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In Digital Circuit s, 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. HISTORY The first electronic flip-flop was invented in 1919 by William Eccles and F. W. Jordan Radio Review Dez 1919 pages 143 following. 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. Flip-flops can be either simple or clocked. Simple flip-flops consist of two cross-coupled ''inverting'' elements – Transistor s, 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 one such device 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 devicesEarly 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. 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"), '''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, , in terms of the input signal(s) and/or the current output, . SET-RESET FLIP-FLOPS (SR FLIP-FLOPS) See Also: SR latch The most fundamental latch is the simple ''SR latch'' (or simple '' SR Flip-flop ''), where S and R stand for ''set'' and ''reset''. It can be constructed from a pair of cross-coupled NOR (negative OR ) Logic Gate s. The stored bit is present on the output marked Q. Normally, in storage mode, the S and R inputs are both low, and Feedback maintains the Q and Q outputs in a constant state, with Q the complement of Q. If S (''Set'') 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 (''Reset'') is pulsed high while S is held low, then the Q output is forced low, and stays low even after R returns low. TOGGLE FLIP-FLOPS (T FLIP-FLOPS) If the T input is high, the T flip-flop changes state ("toggles") whenever the clock input is strobed. If the T input is low, the flip-flop holds the previous value. This behavior is described by the characteristic Equation : : (or, without benefit of the XOR operator, the equivalent: ) and can be described in a Truth Table : A toggle flip-flop composed of a single RS flip-flop becomes an oscillator, when it is clocked. To achieve toggling, the clock pulse must have exactly the length of half a cycle. While such a pulse generator can be built, a toggle flip-flop composed of two RS flip-flops is the easy solution. Thus the toggle flip-flop divides the clock frequency by 2 ie. if clock frequency is 4 MHz, the output frequency obtained from the flip-flop will be 2 MHz. This 'divide by' feature has application in various types of digital counters. JK FLIP-FLOP The JK flip-flop augments the behavior of the SR flip-flop by interpreting the S = R = 1 condition as a "flip" or toggle command. Specifically, the combination J = 1, K = 0 is a command to set the flip-flop; the combination J = 0, K = 1 is a command to reset the flip-flop; and the combination J = K = 1 is a command to toggle the flip-flop, i.e., change its output to the logical complement of its current value. Setting J = K = 0 does NOT result in a D flip-flop, but rather, will hold the current state. To synthesize a D flip-flop, simply set K equal to the complement of J. The JK flip-flop is therefore a universal flip-flop, because it can be configured to work as an SR flip-flop, a D flip-flop or a T flip-flop. NOTE: The flip flop is positive edge triggered (Clock Pulse) as seen in the timing diagram. The characteristic equation of the JK flip-flop is: and the corresponding truth table is:
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