354 39 Solutions Instructor Manual 11 Hardware Features 8051 Chapter 11

91

Chapter11Hardwarefeaturesof8051

SolutiontoNumerical/DESIGNBASEDEXERCISE:

1. Showthecircuitconnectionsforinterfacing16KofEPROMIC27128and8KofRAMIC6264withthe8051.

Thefirststepininterfacingistoselectorfixtheaddressrangeforthechipstobeused.Asthe27128has16KBofmemoryregisters,itrequires14addresslinestoselectonememorylocationin it.Theaddressmapof the27128 is fixedas0000H3FFFH.Theaddress range for the8KBRAMchip6264isselectedasC000HDFFFH.

Themost significantbitsA14 andA15 areused todecode and select the chip. Fordecodingpurposes,a2to4decoderchip,the74139, isused.The74319hasadual2to4decoderandoneofthemisusedforselectingtheEPROMchip.BitsA14andA15areatlogic0fortheEPROMchipand theY0output ismadeactive low for thecorrespondingA14andA15 inputs.ThisY0signalisusedastheactivelowchipselectinputoftheIC27128.

Microprocessors and Microcontrollers Kumar, Saravanan & Jeevananthan

Oxford University Press 2011

92

BitsA14andA15areatlogic1fortheRAMchipandtheY3outputismadeactivelowfortheA14andA15inputsaslogichigh.ThisY3signalisusedastheactivelowchipselectinputoftheIC6264.TheconnectiondiagramisshowninFigure.The8bitlatchorregisterIC74373isusedtodemultiplexthelowerorderaddressanddatabus.OEisthedatareadenablelineofthe27128andisconnectedtothePENsignaloutputofthe8051.Thedatareadenablelineofthe6264isOE,whichisconnectedtotheP3.7portoutputofthe8051.Similarly,thewriteenableinput6264isconnectedtotheportlineP3.6ofthe8051.

SolutionstoTHINKANDANSWERExercises

1. Whataretheconditionsforexternalmemoryaccessinthe8051?

Connecting the EA pin of the 8051 to logic 1 or +5 V will program the

microcontroller to use the internal program memory for the addresses starting at 0000H.

After the available internal memory is addressed, then the external memory is accessed.

Connecting EA pin to logic 0 will make the microcontroller use only external

memory for all the addresses starting from 0000H.

2. Whatisthepurposeofmultiplexingthelowerorderaddressbuswiththedatabusforexternalmemoryaccess?

The purpose of multiplexing is to reduce the pin count and reduce the number of

pins required.

3. Whataretheadvantagesofseparatedataandprogrammemory(Harvardarchitecture)?The separate data and program memory increases the memory addressing

capability of the microcontroller. Moreover, as the program and data are separate, the

chance of erasing of program memory by mistake as data is removed.

4. Writeadelayroutinefor1msusingTimer0ofthe8051,for12MHzcrystalfrequency.



93

The clock frequency to counter is 12MHz/12; the counter will get the clock pulses

at the 1 MHz rate. So, the counter will be incremented every 1 s. The program

uses 16-bit counter mode. The timer should produce a delay of 1000 s. So, the 16-

bit counter must be initialized to 64535 (i.e., 65535 - 1000). The hexadecimal

equivalent of this decimal value is FC17. Therefore, the lower-order eight bits of

the timer are initialized to 17H and the higher-order eight bits to FCH.

The following DELAY subroutine uses the polled method of waiting for 1msec and

then returns to main program.

DELAY: MOV TMOD, #00000001B

; Set Timer 0 in mode 1 (16-bit operation).

CLR TF0 ; Clear the Timer 0 overflow flag. LOOP: MOV TH0, #0FCH ; Initialize the 16 bits of Timer 0 with the appropriate value. MOV TL0, #17H SETB TR0 ; Start or run Timer 0 by setting the TR0 bit. WAIT: JNB TF0, WAIT ; Read, check, and loop until the overflow bit TF0 in TCON

register is set. CLR TR0 CLR TF0 ; Clear the overflow flag. RET

5. Writearoutineusingatimerofthe8051tocountthecarsmovingonaroadandtogive

asignalwhenthecountvaluereaches100.Tocount theexternalpulses,Timer1 is initializedasacounterby setting theD6bitofTMOD.Mode1ofTimer1 isused inthisexample,as100objectsaretobecountedandthiscanbeaccomplishedbythe8bittimerautoreloadmode.Timer1isloadedwiththevalue155(i.e.,255 100),which inhexadecimalform is9B.Thisgivesan interruptafter100counts.

