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8255 PPI or parallel interface
- 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.