Although 8051 family members (e.g., 8751, 89C51, 89C52, DS89C4xO) come in different packages, such as DIP (dual in-line package), QFP (quad flat package), and LLC (leadless chip carrier), they all have 40 pins that are dedicated to various functions such as I/O, RD, WR, address, data, and interrupts.

EA : The 8051 family members, such as the 8751/52, 89C51/52, or DS89C4xO, all come with on-chip ROM to store programs. In such cases, the EA pin is connected to Vcc. For family members such as the 8031 and 8032 in which there is no on-chip ROM. code is stored on an external ROM and is fetched by the 8031/32. Therefore, for the 8031 the EA pin must be connected to GND to indicate that the code is stored externally. EA. which stands for “external access,” is pin number 31 in the DIP packages. It is an input pin and must be connected to either Vcc or GND. In other words, it cannot be left unconnected.

8031 uses this pin along with PSEN to access programs stored in ROM memory located outside the 8031.

PSEN : This is an output pin. PSEN stands for “program store enable.” In an 8031-based system in which an external ROM holds the program code, this pin is connected to the OE pin of the ROM. This pin is used to enable external program memory. If we use an external ROM for storing the program, then logic 0 appears on it, which indicates Micro controller to read data from the memory.