The 8255PPI is a programmable peripheral interface with 3 8-bit ports (A, B, C). Ports can be configured as input or output in different modes. It has 2 main modes - Bit Set/Reset for individual bit operations and I/O mode with submodes 0, 1, 2 for port configuration. Ports A and B can be used for simple I/O in mode 0 or handshake operations in mode 1. Mode 2 allows bidirectional data transfer on port A using port C pins as handshake signals.

2. Pin Diagram of 8255PPI

3. 8255PPI
•TheINTEL8255is a 40pinIC havingtotal24I/Opins.
consistingof 3numbersof 8–bit parallelI/O ports (i.e.
PORTA,PORTB,PORTC).Theports canbe
programmedto functioneitheras a inputport oras a
output port indifferentoperatingmodes.operatesin
mainly twomodes.
•(1)Bit SetResetMode (BSRMode).
•(2)I/O Mode.

4. Function of Blocks
BLOCK FUNCTION OFBLOCK
Data BusBuffer
Itisusedtointerfacetheinternaldatabusof8255tothesystemdatabusby reading
andwritingoperations.
Read/write
Controllogic
Itacceptstheinputfromtheaddressbusandissuescommandstotheindividual group
blocks.also issues appropriateenablingsignals toaccesstherequired data/control
words/statuswords.
PortA
ItcanbeprogrammedinthreemodesMode0,Mode1andMode2.
PortB
ItcanbeprogrammedinthreemodesMode0andMode1.
PortC
ItcanbeprogrammedforBitSet/resetoperation.

5. Function of Pins
PIN FUNCTIONOFPIN
D0-D7(DataBus)
Thesearebidirectional,tri-statedatabuslinesareconnectedtothesystemdatabus.They areusedto
transferdataand controlwordfrommicroprocessor(8085)to 8255orreceive dataorstatusword
from8255tothe8085.
PA0-PA7(PortA)
Theseare8BitbidirectionalI/O pinsusedtosenddatatooutputdeviceandtoreceivedata frominput
device.It functionsas an 8Bitdataoutputlatch/bufferwhenusedin output modeandasan8Bit
datainputlatch/bufferwhenusedininputmode.
PB0-PB7(PortB)
Theseare8BitbidirectionalI/O pinsusedtosenddatatooutputdeviceandtoreceivedata frominput
device.It functionsas an 8Bitdataoutputlatch/bufferwhenusedin output modeandasan8Bit
datainputlatch/bufferwhenusedininputmode.

6. Function of Pins
PIN FUNCTION OFPIN
PC0-PC7
(PortC)
Theseare8bitbidirectionalI/O pins dividedintotwogroupsPCL(PC3-PC)andPCU(PC7-
PC4).thesegroupscanindividuallytransferdatainoroutwhenprogrammedforsimpleI/O, and
usedashandshakesignalswhenprogrammedforhandshakeor
bidirectionalmodes.
RD
Whenthispinislow,theCPUcanreaddataintheportsorthestatuswordthroughthedata bus
buffer.
WR
Whenthispinislow,theCPUcanwritedataontheportsorinthecontrolregisterthrough thedata
busbuffer.
CS
ThispincanbeenabledfordatatransferoperationbetweentheCPUand8255.
RESET
Thispinisusedtoreset8255.i.econtrolregistergetsclearedandalltheportsaresettothe inputmode.

