Holtek HT66F0175 - Reset and Initialisation; Reset F�Nctions

Rev. 1.50 58 August 28, 2017 Rev. 1.50 59 August 28, 2017
HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
Reset and Initialisation
Aresetfunctionisafundamentalpartofanymicrocontrollerensuringthatthedevicecanbeset
tosomepredeterminedconditionirrespectiveofoutsideparameters.Themostimportantreset
conditionisafterpowerisrstappliedtothemicrocontroller.Inthiscase,internalcircuitrywill
ensurethatthemicrocontroller,afterashortdelay,willbeinawelldefinedstateandreadyto
executetherstprograminstruction.Afterthispower-onreset,certainimportantinternalregisters
willbesettodenedstatesbeforetheprogramcommences.OneoftheseregistersistheProgram
Counter,whichwillberesettozeroforcingthemicrocontrollertobeginprogramexecutionfromthe
lowestProgramMemoryaddress.
Inadditiontothepower-onreset,anotherresetexistsintheformofaLowVoltageReset,LVR,
whereafullresetisimplementedinsituationswherethepowersupplyvoltagefallsbelowa
certainthreshold.AnothertypeofresetiswhentheWatchdogTimeroverflowsandresetsthe
microcontroller.Alltypesofresetoperationsresultindifferentregisterconditionsbeingsetup.
Reset Functions
Therearefivewaysinwhichamicrocontrollerresetcanoccur,througheventsoccurringboth
internallyandexternally.
Power-on Reset
Themostfundamentalandunavoidableresetistheonethatoccursafterpowerisrstappliedto
themicrocontroller.AswellasensuringthattheProgramMemorybeginsexecutionfromtherst
memoryaddress,apower-onresetalsoensuresthatcertainotherregistersarepresettoknown
conditions.AlltheI/Oportandportcontrolregisterswillpowerupinahighconditionensuringthat
allpinswillberstsettoinputs.
V
DD
Power-on Reset
SST Time-out
t
RSTD
Note:t
RSTD
ispower-ondelaywithtypicaltime=50ms
Power-On Reset Timing Chart
Low Voltage Reset – LVR
Themicrocontrollercontainsalowvoltageresetcircuitinordertomonitorthesupplyvoltageof
thedevice.TheLVRfunctionisalwaysenabledwithaspecicLVRvoltage,V
LVR
.Ifthesupply
voltageofthedevicedropstowithinarangeof0.9V~V
LVR
suchasmightoccurwhenchanging
thebattery,theLVRwillautomaticallyresetthedeviceinternallyandtheLVRFbitintheCTRL
registerwillalsobesetto1.ForavalidLVRsignal,alowsupplyvoltage,i.e.,avoltageinthe
rangebetween0.9V~V
LVR
mustexistforatimegreaterthanthatspeciedbyt
LVR
intheLVD/LVR
characteristics.Ifthelowsupplyvoltagestatedoesnotexceedthisvalue,theLVRwillignorethe
lowsupplyvoltageandwillnotperformaresetfunction.TheactualV
LVR
valuecanbeselectedby
theLVSbitsintheLVRCregister.IftheLVS7~LVS0bitshaveanyothervalue,whichmayperhaps
occurduetoadverseenvironmentalconditionssuchasnoise,theLVRwillresetthedeviceafter2~3
f
LIRC
clockcycles.Whenthishappens,theLRFbitintheCTRLregisterwillbesetto1.Afterpower
ontheregisterwillhavethevalueof01010101B.NotethattheLVRfunctionwillbeautomatically
disabledwhenthedeviceentersthepowerdownmode.