82C55 datasheet(5/23 Pages) INTEL

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82C55A

82C55A OPERATIONAL DESCRIPTION

Mode Selection

There are three basic modes of operation that can

be selected by the system software

Mode 0

Basic inputoutput

Mode 1

Strobed Inputoutput

Mode 2

Bi-directional Bus

When the reset input goes ‘‘high’’ all ports will be set

to the input mode with all 24 port lines held at a logic

‘‘one’’ level by the internal bus hold devices (see

Figure 4 Note) After the reset is removed the

82C55A can remain in the input mode with no addi-

tional initialization required This eliminates the need

for pullup or pulldown devices in ‘‘all CMOS’’ de-

signs During the execution of the system program

any of the other modes may be selected by using a

single output instruction

This allows a single

82C55A to service a variety of peripheral devices

with a simple software maintenance routine

The modes for Port A and Port B can be separately

defined while Port C is divided into two portions as

required by the Port A and Port B definitions All of

the output registers including the status flip-flops

will be reset whenever the mode is changed Modes

may be combined so that their functional definition

can be ‘‘tailored’’ to almost any IO structure For

instance Group B can be programmed in Mode 0 to

monitor simple switch closings or display computa-

tional results Group A could be programmed in

Mode 1 to monitor a keyboard or tape reader on an

interrupt-driven basis

231256 – 5

Figure 5 Basic Mode Definitions and Bus

Interface

231256 – 6

Figure 6 Mode Definition Format

The mode definitions and possible mode combina-

tions may seem confusing at first but after a cursory

review of the complete device operation a simple

logical IO approach will surface The design of the

82C55A has taken into account things such as effi-

cient PC board layout control signal definition vs PC

layout and complete functional flexibility to support

almost any peripheral device with no external logic

Such design represents the maximum use of the

available pins

Single Bit SetReset Feature

Any of the eight bits of Port C can be Set or Reset

using a single OUTput instruction This feature re-

duces software requirements in Control-based appli-

cations

When Port C is being used as statuscontrol for Port

A or B these bits can be set or reset by using the Bit

SetReset operation just as if they were data output

ports

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