Computer Science Principles V11

Clive W. Humphris

FLIP FLOPS: NAND RS Flip-flop.  

Two cross-coupled NAND gates form the RS flip-flop circuit, driven inputs are labelled Set (S) and Reset (R). As the truth table shows there are four possible input combinations.

The device is in a stable state when S = 1 and R = 0 (set) or when R = 1 and S = 0 (reset). Making both the S and R inputs logic 0 retains the previous state of the Q output with NOT Q taking up the opposite state of Q.

When S = 1 and R = 1, Q and NOT Q will both be forced HIGH. However, if R and S are taken LOW, unpredictable outputs result depending upon which gate switches first. This is known as indeterminate state and a major disadvantage of the simple R-S flip-flop.

By ensuring S and R have opposite logic states the device can be made to flip and retain a defined logic state, which is one bit memory.

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