Asynchronous Input

Asynchronous inputs are those that operate independently of a clock signal and other inputs. They override the state of a circuit and force them into a predefined state.

Asynchronous inputs are an integral part of flip-flops. They are referred to as "SET" and "RESET" or "PRESET" and "CLEAR"...

   SET sets the output Q to "1"

   RESET sets the output Q to "0"

In contrast, the R, S, J, K and D inputs of flip flops are synchronous inputs.

Synchronous inputs have only in combination with a clock signal impact on the state at the output Q.

Application of asynchronous reset

whenever we just turn on the flipflop, then we do not know the output state of the flipflop. Now, imagine one sequential circuit with so many flip-flops. So if we turn on the circuit, then at the power up the output state of the sequential circuit will be totally random.

But if we have preset and clear inputs, then by activating these inputs, we can set or reset the circuit in the specific state or in general, at any time if you want a circuit in a specific state then that can be achieved with the help of the asynchronous inputs. In the case of synchronous reset, the flipflop states depend on the clock input so in order to set any flipflop to a particular value we must wait for the clock to trigger. In the case of asynchronous reset since the output is independent of the clock the flipflops can be easily set to a required value during the power on.