BS85B12-3/BS85C20-3/BS85C20-5
Touch Key Flash MCU with LCD/LED Driver
Reset and Initialisation
Aꢀresetꢀfunctionꢀisꢀaꢀfundamentalꢀpartꢀofꢀanyꢀmicrocontrollerꢀensuringꢀthatꢀtheꢀdeviceꢀcanꢀbeꢀsetꢀ
toꢀsomeꢀpredeterminedꢀconditionꢀirrespectiveꢀofꢀoutsideꢀparameters.ꢀTheꢀmostꢀimportantꢀresetꢀ
conditionꢀisꢀafterꢀpowerꢀisꢀfirstꢀappliedꢀtoꢀtheꢀmicrocontroller.ꢀInꢀthisꢀcase,ꢀinternalꢀcircuitryꢀwillꢀ
ensureꢀthatꢀtheꢀmicrocontroller,ꢀafterꢀaꢀshortꢀdelay,ꢀwillꢀbeꢀinꢀaꢀwellꢀdefinedꢀstateꢀandꢀreadyꢀtoꢀ
executeꢀtheꢀfirstꢀprogramꢀinstruction.ꢀAfterꢀthisꢀpower-onꢀreset,ꢀcertainꢀimportantꢀinternalꢀregistersꢀ
willꢀbeꢀsetꢀtoꢀdefinedꢀstatesꢀbeforeꢀtheꢀprogramꢀcommences.ꢀOneꢀofꢀtheseꢀregistersꢀisꢀtheꢀProgramꢀ
Counter,ꢀwhichꢀwillꢀbeꢀresetꢀtoꢀzeroꢀforcingꢀtheꢀmicrocontrollerꢀtoꢀbeginꢀprogramꢀexecutionꢀfromꢀtheꢀ
lowestꢀProgramꢀMemoryꢀaddress.
AnotherꢀtypeꢀofꢀresetꢀisꢀwhenꢀtheꢀWatchdogꢀTimerꢀoverflowsꢀandꢀresetsꢀtheꢀmicrocontroller.ꢀAllꢀ
typesꢀofꢀresetꢀoperationsꢀresultꢀinꢀdifferentꢀregisterꢀconditionsꢀbeingꢀsetup.ꢀAnotherꢀresetꢀexistsꢀinꢀtheꢀ
formꢀofꢀaꢀLowꢀVoltageꢀReset,ꢀLVR,ꢀwhereꢀaꢀfullꢀreset,ꢀisꢀimplementedꢀinꢀsituationsꢀwhereꢀtheꢀpowerꢀ
supplyꢀvoltageꢀfallsꢀbelowꢀaꢀcertainꢀthreshold.
Reset Functions
Thereꢀareꢀseveralꢀwaysꢀinꢀwhichꢀaꢀmicrocontrollerꢀresetꢀcanꢀoccur,ꢀthroughꢀeventsꢀoccurringꢀbothꢀ
internallyꢀandꢀexternally:
Power-on Reset
Theꢀmostꢀfundamentalꢀandꢀunavoidableꢀresetꢀisꢀtheꢀoneꢀthatꢀoccursꢀafterꢀpowerꢀisꢀfirstꢀappliedꢀtoꢀ
theꢀmicrocontroller.ꢀAsꢀwellꢀasꢀensuringꢀthatꢀtheꢀProgramꢀMemoryꢀbeginsꢀexecutionꢀfromꢀtheꢀfirstꢀ
memoryꢀaddress,ꢀaꢀpower-onꢀresetꢀalsoꢀensuresꢀthatꢀcertainꢀotherꢀregistersꢀareꢀpresetꢀtoꢀknownꢀ
conditions.ꢀAllꢀtheꢀI/Oꢀportꢀandꢀportꢀcontrolꢀregistersꢀwillꢀpowerꢀupꢀinꢀaꢀhighꢀconditionꢀensuringꢀthatꢀ
allꢀpinsꢀwillꢀbeꢀfirstꢀsetꢀtoꢀinputs.
V
D
D
0
.
V
D
9
D
R
S
E
t
t
R
R
T
T
S
S
D
D
+
+
t
t
S
S
S
S
T
t
n
r
e
n
a
R
e
l
e
s
t
Note:ꢀtRSTDꢀisꢀpower-onꢀdelay,ꢀtypicalꢀtime=100ms
Power-On Reset Timing Chart
Low Voltage Reset – LVR
Ifꢀtheꢀsupplyꢀvoltageꢀofꢀtheꢀdeviceꢀdropsꢀtoꢀwithinꢀaꢀrangeꢀofꢀ0.9V~VLVRꢀsuchꢀasꢀmightꢀoccurꢀwhenꢀ
changingꢀtheꢀbattery,ꢀtheꢀLVRꢀwillꢀautomaticallyꢀresetꢀtheꢀdeviceꢀinternally.ꢀTheꢀLVRꢀincludesꢀtheꢀ
followingꢀspecifications:ꢀForꢀaꢀvalidꢀLVRꢀsignal,ꢀaꢀlowꢀvoltage,ꢀi.e.,ꢀaꢀvoltageꢀinꢀtheꢀrangeꢀbetweenꢀ
0.9V~VLVRꢀmustꢀexistꢀforꢀgreaterꢀthanꢀtheꢀvalueꢀtLVRꢀspecifiedꢀinꢀtheꢀA.C.ꢀcharacteristics.ꢀIfꢀtheꢀlowꢀvoltageꢀ
stateꢀdoesꢀnotꢀexceedꢀtLVR,ꢀtheꢀLVRꢀwillꢀignoreꢀitꢀandꢀwillꢀnotꢀperformꢀaꢀresetꢀfunction.ꢀOneꢀofꢀaꢀrangeꢀ
ofꢀspecifiedꢀvoltageꢀvaluesꢀforꢀVLVRꢀcanꢀbeꢀselectedꢀusingꢀconfigurationꢀoptions.ꢀTheꢀLVRꢀfunctionꢀisꢀ
permanentlyꢀonꢀinꢀtheseꢀdevices.
L
R
V
t
R
T
S
D
+
t
S
S
T
I
t
n
r
e
n
a
R
e
l
e
s
t
Note:ꢀtRSTDꢀisꢀpower-onꢀdelay,ꢀtypicalꢀtime=100ms
Low Voltage Reset Timing Chart
Rev. 1.20
54
�ꢀꢁꢀst 10ꢂ 2012
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
