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Section 5 Watchdog Timer (WDT)
Rev. 4.00 Sep. 14, 2005 Page 160 of 982
REJ09B0023-0400
5. When the WDT count overflows, the CPG starts supplying the clock and the processor
resumes operation. The WOVF flag in WTCSR is not set when this happens.
6. Since the WDT continues counting from H'00, set the STBY bit in the STBCR register to 0 in
the interrupt processing program and this will stop the WDT. When the STBY bit remains 1,
the LSI again enters the standby mode when the WDT has counted up to H'80. This standby
mode can be canceled by power-on resets.
5.3.2 Changing the Frequency
To change the frequency used by the PLL, use the WDT. When changing the frequency only by
switching the divider, do not use the WDT.
1. Before changing the frequency, always clear the TME bit in WTCSR to 0. When the TME bit
is 1, an erroneous reset or interval timer interrupt may be generated when the count overflows.
2. Set the type of count clock used in the CKS2 to CKS0 bits of WTCSR and the initial values for
the counter in the WTCNT counter. These values should ensure that the time till count
overflow is longer than the clock oscillation settling time.
3. When the frequency control register (FRQCR) is written, the processor stops temporarily. The
WDT starts counting.
4. When the WDT count overflows, the CPG resumes supplying the clock and the processor
resumes operation. The WOVF in WTCSR is not set when this happens.
5. The counter stops at the values H'00.
6. Before changing the WTCNT after the execution of the frequency change instruction, always
confirm that the value of the WTCNT is H'00 by reading the WTCNT.
5.3.3 Using Watchdog Timer Mode
1. Set the WT/IT bit in the WTCSR to 1, set the reset type in the RSTS bit, set the type of count
clock in the CKS2 to CKS0 bits, and set the initial value of the counter in the WTCNT.
2. Set the TME bit in WTCSR to 1 to start the count in watchdog timer mode.
3. While operating in watchdog timer mode, rewrite the counter periodically to H'00 to prevent
the counter from overflowing.
4. When the counter overflows, the WDT sets the WOVF flag in WTCSR to 1 and generates the
type of reset specified by the RSTS bit. The counter then resumes counting.