latches

A latch is a basic memory element used in digital electronics to store one bit of information. It is a bistable circuit that maintains its output state until an input causes a change. Latches are generally level-sensitive, meaning their outputs can follow inputs while a control input, such as an enable signal, is asserted. When the enable is deasserted, the latch holds its last state.

The most common latches are the SR latch and the gated (or D) latch. An SR latch uses cross-coupled gates and has set (S) and reset (R) inputs. In a version built from NOR gates, setting S to 1 sets the output Q to 1, resetting R to 1 forces Q to 0, and both inputs high is invalid. A version built from NAND gates uses active-low inputs (S̅ and R̅); both inputs low is invalid. In both cases, when S and R return to inactive values, the circuit retains the last state via feedback.

A D latch is a practical extension that eliminates the invalid condition of the SR latch. It passes the data input (D) to the output when the enable signal is active and holds the previous value when the enable is inactive. Internally, a gated arrangement converts D into the appropriate S and R signals to drive an SR latch.

Latches are slower and less robust to timing issues than edge-triggered flip-flops, and they are susceptible to unintended changes if input signals arrive while the latch is enabled. They remain essential in simple storage, level-sensitive memory, and as building blocks in more complex sequential circuits.

In a broader sense, the term latch can also refer to a mechanical fastener that holds one part in place, such as a door latch, though this use is separate from electronic memory elements.