What is the fundamental principle at work in metastability?
The fundamental principle is called “Buridan’s Principle” after the 14th-century French philosopher Jean Buridan, who stated, in essence, that no discrete decision about a continuous variable can be made in a bounded amount of time. The canonical example of this is the donkey placed equidistant from two bales of hay that will starve to death because he can’t decide which one to eat first.
The flip-flop is the electrical manifestation of this principle. A discrete decision is called for at each active clock edge, yet the D input can actually take on any of a continuous set of input values (voltages). It’s the job of the flip-flop to decide at the clock edge whether the input is the “low” state or the “high” state, but if the input is changing at the time of the clock edge, it can’t always make that decision right away. Another way of looking at it is to ask whether the data changed before or after the clock edge.
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