Datasheet

RegisterDescriptions
Power-OnReset
PCA9554
SCPS128B–JULY2006–REVISEDAUGUST2008........................................................................................................................................................ www.ti.com
TheInputPortregister(register0)reflectstheincominglogiclevelsofthepins,regardlessofwhetherthepinis
definedasaninputoranoutputbytheConfigurationregister.Itonlyactsonreadoperation.Writestothese
registershavenoeffect.Thedefaultvalue,X,isdeterminedbytheexternallyappliedlogiclevel.
Beforeareadoperation,awritetransmissionissentwiththecommandbytetoindicatetotheI
2
Cdevicethatthe
InputPortregisterwillbeaccessednext.
Register0(InputPortRegister)Table
BITI7I6I5I4I3I2I1I0
DEFAULTXXXXXXXX
TheOutputPortregister(register1)showstheoutgoinglogiclevelsofthepinsdefinedasoutputsbythe
Configurationregister.Bitvaluesinthisregisterhavenoeffectonpinsdefinedasinputs.Inturn,readsfromthis
registerreflectthevaluethatisintheflip-flopcontrollingtheoutputselection,nottheactualpinvalue.
Register1(OutputPortRegister)Table
BITO7O6O5O4O3O2O1O0
DEFAULT11111111
ThePolarityInversionregister(register2)allowspolarityinversionofpinsdefinedasinputsbytheConfiguration
register.Ifabitinthisregisterisset(writtenwith1),thecorrespondingportpinpolarityisinverted.Ifabitinthis
registeriscleared(writtenwitha0),thecorrespondingportpin'soriginalpolarityisretained.
Register2(PolarityInversionRegister)Table
BITN7N6N5N4N3N2N1N0
DEFAULT00000000
TheConfigurationregister(register3)configuresthedirectionsoftheI/Opins.Ifabitinthisregisterissetto1,
thecorrespondingportpinisenabledasaninputwithhighimpedanceoutputdriver.Ifabitinthisregisteris
clearedto0,thecorrespondingportpinisenabledasanoutput.
Register3(ConfigurationRegister)Table
BITC7C6C5C4C3C2C1C0
DEFAULT11111111
Whenpower(from0V)isappliedtoV
CC
,aninternalpower-onresetholdsthePCA9554inaresetconditionuntil
V
CC
hasreachedV
POR
.Atthatpoint,theresetconditionisreleasedandthePCA9554registersandI
2
C/SMBus
statemachineinitializetotheirdefaultstates.Afterthat,V
CC
mustbeloweredtobelow0.2Vandthenbackupto
theoperatingvoltageforapower-resetcycle.
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