7. Function of Pins
PIN FUNCTIONOFPIN
A0-A1 TheselectionofinputportandcontrolwordregisterisdonebyusingA0andA1pinsIn
conjunctionwithRDandWR pins.
A1 A0 RD WR CS Operations
0 0 0 1 0 PORT A TODATABUS
0 1 0 1 0 PORT B TODATABUS
1 0 0 1 0 PORT CTODATABUS
0 0 1 0 0 DATABUS TOPORTA
0 1 1 0 0 DATABUS TOPORTB
1 0 1 0 0 DATABUS TOPORTC
1 1 1 0 0 DATABUS TOCONTROLREGISTER
x x x x 1 DATABUS TRISTATED
1 1 0 1 0 ILLEGALCONDITION
x x 1 1 0 DATABUS TRISTATED

8. Block Diagram of 8255 PPI
READ/
WRITE
CONTROL
LOGIC
DATABUS
BUFFER
GROUPA
CONTROL
GROUPA
PORTA(8)
GROUPA
PORTC
UPPER(4)
GROUP B
PORTC
LOWER(4)
GROUPB
PORTB
(8)
GROUP B
CONTROL
Bidirectional
DataBus
D7- D0
RD
WR
A0
A1
RESET
cs
8Bit
Internal
DataBus
PA
PA7-PA0
PCU
PC7-PC4
PCL
PC3-PC0
PB
PB7-PB0
DATABUS
BUFFER
POWER
SUPPLIES
+5V
Ground

9. CS,A1,A2
CS A1 A2 RESULT
0 0 0 PORT-A
0 0 1 PORT-B
0 1 0 PORT-C
0 1 1 CONTROL
REGISTER
1 X X NO
SELECTION

10. Control Word Format in BSRMode
D7 D6 D5 D4 D3 D2 D1D1 D0
BIT SET/RESETFLAG
0=ACTIVE
BITSET/RESET
1=SET
0=RESET
BITSELECT
0 1 2 3 4 5 6 7
0 1 0 1 0 1 0 1 B0
0 0 1 1 0 0 1 1 B1
0 0 0 0 1 1 1 1 B2
DON’TCARE

11. Control Word Format in I/OMode
D7 D6 D5 D4 D3 D2 D1 D0 GROUP B
PORT C(LOWER)1=I/P,
0=O/P
PORT B
1=I/P, 0=O/P
MODESELECTION
0=MODE 0
1=MODE1
GROUP A
PORT C(UPPER)1=I/P,
0=O/P
PORTA
1=I/P, 0=O/P
MODESELECTION
00=MODE 0
01=MODE 1
1X =MODE2
MODE SETFLAG
1=ACTIVE

13. Operating Modes Of8255
• There are two main operational modes of8255:
(1)Input/output mode,
(2)Bit set/reset mode (BSRMode).
I/O mode againclassified into three types
• (1) Mode 0,
• (2) Mode 1,
• (3) Mode 2.

14. MODE 0
• In this mode, the ports can be used for simple input/output operations
without handshaking.
• If both port A and B are initialized in mode 0,the two halves of port Ccan
be either used together as an additional 8-bit port, or they can be used as
individual 4-bitports.
• Since the two halves of port Care independent, they may be used such that
one-half is initialized as an input port while the other half is initialized as an
output port.
Themode 0has followingfeatures:
• O/p arelatched.
• I/p are buffered notlatched.
• Port do not have handshake or interruptcapability.

15. MODE 1
• When we wish to use portAor port B for handshake (strobed) input or
output operation, we initialize that port in mode 1.
• Forport B in this mode (irrespective of whether is acting as an input port
or output port), PC0,PC1 and PC2pins function as handshake lines.
Themode 1has followingfeatures:
• Twoports i.e. portAand B can be use as 8-biti/o port.
• Each port uses three lines of port cas handshake signal and remaining two
signals can be function as i/oport.
• Interrupt logic issupported.
• Input and Output data arelatched.

16. MODE 2
•Only groupA can be initialized in thismode.
•PortA can be used for bidirectional handshake data
transfer.This means that data can be input or outputon
the same eight lines (PA0-PA7).
•Pins PC3- PC7are used as handshake lines for portA.
•Theremaining pins of port C(PC0 - PC2) can be usedas
input/output lines if group B is initialized in mode 0.
•In this mode, the 8255may be used to extend thesystem
bus to a slavemicroprocessor.