What is the architecture of 8255A PROGRAMMABLE PERIPHERAL INTERFACE?

The 8255A is generally seen as 8-bit bidirectional data buffer, which is specially designed to transfer the data with the execution of input output instructions requested by the CPU. It has the ability to use with almost any microprocessor.
It consists of three 8-bit bidirectional I/O ports (24I/O lines) which can be configured with their different functional characteristics, each possessing unique features to upgrade the flexibility of 8255.

Ports of 8255A

8255A consists of three ports, i.e., PORT A, PORT B, and PORT C.
  • Port A contains one 8-bit output latch/buffer and one 8-bit input buffer possessing both pull-up and pull-down devices present in Port A.
  • Port B is similar to PORT A.
  • Port C can be split into two parts, i.e. PORT C lower (PC0-PC3) and PORT C upper (PC7-PC4) with the help of control word.
These three ports are further classified into two groups, i.e. Group A includes PORT A and upper PORT C. Group B includes PORT B and lower PORT C. These two groups can be programmed in three different modes, i.e. the first mode is named as mode 0, the second mode is named as Mode 1 and the third mode is named as Mode 2.

Operating Modes

8255A has three different operating modes −
  • Mode 0 − In this mode, Port A and B is used as two 8-bit ports and Port C as two 4-bit ports. Each port can be programmed with the help of either input mode or output mode. The outputs are latched and inputs are not latched in this mode. Ports do not possess interrupt capability.
  • Mode 1 − In this mode, Port A and B are used as 8-bit I/O ports. They can be configured as either input or output ports. Each port of this mode uses three lines taken from port C as handshake signals. Inputs and outputs are latched.
  • Mode 2 − In this mode, Port A can be configured as the bidirectional port and Port B can be available in Mode 0 or Mode 1. Five signals are used by Port A uses from Port C as handshake signals for transfer of data. The remaining three signals from Port C can be used either as simple I/O or as handshake for port B.

Features of 8255A

The prominent features of 8255A are as follows −
  • It consists of 3 8-bit IO ports i.e. PA, PB, and PC to enhance the flexibility of 8225.
  • Address/data bus must be externally demux’d.
  • It is TTL compatible.
  • It has improved DC driving capability.

8255 Architecture

The following figure shows the architecture of 8255A −

8255_architec
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