UPD784938 Datasheet, PDF (392/733 Pages) NEC – 16-Bit Single-Chip Microcontrollers

CHAPTER 13 WATCHDOG TIMER
13.4.2 Cautions on µPD784938 Subseries watchdog timer
(1) The watchdog timer mode register (WDM) can only be written to with a dedicated instruction (MOV WDM, #byte).
(2) The same value should be written each time in writes to the watchdog timer mode register (WDM) to set (to 1) the RUN
bit. The contents written at the first time cannot be changed even if a different value is written.
(3) Once the RUN bit has been set (to 1), it cannot be reset (to 0) by software.
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Preliminary User’s Manual U13987EJ1V0UM00