13. Watchdog Timer - Renesas M16C/62P Series Hardware Manual

M16C/62P Group (M16C/62P, M16C/62PT)

13. Watchdog Timer

The watchdog timer is the function of detecting when the program is out of control. Therefore, we recommend using
the watchdog timer to improve reliability of a system. The watchdog timer contains a 15-bit counter which counts
down the clock derived by dividing the CPU clock using the prescaler. Whether to generate a watchdog timer interrupt
request or apply a watchdog timer reset as an operation to be performed when the watchdog timer underflows after
reaching the terminal count can be selected using the PM12 bit of PM1 register. The PM12 bit can only be set to "1"
(reset). Once this bit is set to "1," it cannot be set to "0" (watchdog timer interrupt) in a program. Refer to 5.4
Watchdog Timer Reset for the details of watchdog timer reset.
When the main clock source is selected for CPU clock, on-chip oscillator clock, PLL clock, the divide-by-N value for
the prescaler can be chosen to be 16 or 128. If a sub-clock is selected for CPU clock, the divide by- N value for the
prescaler is always 2 no matter how the WDC7 bit is set. The period of watchdog timer can be calculated as given
below. The period of watchdog timer is, however, subject to an error due to the prescaler.
With main clock chosen for CPU clock, on-chip oscillator clock, PLL clock
Watchdog timer period
With sub-clock chosen for CPU clock
Watchdog timer period
For example, when CPU clock = 16 MHz and the divide-by-N value for the prescaler= 16, the watchdog timer period
is approx. 32.8 ms.
The watchdog timer is initialized by writing to the WDTS register. The prescaler is initialized after reset. Note that the
watchdog timer and the prescaler both are inactive after reset, so that the watchdog timer is activated to start counting
by writing to the WDTS register.
In stop mode, wait mode and hold state, the watchdog timer and prescaler are stopped. Counting is resumed from the
held value when the modes or state are released.
Figure 13.1 shows the Watchdog Timer Block Diagram. Figure 13.2 shows the WDC and WDTS Register.
CPU
clock
HOLD
Write to WDTS register
Internal RESET signal
("L" active)
CM07: Bit in CM0 register
WDC7: Bit in WDC register
PM12: Bit in PM1 register
PM22: Bit in PM2 register
Figure 13.1 Watchdog Timer Block Diagram
Rev.2.41 Jan 10, 2006
REJ09B0185-0241
http://www.xinpian.net
Prescaler dividing (16 or 128) × Watchdog timer count (32768)
=
Prescaler dividing (2) × Watchdog timer count (32768)
=
Prescaler
CM07 = 0
WDC7 = 0
1/16
CM07 = 0
WDC7 = 1
1/128
CM07 = 1
1/2
On-chip oscillator clock
Page 124 of 390
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CPU clock
CPU clock
PM22 = 0
Watchdog timer
PM22 = 1
13. Watchdog Timer
Watchdog timer
PM12 = 0
interrupt request
PM12 = 1
Reset
Set to
"7FFFh"
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