BS85B12-3/BS85C20-3/BS85C20-5
Touch Key Flash MCU with LCD/LED Driver
Watchdog Timer Operation
InꢀtheseꢀdevicesꢀtheꢀWatchdogꢀTimerꢀsuppliedꢀbyꢀtheꢀfLIRCꢀoscillatorꢀandꢀisꢀthereforeꢀalwaysꢀon.ꢀTheꢀ
WatchdogꢀTimerꢀoperatesꢀbyꢀprovidingꢀaꢀdeviceꢀresetꢀwhenꢀitsꢀtimerꢀoverflows.ꢀThisꢀmeansꢀthatꢀinꢀ
theꢀapplicationꢀprogramꢀandꢀduringꢀnormalꢀoperationꢀtheꢀuserꢀhasꢀtoꢀstrategicallyꢀclearꢀtheꢀWatchdogꢀ
TimerꢀbeforeꢀitꢀoverflowsꢀtoꢀpreventꢀtheꢀWatchdogꢀTimerꢀfromꢀexecutingꢀaꢀreset.ꢀThisꢀisꢀdoneꢀusingꢀ
theꢀclearꢀwatchdogꢀinstructions.ꢀꢀIfꢀꢀtheꢀꢀprogramꢀꢀmalfunctionsꢀꢀforꢀꢀwhateverꢀreason,ꢀjumpsꢀtoꢀ
anꢀunkownꢀlocation,ꢀorꢀentersꢀanꢀendlessꢀloop,ꢀtheseꢀclearꢀinstructionsꢀwillꢀnotꢀbeꢀexecutedꢀinꢀtheꢀ
correctꢀmanner,ꢀinꢀwhichꢀcaseꢀtheꢀWatchdogꢀTimerꢀwillꢀoverflowꢀandꢀresetꢀtheꢀdevice.
Underꢀnormalꢀprogramꢀoperation,ꢀaꢀWatchdogꢀTimerꢀtime-outꢀwillꢀinitialiseꢀaꢀdeviceꢀresetꢀandꢀsetꢀ
theꢀstatusꢀbitꢀTO.ꢀHowever,ꢀifꢀtheꢀsystemꢀisꢀinꢀtheꢀSLEEPꢀorꢀIDLEꢀMode,ꢀwhenꢀaꢀWatchdogꢀTimerꢀ
time-outꢀoccurs,ꢀtheꢀTOꢀbitꢀinꢀtheꢀstatusꢀregisterꢀwillꢀbeꢀsetꢀandꢀonlyꢀtheꢀProgramꢀCounterꢀandꢀStackꢀ
Pointerꢀwillꢀbeꢀreset.ꢀThreeꢀmethodsꢀcanꢀbeꢀadoptedꢀtoꢀclearꢀtheꢀcontentsꢀofꢀtheꢀWatchdogꢀTimer.ꢀ
Theꢀfirstꢀisꢀanꢀexternalꢀhardwareꢀreset,ꢀtheꢀsecondꢀisꢀusingꢀtheꢀWatchdogꢀTimerꢀsoftwareꢀclearꢀ
instructionsꢀandꢀtheꢀthirdꢀisꢀviaꢀaꢀHALTꢀinstruction.ꢀTheꢀWatchdogꢀTimerꢀisꢀclearedꢀusingꢀaꢀsingleꢀ
CLRꢀWDTꢀinstruction.
Theꢀmaximumꢀtimeꢀoutꢀperiodꢀisꢀwhenꢀtheꢀ215ꢀdivisionꢀratioꢀisꢀselected.ꢀAsꢀanꢀexample,ꢀwithꢀtheꢀ
LIRCꢀoscillatorꢀasꢀitsꢀsourceꢀclock,ꢀthisꢀwillꢀgiveꢀaꢀmaximumꢀwatchdogꢀperiodꢀofꢀaroundꢀ1ꢀsecondꢀ
forꢀtheꢀ215ꢀdivisionꢀratio,ꢀandꢀaꢀminimumꢀtimeoutꢀofꢀ7.8msꢀforꢀtheꢀ28ꢀdivisionꢀration.
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Watchdog Timer
Rev. 1.20
53
�ꢀꢁꢀst 10ꢂ 2012
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
