7.25 If the output goes metastable near V
DD/2, the flip-flop will indeed produce a good
high output. However, when it resolves from metastability, the output might sud-
denly fall low. Because the resolution time can be unbounded, the clock-to-Q delay
of the synchronizer is also unbounded. The problem with synchronizers is not that
their output takes on an illegal logic level for a finite period of time (all logic gates
do that while switching), but rather that the delay for the output to settle to a correct
value cannot be bounded. With high probability it will eventually resolve, but with-
out knowing more about the internal characteristics of the flip-flop, it is dangerous
to make assumptions about the probability.
Chapter 8
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