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Purpose of port 3 and port 2 of 8051

Short answer

Port 2 supplies the high address byte (A8-A15) during external memory access; otherwise it is general-purpose I/O. Port 3’s alternate functions are serial (RxD, TxD), external interrupts (INT0, INT1), timer inputs (T0, T1), and external memory read/write strobes (RD, WR).

Describe the different purpose of port 3 and port 2 of 8051 microcontroller.

Port 2 (P2.0-P2.7)

  • An 8-bit bidirectional I/O port with internal pull-ups. Writing 1 makes a pin an input.
  • Alternate function: when external program or data memory is used, Port 2 outputs the high-order address byte A8-A15. P0 gives the low byte A0-A7 (multiplexed with data). Then P2 cannot be used for ordinary I/O.
  • With no external memory it is a general-purpose I/O port, e.g. for LEDs or an LCD.

Port 3 (P3.0-P3.7)

  • An 8-bit bidirectional I/O port with internal pull-ups. Each pin has a special alternate function, used when the corresponding feature is enabled:
PinNameFunction
P3.0RXDSerial data input
P3.1TXDSerial data output
P3.2INT0External interrupt 0
P3.3INT1External interrupt 1
P3.4T0Timer/counter 0 external input
P3.5T1Timer/counter 1 external input
P3.6WRExternal data memory write strobe
P3.7RDExternal data memory read strobe

So P2 mainly supplies the upper address lines, and P3 provides the control and special signals (serial, interrupts, timer inputs and read/write strobes).

✓ Answer checked by Claude Opus 5.5 (high effort)

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