CMP82C55AZ datasheet(5/26 Pages) RENESAS

5 / 26 page

MS82C55A, MQ82C55A, MP82C55A

FN6140 Rev 2.00

Page 5 of 26

June 15, 2006

Operational Description

Mode Selection

There are three basic modes of operation than can be selected

by the system software:

Mode 0 - Basic Input/Output

Mode 1 - Strobed Input/Output

Mode 2 - Bidirectional Bus

When the reset input goes “high”, all ports will be set to the

input mode. After the reset is removed, the 82C55A can

remain in the input mode with no additional initialization

required. The control word register will contain 9Bh. During the

execution of the system program, any of the other modes may

be selected using a single output instruction. This allows a

single 82C55A to service a variety of peripheral devices with a

simple software maintenance routine. Any port programmed as

an output port is initialized to all zeros when the control word is

written.

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 computational results, Group

A could be programmed in Mode 1 to monitor a keyboard or

tape reader on an interrupt-driven basis.

The mode definitions and possible mode combinations 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 efficient 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 Set/Reset Feature (Figure 5)

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

single Output instruction. This feature reduces software

requirements in control-based applications.

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 output ports.

FIGURE 3. BASIC MODE DEFINITIONS AND BUS INTERFACE

DATA BUS

8I/O

PB7-PB0

4I/O

PC3-PC0

4I/O

PC7-PC4

8I/O

PA7-PA0

CONTROL BUS

ADDRESS BUS

RD, WR

82C55A

D7-D0

A0-A1

MODE 0

8I/O

PB7-PB0

CONTROL

8I/O

PA7-PA0

MODE 1

OR I/O

CONTROL

OR I/O

8I/O

PB7-PB0

BI-

PA7-PA0

MODE 2

CONTROL

DIRECTIONAL

FIGURE 4. MODE DEFINITION FORMAT

D7 D6 D5 D4 D3 D2 D1 D0

PORT C (LOWER)

1 = INPUT

0 = OUTPUT

PORT B

1 = INPUT

0 = OUTPUT

MODE SELECTION

0 = MODE 0

1 = MODE 1

GROUP B

PORT C (UPPER)

1 = INPUT

0 = OUTPUT

PORT A

1 = INPUT

0 = OUTPUT

MODE SELECTION

00 = MODE 0

01 = MODE 1

GROUP A

1X = MODE 2

MODE SET FLAG

1 = ACTIVE

CONTROL WORD

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