Pin diagram for 8051 micro controller has show below.
ALE/PROG: Address Latch Enable output pulse for latching the low byte of the address during accesses to external memory. ALE is emitted at a constant rate of 1/6 of the oscillator frequency, for external timing or clocking purposes, even when there are no accesses to external memory. (However, one ALE pulse is skipped during each access to external Data Memory.) This pin is also the program pulse input (PROG) during EPROM programming.
PSEN: Program Store Enable is the read strobe to external Program Memory. When the device is executing out of external Program Memory, PSEN is activated twice each machine cycle (except that two PSEN activations are skipped during accesses to external Data Memory). PSEN is not activated when the device is executing out of internal Program Memory.
EA/VPP: When EA is held high the CPU executes out of internal Program Memory (unless the Program Counter exceeds 0FFFH in the 80C51). Holding EA low forces the CPU to execute out of external memory regardless of the Program Counter value. In the 80C31, EA must be externally wired low. In the EPROM devices, this pin also receives the programming supply voltage (VPP) during EPROM programming.
XTAL1: Input to the inverting oscillator amplifier.
XTAL2: Output from the inverting oscillator amplifier.
Port 0: Port 0 is an 8-bit open drain bidirectional port. As an open drain output port, it can sink eight LS TTL loads. Port 0 pins that have 1s written to them float, and in that state will function as high impedance inputs. Port 0 is also the multiplexed low-order address and data bus during accesses to external memory. In this application it uses strong internal pullups when emitting 1s. Port 0 emits code bytes during program verification. In this application, external pullups are required.
Port 1: Port 1 is an 8-bit bidirectional I/O port with internal pullups. Port 1 pins that have 1s written to them are pulled high by the internal pullups, and in that state can be used as inputs. As inputs, port 1 pins that are externally being pulled low will source current because of the internal pullups.
Port 2: Port 2 is an 8-bit bidirectional I/O port with internal pullups. Port 2 emits the high-order address byte during accesses to external memory that use 16-bit addresses. In this application, it uses the strong internal pullups when emitting 1s.
Port 3: Port 3 is an 8-bit bidirectional I/O port with internal pullups. It also serves the functions of various special features of the 80C51 Family as follows:
Port Pin Alternate Function
P3.0 RxD (serial input port)
P3.1 TxD (serial output port)
P3.2 INT0 (external interrupt 0)
P3.3 INT1 (external interrupt 1)
P3.4 T0 (timer 0 external input)
P3.5 T1 (timer 1 external input)
P3.6 WR (external data memory write strobe)
P3.7 RD (external data memory read strobe)
VCC: Supply voltage
VSS: Circuit ground potential