Theprogramiswrittenintwoparts.Themainprograminitializesthetimerandrunsit.Afterthat,themainprogramdoesnothing.Detectingthecountvalueof100is



94

doneautomatically,bygeneratinganinterruptandthengivinglogic1outputonportpinisdoneintheInterruptserviceroutine.

Mainprogram:

MAIN: MOV TMOD, #01110000B ;SetTimer1inmode1(counter operation).

MOV TH1, #09BH ;LoadtheTimer1count.

MOV IE, #10001000B ;EnableTimer/Counter1interrupt

SETB TR1 ;StartTimer/Counter1.

LOOP: LJMP LOOP ;Loopanddonothing.

Interruptserviceroutine:

ISR_TIMER1:

CLR TR1 ;StopTimer1tobesafe.

SETB P1.0 ;SethighLSBofP1.

RETI ;Returnfrominterrupt.

6. WritetheInterruptPrioritywordformakingserialportandexternalinterrupt1ashigh

priorityandotherinterruptsaslowpriorityones.InterruptPrioritywordformatBit D7 D6 D5 D4 D3 D2 D1 D0



95

position

Name EA PS PT1 PX1 PT0 PX0

1 0 0 1 0 1 0 0

Explanation

Enable Interrupt

s Made 1

to enable

all interrupt

s

Undefined

Undefined

Set Serial

interrupt as high

priority

Timer 1 interrup

t priority

Set Extern

al 1 interrup

t as high

priority

Timer 0 interrup

t priority

External 0

interrupt

priority

CorrespondingInterruptprioritywordis94H.7. Writethecontrolwordformaskingexternalinterruptsinan8051basedsystem.InterruptEnableRegisterformatBit

positionD7 D6 D5 D4 D3 D2 D1 D0

Name EA ES ET1 EX1 ET0 EX0

1 0 0 1 1 0 1 0

Explanatio

nGlobal

interruptenable/disable

Undef

inedUndefine

d

Enableserialinterru

pt

EnableTimer1interru

pt

Disableexterna

l1interru

pt

EnableTimer0interru

pt

Disableexterna

l0interru

pt

8. Writethecontrolwordformatforsettingtheserialportinmode1.



96

BitpatternsforSCONregister

Bit NameValue Explanationoffunction

D7 SM0 0

D6 SM1 1Serialportmodeselectbits

D5 SM2 0 Multiprocessorcommunicationsenablebit

D4 REN 1 ReceiverenableThisbitmustbeset,toreceivecharacters.

D3 TB8 0 Transmitbit8The9thbittotransmitinmodes2and3

D2 RB8 0 Receivebit8The9thbitreceivedinmodes2and3

D1 TI 0 TransmitInterruptflagSetwhenabytehasbeencompletelytransmitted

D0 RI 0 ReceiveInterruptflagSetwhenabytehasbeencompletelyreceived

CorrespondingSCONvalueis01010000Bi.e.50H.9. CalculatethereloadvalueofTimer1forachievingabaudrateof4800inthe8051,fora

crystalfrequencyof11.0592MHz.TherelationbetweenthebaudrateandtheTH1timerreloadvalueisgivenbelow.

TH1=256((Clockfrequency/384)/Baud)ifSMODinPCONSFRis0.

TH1=256((Clockfrequency/192)/Baud)ifSMODinPCONSFRis1Accordingly,for4800baudrate,thereloadvalueisF9HifSMODbitis0andF3HifSMODbitis1.



97

10. Writean8051ALPtotransmitHelloWorldseriallyat9600baudforacrystalfrequencyof11.0592MHz.Following program assumes that the ASCII code for the

characters Hello World are stored consecutively in the

internal memory 40H onwards.

MAIN: ;SetupTimer1todrivebaudrateof9600.

MOV TMOD, #00100000B;SetTimer1inmode2(8bittimer).

MOV TH1, #0FDH ;TimeTimer1for9600baud.

SETB TR1 ;EnableTimer1forfreerun.

MOV SCON, #01000000B;Initializeserialportformode1operation.

MOV R0, #40H ;Initializememorypointer.

MOV R1, #0BH ;IntializeacounterforthenumberofcharactersinthewordsHelloWorld.

SEND: MOV SBUF,@R0 ;GetthedatafrommemoryandsendittoSBUFfortransmission.

LOOP: JNB TI, LOOP ;TestTIflagtocheckwhetherdatahasbeensent.

CLR TI ;ClearTI.

INC R0 ;Pointtothenextdata.

DJNZ R1, SEND ;Loopagaintosenddata,ifnotcompleted.

END



98



Documents

354 39 Solutions Instructor Manual 11 Hardware Features 8051 Chapter 11