Clocking causes the flip-flop either to change or to retain its output signal based upon the values of the input signals at the transition. Simple flip-flops can be built around a single 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.Ĭlocked devices are specially designed for synchronous systems such devices ignore their inputs except at the transition of a dedicated clock signal (known as clocking, pulsing, or strobing). In the context of hardware description languages, the simple ones are commonly described as latches, while the clocked ones are described as flip-flops. Implementation Ī traditional (simple) flip-flop circuit based on bipolar junction transistorsįlip-flops can be either simple (transparent or asynchronous) or clocked (synchronous). Nelson used the notations " j-input" and " k-input" in a patent application filed in 1953. In designing a logical system, Nelson assigned letters to flip-flop inputs as follows: #1: A & B, #2: C & D, #3: E & F, #4: G & H, #5: J & K. Flip-flops in use at Hughes at the time were all of the type that came to be known as J-K. Lindley explains that he heard the story of the JK flip-flop from Eldred Nelson, who is responsible for coining the term while working at Hughes Aircraft. They differ slightly from some of the definitions given below. Lindley was at the time working at Hughes Aircraft under Eldred Nelson, who had coined the term JK for a flip-flop which changed states when both inputs were on (a logical "one"). Lindley, an engineer at the US Jet Propulsion Laboratory, the flip-flop types detailed below (SR, D, T, JK) were first discussed in a 1954 UCLA course on computer design by Montgomery Phister, and then appeared in his book Logical Design of Digital Computers. Early flip-flops were known variously as trigger circuits or multivibrators.Īccording to P. The design was used in the 1943 British Colossus codebreaking computer and such circuits and their transistorized versions were common in computers even after the introduction of integrated circuits, though flip-flops made from logic gates are also common now. It was initially called the Eccles–Jordan trigger circuit and consisted of two active elements ( vacuum tubes). The first electronic flip-flop was invented in 1918 by the British physicists William Eccles and F. When a level-triggered latch is enabled it becomes transparent, but an edge-triggered flip-flop's output only changes on a single type (positive going or negative going) of clock edge.įlip-flop schematics from the Eccles and Jordan patent filed 1918, one drawn as a cascade of amplifiers with a positive feedback path, and the other as a symmetric cross-coupled pair The terms "edge-triggered", and "level-triggered" may be used to avoid ambiguity. Using either terminology, the term "flip-flop" refers to a device that stores a single bit of data, but the term "latch" may also refer to a device that stores any number of bits of data using a single trigger. Using this terminology, a level-sensitive flip-flop is called a transparent latch, whereas an edge-triggered flip-flop is simply called a flip-flop. Recently, some authors reserve the term flip-flop exclusively for discussing clocked circuits the simple ones are commonly called transparent latches. The term flip-flop has historically referred generically to both level-triggered and edge-triggered circuits that store a single bit of data using gates. It can also be used for counting of pulses, and for synchronizing variably-timed input signals to some reference timing signal.įlip-flops can be either level-triggered (asynchronous, transparent or opaque) or edge-triggered ( synchronous, or clocked). When used in a finite-state machine, the output and next state depend not only on its current input, but also on its current state (and hence, previous inputs). Such data storage can be used for storage of state, and such a circuit is described as sequential logic in electronics. A flip-flop is a device which stores a single bit (binary digit) of data one of its two states represents a "one" and the other represents a "zero". Flip-flops and latches are fundamental building blocks of digital electronics systems used in computers, communications, and many other types of systems.įlip-flops and latches are used as data storage elements. It is the basic storage element in sequential logic. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. In electronics, a flip-flop or latch is a circuit that has two stable states and can be used to store state information – a bistable multivibrator. An animated interactive SR latch ( R1, R2 = 1 kΩ R3, R4 = 10 kΩ).