flip flops (bistable gates)

Flip-flops, also called bistable gates, are digital logic circuits that can be in one of two states. Flip-flops maintain their state indefinitely until an input pulse called a trigger is received. When a trigger is received, the flip-flop outputs change state according to defined rules and remain in those states until another trigger is received. Flip-flop circuits are interconnected to form the logic gates for the digital integrated circuits (IC s) used in memory chips and microprocessors. Flip-flops can be used to store one bit, or binary digit, of data. The data may represent the state of a sequencer, the value of a counter, an ASCII character in a computer's memory or any other piece of information.

There are several different kinds of flip-flop circuits, with designators such as T (toggle),  S-R (set/reset) J-K (possibly named for Jack Kilby) and D (delay). A flip-flop typically includes zero, one, or two input signals as well as a clock signal and an output signal. Some flip-flops also include a clear input signal to reset the current output.

The first electronic flip-flop was invented in 1919 by W. H. Eccles and F. W. Jordan. It used vacuum tubes and was initially called the Eccles-Jordan trigger circuit.  

This tutorial from Columbia Gorge Community College explains three applications for flip flops.

See also: sequential logic, logic gates