Intel 8255A – Programmable Peripheral Interface Pin Diagram

Intel 8255A – Programmable Peripheral Interface used when we want control more than eight peripheral device. Suppose we want to control various device at a time like keyboard, printer, 7 segment display etc. then we need eight data bus of microprocessor at a time but here we have three peripheral so we require eight data bus for the keyboard, eight data bus for the printer eight data bus for the seven segment display then here we required 24 data bus that is not possible to control by the any processor so we required a device who control these 24 data bus and this control by the processor. Hence we required programmable peripheral interface that program to control the peripheral which pin diagram the shown in the figure below

Now let’s discuss the function description of the pins in the Intel 8255A – Programmable Peripheral Interface Pin Diagram

Bus

It is a tri-state 8-bit buffer that connects the microprocessor to the system data bus. With the help of these data bus data is transferred or received according to instructions from the CPU. To control the data flow Control words and status information write via programming and share over the bus.

Read / Write control logic

This block is responsible for controlling the internal / external transfer of data / control / status words. It accepts input from the CPU address and the control bus and then issues commands to both control groups. Group A and Group B.

CS’

It stands for Chip Select. The low level of this input selects the chip and allows communication between the 8255A and the CPU. It is connected to the decode address, and A0 and A1 are connected to the address line of the microprocessor.

Their results depend on the following conditions:

CS

A1 A0 Result

0

0 0

PORT A

0

0 1 PORT B

0

1 0 PORT C
0 1 1

Control Register

1 X X

No Selection

WR’

It stands for writing. This control signal enables the writing process. When this signal goes low, the microprocessor writes to the selected I / O port or control register.

RESET

This is an effective high signal. It clears the control registers and puts all ports in input mode.

RD’

It is about reading. This control signal enables the reading process. When the signal is low, the microprocessor reads data from the selected I / O port of the 8255.

A0 and A1

These input signals operate with RD, WR and one of the control signals. The following table shows the different signals and their results.

A1 A0 RD WR CS Result
0 0 0 1 0 Input Operation
PORT A → Data Bus
0 1 0 1 0 PORT B → Data Bus
1 0 0 1 0 PORT C → Data Bus
0 0 1 0 0 Output Operation
Data Bus → PORT A
0 1 1 0 0 Data Bus → PORT A
1 0 1 0 0 Data Bus → PORT B
1 1 1 0 0 Data Bus → PORT D