Control of ALE pin of 51 MCU

ALE/PROG: Address Latch Enable output level used to latch the status byte of the address when accessing external memory.

During FLASH programming, this pin is used to input programming pulses.

Normally, the ALE terminal outputs a positive pulse signal with a constant frequency period, which is 1/6 of the oscillator frequency. Therefore, it can be used as a pulse for external output or for timing purposes. However, it should be noted that whenever it is used as an external data storage, an ALE pulse will be skipped. If you want to disable the output of ALE, you can set it to 0 at the SFR8EH address. At this time, ALE will only work when executing MOVX, MOVC instructions. In addition, the pin is slightly pulled high. If the microprocessor is in the external execution state ALE is disabled, the setting is invalid.

When accessing external memory, ALE is used as the control signal to latch the lower 8-bit byte of the extended address. When accessing external data memory, ALE outputs a positive pulse at one-twelfth of the oscillation frequency. At the same time, this pin is also the programming pulse input terminal when programming EPROM.

But this pin has another very useful function that is often overlooked by many people, that is, when not accessing external data memory, ALE outputs a fixed positive pulse at one-sixth of the oscillation frequency. One machine cycle of 8051 = 6 state cycles = 12 oscillation cycles. If a 6MHz crystal oscillator is used, ALE will send a fixed positive pulse of 1MHz. Therefore, it can be used as an external clock or timing. If we apply this function to practice, it may simplify our design and reduce production costs.

The ALE pin becomes valid only when using the MOVX and MOVC instructions (these instructions use the address of external RAM or ROM. These instructions have a characteristic: the address and data appear at port P0 in a time-sharing manner). When using C to write a program, to make it valid, you can use the method of accessing the internal RAM address. For example: uVariable = * ((char *) 0x12C), give the content of the 0x12C address to the uVariable variable. The valid pins in this process are ALE and RD.

The signal generation of this signal line is realized by the MCU hardware circuit and cannot be controlled manually.
In some MCUs with built-in TOM, the ALE signal output can be turned off to reduce EMI.