R8C11 datasheet(79/227 Pages) RENESAS

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R8C/11 Group

11. Watchdog Timer

Rev.1.20

Jan 27, 2006

page 68 of 204

REJ09B0062-0120

11. Watchdog Timer

The watchdog timer is the function of detecting when the program is out of control. Therefore, we recom-

mend 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 per-

formed when the watchdog timer underflows after reaching the terminal count can be selected using the

PM12 bit in the 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 Section 5.3, “Watchdog Timer

Reset” for details.

The divide-by-N value for the prescaler can be chosen to be 16 or 128 with the WDC7 bit in the WDC

however, subject to an error due to the prescaler.

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.

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. After that, the watchdog timer is initialized by

writing to the WDTR register and the counting continues.

In stop mode and wait mode, the watchdog timer and prescaler are stopped. Counting is resumed from

the held value when the modes or state are released.

Figure 11.1 shows the block diagram of the watchdog timer. Figure 11.2 shows the watchdog timer-

related registers.

Prescaler dividing (16 or 128) X Watchdog timer count (32768)

CPU clock

Watchdog timer period =

Write to WDTR register

Internal

reset

signal

PM12 = 0

Watchdog timer

Set to

“7FFF16”

CPU clock

1/128

1/16

WDC7 = 1

WDC7 = 0

Prescaler

PM12 = 1

Watchdog timer

interrupt request

Watchdog

timer Reset

Figure 11.1 Watchdog Timer Block Diagram

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