Section 9 Timers
Rev. 7.00 Mar 10, 2005 page 309 of 652
REJ09B0042-0700
9.6.3 Timer Operation
The watchdog timer has an 8-bit counter (TCW) that is incremented by clock input. The input
clock is selected by the WDCKS in port mode register 2 (PMR2): on the H8/38024, H8/38024S,
and H8/38024R Group, φ/8192 is selected when WDCKS is cleared to 0, and φw/32 when set to 1.
On the H8/38124 Group, if WDCKS is cleared to 0 the clock selection is specified by the setting
of timer mode register W (TMW), and if WDCKS is set to 1 the φw/32 clock source is selected.
When TCSRWE = 1 in TCSRW, if 0 is written in B2WI and 1 is simultaneously written in
WDON, TCW starts counting up. (Write access to TCSRW is required twice to turn on the
watchdog timer. However, on the H8/38124 Group WDON is set to 1 after a reset is cancelled,
TCW starts to be incremented even without gaining write access to TCSRW.) When the TCW
count value reaches H'FF, the next clock input causes the watchdog timer to overflow, and an
internal reset signal is generated one base clock (φ or φ
SUB
) cycle later. The internal reset signal is
output for 512 clock cycles of the φ
OSC
clock. It is possible to write to TCW, causing TCW to
count up from the written value. The overflow period can be set in the range from 1 to 256 input
clocks, depending on the value written in TCW.
Figure 9.18 shows an example of watchdog timer operations.
H'F8
TCW overflow
Start
H'F8 is written
in TCW
H'F8 is written in TCW
Reset
Internal reset
signal
512 φ
OSC
clock cycles
H'FF
H'00
TCW count
value
Example: φ = 2 MHz and the desired overflow period is 30 ms.
The value set in TCW should therefore be 256 − 8 = 248 (H'F8).
2 • 10
6
• 30 • 10
−3
= 7.3
8192
Figure 9.18 Typical Watchdog Timer Operations (Example)
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