The so-called magnetic latching relay means that the state of the output terminal does not change with the state of the input terminal, and the input state is only saved to the output when there is a latch signal, and does not change until the next latch signal arrives.

Latches are mostly used in integrated circuits. They are used as storage elements in sequential circuits in digital circuits. In some arithmetic circuits, latches are sometimes used as data registers.
After being packaged as an independent product, it can also be used separately, and the data is effectively delayed than the clock signal is valid. This means that the clock signal comes first, and the data signal comes later.

In some applications, an external latch is required on the I/O port of the microcontroller. For example, when the single-chip microcomputer is connected to an off-chip memory, a latch must be connected to achieve address multiplexing. Assuming that the 8 I/O pins of the MCU port are used for both address signals and data signals, then the address can be latched with a latch.

When accessing external memory, P0 port and P2 port are used as an address bus, and P0 port is often connected to a latch and then to a memory. To prevent conflicts between buses. The P2 port is directly connected to the memory. Because the internal timing sequence of the microcontroller can only lock the address of the P2 port, if the latch is not used when the P0 port is used to transmit data, the address will change.

It is very helpful for us to look at the timing diagram of 8051 MCU bus operation. Since the data bus and the address bus share the P0 port, time-sharing multiplexing is required. The address information is sent first, and the ALE enable latch locks the address information into the address end of the peripheral, and then sends the data information and read and write enable signals, and read and write operations at the specified address
Use latches to distinguish the address and data of the single-chip microcomputer. The 8051 series of single-chip microcomputers use more. There are also some single-chip microcomputers with internal address latch functions. For example, the 8279 does not need a latch.