010 16:25 On Tue, 25 May 2010 10:42:46 -0700 (PDT), Gabor <gabor(a)alacron.com> wrote: >On May 25, 9:30�am, glen herrmannsfeldt <g...(a)ugcs.caltech.edu> wrote: >> Gabor <ga...(a)alacron.com> wrote: >> >> (snip, I wrote) >> >> >> The one that I would wonder about is, if the metastable input >> >> was oscillating at a high frequency, that it might capacitively >> >> couple through. >> >> (snip) >> >> > I don't think that metastability generally causes oscillation. >> >> I believe that it isn't usual, but I am not sure that it >> isn't possible. >> >> > I would think that as soon as the signals swing one way or the >> > other they should stabilize. �Think of a coin landing on its >> > edge. �In the metastable case it stands upright for some time >> > before leaning toward heads or tails, but once it leans one >> > way or the other it accelerates to its resting position. >> >> Consider the case where the coin is spinning? �Or bouncing >> around on the table before settling down? �But maybe the >> analogy isn't perfect. >> >> -- glen > >I was going by the posts about FPGA fabric flip-flops. In the >many discussions on metastability, the gurus seemed to say that >oscillation is not a typical manifestation in these structures. There may be no oscillation on the metastable FF itself, but remember that it is probably driving all its loads into their linear region. One or more of those loads (e.g. an inverter driven from it) may then oscillate, but the metastable FF itself will not. (In any case, metastable events with any significant probability is easily dealt with in FPGA as has been discussed many times before) - Brian |