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Programmable Peripheral Interface

ver 1.00

OVERVIEW

The D8255 is a programmable I/O device

which is designed for use with all Intel and

most other microprocessors.

It provides 24 I/O pins which may be indi-

vidually programmed in 2 groups of 12 and

used in 3 major modes of operation:

tional configuration provides simple input

and output operations for each of the

three ports. No „handshaking'' is required,

data is simply written to or read from a

specified port. Mode 0 Basic Functional

Definitions:

Two 8-bit ports and two 4-bit ports.

16 different Input/Output configurations

are possible in this Mode

MODE 1 - Strobed Input/Output. This

functional configuration provides a means

for transferring I/O data to or from a

specified port in conjunction with strobes

or „handshaking'' signals. In mode 1, Port

A and Port B use the lines on Port C to

generate or accept these „handshaking''

signals. Mode 1 Basic functional Defini-

tions

:

Two Groups (Group A and Group B).

Each group contains one 8-bit data port and

one 4-bit control/data port.

The 8-bit data port can be either input or out-

put Both inputs and outputs are latched.

The 4-bit port is used for control and status of

the 8-bit data port.

MODE 2 - Strobed Bidirectional Bus I/O.

This functional configuration provides a

means for communicating with a periph-

eral device or structure on a single 8-bit

bus for both transmitting and receiving

data (bidirectional bus I/O). „Handshak-

ing'' signals are provided to maintain

proper bus flow discipline in a similar

manner to MODE 1. Interrupt generation

and enable/disable functions are also

available. MODE 2 Basic Functional Defi-

nitions:

Used in Group A only.

One 8-bit, bi-directional bus port (Port A) and

a 5-bit control port (Port C).

The 5-bit control port (Port C) is used for con-

trol and status for the 8-bit, bi-directional bus

port (Port A)

The functional configuration of the D8255 is

programmed by the system software so that

normally no external logic is necessary to

interface peripheral devices or structures.

The control word register can be both written

and read as shown in the address decode

table in the pin descriptions.