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III PERIPHERAL BLOCK: WATCHDOG TIMER
S1C33210 FUNCTION PART EPSON B-III-5-3
I/O Memory of Watchdog Timer
Table 5.1 shows the control bits of the watchdog timer.
Table 5.1 Control Bits of Watchdog Timer
NameAddressRegister name Bit Function Setting Init. R/W Remarks
WRWD
–
D7
D6–0
EWD write protection
–
0
–
R/W
–
0 when being read.
0040170
(B)
–
1
Write enabled
0
Write-protect
Watchdog
timer write-
protect register
–
–
–
EWD
–
D7–2
D1
D0
–
Watchdog timer enable
–
–
0
–
–
R/W
–
0 when being read.
0 when being read.
0040171
(B)
1
NMI enabled
0
NMI disabled
Watchdog
timer enable
register
WRWD: EWD write protection (D7) / Watchdog timer write-protect register (0x40170)
Enables writing to the EWD register.
Write "1": Writing enabled
Write "0": Write-protected
Read: Valid
The EWD bit is write-protected to prevent unwanted modifications. Writing to this bit is enabled for only one
writing by setting WRWD to "1". WRWD is reset back to "0" by writing to EWD, so EWD is write-protected again.
If WRWD is reset to "0" when EWD is write-enabled (WRWD = "1"), EWD becomes write-protected again.
At initial reset, WRWD is set to "0" (write-protected).
EWD: NMI enable (D1) / Watchdog timer enable register (0x40171)
Controls the generation of a nonmaskable interrupt (NMI) by the watchdog timer.
Write "1": NMI is enabled
Write "0": NMI is disabled
Read: Valid
The watchdog timer's interrupt signal is masked by writing "0" to EWD, so a nonmaskable interrupt (NMI) to the
CPU is not generated. If EWD is set to "1", an NMI is generated by the 16-bit programmable timer 0 comparison B
signal.
Writing to EWD is valid only when WRWD = "1".
Even when EWD is set to "0", the 16-bit programmable timer 0 does not stop counting. Therefore, if the NMI has
been temporarily disabled, be sure to reset the 16-bit programmable timer 0 before setting the EWD register back to
"1".
At initial reset, EWD is set to "0" (NMI disabled).
Programming Notes
(1) If the watchdog timer's NMI is enabled, the watchdog timer must be reset in the software before the 16-bit
programmable timer 0 outputs the comparison B signal.
(2) Even when EWD is set to "0", the 16-bit programmable timer 0 does not stop counting. Therefore, if the NMI
has been temporarily disabled, be sure to reset the 16-bit programmable timer 0 before setting EWD back to
"1".