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Metastability Theory

A pencil balancing on a razor blade can be metastable, but normally flops to one side or the other. A bistable is two inverters connected in a ring. This has two stable states, but there is also a metastable state. If a D-type is clocked while its input is changing, it might be set close to its metastable state and then drift to one level or the other. Sometimes, it will take a fair fraction of a clock period to settle. The oscillogram shows metastable waveforms at the output of a D-type when set/hold times are sometimes violated.

41: (C) 2008-11, DJ Greaves, University of Cambridge, Computer Laboratory.