8051 Pin Functions — P3.1 Alternate Role For the 8051 microcontroller, what is the alternate function of port pin P3.1?

Introduction / Context:
Port 3 pins on the 8051 double as special function signals. Correctly recognizing these saves hardware and enables on-chip peripherals. P3.0 and P3.1 are particularly important for the built-in UART.

Given Data / Assumptions:

• P3.0 is RXD (receive data) and P3.1 is TXD (transmit data).
• Standard UART operation is assumed.
• No external memory control signals are mapped to P3.1.

Concept / Approach:
The 8051 serial port uses two pins: RXD for input and TXD for output. TXD is mapped to P3.1, which means P3.1 drives the serial transmit data stream when the UART is enabled.

Step-by-Step Solution:
1) Recall Port 3 alternates: P3.0=RXD, P3.1=TXD, P3.2=INT0, P3.3=INT1, P3.4=T0, P3.5=T1, P3.6=WR, P3.7=RD.2) Match P3.1 with TXD.3) Therefore its function is serial port output.

Verification / Alternative check:
Any 8051 datasheet pin table confirms P3.1=TXD.

Why Other Options Are Wrong:

• serial port input: That is P3.0 (RXD), not P3.1.
• memory write strobe: WR is P3.6.
• memory read strobe: RD is P3.7.

Common Pitfalls:
Swapping RXD and TXD pins or confusing serial pins with external memory control signals.

Final Answer:
serial port output