Datasheet

Table Of Contents
Section 12 Watchdog Timer (WDT)
REJ09B0140-0900 Rev. 9.00 Page 373 of 846
Sep 16, 2010
H8S/2215 Group
12.3 Operation
12.3.1 Watchdog Timer Mode
To use the WDT as a watchdog timer, set the WT/IT bit in TCSR and the TME bit to 1.
TCNT does not overflow while the system is operating normally. Software must prevent TCNT
overflows by rewriting the TCNT value (normally be writing H'00) before overflows occurs.
When the RSTE bit of the RSTCSR is set to 1, and if the TCNT overflows, an internal reset signal
for this LSI is issued. In this case, select power-on reset or manual reset by setting the RSTS bit of
the RSTCSR to 0.
If a reset caused by a signal input to the RES pin occurs at the same time as a reset caused by a
WDT overflow, the RES pin reset has priority and the WOVF bit in RSTCSR is cleared to 0. The
internal reset signal is output for 518 states.
When the TCNT overflows in watchdog timer mode, the WOVF bit of the RSTCSR is set to 1. If
the RSTE bit of the RSTCSR has been set to 1, an internal reset signal for the entire LSI is
generated at TCNT overflow.
TCNT value
H'00
Time
H'FF
WT/IT = 1
TME = 1
H'00 written
to TCNT
WT/IT = 1
TME = 1
H'00 written
to TCNT
518 states (WDT0)
Internal reset signal*
Legend:
WT/IT: Timer mode select bit
TME: Timer enable bit
Overflow
WOVF = 1
Note: * With WDT0, the internal reset signal is generated only when the RSTE bit is set to 1.
Internal reset
generated
Figure 12.2 Operation in Watchdog Timer Mode