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CHAPTER 10 WATCHDOG TIMER
10.1 Overview of the Watchdog Timer
The watchdog timer is a 2-bit counter that uses the output of the timebase timer
counter as the count clock. After the watchdog timer is activated, the CPU is reset
within a specified interval unless the watchdog timer is cleared.
s Watchdog Timer Function
The watchdog timer is provided to handle program runaways.
The watchdog timer, once
activated, must continue to be cleared within every specified interval. If the program results in
an endless loop and the watchdog timer is not cleared within the minimum time shown in
Table10.1-1 "Intervals for the Watchdog Timer" a watchdog reset is issued to the CPU, sending it into
the reset status. Specify the watchdog timer interval in the interval setting bits (WT1, WT0) of
the watchdog timer control register (WDTC).
For more information on a watchdog timer interval, see Section
10.4 "Operation of the
Watchdog Timer".
Note:
The watchdog counter consists of a 2-bit counter that uses the carry signals of the timebase
timer as count clocks.
Therefore, if the timebase timer is cleared, the watchdog reset
generation time may become longer than the time set.
Reference:
The watchdog timer, after being activated, can be stopped using a power-on reset or a reset
from the watchdog timer. The watchdog timer can be cleared with an external reset, an
internal reset, writing to the watchdog control bit (WTE) of the watchdog timer control register
(WDTC), or transition to sleep or stop mode. However, the watchdog function is enabled
and not stopped.
Table 10.1-1 Intervals for the Watchdog Timer
WT1
WT0
Interval
Minimum (*1)
Maximum (*1)
Oscillation clock cycle count
0
Approx. 3.58 ms
Approx. 4.61 ms
214±211 cycle
0
1
Approx. 14.33 ms
Approx. 18.3 ms
216±213 cycle
1
0
Approx. 57.23 ms
Approx. 73.73 ms
218±215 cycle
1
Approx. 458.75 ms
Approx. 589.82 ms
221±218 cycle
*1 Value during operation of the 4 MHz oscillation clock