Most simple type of flip flop is **S R Flip Flop**. It has two inputs S and R and two outputs Q and . The state of this latch is determined by condition of Q. If Q is 1 the latch is said to be SET and if Q is 0 the latch is said to be RESET. This **S R Latch** or Flip flop can be designed either by two cross-coupled NAND gates or two-cross coupled NOR gates. When we design this latch by using NOR gates, it will be an active high S-R latch. That means it is SET when S = 1. When we design this latch by using NAND gates, it will be an active low S-R latch. That means it is SET when S = 0. **S R Flip Flop** is also called **SET RESET Flip Flop**.

Figure below shows the logic circuit of **S R latch**.

In the above logic circuit if S = 1 and R = 0, Q becomes 1. Let us explain how.

- NOR gate always gives output 0 when at least one of the inputs is 1.
- So when S is applied as 1 the output of gate G2 i.e.
is 0 irrespective of the condition of second input Q to the gate. - Now
is input of gate G1 so both the inputs of G1 become 0 as R is already 0. So, output of G1 is now or 1. - So whatever may be the previous condition of Q, it always becomes Q = 1 and
= 0 when, S = 1 and R = 0. This is called SET condition of the latch.

In the above logic circuit if S = 0 and R = 1, Q becomes 0. Let us explain how.

- As we already said, a NOR gate always gives output 0 when at least one of the inputs is 1.
- So when R is applied as 1, the output of gate G1 i.e. Q is 0 irrespective of the condition of second input
to the gate. - So, whatever may be the previous condition of Q, it always becomes 0 this 0 is then fed back to input of gate G2. As here S is already 0, both inputs of G2 are 0. Hence output of G2 i.e.
will be 1. So, Q = 0 and = 1 when, S = 0 and R = 1. This is called RESET condition of the latch.

In the above logic circuit if S = 0 and also R = 0, Q remains same as it was. Let us explain how.

- First suppose Q is previously 1.
- Now the inputs of G2 are 0 and 1 as S=0 and Q=1. So output of G2 i.e.
is or 0. - Now both inputs of G1 are 0 as R=0 and
=0. So output of G1 i.e. Q is or 1. - Now suppose Q is previously 0.
- Now both inputs of G2 are 0 and 1 as S = 0 and Q = 0. So output of G2 i.e.
is or 1. - Now the inputs of G1 are 0 and 1 as R=0 and
= 1. So output of G1 i.e. Q is or 0. - So it is proved that Q remains same as it is when S = 0 and also R = 0 in S R latch or flip flop.

In the above logic circuit if S = 1 and also R = 1, the condition of Q is totally unpredictable. Let us explain how.

- First suppose Q is previously 1.
- Now both inputs of G2 are 1 as S = 1 and Q = 1. So output of G2 i.e.
isor 0. - Now the inputs of G1 are 1 and 0 as R = 1 and
= 0. So output of G1 i.e. Q is or 0. That means Q is changed. - Now Q is 0. So inputs of G2 are 1 and 0 as S = 1 and Q = 0. So output of G2 i.e.
is or 0. That means is unchanged. - Now the inputs of G1 are 1 and 0 as R = 1 and
= 0. So output of G1 i.e. Q is or 0. That means Q is unchanged.

So, when both S and R are 1, it becomes unpredictable whether the value of output Q will be changed or unchanged. This condition of S R latch normally avoided. As the latch is SET when S = 1(HIGH), the latch is called Active High S R Latch. There is other type of latch which is SET when, S = 0 (LOW), and this latch is known as Active Low S R Latch.