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Functional Controller Module (FCM)
Advance Information
MC68HC(9)08LJ60 — Rev. 1.0
144
Functional Controller Module (FCM)
MOTOROLA
NOTE:
This section assumes that a 32.000-kHz oscillator or a 38.400-kHz
oscillator is used, according to the software option selected. These
crystals will generate precise interrupts. Crystals of a frequency two
times these frequencies can also be used to generate accurate RTC
clock control. Any other oscillator frequency will not provide accurate
100 Hz and 1 Hz counters.
8.5.3 COP Watchdog Timer Submodule
The computer operating properly (COP) or watchdog timer subsystem is
a software selectable feature which will generate a system reset if not
serviced within the specified watchdog timeout period. The watchdog
timer counter chain is derived from an output of the timebase circuit. This
input signal is divided to give the watchdog timer reset rate selected by
the first write to the COP select bits in the timebase control register.
A watchdog timer reset is performed by writing any data to address
$FFFF. This will reset the counter chain and begin the timeout
countdown again. The watchdog timer counter chain is also cleared
when the MCU is in reset.
The value of the two watchdog timer rate select bits in the timebase
control register (TBCR) determines the watchdog timer timeout rate.
These bits can be written only on the first write to this register after a
reset. If these bits are never written to, the watchdog timer reset rate will
NOTE:
Although these bits default to 0, the user should write to these bits to
prevent subsequent writes from changing the timeout rate. A bit set/clear
for any bit in this register is executed as a read-modify-write of this
register. If used as the first write to this register, further writes to
CRS[1:0] would not be valid, and the default value would still be set.
The CPU clock halts during wait mode, but the oscillator and the
watchdog timer system are still active. The software should exit wait
mode to service the watchdog timer system before the COP timeout
period.
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