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a trigger circuit (seeFLIP-FLOP CIRCUIT) that can remain for an arbitrary length of time in one of two (or, less frequently, of more than two) stable states and can be abruptly switched from one state to the other by the application of an input signal.
A flip-flop has two outputs: a main output and an inverse output. Specific output signals of differing levels correspond to each flip-flop state. In one state a high-level signal is formed at the main output, and a low-level signal is formed at the inverse output. In the other state the high-level and low-level signals are formed at the inverse and main outputs, respectively.
A flip-flop is characterized by the following important parameters: the speed, the response time, and the levels of the input and output signals. The speed is defined as the maximum possible number of switching operations per unit time. The response time is defined as the time required for the flip-flop to go from one state to the other; this time characterizes the time lag between the output signal and the input signal. The level of the input signal is understood to be the minimum signal value required to switch the flip-flop. The level of the output signal in most flip-flops is not lower than the level of the input signal; this situation makes it possible to connect flip-flops in series without intermediate amplification.
The most widely used types of flip-flops are electronic devices that incorporate such components as electron tubes, gas discharge devices, semiconductor diodes, various types of transistors, and, especially, integrated microcircuits. Flip-flops based on magnetic elements, pneumatic or hydraulic control devices, and other elements are also sometimes used. According to the nature of the input signals, flip-flops with potential inputs (direct and inverse) and with dynamic inputs (also direct and inverse) are distinguished. Flip-flops with potential inputs respond to a high-level signal at the direct input and a low-level signal at the inverse input. Flip-flops with dynamic inputs respond to differences between input signals, that is, to variations in signal levels. Such flip-flops respond to a positive difference at the direct input and to a negative difference at the inverse input.
A description of the most frequently used types of flip-flops follows. Flip-flops with a counting input (T flip-flops) change their state with each input signal. Flip-flops with two conditioning inputs (R-S flip-flops) change their state only when a control signal is applied to a specific input (the R or the S input); in this scheme the repeated action of a signal at the same input does not change the state of the flip-flop. Universal flip-flops (JK flip-flops) combine the properties of the T flip-flop and the R-S flip-flop. In delay flip-flops (D flip-flops) the state and the output signal corresponding to that state duplicate the input signal. In addition to flip-flops of these types there are also combination flip-flops, which are universal multifunction devices with large numbers of inputs.
The flip-flops described above are called symmetrical; asymmetrical flip-flops, or Schmitt triggers, are also used. An asymmetrical flip-flop goes from one state to the other when the input signal reaches a level called the response threshold and returns to the original state when the input signal decreases to a certain level. There are also multistable flip-flops, which have more than two stable states.
Flip-flops of various types are used in digital computers and in automation. With the use of flip-flops, one can construct such devices as digital automatic machines with program control for discrete data processing (for example, counters, scalers, registers of various kinds, decoders, and adders), pulse shapers, and digital frequency dividers. In digital automation flip-flops function as elementary automatic devices with a memory and two states corresponding to the two possible values of the binary logic variable (x = 0 and x = 1). Such flip-flops are classified as asynchronous or synchronous. Synchronous, or clocked, flip-flops operate only when periodic clock signals, usually of the meander type, that synchronize the operation of the flip-flop are applied to the inputs. Synchronous flip-flops are subdivided into single-cycle and double-cycle types. The latter type is a system consisting of two flip-flops that execute the same logic operation, but with a time shift equal to the duration of half a cycle of the input clock signal. This duplication of flip-flop operation is required for time division of the reception of the data conveyed by the input signals and for transmission of data from the flip-flop outputs to other components of the device or to its input.
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Early literature refers to the "Eccles-Jordan circuit" and the "Eccles-Jordan binary counter", using two vacuum tubes as the active (amplifying) elements for each bit of information storage. Later implementations using bipolar transistors could operate at up to 20 million state transitions per second as early as 1963.