Datasheet

RegisterDescriptions
Power-OnReset
PCA9554A
SCPS127D–SEPTEMBER2006–REVISEDAUGUST2008........................................................................................................................................... www.ti.com
TheInputPortregister(register0)reflectstheincominglogiclevelsofthepins,regardlessofwhetherthepinis
definedasaninputoranoutputbytheConfigurationregister.Itonlyactsonreadoperation.Writestothese
registershavenoeffect.Thedefaultvalue,X,isdeterminedbytheexternallyappliedlogiclevel.
Beforeareadoperation,awritetransmissionissentwiththecommandbytetolettheI
2
Cdeviceknowthatthe
InputPortregisterwillbeaccessednext.
Register0(InputPortRegister)
BITI7I6I5I4I3I2I1I0
DEFAULTXXXXXXXX
TheOutputPortregister(register1)showstheoutgoinglogiclevelsofthepinsdefinedasoutputsbythe
Configurationregister.Bitvaluesinthisregisterhavenoeffectonpinsdefinedasinputs.Inturn,readsfromthis
registerreflectthevaluethatisintheflip-flopcontrollingtheoutputselection,nottheactualpinvalue.
Register1(OutputPortRegister)
BITO7O6O5O4O3O2O1O0
DEFAULT11111111
ThePolarityInversionregister(register2)allowspolarityinversionofpinsdefinedasinputsbytheConfiguration
register.Ifabitinthisregisterisset(writtenwith1),thecorrespondingportpinpolarityisinverted.Ifabitinthis
registeriscleared(writtenwitha0),thecorrespondingportpinoriginalpolarityisretained.
Register2(PolarityInversionRegister)
BITN7N6N5N4N3N2N1N0
DEFAULT00000000
TheConfigurationregister(register3)configuresthedirectionsoftheI/Opins.Ifabitinthisregisterissetto1,
thecorrespondingportpinisenabledasaninputwithhighimpedanceoutputdriver.Ifabitinthisregisteris
clearedto0,thecorrespondingportpinisenabledasanoutput.
Register3(ConfigurationRegister)
BITC7C6C5C4C3C2C1C0
DEFAULT11111111
Whenpower(from0V)isappliedtoV
CC
,aninternalpower-onresetholdsthePCA9554Ainaresetcondition
untilV
CC
hasreachedV
POR
.Atthatpoint,theresetconditionisreleasedandthePCA9554Aregistersand
I
2
C/SMBusstatemachinewillinitializetotheirdefaultstates.Afterthat,V
CC
mustbeloweredtobelow0.2Vand
thenbackuptotheoperatingvoltageforapower-resetcycle.
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Not Recommended for New Designs