R8C/1A Group, R8C/1B Group
13. Watchdog Timer
Rev.1.30
Dec 08, 2006
Page 106 of 315
REJ09B0252-0130
13.1
Count Source Protection Mode Disabled
The count source of the watchdog timer is the CPU clock when count source protection mode is disabled. Table
13.2 lists the Watchdog Timer Specifications (with Count Source Protection Mode Disabled).
NOTES:
1. The watchdog timer is reset when 00h is witten to the WDTR register before FFh. The prescaler is
reset after the MCU is reset. Some errors in the period of the watchdog timer may be caused by the
prescaler.
2. The WDTON bit cannot be changed by a program. To set the WDTON bit, write 0 to bit 0 of address
0FFFFh with a flash programmer.
Table 13.2
Watchdog Timer Specifications (with Count Source Protection Mode Disabled)
Item
Specification
Count source
CPU clock
Count operation
Decrement
Period
Division ratio of prescaler (n) × count value of watchdog timer (32768)
(1)
CPU clock
n: 16 or 128 (selected by WDC7 bit in WDC register)
Example: When the CPU clock frequency is 16 MHz and prescaler
divides by 16, the period is approximately 32.8 ms.
Count start conditions
The WDTON bit
(2)
in the OFS register (0FFFFh) selects the operation of
the watchdog timer after a reset.
• When the WDTON bit is set to 1 (watchdog timer is in stop state after
reset).
The watchdog timer and prescaler stop after a reset and the count
starts when the WDTS register is written to.
• When the WDTON bit is set to 0 (watchdog timer starts automatically
after exiting).
The watchdog timer and prescaler start counting automatically after
reset.
Reset condition of watchdog
timer
• Reset
• Write 00h to the WDTR register before writing FFh.
• Underflow
Count stop condition
Stop and wait modes (inherit the count from the held value after exiting
modes)
Operation at time of underflow • When the PM12 bit in the PM1 register is set to 0.
Watchdog timer interrupt
• When the PM12 bit in the PM1 register is set to 1.
Watchdog timer reset (Refer to
6.5 Watchdog Timer Reset
.)