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12.2 Control Register
• Watchdog timer mode register (WDM)
The WDM is the 8-bit register that controls watchdog timer operation.
To prevent the watchdog timer from erroneously clearing this register due to an inadvertent program loop, this
register is only written by a special instruction. This special instruction has a special code format (4 bytes) in the
MOV WDM #byte instruction. Writing takes place only when the third and fourth op codes are mutual 1's
complements. If the third and fourth op codes are not mutual 1's complements and not written, the operand error
interrupt is generated. In this case, the return address saved in the stack is the address of the instruction that
caused the error. Therefore, the address that caused the error can be identified from the return address saved
in the stack.
If returning by simply using the RETB instruction from the operand error, an infinite loop results.
Since an operand error interrupt is generated only when the program inadvertently loops (the correct special
instruction is only generated when MOV WDM #byte is described in the RA78K4 NEC assembler), make the
program initialize the system.
Other write instructions (MOV WDM, A; AND WDM, #byte instruction, SET1 WDM, etc.) are ignored and nothing
happens. In other words, WDM is not written, and interrupts, such as operand error interrupts, are not generated.
After a system reset (RESET input), when the watchdog timer starts (when the RUN bit is set to one), the WDM
contents cannot change. Only a reset can stop the watchdog timer. The watchdog timer can be cleared by a special
instruction.
WDM can be read by 8-bit data transfer instructions.
RESET input sets WDM to 00H.
Figure 12-2 shows the WDM format.
226
CHAPTER 12 WATCHDOG TIMER
User's Manual U12697EJ3V0UM
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