SR latch

In electronics, a latch is a kind of bistable multivibrator, an electronic circuit that has two stable states and therefore can store one bit of information. Today the word is mainly used for simple transparent storage elements, while slightly more advanced non-transparent (or clocked) devices are described as flip-flops. Informally, as this distinction is quite new, the two words are sometimes used interchangeably.[citation needed]

This is an alternate model of the simple SR latch built with NAND (not AND) logic gates. Set and reset now become active low signals, denoted S and R respectively. Otherwise, operation is identical to that of the SR latch. Historically, SR-latches have been predominant despite the notational inconvenience of active-low inputs. This is because NAND gates are cheaper to produce than NOR gates in the diode-transistor logic (DTL) and transistor-transistor logic (TTL) families, which were the basis of early integrated circuits before the complementary metal–oxide semiconductor (CMOS) family attained widespread use. Since the 1970s and still as of 2010[update], most integrated circuits are built using CMOS technology, where NAND gates are also preferred.

SR latch operation

S       R       Action

0       0       Restricted combination

0       1       Q = 1

1       0       Q = 0

1       1       Keep state