Asynchronous or Direct Inputs

Asynchronous or Direct Inputs

• For the flip-flops discussed so far, the SR, D, JK, and T, the inputs are called synchronous inputs because data on these inputs are transferred to the flip-flop's output only on the triggering edge of the clock pulse; that is, the data are transferred synchronously with the clock.

• When power is turn ON, the state of the flip-flop is uncertain. It may come to set (Q = 1) or reset (Q = 0) state. In many applications, it is necessary to initially set or reset the flip-flop.

Such initial state of flip-flop can be accomplished by using the direct or asynchronous inputs of the flip-flop. These inputs are :  They can be applied at any time between clock pulses and are not in synchronism with the clock.

• The Fig. 4.7.1 shows the SR and D flip-flops with preset and clear inputs. These are active-low inputs and thus when the circuit operates in accordance with the truth table of SR flip-flop. If , the flip-flop is reset and the flip-flop is set.

Note : Condition P = C = 0 must not be used, since this leads to an uncertain state.