82C55 datasheet(3/23 Pages) INTEL

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82C55A

82C55A FUNCTIONAL DESCRIPTION

General

The 82C55A is a programmable peripheral interface

device designed for use in Intel microcomputer sys-

tems Its function is that of a general purpose IO

component to interface peripheral equipment to the

microcomputer system bus The functional configu-

ration of the 82C55A is programmed by the system

software so that normally no external logic is neces-

sary to interface peripheral devices or structures

Data Bus Buffer

This 3-state bidirectional 8-bit buffer is used to inter-

face the 82C55A to the system data bus Data is

transmitted or received by the buffer upon execution

of input or output instructions by the CPU Control

words and status information are also transferred

through the data bus buffer

ReadWrite and Control Logic

The function of this block is to manage all of the

internal and external transfers of both Data and

Control or Status words It accepts inputs from the

CPU Address and Control busses and in turn issues

commands to both of the Control Groups

Group A and Group B Controls

The functional configuration of each port is pro-

grammed by the systems software In essence the

CPU ‘‘outputs’’ a control word to the 82C55A The

control word contains information such as ‘‘mode’’

‘‘bit set’’ ‘‘bit reset’’ etc

that initializes the func-

tional configuration of the 82C55A

Each of the Control blocks (Group A and Group B)

accepts ‘‘commands’’ from the ReadWrite Control

Logic receives ‘‘control words’’ from the internal

data bus and issues the proper commands to its as-

sociated ports

Control Group A - Port A and Port C upper (C7 – C4)

Control Group B - Port B and Port C lower (C3 – C0)

The control word register can be both written and

read as shown in the address decode table in the

pin descriptions Figure 6 shows the control word

format for both Read and Write operations When

the control word is read bit D7 will always be a logic

‘‘1’’ as this implies control word mode information

Ports A B and C

The 82C55A contains three 8-bit ports (A B and C)

All can be configured in a wide variety of functional

characteristics by the system software but each has

its own special features or ‘‘personality’’ to further

enhance the power and flexibility of the 82C55A

Port A

One 8-bit data output latchbuffer and one

8-bit input latch buffer Both ‘‘pull-up’’ and ‘‘pull-

down’’ bus hold devices are present on Port A

Port B

One 8-bit data inputoutput latchbuffer

Only ‘‘pull-up’’ bus hold devices are present on Port

Port C

One 8-bit data output latchbuffer and one

8-bit data input buffer (no latch for input) This port

can be divided into two 4-bit ports under the mode

control Each 4-bit port contains a 4-bit latch and it

can be used for the control signal outputs and status

signal inputs in conjunction with ports A and B Only

‘‘pull-up’’ bus hold devices are present on Port C

See Figure 4 for the bus-hold circuit configuration for

Port A B and C

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