h8s-2646 Renesas Electronics Corporation., h8s-2646 Datasheet - Page 457

• Manufacturer Part Number: h8s-2646
• Description: Renesas 16-bit Single-chip Microcomputer H8s Family/h8s/2600 Series
• Manufacturer: Renesas Electronics Corporation.

12.3
12.3.1
To use the WDT as a watchdog timer, set the WT/IT bit in TCSR and the TME bit to 1. Software
must prevent TCNT overflows by rewriting the TCNT value (normally by writing H'00) before
overflow occurs. This ensures that TCNT does not overflow while the system is operating
normally. If TCNT overflows without being rewritten because of a system malfunction or other
error, an internal reset is issued, in the case of WDT0, if the RSTE bit in RSTCSR is set to 1. This
is illustrated in figure 12.4 (a).
The internal reset signal is output for 518 states.
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.
In the case of WDT1, the chip is reset, or an NMI interrupt request is generated, for 516 system
clock periods (516φ) (515 or 516 clock periods when the clock source is φSUB (PSS = 1)). This is
illustrated in figure 12.4 (b).
An NMI request from the watchdog timer and an interrupt request from the NMI pin are both
treated as having the same vector. So, avoid handling an NMI request from the watchdog timer
and an interrupt request from the NMI pin at the same time.
Operation
Watchdog Timer Operation
Legend:
WT/IT
TME
Note: * The internal reset signal is generated only if the RSTE bit is set to 1.
Internal reset signal *
: Timer mode select bit
: Timer enable bit
Figure 12.4 (a) WDT0 Watchdog Timer Operation
H'FF
H'00
TCNT value
WT/IT=1
TME=1
Write H'00'
to TCNT
internal reset is
generated
Overflow
WOVF=1
518 states
Rev. 5.00 Sep 22, 2005 page 431 of 1136
WT/IT=1
TME=1
Section 12 Watchdog Timer
Write H'00'
to TCNT
REJ09B0257-0500
Time