TABLEOFCONTENTS:
1.SPI:WhatisSerialPeripheralInterfaceProtocol
2.WhyisSPImorepreferredmoreforPCB
3.Daisy-chainedslaveconfiguration
4.CommunicationinSPI
5.ClockpolarityandphaseinSPI
SPI:WhatisSerialPeripheralInterfaceProtocol
HowDoesCommunicationinSPIWork?
Thecommunicationisinitiatedbythemasterallthetime.The
masterfirstarrangestheclockbyusingafrequency,whichis
lessthanorequaltothemaximumfrequencythattheslave
devicesupports.Now,thisSPImastercontrolsthedata
transferbygeneratingtheclocksignal(SCLK).Themaster
thenselectthedesiredslaveforcommunicationbypullingthe
chipselect(SS)lineofthatparticularslave-peripheralto"low"
stateandactivatestheparticularslaveitwantsto
communicatewithbyusingslave-selectsignal(SS).Onceslaveisselectedthenreceivesortransmitsdatavia
thetwodatalines.Amaster,usuallythehostmicrocontroller,alwaysprovidesclocksignaltoalldevicesona
buswhetheritisselectedornot.Theslavesonthebusthathasnotbeenactivatedbythemasterusingitsslave
selectsignalwilldisregardtheinputclockandMOSIsignalsfromthemaster,andmustnotdriveMISO.That
meansthemasterselectsonlyoneslaveatatime.
Theusageoftheseeachfourpinsmaydependonthedevices.Forexample,SDIpinmaynotbepresentifa
devicedoesnotrequireaninput(ADCforexample),orSDOpinmaynotbepresentifadevicedoesnotrequirean
output(LCDcontrollersforexample).Ifamicrocontrolleronlyneedstotalkto1SPIPeripheraloroneslave,then
theCSpinonthatslavemaybegrounded.Withmultipleslavedevices,anindependentSSsignalisneededfrom
themasterforeachslavedevice.
WhatisthesignificanceofTri-stateoutputinSPI?
Indigitalelectronicsthree-state,tri-state,or3-statelogicallowsanoutputporttoassumeahighimpedancestate
inadditiontothe0and1logiclevels,effectivelyremovingtheoutputfromthecircuit.Thisallowsmultiplecircuits
tosharethesameoutputlineorlines(suchasabuswhichcannotlistentomorethanonedeviceatatime.Most
devices/peripheralshavetri-stateoutputs,whichgoestohighimpedancestate(disconnected)whenthedeviceis
notselected.Deviceswithoutthesetri-stateoutputscannotshareSPIbuswithotherdevices,becausesuch
slave''schip-selectmaynotgetactivated.
Now,ifawaitingperiodisrequired(suchasforanalog-to-digitalconversion)thenthemastermustwaitforatleast
thatperiodoftimebeforestartingtoissueclockcycles.InthemasterSPI,thebitsaresentoutoftheMOSIpin
andreceivedintheMISOpin.ThebitstobeshiftedoutarestoredintheSPIdataregister,SP0DR,andaresent
outmostsignificantbit(bit7)first.Whenbit7ofthemasterisshiftedoutthroughMOSIpin,abitfrombit7ofthe
slaveisbeingshiftedintobit0ofthemasterviatheMISOpin.After8clockpulsesorshifts,thisbitwilleventually
endupinbit7ofthemaster.TheleastsignificantbitcanbesentoutfirstbysettingtheLSBFbitto1intheSPI
ControlRegister.TheclockthatcontrolshowfastthebitsareshiftedoutandintoSP0DRisthesignalSCLK.The
frequencyofthisclockcanbecontrolledbytheSPIbaudrateregister,SP0BR.TheSSpinmustbelowtoselect
aslave.AcompleteSPIsystemisshownbelow.
Afullduplexmeansdatatransmissioncanoccurinbothdirectionsduringeachclockcycle.So,whenthemaster
sendsabitontheMOSIline;theslavereadsitfromthatsamelineandtheslavesendsabitontheMISOline;
themasterreadsitfromthatsameline.Now,tomakedatatransferorganizedshiftregistersareusedwithsome
givenwordsizesuchas8bit(canbemore)inbothmasterandslave.Theyareconnectedinaring.Whilemaster
shiftsregistervalueoutthroughMOSIline,theslaveshiftsdataintoitsshiftregister.
DataareusuallyshiftedoutwiththeMSB-mostsignificantbitfirst,whileshiftinganewLSB-leastsignificantbit
intothesameregister.Afterthatregisterhasbeenshiftedout,themasterandslavehaveexchangedtheirregister
values.Theneachdevicetakesthatvalueanddoesthenecessaryoperationwithit(forexample,writingitto
memory).Iftherearemoredatatobeexchanged,theshiftregistersareloadedwithnewdataandtheprocessis
repeated.Whentherearenomoredatatobetransmitted,themasterstopsitsclock.Normally,itthenrejectsthe
slave.
Thereisa"multiplebytestreammode"availablewithSPIbusinterface.Inthismodethemastercanshiftbytes
continuously.Inthiscase,theslaveselect(SS)iskeptlowuntilallstreamprocessgetsfinished.
SPIdevicessometimesuseanothersignallinetosendaninterruptsignaltoahostCPU.Someoftheexamples
forthesetypeofsignalsarepen-downinterruptsfromtouch-screensensors,thermallimitalertsfromtemperature
sensors,alarmsissuedbyrealtimeclockchips,andheadsetjackinsertionsfromthesoundcodecinacell
phone.
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