Introduction to 8031 ​​MCU Control Line

l ALE//PROG: ALE-address latch valid signal output. During access to the off-chip program memory, this signal appears twice per machine cycle (frequency = fosc/6). Its falling edge is used to control the latch to latch the low 8-bit address signal output from port P0. Even if the off-chip program memory is not accessed, this signal appears at the above frequency. However, during access to the off-chip data memory, the ALE pulse will skip one (during DPH or P2 output and data input or output). 

/PROG-For models with EPROM on the chip, this pin is used as the input terminal of the programming pulse during programming. 

l /PSEN: Off-chip program memory read select signal output terminal. During the period of fetching instructions or constants from the off-chip program memory, this signal is valid twice per machine cycle. During access to the off-chip data memory, the /PSEN signal does not appear. 

l RST/Vpp: RST-reset terminal. When the microcontroller oscillator is working, a high level that lasts for two machine cycles appears at this terminal to achieve a reset operation. When powered on, considering that the oscillator has a certain start-up time, the high level on this pin must last for more than 10ms to ensure an effective reset. 

Vpp-During the Vcc power-off period, if this pin is connected to the backup power supply Vpp (5+-0.5V), the data of the on-chip RAM can be saved. 

l /EA/VDD: /EA=0-Off-chip program memory selection terminal (input), while the on-chip program memory is ignored. For chips with on-chip program memory (such as 8051 4KB), to make EA=1, when the program code address (PC value) is between 0000~0FFFH, the CPU fetches instructions to access the on-chip program memory. If the PC value is greater than 0FFFH, it automatically switches to access the off-chip program memory.

VDD: For chips containing EPROM on board, this pin inputs 21V programming power during programming.