Intel 8255 (Programmable Peripheral Interface)

Intel 8255 is a PPI (Programmable Peripheral Interface) and is used to interface Peripheral devices to microcomputers. It can be programmed in a variety of ways as per Programmer Requirements. It has two versions 8255A and 8255B. Both of them are similar in general description but are different in their electrical characteristics.

Intel 8255 Consists of Three 8 Bit Ports: Port A, Port B and Port C. The Port C is further divided into two 4 Bit Ports: Port C_UPPER and C_LOWER. These three ports are divided into two groups for the purpose of Programming:

Group A: This Includes Port A (PA₀-PA₇) and 4 MSBs of Port C (PC₄-PC₇).

Group B: This Includes Port B (PB₀-PB₇) and 4 LSBs of Port C (PC₀-PC₃).

Features of 8255A (PPI)

It is a 40 pin IC.
It Operates on single +5V_DC supply.
Power Dissipation=1 watt.
Ambient Temp. =0- 70 °C
Input Low Voltage (V_IL) = Min. 0.5V, Max. 0.8V.
Input High Voltage (V_IH) =Min. 2V, Max. V_CC.
Output Low Voltage (V_OL) = 0.45V
Output High Voltage (V_OH)= 2.4V
Voltage on any pin 0.5V to 7V.
Darlington Drive Current= Min. 1mA, Max. 4mA.

Pin Description of 8255A

Pin	Description
D0-D7	8 Bit Bidirectional Data Bus and goes to tri state condition if CS is high.
	Chip Select Signal. 8255 is Selected if this pin status is low.
A0-A1	Used to select port from which data is to be transmitted or to which data is to be transmitted.
	Port/Control Word Register	Port/Control Word Register Address
	Port A	00
	Port B	01
	Port C	02
	Control Word Register	03
	To indicate read operation
	To indicate write operation.
PA0-PA7	8 Pins of Port A
PB0-PB7	8 Pins of Port B
PC0-PC3	4 Pins of Port C LOWER
PC4-PC7	4 Pins of Port C UPPER
VCC	+5V DC Supply.
GND	Ground Signal

Operating Modes of 8255A

Three Ports of Intel 8255A can be programmed in following three operating modes:

Mode 0 or Input /Output Mode.

Mode 1 or Strobed Input /output Mode.

Mode2 or Bi-directional Bus Mode

Mode0 or Input /Output Mode: In mode is each of four Ports (i.e. Port A, Port B, Port C_LOWER and Port C_UPPER) can be programmed as either an input or output port. If any of the port is working in mode 0 then read and write operation on that port can be performed without any control signal.

Mode1 or Strobed Input /Output Mode: Only Port A and B can operate in this mode. If Port A and Port B are programmed in mode 0, then 6 pins of Port C are used for their control. Pins PC₀-PC₂are used to control Port B which can be used either input or output port. PC₃-PC₅ are used as control signal for Port A. If Port A acts as input port, pins PC₃-PC₅ are used. If Port A acts as Output port than PC₃, PC₆, PC₇ are used as control signals.

Mode2 or Strobed Bi-directional Bus I/O: Only port A can be programmed to work in this mode. PC3-PC7 are used as control signals for the transfer of data. In mode 2 Port A can be used to transmit data or to receive data from peripherals. The device can strobe data into port by using STB signal.