8255 is a popularly used parallel, programmable input-output device. It can be used to transfer data under various condition from simple input-output to interrupt input-output. This is economical, functional, flexible but is a little complex and general purpose i/o device that can be used with almost any microprocessor.

8255 pin diagram –
It has 24 pins that can be grouped in two 8-bit parallel ports: A and B called Port A(PA) and Port B(PB) with the remaining eight known as Port C(PC). Port C can be further divided into groups of 4-bits ports named Cupper(Cu) and Clower(Cl). There are 40 pins and operates in +5 regulated power supply.

Modes of 8255 – It works in two modes:

1. Bit set reset (BSR) mode
2. Input/output (I/O) mode

To know in which mode the interface is working we need to know the value of Control word. Control word is a part of control register in 8255 which specify an I/O function for each port. This is format of control word 8255.

If the most significant bit of control word or D7 is 1 then 8255 works in I/O mode else, if it’s value is 0 it works in BSR mode.

1. BSR Mode – When MSB of the control register is zero(0), 8255 works in Bit Set-Reset mode.in this only PC bit are used for set and reset.
2. I/O Mode – When MSB of the control register is one(1), 8255 works in Input-Output mode.it is further divided into three categories.
3. Mode 0 – In this mode all three ports (PA, PB, PC) can work as simple input function or output function also in this mode there is no interrupt handling capabilities.
4. Mode 1 – In this either port A or port B can work and port C bits are used as Handshake signal before actual data transmission plus it has interrupt handling capabilities.
5. Mode 2 – In this only port A works and port B can work either in Mode 0 or Mode 1 and the 6 bits of port C are used as Handshake signal plus it also has to interrupt handling capability.

To communicate with peripherals through 8255 three steps are necessary:

1. Determine the addresses of Port A, B, C and Control register according to Chip Select Logic and the Address lines A0 and A1.
2. Write a control word in control register.
3. Write I/O intructions to communicate with peripherals through port A, B, C.

The common applications of 8255 are:

• Traffic light control
• Generating square wave
• Interfacing with DC motors and stepper motors

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