8051 ports and alternate functions — what is the alternate function of port pin P3.0 (labelled RXD)? Choose the correct role of P3.0 within the on-chip serial interface.

Introduction / Context:
Port 3 pins on the 8051 have multiplexed alternate functions that connect to core peripherals. Correctly mapping each pin is crucial when configuring serial communications and external memory/interrupt interfaces.

Given Data / Assumptions:

• P3.0 is labelled RXD
• Standard 8051 pinout and peripheral mapping
• On-chip UART supports full-duplex serial I/O

Concept / Approach:
RXD stands for receive data. In the 8051, P3.0 serves as the serial port receive input to the UART. TXD (transmit data) is on P3.1. Other Port 3 pins are associated with external memory strobes and interrupts.

Step-by-Step Solution:

Verification / Alternative check:
Any standard 8051 pin diagram shows P3.0 as the receive data input for the serial interface, confirming the alternate function as serial input.

Why Other Options Are Wrong:

• Serial port output: that is TXD on P3.1.
• Memory write strobe: mapped to P3.6 (/WR).
• Memory read strobe: mapped to P3.7 (/RD).
• External interrupt 0: mapped to P3.2 (/INT0), not P3.0.

Common Pitfalls:

• Swapping TXD and RXD assignments.
• Assuming all Port 3 pins are general-purpose when peripherals are enabled.

Final Answer:
serial port input