8051 Microcontroller Pin Description:
Pins 1-8: Port Each of these pins can be configured as an input or an output.
Pin 9: RS A logic one on this pin disables the microcontroller and clears
the contents of most registers. In other words, the positive voltage on this
pin resets the microcontroller. By applying logic zero to this pin, the program
starts execution from the beginning.
Pins10-17: Port 3 Similar to port 1, each of these pins can serve as general input
or output. Besides, all of them have alternative functions:
Pin 10: RXD Serial asynchronous communication input or Serial synchronous
communication output.
Pin 11: TXD Serial asynchronous communication output or Serial synchronous
communication clock output.
Pin 12: INT0 Interrupt 0 input.
Pin 13: INT1 Interrupt 1 input.
Pin 14: T0 Counter 0 clock input.
Pin 15: T1 Counter 1 clock input.
Pin 16: WR Write to external (additional) RAM.
Pin 17: RD Read from external RAM.
Pin 18, 19: X2, X1 Internal oscillator input and output. A quartz crystal which specifies operating frequency is usually connected to these pins. Instead of it, miniature ceramics resonators can also be used for frequency stability. Later versions of microcontroller operate at a frequency of 0 Hz up to over 50 Hz.
Pin 20: GND Ground.
Pin 21-28: Port 2 If there is no intention to use external memory then these port
pins are configured as general inputs/outputs. In case external memory is used,
the higher address byte, i.e. addresses A8-A15 will appear on this port. Even
though memory with capacity of 64Kb is not used, which means that not all eight
port bits are used for its addressing, the rest of them are not available as
inputs/outputs.
Pin 29: PSEN If external ROM is used for storing program then a logic zero (0)
appears on it every time the microcontroller reads a byte from memory.
Pin 30: ALE Prior to reading from external memory, the microcontroller puts
the lower address byte (A0-A7) on P0 and activates the ALE output. After
receiving signal from the ALE pin, the external register (usually 74HCT373 or
74HCT375 add-on chip) memorizes the state of P0 and uses it as a memory chip
address. Immediately after that, the ALU pin is returned its previous logic
state and P0 is now used as a Data Bus. As seen, port data multiplexing is
performed by means of only one additional (and cheap) integrated circuit. In
other words, this port is used for both data and address transmission.
Pin 31: EA By applying logic zero to this pin, P2 and P3 are used for data
and address transmission with no regard to whether there is internal memory or
not. It means that even there is a program written to the microcontroller, it
will not be executed. Instead, the program written to external ROM will be
executed. By applying logic one to the EA pin, the microcontroller will use
both memories, first internal then external (if exists).
Pin 32-39: Port 0 Similar to P2, if external memory is not used, these pins can be
used as general inputs/outputs. Otherwise, P0 is configured as address output
(A0-A7) when the ALE pin is driven high (1) or as data output (Data Bus) when
the ALE pin is driven low (0).
Pin 40: VCC +5V power supply.
Pin 40: VCC +5V power supply.
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