Simplercircuits: 8051 Micro Controller

.Pins No. 1 to 8 are Port 1. Each of these pins can be configured as input or output.To configure a Port pin as Input one must write 1 to it by programming and to configure a port pin as output one must write 0 to its latch using programming.

Pin N0. 9 is reset, a high on this pin will reset the microcontroller.On applying Zero to this pin microcontroller starts executing the program from the beginning.

Pins no. 10 to 17 constitute port 3. These pins have alternate functions as denoted in the pin diagram.These are:

RXD,Pin 10: This pin is used as Serial asynchronous Communication Input or Serial Synchronous Communication Output.
TXD,Pin11: This pin is used as Serial asynchronous Communication Output or Serial Synchronous Communication Clock Output.
INT0,Pin12: This pin is external interrupt 0 input. It is active low.
INT1,Pin13: This pin is external interrupt 1 input. It is active low.
T0,Pin 14: Counter 0 clock input.
T1,Pin15: Counter 1 clock input.
WR,Pin16:Write to external RAM.
RD,Pin17:Read from external RAM.

Pins 21 to 28 are Port 2 lines.These pins are used for addressing external memory ,when used as they also serve as higher order address lines.Microcontroller is capable of addressing 64K external memory as it has 16 address line. It is not necessary that always all the 16 address lines are used.

Pin 29 is Program Store Enable(PSEN). If external ROM is used then a logic 0 appears on it every time the microcontroller wants to read from external memory.

Pin 30 is ALE(Address Latch Enable). It is used to latch the lower order address.

Pin no. 31 is External Access(EA). On applying Logic zero to this pin, microcontroller ignores the internal code memory and execute the program from external memory only. When a high is applied to it, microcontroller first uses the internal ROM and then External ROM(if available).

Pins 32 to 39 constitute Port 0 and lower order Address Bus when ALE is 1 or Data Bus when ALE=0.

Simplercircuits