82C55A Datasheet, PDF (5/23 Pages) Intel Corporation – CHMOS PROGRAMMABLE PERIPHERAL INTERFACE

82C55A
82C55A OPERATIONAL DESCRIPTION
Mode Selection
There are three basic modes of operation that can
be selected by the system software
Mode 0
Mode 1
Mode 2
Basic input output
Strobed Input output
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 I O 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
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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 I O 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
231256 –5
Figure 5 Basic Mode Definitions and Bus
Interface
Single Bit Set Reset 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 status control for Port
A or B these bits can be set or reset by using the Bit
Set Reset operation just as if they were data output
ports
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