Watchdog Timer
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Preliminary Specifications REV.E
Specifications in this manual are tentative and subject to change.
Mitsubishi microcomputers
M30220 Group
SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
53
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 X
IN
is selected for the
BCLK
,
bit 7 of the watchdog timer control register (address 000F
16
) selects the prescaler division ratio (by
16 or by 128). When X
CIN
is selected as the BCLK, the prescaler is set for division by 2 regardless of bit 7
of the watchdog timer control register (address 000F
16
). 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 000E
16
)
RESET
Watchdog timer
interrupt request
Watchdog timer
Set to
“7FFF
16
”
1/128
1/16
“CM07 = 0”
“WDC7 = 1”
“CM07 = 0”
“WDC7 = 0”
“CM07 = 1”
1/2
Prescaler
For example, suppose that BCLK runs at 10 MHz and that 16 has been chosen for the dividing ratio of the
prescaler, then the watchdog timer's period becomes approximately 52.4 ms.
The watchdog timer is initialized by writing to the watchdog timer start register (address 000E
16
) 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 000E
16
).
Figure 1.11.1 shows the block diagram of the watchdog timer. Figure 1.11.2 shows the watchdog timer-
related registers.
With X
IN
chosen for BCLK
Watchdog timer period =
prescaler dividing ratio (16 or 128) X watchdog timer count (32768)
BCLK
Figure 1.15.1. Block diagram of watchdog timer
With X
CIN
chosen for BCLK
Watchdog timer period =
prescaler dividing ratio (2) X watchdog timer count (32768)
BCLK