•    The INTEL 805 F is an 8-bit microcontroller with 128-byte internal RAM and 4kb internal ROM.

•    The INTEL 8031 is the same, as 8051 except that it does not have internal ROM.

•    The 8051 is a pin 40 pin IC available in the Dual-In line package (DIP).

•    Requires a single power supply of +5V.

•    Its maximum internal clock frequency rating is 12 MHz.

Pin Diagram for 8051/8031 microcontroller

Signals of 8031/8051 microcontroller

Alternate functions of port pins

Ports: (pins 1 to 8, pin 10 to 17, pin 21 to 28 and pin 32 to 39)

•    The 8031/8051 microcontroller has 32 I/O pins and they are organized as four numbers of 8-bit parallel ports.

•    The ports are denoted as port-0, port-1, port-2 and port-3. Each port can be used as either an 8-bit parallel port or 8 numbers of 1-bit ports.

•    The ports behave as latches during output operation and behave as buffers during an input operation.

•    Port-1 can be used only for I/O operation

•    When external memory is employed, the port-0 function as multiplexed low-byte address or data lines, and port-2 functions as high-byte address lines. Therefore for accessing external memory the microcontroller uses a 16-bit address and accesses the memory in bytes. Hence the addressable memory space is 64 kb (216 = 64kb).

•    The 8031/8051 allows the external memory to be organized as two banks of 64 kb. One is program/code memory and the other is data memory.

PSEN (low signal): pin 29

•    The signal PSEN (low) is used as reading control/enable for program memory.

RD (low signal) and WR (low signal): pin 17 and pin 16

•    The port pin P3.7 function as read control and port pin P3.6 functions as write control for data memory.

•    When two external memory banks are not desirable, the PSEN (low) and RD (low) should be externally ANDed to provide a single read control signal. In such cases, the controller will access a common memory space (of maximum capacity of 64 kb) for the program and data.

•    ALE is used to demultiplex the low-byte address or data using an external latch.

EA (Low)/Vpp: pin 31

•    When the microcontroller access program from external memory, then this pin is low. ie. EA (low) is enabled.

•    When the microcontroller access program from internal memory, then this pin is high. At that time this pin is used to supply programming voltage +12V to EPROM/ROM.


•    The XTAL 1 and XTAL2 pins are provided for external quartz crystal connection, in order to generate the required clock for the microcontroller. The maximum frequency of quartz crystal that can be connected to the 8031/8051 microcontroller is 12 MHz.

RST (low): pin 9

•    The RST(low) signal is used to reset the microcontroller in order to bring the controller to a known state.

Contents of the register after reset

INTERRUPTS: pin 12 to 15

•    The 803 1/8051 has five interrupts.

•    In these two interrupts are external interrupts as INT0 (Low), and INT1 (Low), and the remaining three are internal interrupts as timer-0, timer-1, and serial port.

•    All interrupts are maskable and vectored interrupts.

Vector address and priority of interrupts

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