Prof. Nitin Ahire 1

I/O Interfacingwith 8085

Prof. Nitin Ahire

XIE, Mahim

Prof. Nitin Ahire 2

fundamentals of I/O devices

• I/p Port

I/P PORTBufferD0-D7

D0-D7

Enable

Data from Keyboard

Prof. Nitin Ahire 3

fundamentals of I/O devices

• O/p Port

O/P PORTlatchD0-D7

D0-D7

Enable

Data to Display

Prof. Nitin Ahire 4

I/O interfacing techniques

• Up support I/O interface tech.

• It partitions memory from I/O, via software

instruction like IN add, OUT add

• When these instructions decoded by the processor it generate appropriate control signals IO/M^

• In 8085 it is possible to connect 256 I/O ports and 64Kb memory

Prof. Nitin Ahire 5

I/O interfacing techniques

• I/O devices can be interfaced in two ways

1) I/O mapped I/O

2) Memory mapped I/O

Prof. Nitin Ahire 6

I/O interfacing techniques

• Memory mapped I/O1) In this device add is 16 bit

( A0-A15)2) MEMR^ and MEWR^

control signals are used3) Instructions are LDA add,

STA Add, MOV A,M 4) Data trans. Bet reg and

I/O devices 5) No. of I/O devices

interface= 65536 ( Theoretically)

• I/O mapped I/P1) In this device add is 8

bit (A0-A7)2) IOR^ and IOW^ control

signals are used3) Instruction are IN Add,

OUT Add 4) Data trans. Bet acc and

I/O devices5) No. of I/O devices

interface= 256 only

Prof. Nitin Ahire 7

I/O device selection for add (80h)

Decoder

A0

A3

A2A1

A6A5A4

A7

G1^

G2^

GY0

OR

RD^/WR^

IO/M^

BufferOr

latch

OR

NOTOR

To I/O device

Data bus

Enable IOR^/IOW^

Data bus

Prof. Nitin Ahire 8

• IN 80h ( here 80h is the address of i/p switches)

8 0

A15/A7 A14/A6 A9/A1A10/A2A11/A3A12/A4A13/A5 A8/A0

1 0 00000 0

Prof. Nitin Ahire 9

I/O device selection for add (80h)

3:8 Decoder

A0

A3

A2A1

A6A5A4

A7

G1^

G2^

GY0

OR

RD^/WR^

IO/M^

BufferOr

latch

OR

NOTOR

Data transfer to Accumulator

Enable IOR^/IOW^

I/P switches S0-S7

Select lines

Data bus

Prof. Nitin Ahire 10

I/O device selection for add (81h)

3:8 Decoder

A0

A3

A2A1

A6A5A4

A7

G1^

G2^

GY1

OR

RD^/WR^

IO/M^

BufferOr

latch

OR

NOTOR

Data transfer to Accumulator

Enable IOR^/IOW^

0/P Display devices

Select lines

Data bus