LATCHES

The latch is a type of memory storage device that has 2 stable 
states (bistable).

•Latches are basically similar to flip-flops because they are 
bistable devices that can reside in either of two states using
 a feedback arrangement in which the outputs are connected
 back to the opposite inputs.

•The main difference between latches and flip-flops is the 
method used for changing their state.

S-R (SET-RESET) Latch

•A latch is a type of bistable logic device or multivibrator.

•An active HIGH input S-R (SET-RESET) latch is formed
 with 2 crossed coupled NOR Gates.

•An active LOW input S’-R’ latch is formed with 2 crossed
 coupled NAND Gates.

•The output of each gate is connected to an input of the opposite gate.

•The wiring arrangement produces the regenerative feedback that is 
characteristic of all latches and flip-flops.

                                                                                             

Application Example: The Latch as a Contact-

Bounce Eliminator

•When the pole of a switch strikes the

contact upon switch closure, it physically

vibrates or bounces several times before

finally making a solid contact.

•Although these bounces are very short in

duration, they produce voltage spikes that

are often not acceptable in a digital system.

•A S’-R’ latch can be used to eliminate the

effects of switch bounce.

The Gated S-R Latch

A gated latch requires an enable input, EN

(G is also used to designate an enable input).

•The S and R inputs control the state to

which the latch will go when a HIGH level is

applied to the EN input.

•The latch will not change until EN is HIGH,

but as long as it remains HIGH, the output is

controlled by the state of the S and R

inputs.

Gated D Latch

•The gated D Latch differs from the S-R

circuit because it has only one input in

addition to EN.

•The input is called D (data) input.

•When the D input is HIGH and the EN

input is HIGH, the latch will set.