M16CSLASH62M datasheet(85/615 Pages) RENESAS

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Mitsubishi microcomputers

M16C / 62A Group

SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER

Watchdog Timer

1-66

Watchdog Timer

The watchdog timer has 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 is a 15-bit counter

which down-counts the clock derived by dividing the BCLK using the prescaler. A watchdog timer interrupt

is generated when an underflow occurs in the watchdog timer. When XIN is selected for the BCLK

, bit 7 of

the watchdog timer control register (address 000F16) selects the prescaler division ratio (by 16 or by 128).

When XCIN is selected as the BCLK, the prescaler is set for division by 2 regardless of bit 7 of the watchdog

timer control register (address 000F16). Thus the watchdog timer's period can be calculated as given

below. The watchdog timer's period is, however, subject to an error due to the prescaler.

BCLK

Write to the watchdog timer

start register

(address 000E16)

RESET

Watchdog timer

interrupt request

Watchdog timer

Set to

“7FFF16”

1/128

1/16

“CM07 = 0”

“WDC7 = 1”

“CM07 = 0”

“WDC7 = 0”

“CM07 = 1”

HOLD

1/2

Prescaler

For example, suppose that BCLK runs at 16 MHz and that 16 has been chosen for the dividing ratio of the

prescaler, then the watchdog timer's period becomes approximately 32.8 ms.

The watchdog timer is initialized by writing to the watchdog timer start register (address 000E16) and when

a watchdog timer interrupt request is generated. The prescaler is initialized only when the microcomputer is

reset. After a reset is cancelled, the watchdog timer and prescaler are both stopped. The count is started by

writing to the watchdog timer start register (address 000E16). 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 1.12.1 shows the block diagram of the watchdog timer. Figure 1.12.2 shows the watchdog timer-

related registers.

With XIN chosen for BCLK

Watchdog timer period =

prescaler dividing ratio (16 or 128) X watchdog timer count (32768)

BCLK

Figure 1.12.1. Block diagram of watchdog timer

With XCIN chosen for BCLK

Watchdog timer period =

prescaler dividing ratio (2) X watchdog timer count (32768)

BCLK

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