Q23Microprocessor and Microcontroller

The Intel 8255 PPI is a highly versatile, programmable peripheral interface IC universally utilized to mathematically expand the limited I/O capabilities of the 8085 microprocessor. It provides exactly 24 programmable I/O pins, which can be dynamically configured via software as input or output, drastically simplifying the physical interfacing of keyboards, ADC/DACs, and stepper motors.

Internal Port Architecture

The 24 pins are mathematically divided into three completely distinct 8-bit ports:

• Port A (8 bits): A highly capable port supporting simple I/O, strobed handshaking I/O, and massive bidirectional data transfer.
• Port B (8 bits): Supports simple I/O and strobed handshaking I/O, but completely lacks bidirectional capability.
• Port C (8 bits): This port is unique. It can operate as a standard 8-bit simple I/O port, or it can be violently mathematically severed in half by the hardware: Port C-Upper (4 bits) and Port C-Lower (4 bits). These halves are primarily utilized to provide the necessary hardware handshaking control signals (Strobe, Acknowledge) specifically for Ports A and B.

Operational Modes of the 8255

The 8255 is dynamically configured by writing a specific 8-bit Control Word into its internal Control Register. It operates in two primary modes:

• 1. Bit Set/Reset (BSR) Mode: This mode explicitly targets ONLY Port C. It allows the programmer to individually SET (1) or RESET (0) any single specific pin out of the 8 pins of Port C without mathematically affecting the other 7 pins. Highly useful for generating precise control pulses.
• 2. I/O Modes (Modes 0, 1, 2): - Mode 0 (Simple I/O): Ports A, B, and both halves of C act as basic, unlatched input or latched output ports. No hardware handshaking is utilized. - Mode 1 (Strobed I/O): Ports A and B act as high-speed data ports. Crucially, Port C is hijacked to provide the strict hardware handshaking signals (like STB, ACK) required to safely transfer data to slow external devices like printers. - Mode 2 (Bidirectional I/O): This mode exclusively utilizes Port A. Port A becomes a massive bidirectional data bus (data flows in both directions on the same pins), while Port C provides the highly complex 5-wire handshaking required to orchestrate this bidirectional traffic.