VCC – 5V Supply

Port 0(p0.0 to p 0.7): It is eight bit bidirectional I/O port. During external memory access, it functions as multiplexed data and law order address bus AD0-AD7.

Port 1(P1.0 to P 1.7): It is eight bit bidirectional I/O port, when logic 1 then it works as Input mode.

Port 2(P2.0 to P2.7): It is eight bit bidirectional I/O port. During external memory access it works as higher order address bus.(A8-A15).

Port 3 (P 3.0 to port 3.7): It is eight bit  I/O port. Each pin can be used as special I/O pin.

P3.0/RXD: Use of RXD port is to receive serial data of serial communication.

P3.1- TXD: O/P signal of serial port through this signal data is transmitted.

P3.3/P3.2 (INTO): it is external hardware interrupt input output signal.

P3.4-TO: It is I/P signal to internal timer -0 circuit external clock pulse can connect to timer -0 through this I/P signal.

P (3.5): T1: It is I/P signal internal timer-1 circuit external clock pulse can connect to timer-1 through this O/P signal.

P (3.6) (WR (BAR): It is active low weight O/P control signal when WR (bar)=0 then performs write operation.

P(3.7) RD (bar): Active low O/P control signal, when  RD(bar)=0 then performs  read operations from external  RAM .

XTAL1 and XTAL2: These are two I/P lines for a chip oscillator and a clock generator circuit .8051 microcontroller when  (EA)=0 then microcontroller access external program memory(ROM)only, when (EA)=1 then it access both internal and external program memories.

PSEN (bar): Active low O/P signal, it is used to enable external memory program memory (ROM). When PSEN (bar) = 0 then external memory program is enable and microcontroller read content of external location.

ALE: Address latch Enable:  It is active high O/P signal, pin 30 in microcontroller is called address latch enable and used when multiple memory chips are connected and only one of them is required use also for de-multiplex the address and data signal of port 0 while external memory interfacing. In one machine cycle 2 ale is required.

Reset:  It is active high I/P signal. It shall be maintained high for at least two machines agent while oscillator is running the 8051 microcontroller resets.