Flip-Flop

History

The first electronic flip-flop was invented in 1918 by William Eccles and F. W. Jordan.^[1][2] 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 to one of the back coupled amplifiers outputs during the trigger process within the circuit (as such, it may be considered a case of onomatopoeia).^{[citation needed]} This original electronic flip-flop—a simple two-input bistable circuit without any dedicated clock (or even gate) signal, was transparent, and thus a device that would be labeled as a "latch" in many circles today.

Implementation

Flip-flops can be either simple (transparent) or clocked. Simple flip-flops can be built around a pair of cross-coupled inverting elements: vacuum tubes, bipolar transistors, field effect transistors, inverters, and inverting logic gates have all been used in practical circuits — perhaps augmented by some gating mechanism (an enable/disable input). The more advanced clocked (or non-transparent) devices are specially designed for synchronous (time-discrete) systems; such devices therefore ignore 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^[3] where two basic flip-flops (plus some additional logic) collaborate 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" or "delay"^[4]), T ("toggle"), and JK types are the common ones; all of which may be synthesized 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, Q_next, in terms of the input signal(s) and/or the current output, Q. next

sumber: http://en.wikipedia.org/wiki/Flip-flop_(electronics)