Datasheet

Section 12 Watchdog Timer (WDT)
Rev. 3.00 Sep. 28, 2009 Page 243 of 710
REJ09B0384-0300
TCNT value
H'00
Time
H'FF
WT/IT = 1
TME = 1
Write H'00 to
TCNT
WT/IT = 1
TME = 1
Write H'00 to
TCNT
518 system clocks
Internal reset signal
WT/IT:
TME:
OVF:
Overflow
OVF = 1*
Timer mode select bit
Timer enable bit
Overflow flag
Note: * After the OVF bit becomes 1, it is cleared to 0 by an internal reset.
The XRST bit is also cleared to 0.
Figure 12.2 Watchdog Timer Mode (RST/NMI = 1) Operation
12.4.2 Interval Timer Mode
When the WDT is used as an interval timer, an interval timer interrupt (WOVI) is generated each
time the TCNT overflows, as shown in figure 12.3. Therefore, an interrupt can be generated at
intervals. When the TCNT overflows in interval timer mode, an interval timer interrupt (WOVI) is
requested at the same time the OVF bit of TCSR is set to 1. The timing is shown in figure 12.4.
TCNT value
H'00
Time
H'FF
WT/IT = 0
TME = 1
WOVI
Overflow Overflow Overflow Overflow
WOVI : Interval timer interrupt request occurrence
WOVI WOVI
WOVI
Figure 12.3 Interval Timer Mode Operation