Lab 2 Instructionsx Instructions

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Real‐TimeProgramming,Lab2Instructions  March2017,ÖrebroUniversity

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Lab 2: Breathing LED, Buttons

Get familiar with and implement PWM, practice with buttons

FarhangNemati,March2017

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Real‐TimeProgramming,Lab2Instructions  March2017,ÖrebroUniversity

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InthislabyouwilllearnandpracticetheconceptofPulseWidthModulation(PWM).Thenyouwilluse

ittoimplementabreathingLED.YouwilllateronneedPWMinLab4aswell.Youwillalsolearnhowtowork

withbuttons.

A. Breathing LED

A.1.

ReusetheblinkingLEDprogramyouimplementedinLab1.Insteadofsleep()useclock_nanosleep()

functiontohavemoreprecisedelays.ChangetheprogramsothattheLEDisturnedonfor20ms(milliseconds)

anditisoffforanother20ms.Lettheprogramdothecycleforenoughnumberoftimes(e.g.,2000times).Now

changetheprogramsothattheLEDisturnedonfor10msandisturnedofffor20ms.Finallychangetheprogram

sothattheLEDisonfor5msandisofffor20ms.Whatisthedifferenceamongthealternativeswithregardto

theshiningstrength(brightness)oftheLED?Why?

PulseWidthModulation

The mainuseofPWMistocontrolthepowersuppliedtoelectricaldevices,especiallytoinertialloads

suchasmotorsetc.Assumeamotorisgoingtobecontrolledbyanoutputthatsuppliespowertothemotor.

Whentheoutputishighthemotorwillrunwithaconstantspeed.Whatwouldyoudosothatthemotorruns

withhalfofthisspeed?Iftheoutputishighforadurationoftime,sayt1andislowfort1timeunitsthemotor

willrunfort1timeunitsanditwillstopforanothert1timeunits.Ifthisprocessiscontinuedthemotorwillbe

runningfor50%oftime,thusitwillhave50%offullspeed.

Forexampleifthepowerisonfor1secondandisofffor1secondthemotorwillruneveryothersecond

anditsaveragespeedwouldbe50%ofitsfullspeed.However,thedurationof1secondistoolongandonecan

clearlyseethatrunningandstoppingofthemotoristoodiscrete.Ifthefrequencyofrunningandstoppingthe

motor(turningonandoffthepoweroutput)isdecreasedenoughonewillnotnoticethatrunningandstopping

isdiscretebutitwillseemmorecontinues.Forexampleifthepoweroutputisonfor10msandisofffor10ms

themotorwillseemtoruncontinuously.Itwillstillrunwith50%ofitsfullspeedbecauseitrunsforonly50%

oftime.Itseemscontinuesbecauseweincreasedthefrequencyofturningonandoff.Whatwedoistohave

pulsesof20msandduringitthemotorisonfor10msandofffor10ms,i.e.,for50%ofthepulsethemotoris

onanditisoffforanother50%ofeachpulse.Whatwouldyoudotorunthemotorwith25%ofitsfullspeed?

APWMhasafrequencywhichindicatesthelengthofthepulse.ForexampleifthefrequencyofPWMis

100HZ(100pulsespersecond)itmeansthelengthofeachpulseis1/100second=10ms.Thedurationoftime

duringwhichtheoutputishigh(forexample25%)iscalleddutypercent.Forthemotorexamplementioned

above,tocontrolthespeedofamotorwecanhaveaPWMthathasapulselengthof20ms(thefrequencyis

50HZ).Ifwewantthemotortorunwith25%offullspeedwehavetosetthedutypercentto25%.Thismeans

duringofeachpulse(20ms)theoutputisonfor25%ofthepulse(for5ms)andisofffor75%ofthepulse(15ms).

A.2.

ImplementPWM.Downloadthecode(pwm.handpwm.c)fromBlackboard.Implementfunctions

pwmCreate()andpwmPulse()inpwm.c.pwmCreate()isusedtocreateaPWMonaGPIOpinwithafrequencyand

pwmPulse()isusedtoissueadutypercentonthepin.WhenyoucreateaPWMonaGPIOwithfrequency,sayf,

thepinwillbesettoanoutputpinandwhenpwmPulse()iscalledforagivendutypercent,sayd,theGPIOpin

Real‐TimeProgramming,Lab2Instructions  March2017,ÖrebroUniversity

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shouldbehighford%ofthelengthofapulsethathasfrequencyfanditshouldbelowfor(100‐d)%ofthe

lengthofthepulse.Thelengthofapulsewithfrequencyoffis1/fsecond.CallpwmDestroy()onanyPWMyou

createafteryouarefinishedwithit



A.3.

UsethePWMforimplementingabreathingLED.ThebrightnessofaLEDhastograduallyincreasefrom

0%(off)to100%(fullyshining)andthenthebrightnesshastograduallydecreasefrom100%backto0%.Repeat

thisinaloop.ChooseagoodenoughfrequencyforPWMsothatit’scontinuousenoughthatitlookslikethe

LEDisbreathing.



B. Using Buttons

Thereare2buttons(atleast)alreadyinstalledonyourbreadboard.ReadSection“UsingaGPIOpinfor

buttonsandswitches”inthedocumentforGeneralInstructions(itisonBlackboard).Usepull‐upbuttonshere.

Rememberthatthevalueofaninputpinthatapull‐upbuttonisconnectedtobydefaultishigh.Whenthe

buttonispushedthevalueoftheinputpinbecomeslow.Onepinofeachbuttonisalreadyconnectedtothe

GND.TheotherpinhastobeconnectedtoaGPIOpinwhichhastobecreatedasapull‐upinputpin.

B.1.

ImplementaprogramwhereeachoftheLEDsarecontrolledbyoneofthebuttons.Whenabuttonis

pusheditscorrespondingLEDhastobeturnedon.Whenthebuttonispushedagain,theLEDshouldturnoff.

B.2.

InthisSectionyouwillreusethePWMyouimplementedinSectionA.Implementaprogramwherethe

brightnessofaLEDsiscontrolledbyabutton.EachLEDhastohave5states;state1wheretheLEDisturned

off,state2wheretheLEDshineswith25%ofitsfullbrightness,state3wheretheLEDshines50%,state4where

theLEDshines75%,andstate5wheretheLEDshinesfully(100%).BydefaulttheLEDhastobeoff.Whenits

correspondingbuttonispushed,theLEDshouldgotostate1,andwhenthebuttonispushedagainitwillgoto

state2,andtostate3whenthebuttonispushedforthethirdtime,tostate4whenthebuttonispushedfor

4thtime,tostate5whenitispushedfor5thtime,andfinallyitshouldgobacktostate1(turnedoff)whenthe

buttonispushedforthe6thtime.It’senoughifyouimplementthisonlyforoneLEDandonecorresponding

button.

Examination

Topassthelabhandinashortreportandyourcode(onlyinBlackboard!).Youalsoneedto

demonstratetheprogramsforthelabassistantinoneofthelabsessions.Thelabassistantmayaskyou

questionsrelatedtolabtask.

