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Section 5 Watchdog Timer (WDT)
Rev. 4.00 Sep. 14, 2005 Page 161 of 982
REJ09B0023-0400
5.3.4
Using Interval Timer Mode
When operating in interval timer mode, interval timer interrupts are generated at every overflow of
the counter. This enables interrupts to be generated at set periods.
1. Clear the WT/
IT
bit in the WTCSR to 0, 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 interval timer mode.
3. When the counter overflows, the WDT sets the IOVF in WTCSR to 1 and an interval timer
interrupt request is sent to INTC. The counter then resumes counting.
5.4
Precautions to Take when Using the WDT
Pay attention to the following points when using the WDT in either the interval timer or watchdog
timer mode.
1. Timer tolerance
After timer operation has started, the period from the power-on reset point to the first count up
timing of the WTCNT varies depending on the time period that is set by the TME bit of the
WTCSR register. The shortest such time period is thus one cycle of the peripheral clock, p
φ
,
while the longest is the result of frequency division according to the value in CKS2 to CKS0.
The timing of subsequent incrementation is in accord with the selected frequency divisor. This
time difference is referred to as timer variation. This also applies to the timing of the first
incrementation after the WTCNT register has been written to during timer operation.
2. Do not set WTCNT to H'FF
When the value in WTCNT reaches H'FF, the WDT assumes that an overflow has occurred.
Accordingly, when H'FF is placed in WTCNT, an interval timer interrupt or WDT reset will
occur immediately, regardless of the current clock selection by bits CKS2 to CKS0.
Summary of Contents for HD6417641
Page 2: ...Rev 4 00 Sep 14 2005 Page ii of l...
Page 7: ...Rev 4 00 Sep 14 2005 Page vii of l...
Page 11: ...Rev 4 00 Sep 14 2005 Page xi of l USB Universal serial bus WDT Watch dog timer...
Page 12: ...Rev 4 00 Sep 14 2005 Page xii of l...
Page 28: ...Rev 4 00 Sep 14 2005 Page xxviii of l...
Page 204: ...Section 4 Clock Pulse Generator CPG Rev 4 00 Sep 14 2005 Page 154 of 982 REJ09B0023 0400...
Page 212: ...Section 5 Watchdog Timer WDT Rev 4 00 Sep 14 2005 Page 162 of 982 REJ09B0023 0400...
Page 228: ...Section 6 Power Down Modes Rev 4 00 Sep 14 2005 Page 178 of 982 REJ09B0023 0400...
Page 246: ...Section 8 X Y Memory Rev 4 00 Sep 14 2005 Page 196 of 982 REJ09B0023 0400...
Page 318: ...Section 11 User Break Controller UBC Rev 4 00 Sep 14 2005 Page 268 of 982 REJ09B0023 0400...
Page 454: ...Section 12 Bus State Controller BSC Rev 4 00 Sep 14 2005 Page 404 of 982 REJ09B0023 0400...
Page 504: ...Section 14 U Memory Rev 4 00 Sep 14 2005 Page 454 of 982 REJ09B0023 0400...
Page 566: ...Section 17 Compare Match Timer CMT Rev 4 00 Sep 14 2005 Page 516 of 982 REJ09B0023 0400...
Page 868: ...Section 21 A D Converter Rev 4 00 Sep 14 2005 Page 818 of 982 REJ09B0023 0400...
Page 914: ...Section 23 I O Ports Rev 4 00 Sep 14 2005 Page 864 of 982 REJ09B0023 0400...
Page 956: ...Section 24 List of Registers Rev 4 00 Sep 14 2005 Page 906 of 982 REJ09B0023 0400...
Page 1016: ...Section 25 Electrical Characteristics Rev 4 00 Sep 14 2005 Page 966 of 982 REJ09B0023 0400...
Page 1024: ...Appendix Rev 4 00 Sep 14 2005 Page 974 of 982 REJ09B0023 0400...
Page 1032: ...Rev 4 00 Sep 14 2005 Page 982 of 982 REJ09B0023 0400 X X Y data addressing 52 X Y memory 193...
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