Flip-flop
From Biomatics.org
In digital circuits, the flip-flop, latch, or bistable multivibrator is an electronic circuit which has two stable states and thereby is capable of serving as one bit of memory. A flip-flop is controlled by one or two control signals and/or a gate or clock signal. The output often includes the complement as well as the normal output. As flip-flops are implemented electronically, they naturally also require power and ground connections.
Flip-flops can be either simple or clocked. Simple flip-flops consist of two cross-coupled inverting elements – transistors, or NAND, or NOR-gates – perhaps augmented by some enable/disable (gating) mechanism. Clocked devices are specially designed for synchronous (time-discrete) systems and therefore ignores its inputs except at the transition of a dedicated clock signal (known as clocking, pulsing, or strobing). This causes the flip-flop to either change or retain its output signal based upon the values of the input signals at the transition. Some flip-flops change output on the rising edge of the clock, others on the falling edge.
Clocked flip-flops are typically implemented as master-slave devices* where two basic flip-flops (plus some additional logic) collaborates to make it insensitive to spikes and noise between the short clock transitions; they nevertheless also often include asynchronous clear or set inputs which may be used to change the current output independent of the clock.
Flip-flops can be further divided into types that have found common applicability in both asynchronous and clocked sequential systems: the SR ("set-reset"), D ("data"), T ("toggle"), and JK types are the common ones; all of which may be synthetisized from (most) other types by a few logic gates. The behavior of a particular type can be described by what is termed the characteristic equation, which derives the "next" (i.e., after the next clock pulse) output, Qnext, in terms of the input signal(s) and/or the current output, Q.
The first electronic flip-flop was invented in 1919 by William Eccles and F. W. Jordan [1]. It was initially called the Eccles-Jordan trigger circuit and consisted of two active elements (radio-tubes). The name flip-flop was later derived from the sound produced on a speaker connected with one of the backcoupled amplifiers output during the trigger process within the circuit.
* Early master-slave devices actually remained (half) open between the first and second edge of a clocking pulse; today most flip-flops are designed so they may be clocked by a single edge as this gives large benefits regarding noise immunity, without any significant downsides.
