Saturday, 20 July 2013

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 CUPPER and CLOWER. 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 +5VDCsupply.
Power Dissipation=1 watt.
Ambient Temp. =0- 70 °C
Input Low Voltage (VIL) = Min. 0.5V, Max. 0.8V.
Input High Voltage (VIH) =Min. 2V, Max. VCC.
Output Low Voltage (VOL) = 0.45V
Output High Voltage (VOH)= 2.4V
Voltage on any pin 0.5V to 7V.
Darlington Drive Current= Min. 1mA, Max. 4mA.

Pin Description of 8255A

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.
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
Port B
Port C
Control Word Register

To indicate read operation

To indicate write operation.
8 Pins of Port A
8 Pins of Port B
4 Pins of Port C LOWER
4 Pins of Port C UPPER
+5V DC Supply.
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 CLOWER and Port CUPPER) 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.

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