8051 microcontroller — Port 3 alternate functions What is the alternate (special) function of port pin P3.4 on the 8051 family devices?

Introduction / Context:
Each pin of Port 3 on the 8051 serves a dual role: a general-purpose I/O and a dedicated peripheral signal. Knowing these alternate functions is essential for interfacing timers, serial ports, and external memory correctly.

Given Data / Assumptions:

• P3.4 is one of the Port 3 pins with an alternate function.
• Standard 8051 pin mapping is assumed (P3.0–P3.7 have fixed special roles).
• We are identifying the specific peripheral function attached to P3.4.

Concept / Approach:
In the classic 8051, Port 3 signals are: P3.0 = RXD, P3.1 = TXD, P3.2 = INT0, P3.3 = INT1, P3.4 = T0, P3.5 = T1, P3.6 = /WR, P3.7 = /RD. Therefore, P3.4 is the external input for Timer 0 (T0).

Step-by-Step Solution:

Verification / Alternative check:
Cross-reference any canonical 8051 pinout; P3.4 is universally shown as T0 (Timer 0 input).

Why Other Options Are Wrong:

• Timer 1: mapped to P3.5 (T1), not P3.4.
• interrupt 0: mapped to P3.2 (INT0).
• interrupt 1: mapped to P3.3 (INT1).

Common Pitfalls:
Memorizing P3.x functions out of order or confusing T0/T1 with INT0/INT1. Always remember the ascending pairings: INT0 (P3.2), INT1 (P3.3), T0 (P3.4), T1 (P3.5).

Final Answer:
Timer 0