Asynchronous Flip-Flop Inputs

Chapter 10 - Multivibrators

PDF Version

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 an 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:

 

Asynchronous flip flops inputs diagram three

 

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.

 

Asynchronous flip flops inputs diagram two

 

Sometimes the designations “PRE” and “CLR” will be shown with inversion bars above them, to further denote the negative logic of these inputs:

 

Asynchronous flip flops inputs diagram one

 

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.