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8.4 Operation of Watchdog Timer
8.4.1 Controlling operation of watchdog timer
1.
When the watchdog timer is used, its operation is specified by the option byte (000C0H).
• Enable counting operation of the watchdog timer by setting bit 4 (WDTON) of the option byte (000C0H) to 1
(the counter starts operating after a reset release) (for details, see CHAPTER 22).
WDTON
0
Counter operation disabled (counting stopped after reset)
1
Counter operation enabled (counting started after reset)
• Set an overflow time by using bits 3 to 1 (WDCS2 to WDCS0) of the option byte (000C0H) (for details, see
8.4.2 and CHAPTER 22).
• Set a window open period by using bits 6 and 5 (WINDOW1 and WINDOW0) of the option byte (000C0H)
(for details, see 8.4.3 and CHAPTER 22).
2.
After a reset release, the watchdog timer starts counting.
3.
By writing "ACH" to WDTE after the watchdog timer starts counting and before the overflow time set by the
option byte, the watchdog timer is cleared and starts counting again.
4.
After that, write WDTE the second time or later after a reset release during the window open period. If WDTE
is written during a window close period, an internal reset signal is generated.
5.
If the overflow time expires without "ACH" written to WDTE, an internal reset signal is generated.
A internal reset signal is generated in the following cases.
• If a 1-bit manipulation instruction is executed on the watchdog timer enable register (WDTE)
• If data other than "ACH" is written to WDTE
Cautions 1. When data is written to WDTE for the first time after reset release, the watchdog timer is
cleared in any timing regardless of the window open time, as long as the register is written
before the overflow time, and the watchdog timer starts counting again.
2. If the watchdog timer is cleared by writing "ACH" to WDTE, the actual overflow time may be
different from the overflow time set by the option byte by up to 2/f
3. The watchdog timer can be cleared immediately before the count value overflows.
<Example>
292
CHAPTER 8 WATCHDOG TIMER
Watchdog Timer Counter
When the overflow time is set to 2
3FH.
User's Manual U17854EJ9V0UD
seconds.
IL
10
/f
, writing "ACH" is valid up to count value
IL
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