Peripheral Programmable Interface

8/17/2019 Peripheral Programmable Interface

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PROGRAMMABLE

PERIPHERAL INTERFACE

-8255

Features: It is a programmable device.

It has 24 I/O programmable pins like PA,P,P!"#$% pins&.

• ' ' ( compatible.

• Improved dc driving capabilit)


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Pin Diagram


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Function of pins:

*ata bus"*+$*):'hese are %$bit bi$directionalbuses, connected to %+%- data bus ortranserring data.

!: 'his is Active (o0 signal. 1hen it is lo0,then data is transer rom %+%-.

ead: 'his is Active (o0 signal, 0hen it is (o0

read operation 0ill be start.

1rite: 'his is Active (o0 signal, 0hen it is (o0

1rite operation 0ill be start.


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A1A1 A0A0 SelectSelect

00 00 PAPA

00 11 PBPB

11 00 PCPC

11 11 ControlControl

reg.reg.

Address "A+$A3&:'his is used to select the

ports. like this


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': 'his is used to reset the device. 'hatmeans clear control registers.

PA+$PA:It is the %$bit bi$directional I/O pinsused to send the data to peripheral or

or to receive the data rom peripheral.

P+$P:imilar to PA

P!+$P!:'his is also %$bit bidirectional I/O pins.

'hese lines are divided into t0o groups.3. P!+ to P!#"(o0er 5roups&

2. P!4 to P! "6igher groups&

'hese t0o groups 0orking in separatel) using 4

data7s.


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Block Diagram


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Data Bus buffer : It is a %$bit bidirectional *ata bus.

8sed to interace bet0een %2-- data bus0ith s)stem bus.

'he internal data bus and Outer pins *+$*

pins are connected in internall).

'he direction o data buer is decided b)ead/!ontrol (ogic.


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Read/Write Control Logic: This is getting the input signals from

control bus and Address bus

Control signal are RD and WR.

Address signals are A0A!and C".

#$%% operation is enabled or disabled b&

C".


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5roup A and 5roup control: 5roup A and get the !ontrol

ignal rom !P8 and send the command to theindividual control blocks.

9 5roup A send the control signal to port A and Port! "8pper& P!$P!4.

9 5roup send the control signal to port and Port! "(o0er& P!#$P!+.

9 PO' A:9 'his is a %$bit buered I/O latch.

9 It can be programmed b) mode + , mode 3, mode2 .


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PO' : 'his is a %$bit buer I/O latch.

It can be programmed b) mode + and mode 3.

PO' !: 'his is a %$bit 8nlatched buer Input and an

Output latch.

It is split$ed into t0o parts.

It can be programmed b) bit set/reset

operation.


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Operation modes:

I' '/' O*: 'he PO' ! can be et or eset b) sending O8'instruction to the !O;'O( registers.

I/O O*: O* +"imple input / Output&: In this mode , port A, port and port ! is used as

individuall) "impl)&.

Features: Outputs are latched , Inputs are buered not

latched. Ports do not have 6andshake or interrupt

capabilit).


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O* 3 :"Input/output 0ith 6and shake&

In this mode, input or output is transerred b)

hand shaking ignals.

6andshaking signals is used to transer databet0een 0hose data transer is not same.

Computer

Printer DATA BUS STB ACK

Bus


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O* 2:bi$directional I/O data transer:

'his mode allo0s bidirectional data transer over

a single %$bit data bus using handshake signals. 'his eature is possible onl) 5roup A

Port A is 0orking as %$bi) bidirectional.

P!#$P! is used or handshaking purpose. 'he data is sent b) !P8 through this port , 0hen

the peripheral re=uest it.

!O;'O( 1O* FOA': In the I;P8' mode , 1hen ' is 6igh all

24 pins "#$ports& be a input mode.


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i.e all lip lops are cleared and the interrupts are

rest.

'his condition is maintained even ater 'goes lo0.

'his can be avoid b) 0riting single control 0ord

to the control registers , 0hen re=uired.


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FO I' '/' O*:

'his is bit set/reset control 0ord ormat.

> > >

*on7t care

it select

+

3

2

D' D( D% D) D* D$ D! D0

0 ! $ * ) % ( '

+ 3 + 3 + 3 + 3

+ + 3 3 + + 3 3

+ + + + 3 3 3 3

B!TS"T#$"S"T%&S"T'&$"S"T

B!T S"T#$"S"T F(A

&' Acti*e


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P!+$P! is set or reset as per the status o *+.

A 0ord is 0ritten or each bit

>>+333.

P!4 is eset then control register 0ill be+>>>+3+++.

> is a don7t care.


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FO I/O O*:

'he mode ormat or I/O as sho0n in igure

D

7

D

6

D

5

D

4

D

3

D

2

D

1

D

0

roup A

Port C Upper

%&!nput

'&Output

Port A

%&!nput

'&Output

+odeselection

''&mode '

'%&mode %

%,&mode -

roup B

Port C (o.er

%&!nput

'&Output

Port B

%&!nput

'&Output

+ode selection

'&mode '

%&mode %

+ode set/ag&%&Acti*e


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'he control 0ord or both mode is same.

it * is used or speci)ing 0hether 0ord

loaded in to it set/reset mode or odedeinition 0ord.

*?3?ode deinition mode.

*?+?it set/eset mode.


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T0ank ou

@A


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Prepared 1

Prof2$2K2Kumar

Peripheral Programmable Interface

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