intd.
87C51GE HARDWAREDESCRIPTION
7.5 HighSpeedOutputMode
The High SpeedOutput (1-ISO)mode togglesa CEXn
pin whena matchoccursbetweenthe PCA timer and a
pm-loadedvaluein a module’smmpare registers.For
this mod%the TOOnbit needsto be set in additionto
the ECOMn and MATn bits in the CCAPMn mode
register. Bysettingor clearirr
g the pin in software,the
user can selectwhetherthe CEXnpin willchangefrom
a logicalOto a logical1or viceversa.Theuser alsohas
the optionofflaggingan interrupt whena match event
occurs by settingthe ECCFnbit. SeeFigure 21.
The HSO mode is more accurate than togglingport
pins in software because the toggle occurs before
branching to an interrupt. That N interrupt latency
will not effectthe accuracyof the output. In fact, the
interrupt is optional.Onlyif the user wants to change
the timeforthe neat toggleis it necemary
to updatethe
compareregisters.Otherwise,the nexttogglewilloccur
when the PCA timer rolls over and matches the last
comnarevalue.
Without any CPU intervention,the fastest waveform
the PCAcangeneratewith the HSOmodeis a 30.5Hz
signalat 16MHz.
7.6 WatchdogTimerMode
A WatchdogTimer is a circuit that automaticallyin-
vokesa reset unless the system beingwatched sends
regularhold4f signalsto the Watchdog.Thesecircuits
are used in applicationsthat arc subjectto electrical
noi~ power glitches,electrostaticdischarg~ etc., or
wherehighreliabilityis required.
The Watchdog Timer function is only available on
PCA Module4. If a WatchdogTimer is not needed,
Module4 can still be used in othermodes.
Asa Watchdogtimer, everytimethe countin the PCA
timer matchesthe valuestored in module4’scompare
registers,an internal reset is generated(seeFigure 22).
Thebit that selectsthis modeis WDTEin the CMOD
~— —..
register. Module4 must be set up in either compare
modeas a “SoftwareTimer” or HighSpeedOutput.
TERRUF7
llMCR/%
CEXnPIN
270297-23
Figure21.PCA16-BitComparatorMode:HighSpeedOutput
6-30
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