SR Flip-Flop

SR flip-flop operates with only positive clock transitions or negative clock transitions. Whereas, SR latch operates with enable signal. The circuit diagram of SR flip-flop is shown in the following figure.

his circuit has two inputs S & R and two outputs Q(t) & Q(t)’. The operation of SR flipflop is similar to SR Latch. But, this flip-flop affects the outputs only when positive transition of the clock signal is applied instead of active enable.

State table of SR flip-flop.

S R Q(t+1)
0 0 Q(t)
0 1 0
1 0 1
1 1 -

Here, Q(t) & Q(t + 1) are present state & next state respectively. So, SR flip-flop can be used for one of these three functions such as Hold, Reset & Set based on the input conditions, when positive transition of clock signal is applied.

Characteristic table of SR flip-flop.

Therefore, SR Latch performs three types of functions such as Hold, Set & Reset based on the input conditions.

S R Q(t) Q(t+1)
0 0 0 0
0 0 1 1
0 1 0 0
0 1 1 0
1 0 0 1
1 0 1 1
1 1 0 x
1 1 1 x

By using three variable K-Map, we can get the simplified expression for next state, Q(t + 1). The three variable K-Map for next state, Q(t + 1) is shown in the following figure.

The maximum possible groupings of adjacent ones are already shown in the figure. Therefore, the simplified expression for next state Q(t + 1) is

          Q(t+1)=S+R′Q(t)