MCS51 microcontroller internal structure block diagram and pin diagram

Crystal Oscillator

Quartz Oscillator 3225 20M OSC

1. MCS-51 Microcontroller Internal Structure Block Diagram

2. MCS-51 Pin Diagram

3. Pin Description
VCC (Pin 40): Connects to the positive terminal of the +5V power supply.
VSS (Pin 20): Connects to the ground of the +5V power supply.
XTAL1 (Pin 19): Connects one end of the external crystal oscillator. Internally, it serves as the input to an inverting amplifier that forms the on-chip oscillator. When using an external clock signal, this pin is grounded for HMOS microcontrollers, while for CHMOS microcontrollers, it acts as the input for the external oscillator signal.
P0 Port (Pins 39–32): P0.0 to P0.7 collectively form the P0 port. It can function as a quasi-bidirectional I/O port when no external memory or I/O expansion is connected. However, when external memory or expanded I/O is used, the P0 port is time-multiplexed into the lower 8-bit address bus and a bidirectional data bus.
P1 Port (Pins 1–8): P1.0 to P1.7 make up the P1 port, which functions as a quasi-bidirectional I/O port. In the 52 sub-series, P1.0 and P1.1 have secondary functions: P1.0 can be used as the T2 input for Timer/Counter 2, and P1.1 can serve as the T2EX control signal for Timer/Counter 2.
P2 Port (Pins 21–28): P2.0 to P2.7 form the P2 port, typically used as a quasi-bidirectional I/O port. When connecting to external memory or expanded I/O with an addressing range exceeding 256 bytes, the P2 port functions as the upper 8-bit address bus.
P3 Port (Pins 10–17): P3.0 to P3.7 are the P3 port. In addition to acting as a quasi-bidirectional I/O port, each pin has a secondary function and can be independently configured as either an I/O pin or a secondary function. The secondary functions of the P3 port are listed in Table 2.1.