Chapter 2

Latches
A latch is a temporary storage device that has two
stable states (bistable). It is a basic form of memory.




The S-R (Set-Reset) latch is the most basic type. It can be constructed
from NOR gates or NAND gates. With NOR gates, the latch responds to
active-HIGH inputs; with NAND gates, it responds to active-LOW inputs.

, Latches
The active-HIGH S-R latch is in a stable (latched) condition
when both inputs are LOW.
0 R 1
0
Assume the latch is initially RESET Q
(Q = 0) and the inputs are at their Latch
inactive level (0). To SET the latch initially
(Q = 1), a momentary HIGH signal RESET
0
1
is applied to the S input while the R Q
0 S
remains LOW.
0 R 1
0
To RESET the latch (Q = 0), a Q
momentary HIGH signal is Latch
applied to the R input while the S initially
remains LOW. SET
1
0
Q
0 S

, Latches
The active-LOW S-R latch is in a stable (latched) condition
when both inputs are HIGH.
Assume the latch is initially RESET 1 S 1
0
Q
(Q = 0) and the inputs are at their
Latch
inactive level (1). To SET the latch initially
(Q = 1), a momentary LOW signal RESET
is applied to the S input while the R 1
0
1 R Q
remains HIGH.
To RESET the latch a momentary 1 S 1
0
Q
LOW is applied to the R input
Latch
while S is HIGH. initially
Never apply an active set and 1 SET
0
Q
reset at the same time (invalid). 1R

, Latches