Microprocessor | 8255 (programmable peripheral interface)

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 instructions 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

Advantages:

Flexibility: A microprocessor with an 8255 PPI allows for flexible input/output (I/O) operations. The PPI can be programmed to operate in a variety of modes, and the microprocessor can handle the data transfer between the PPI and other devices.

Improved system performance: The use of a microprocessor with an 8255 PPI can improve system performance by offloading I/O operations from the CPU. This allows the CPU to focus on other tasks while the PPI handles the data transfer.

Easy to interface with other devices: The 8255 PPI can be easily programmed and interfaced with other devices, which makes it easy to use in a variety of applications.

Low cost: The 8255 PPI is a relatively low-cost component, which makes it an affordable option for many different applications.

Disadvantages:

Complexity: Using a microprocessor with an 8255 PPI can add complexity to a system, particularly for novice programmers. The PPI requires programming, and the microprocessor must be programmed to handle the data transfer between the PPI and other devices.

Limited functionality: While the 8255 PPI is versatile, it has limited functionality compared to newer I/O interface components. It is not capable of high-speed data transfer and has limited memory capacity.

Limited number of ports: The 8255 PPI provides only three 8-bit ports, which may not be sufficient for some applications that require more I/O ports.

Obsolete technology: While the 8255 PPI is still used in some applications, it is considered an older technology and is being replaced by newer, more advanced I/O interface components.