Latch
A latch (engl. For snap lock , latch ), also known as state-controlled flip-flop referred to, is a particular embodiment of an electronic one-bit data memory and forms (e) the basis for wide ranges of digital hardware , and in particular of CPU - and O - Crisps.
In addition to the data input, a latch has at least one second input (called enable or gate ) that controls the operating mode: In the transparent phase, the output of the latch immediately follows the data input; in the hold phase, the previous state is frozen. In addition to this control input, there may be others depending on the version, e.g. B. a reset input which, if there are several latches in a chip, acts globally for all of these individual latches.
In contrast to edge-triggered flip-flops, a latch is transparent during the entire active clock phase, i. H. Input changes can affect the output immediately.
General classification
Latches are sometimes called “latch registers”, but they are not registers in the true sense of the word, which are generally defined as edge-triggered flip-flops.
In the TTL series of logic chips there are u. a. the latches 7475, 74100, 74116, 74373, 74375 (see also the list of integrated circuits of the 74xx family ). There are also other variants, e.g. Sometimes with more inputs: Intel 8212 , Intel 8282 , Intel 8283 .
In today's microelectronics, which work almost exclusively with edge control, latches are steadily losing importance. However, IBM and Intel are still developing most circuits with latches as a basic element.
Another relationship in analog technology is the sample & hold circuit .
Latching as an error phenomenon
If you use the term as a verb latch , you often mean a malfunction of a digital hardware circuit . It gets stuck in a certain state ("this state is latched"), although the input signals actually require a change in the output state. The cause of such sticking can be overloading of individual circuit parts, a race condition or thermal influence.