7/27/2019 Activity 3 - Timer0

1/3

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7/27/2019 Activity 3 - Timer0

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Figure3showstheinternalblockdiagramofTIMER0moduleofPICmicro.Thesuppliedclockcanbethe

oscillatorprovidedoranotherexternalclock.ThiscanbechosenbyswitchingT0CS. The inputedclock

canthenbemodifiedbytheprogrammableprescalebyswitchingPSA.

Asanexample,activity3 configuresTimer0with thePrescaler foramaximumdelayonTimer0.The

prescalerwilldividetheprocessorclockby256andTimer0willdividethatby256again.Thus,Timer0

Flagwillbesetevery65536s(0.0000001second*256*256),orabout15timesasecond.Themain

programsitsinaloopwaitingfortherolloverandwhenitdoes,itincrementsthedisplayandthenloops

back.

TocalculatefortheTIMER0Periodoverflow, it istorealizethatbyusingtheclockoftheoscillatorthe

timeisdividedby4(Fosc/4).Theequationbelowsummarizesthecalculation.

4 256 0

FoscFout

prescale TMR

and1

ToutFout

#i ncl ude

__conf i g( _HS_OSC & _PWRTE_OFF & _CP_OFF & _WDT_OFF)

cbl ock 0x0CDi spl ay

endc

ORG 0

BSF STATUS, RP0MOVLW b' 00000111' ; conf i gur e Ti mer 0. Sour ced f r omt he

; Pr ocessor cl ockMOVWF OPTI ON_REG ; Maxi mum Pr escal erCLRF TRI SB ; Make PORTB al l out putCLRF Di spl ayBCF STATUS, RP0

For everLoopBTFSS I NTCON, T0I F ; wai t here unt i l Ti mer 0 r ol l s overGOTO For ever LoopBCF I NTCON, T0I F ; f l ag must be cl ear ed i n sof t war eI NCF Di spl ay, f ; i ncrement di spl ay var i abl eMOVF Di spl ay, w ; send t o t he LEDsMOVWF PORTBGOTO For ever Loop

END

Figure4.TIMER0Example

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Aswiththeexampleabove,with4MHzoscillator,aprescaleof256andhavingTMR0registertocount

from0255(256steps,settingittozero).Theoverflowfrequencyis,

415.25879

4 256 256

MHzFout Hz

TheTIMER0overflowperiodis

10.0655359 65.536

15.25879Tout ms

Hz

Notethatthe8bitTMR0registercanbewrittentoprovideavariedTIMER0timeperiodoutput. Ifan

externalclockisconnectedtoT0CKI(also,settingthepropervalueofT0CS),theFoscwillnotbedivided

by4.

PleaserefertoPIC16F84adatasheetandPICmicroMidRangeMCUFamilyReferenceManualformore

information

about

TIMER0.

Problems:Solvethefollowingproblem.Verifyyouranswersusingassemblylanguage.1. ConfiguretheprescaletogenerateaTMR0interruptthatwillincrementthecountervariablefor

eachofthefollowingperiodsexactlyassumingusinganoscillatorof4MHz.

a. 8.192msb. 1.024ms

2. Create/modifytheexampleassemblyprogramabovethatwillincrementtoexactly1second.