All digital designers use the name flip-flop for a sequential device that noramlly samples its inputs and changes its outputs only at times determined by a clocking signal. On the other hand, most digital designers use the name latch for a sequential device that watches all of its inputs continuously and changes its outputs at any time, independent of a clocking signal. Some text books and digital designers may (incorrectly) use the name "flip-flop" for a device that we call a "latch".
-- 在fpga中一般避免用latch，因為在FPGA中觸發器資源豐富，不用白不用，latch由於是電平觸發的， 相對觸發器來說容易產生毛刺，電路不穩定。latch的優點是完成同一個功能所需要的門較觸發器要少，所以在asic中用的較多。
（1）An electronic circuit that alternates between two states. When current is applied, it changes to its opposite state (0 to 1 or 1 to 0). Made of several transistors, it is used in the design of static memories and hardware registers.
（2）<hardware> A digital logic circuit that can be in one of two states which it switches (or "toggles") between under control of its inputs. It can thus be considered as a one bit memory. Three types of flip-flop are common: the SR flip-flop, the JK flip-flop and the D-type flip-flop (or latch).
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.
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