82C55A datasheet(3/26 Pages) HARRIS

3 / 26 page

Functional Description

Data Bus Buffer

This three-state bi-directional 8-bit buffer is used to interface

the 82C55A to the system data bus. Data is transmitted or

received by the buffer upon execution of input or output

instructions by the CPU. Control words and status informa-

tion are also transferred through the data bus buffer.

Read/Write and Control Logic

The function of this block is to manage all of the internal and

external transfers of both Data and Control or Status words.

It accepts inputs from the CPU Address and Control busses

and in turn, issues commands to both of the Control Groups.

(CS) Chip Select. A “low” on this input pin enables the

communcation between the 82C55A and the CPU.

(RD) Read. A “low” on this input pin enables 82C55A to send

the data or status information to the CPU on the data bus. In

essence, it allows the CPU to “read from” the 82C55A.

(WR) Write. A “low” on this input pin enables the CPU to

write data or control words into the 82C55A.

(A0 and A1) Port Select 0 and Port Select 1. These input

signals, in conjunction with the RD and WR inputs, control

the selection of one of the three ports or the control word

bits of the address bus (A0 and A1).

(RESET) Reset. A “high” on this input initializes the control

mode. “Bus hold” devices internal to the 82C55A will hold

the I/O port inputs to a logic “1” state with a maximum hold

current of 400

µA.

Group A and Group B Controls

The functional conﬁguration of each port is programmed by

the systems software. In essence, the CPU “outputs” a con-

trol word to the 82C55A. The control word contains

information such as “mode”, “bit set”, “bit reset”, etc., that ini-

tializes the functional conﬁguration of the 82C55A.

Each of the Control blocks (Group A and Group B) accepts

“commands” from the Read/Write Control logic, receives

“control words” from the internal data bus and issues the

proper commands to its associated ports.

Control Group A - Port A and Port C upper (C7 - C4)

Control Group B - Port B and Port C lower (C3 - C0)

The control word register can be both written and read as

shown in the “Basic Operation” table. Figure 4 shows the

control word format for both Read and Write operations.

When the control word is read, bit D7 will always be a logic

“1”, as this implies control word mode information.

82C55A BASIC OPERATION

INPUT OPERATION

(READ)

00010

Port A

→ Data Bus

01010

Port B

→ Data Bus

10010

Port C

→ Data Bus

11010

Control Word

→ Data Bus

OUTPUT OPERATION

(WRITE)

00100

Data Bus

→ Port A

01100

Data Bus

→ Port B

10100

Data Bus

→ Port C

11100

Data Bus

→ Control

DISABLE FUNCTION

XXXX

Data Bus

→ Three-State

Data Bus

→ Three-State

FIGURE 1. 82C55A BLOCK DIAGRAM. DATA BUS BUFFER,

READ/WRITE, GROUP A & B CONTROL LOGIC

FUNCTIONS

GROUP A

PORT A

(8)

GROUP A

PORT C

UPPER

(4)

GROUP B

PORT C

LOWER

(4)

GROUP B

PORT B

(8)

GROUP B

CONTROL

GROUP A

CONTROL

DATA

READ

WRITE

CONTROL

LOGIC

RESET

D7-D0

POWER

SUPPLIES

+5V

GND

BI-DIRECTIONAL

DATA BUS

I/O

PA7-

I/O

PC7-

I/O

PC3-

I/O

PB7-

BUFFER

BUS

PB0

PC0

PC4

PA0

8-BIT

INTERNAL

DATA BUS

82C55A

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