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10.7. Asynchronous flip-flop inputs



When the preset input is activated, the flip-flop will be set (q="1", not-q="0") regardless of any of the synchronous inputs or the clock. When the clear input is activated, the flip-flop will be reset (q="0", not-q="1"), regardless of any of the synchronous inputs or the clock. So, what happens if both preset and clear inputs are activated? Surprise, surprise: we get an invalid state on the output, where Q and not-Q go to the same state, the same as our old friend, the S-R latch! Preset and clear inputs find use when multiple flip-flops are ganged together to perform a function on a multi-bit binary word, and a single line is needed to set or reset them all at once.





  • Asynchronous inputs on a flip-flop have control over the outputs (Q and not-Q) regardless of clock input status.
  • These inputs are called the preset (PRE) and clear (CLR). The preset input drives the flip-flop to a set state while the clear input drives it to a reset state.
  • It is possible to drive the outputs of a J-K flip-flop to an invalid condition using the asynchronous inputs, because all feedback within the multivibrator circuit is overridden.