Flip Flop

"Flip-flop" is the common name given to two-state devices which offer basic memory for sequential logic operations. Flip-flops are heavily used for digital data storage and transfer and are commonly used in banks called "registers" for the storage of binary numerical data.

RS (Reset-Set) flip-flop:

The fundamental latch is the simple RS flip-flop (also commonly known as SR flip-flop), where R and S stand for reset and set, respectively. It can be constructed from a pair of cross-coupled NAND or NOR logic gates. The stored bit is present on the output marked Q.

Normally, in storage mode, the R and S inputs are both low, and feedback maintains the Q and Q outputs in a constant state, with Q the complement of Q. If S is pulsed high while R is held low, then the Q output is forced high, and stays high even after S returns low; similarly, if R is pulsed high while S is held low, then the Q output is forced low, and stays low even after R returns low.

The next-state equation of the RS flip-flop is

where Q is the current state. Q_next becomes Q (the stored value) at clock edge.

RS Flip-Flop operation (BUILT WITH NOR GATES)
Characteristic table			Excitation table
S	R	Action	Q(t)	Q(t+1)	S	R	Action
0	0	Keep state	0	0	0	X	No change
0	1	Q = 0	1	0	0	1	reset
1	0	Q = 1	0	1	1	0	set
1	1	Race Condition	1	1	X	0	No Change