Datasheet

Section 15 Watchdog Timer
Rev. 1.50 Sep. 18, 2007 Page 363 of 584
REJ09B0240-0150
15.3 Operation
The watchdog timer is provided with an 8-bit counter. After the reset state is released, TCWD
starts counting up. When the TCWD count value overflows H'FF, an internal reset signal is
generated. The internal reset signal is output for a period of 256 φ
RC
clock cycles. As TCWD is a
writable counter, it starts counting from the value set in TCWD. An overflow period in the range
of 1 to 256 input clock cycles can therefore be set, according to the TCWD set value. When the
watchdog timer is not used, stop TCWD counting by writing 0 to B2WI and WDON
simultaneously while the TCSRWE bit in TCSRWD is set to 1. (To stop the watchdog timer, two
write accesses to TCSRWD are required.)
Figure 15.2 shows an example of watchdog timer operation.
Example: With 30ms overflow period when φ = 4 MHz
4 × 10
6
× 30 × 10
–3
= 14.6
8192
TCWD overflow
H'FF
H'00
Internal reset
signal
H'F1
TCWD
count value
H'F1 written
to TCWD
H'F1 written to TCWD Reset generated
256
φ
RC
clock cycles
Therefore, 256 – 15 = 241 (H'F1) is set in TCW.
Figure 15.2 Watchdog Timer Operation Example