latch

Latch is a device that maintains a binary state after an input signal has been removed. In electronics, a latch is a bistable circuit that stores one bit of information. Mechanical latches are simple fasteners that hold a door, lid, or gate in place.

Electronic latches are built from feedback between two active elements, typically logic gates such as NOR or NAND gates. A basic SR latch has two inputs, set and reset, and two outputs that are complements of each other. When S is asserted, the latch output Q becomes 1; when R is asserted, Q becomes 0. If both inputs are asserted, the circuit may enter an undefined or invalid state depending on the gate type. Latches can be level-sensitive; a D latch adds a data input and an enable signal so that Q follows D while enable is active and holds its value when disabled. More complex latches are used as memory cells in registers and caches. Latches are often described as level-triggered devices, in contrast to edge-triggered flip-flops which respond only to transitions, not to a sustained level. In practice, latches are built from cross-coupled logic gates and can be realized in various technologies, including TTL and CMOS. In most common implementations, latches are volatile memory elements that lose their stored state when power is removed.

Applications include simple storage elements in digital circuits, timing adjustment, keyboard debouncing, and masking or gating signals. Mechanical latches include hook-and-eye, deadbolts, and spring-loaded latches used on doors and cabinets.

See also: flip-flop, register, sequential logic.