HD64336900G Datasheet, PDF (213/408 Pages) Renesas Technology Corp – 16-Bit Single-Chip Microcomputer H8 Family/H8/300H Tiny Series

13.3 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 13.2 shows an example of watchdog timer operation.
Example: With 30ms overflow period when φ = 4 MHz
4 × 10⁶ _{× 30 × 10–3 = 14.6}
8192
Therefore, 256 – 15 = 241 (H'F1) is set in TCW.
H'FF
H'F1
TCWD
count value
TCWD overflow
H'00
Internal reset
signal
H'F1 written H'F1 written to TCWD
to TCWD
Reset generated
256 _RC clock cycles
Figure 13.2 Watchdog Timer Operation Example
Rev. 1.00, 11/03, page 185 of 376