M30620FCAFP datasheet(64/274 Pages) MITSUBISHI

64 / 274 page

Mitsubishi microcomputers

M16C / 62A Group

SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER

Watchdog Timer

The watchdog timer has the function of detecting when the program is out of control. 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 calcu-

lated 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).

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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