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82C55A
82C55A OPERATIONAL DESCRIPTION
Mode Selectlon
There are three basic modes of operation that can
be selected by the system software:
Mode 0 - Basic input/output
Mode 1 - Strobed Input/ouput
Mode 2 - Bi-directional Bus
When the res€t input goes "high" all ports will be set
to the input mode with all24 port lines held at a logic
"on€" level by the internal bus hold devices (see
Figure 4 Note). Atter the reset is removed the
82G55A 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 s€lected by using a
single output instruction. This allows a single
82C55A to service a variety ol 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 l/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
intenupt-driven basis.
Figure 5. Basic Mode Definltions and Bus
Interface
Flgure 6. Mode Detlnltlon Format
The mode definitions and possible mode combina-
tions may seem confusing at first but atter a cursory
review of the complete device operation a simple,
logical l/O approach will surface. The design of the
82C55A has taken into account things such as effi-
cient PC board layout, controlsignaldefinition vs PC
layout and complete functional flexibility to support
almost any peripheral device with no extemal logic.
Such design represents the maximum use of the
available pins.
Slngle Blt Sel/Reset Feature
Any of the eight bits of Port C can be Set or Reset
using a single OUTput instruction. This feature re-
duces sottware requirements in Control-based appli-
cations.
When Port C is being used as status/controlfor Port
A or B, these bits can be set or r€set by using the Bit
Set/Reset operation just as if they were data output
POrts.
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