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Copyright 1999-2007 DCD – Digital Core Design. All Rights Reserved.

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Group B – Port B and lower half of Port C

KEY FEATURES

Compatible with industry standard 8255

24 I/O lines

individually programmed in

2 groups of 12:

Group A - Port A and upper half of Port C

3 major modes of operation

Mode 0 – Basic input/output

Mode 1 – Strobed Input/output

Mode 2 – Bi-directional Bus

Control Word Read-Back Capability

Direct Bit Set/Reset Capability

Interrupt control functions

No internal three states busses

Fully synthesizable technology independ-

ent source code.

DESIGN FEATURES

SYNCHRONIZED BEFORE INTERNAL USE

LENT FLIP

FUNCTIONALITY

APPLI C ATIONS

Embedded microprocessor boards

Interface to the printer

I/O component to interface peripheral

equipment to the microcomputer system

bus

SYMBOL

datai(7:0)

datao (7:0)

we

rd

cs

rst

clk

a(1:0)

portai(7:0)

portao (7:0)

portbi(7:0)

portbo (7:0)

portci(7:0)

portco (7:0)

PINS DESCRIPTI O N

PIN

TYPE

DESCRIPTION

clk

input

Global clock

reset

input

Global reset

cs

input

Chip select

rd

input

Processor read strobe

we

input

Processor write strobe

a[1:0]

input

Processor address lines

portai[7:0]

input

Port A input

portbi[7:0]

input

Port B input

portci[7:0]

input

Port C input

datai[7:0]

input

Data bus (input)

datao[7:0]

output

Data bus (output)

portao[7:0]

output

Port A output

portbo[7:0]

output

Port B output

portco[7:0]

output

Port C output

BLOCK DIAGRAM

Data Bus Buffer

– The Data Bus Buffer is

used to interface the D8255 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

information

are

also

transferred

through the data bus buffer.

Read/Write and Control Logic

- The control

logic block manages 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, is-

sues commands to both of the Control

Groups A and B.