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S R Flip Flop S R Latch

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.

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

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S R Flip Flop S R Latch

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

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

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

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. truth table of flip flop


PP. VINODHINI commented on 23/05/2018
The explanation is superb. I can only understand the working of SR latch from this site. Thanks a lot.
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