Datasheet

RegisterDescriptions
Power-OnReset
Reset(RESET)Input
TCA6408A
www.ti.com .............................................................................................................................................................SCPS192C–APRIL2009–REVISEDJULY2009
TheInputPortRegister(register0)reflectstheincominglogiclevelsofthepins,regardlessofwhetherthepinis
definedasaninputoranoutputbytheConfigurationRegister.Theyactonlyonreadoperation.Writestothis
registerhavenoeffect.Thedefaultvalue(X)isdeterminedbytheexternallyappliedlogiclevel.Beforearead
operation,awritetransmissionissentwiththecommandbytetoindicatetotheI
2
CdevicethattheInputPort
Registerwillbeaccessednext.
Register0(InputPortRegister)
BITI-7I-6I-5I-4I-3I-2I-1I-0
DEFAULTXXXXXXXX
TheOutputPortRegister(register1)showstheoutgoinglogiclevelsofthepinsdefinedasoutputsbythe
ConfigurationRegister.Bitvaluesinthisregisterhavenoeffectonpinsdefinedasinputs.Inturn,readsfromthis
registerreflectthevaluethatisintheflip-flopcontrollingtheoutputselection,nottheactualpinvalue.
Register1(OutputPortRegister)
BITO-7O-6O-5O-4O-3O-2O-1O-0
DEFAULT11111111
ThePolarityInversionRegister(register2)allowspolarityinversionofpinsdefinedasinputsbytheConfiguration
Register.Ifabitinthisregisterisset(writtenwith1),thecorrespondingportpin'spolarityisinverted.Ifabitin
thisregisteriscleared(writtenwitha0),thecorrespondingportpin'soriginalpolarityisretained.
Register2(PolarityInversionRegister)
BITN-7N-6N-5N-4N-3N-2N-1N-0
DEFAULT00000000
TheConfigurationRegister(register3)configuresthedirectionoftheI/Opins.Ifabitinthisregisterissetto1,
thecorrespondingportpinisenabledasaninputwithahigh-impedanceoutputdriver.Ifabitinthisregisteris
clearedto0,thecorrespondingportpinisenabledasanoutput.
Register3(ConfigurationRegister)
BITC-7C-6C-5C-4C-3C-2C-1C-0
DEFAULT11111111
Whenpower(from0V)isappliedtoV
CCP
,aninternalpower-onresetholdstheTCA6408Ainaresetcondition
untilV
CCP
hasreachedV
POR
.Atthattime,theresetconditionisreleased,andtheTCA6408Aregistersand
I
2
C/SMBusstatemachineinitializetotheirdefaultstates.Afterthat,V
CCP
mustbeloweredtobelowV
PORF
and
backuptotheoperatingvoltageforapower-resetcycle.
TheRESETinputcanbeassertedtointializethesystemwhilekeepingV
CCP
atitsoperatinglevel.Aresetcanbe
accomplishedbyholdingtheRESETpinlowforaminimumoft
W
.TheTCA6408AregistersandI
2
C/SMBusstate
machinearechangedtotheirdefaultstateonceRESETislow(0).WhenRESETishigh(1),theI/Olevelsatthe
Pportcanbechangedexternallyorthroughthemaster.ThisinputrequiresapullupresistortoV
CCI
,ifnoactive
connectionisused.
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