The normal data inputs to a flip flop (D, S and
R, or J and K) are referred to as synchronous inputs because they
have effect on the outputs (Q and not-Q) only in step, or in sync, with the
clock signal transitions. These extra inputs that I now bring to your
attention are called asynchronous because they can set or reset the
flip-flop regardless of the status of the clock signal. Typically, they're
called preset and clear:
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, just like synchronous inputs, can be engineered to
be active-high or active-low. If they're active-low, there will be an
inverting bubble at that input lead on the block symbol, just like the
negative edge-trigger clock inputs.
Sometimes the designations "PRE" and "CLR" will be shown with inversion
bars above them, to further denote the negative logic of these inputs:
REVIEW: 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.
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