CS82C55A datasheet(5/29 Pages) INTERSIL

5 / 29 page

FN2969.10

November 16, 2006

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

output and status signal inputs in conjunction with ports A

and B. See Figure 2B.

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 with all 24 port lines held at a logic “one” level by

internal bus hold devices. After the reset is removed, the

82C55A can remain in the input mode with no additional

initialization required. This eliminates the need to pull-up or

pull-down resistors in all-CMOS designs. The control word

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.

FIGURE 2A. PORT A BUS-HOLD CONFIGURATION

FIGURE 2B. PORT B AND C BUS-HOLD CONFIGURATION

FIGURE 2. BUS-HOLD CONFIGURATION

MASTER

RESET

OR MODE

CHANGE

INTERNAL

DATA IN

INTERNAL

DATA OUT

(LATCHED)

EXTERNAL

PORT A PIN

OUTPUT MODE

INPUT MODE

RESET

OR MODE

CHANGE

INTERNAL

DATA IN

INTERNAL

DATA OUT

(LATCHED)

EXTERNAL

PORT B, C

OUTPUT MODE

VCC

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

82C55A

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