Rev.1.07
Mar 19, 2009
REJ03B0140-0107
7545 Group
Watchdog Timer
The watchdog timer gives a means for returning to a reset status
when the program fails to run on its normal loop due to a
runaway.
The watchdog timer consists of an 8-bit watchdog timer H and an
8-bit watchdog timer L, being a 16-bit counter.
1. Standard operation of watchdog timer
The watchdog timer is valid by setting “0” to bit 0 of the
Function set ROM data (address FFDA16) of the built-in
QzROM.
When an internal clock is supplied after waiting the oscillation
stabilizing time by timer 1 after system is released from reset, the
watchdog timer starts operation. When the watchdog timer H
underflows, an internal reset occurs. Accordingly, it is
programmed that the watchdog timer control register (address
003916) can be set before an underflow occurs.
When the watchdog timer control register (address 003916) is
read, the values of the high-order 6-bit of the watchdog timer H
and watchdog timer H count source selection bit are read.
2. Initial value of watchdog timer
By a reset or writing to the watchdog timer control register
(address 003916), the watchdog timer H is set to “FF16” and the
watchdog timer L is set to “FF16”.
3. Operation of watchdog timer H count source
selection bit
A watchdog timer H count source can be selected by bit 7 of the
watchdog timer control register (address 003916). When this bit
is “0”, the count source becomes a watchdog timer L underflow
signal. The detection time is 262.144 ms at f(XIN) = 4 MHz.
When this bit is “1”, the count source becomes f(XIN)/16. In this
case, the detection time is 1024
s at f(XIN) = 4 MHz.
This bit is cleared to “0” after reset.
4. STP instruction function selection bit
The function of the STP instruction can be selected by the bit 1 in
FSROM. This bit cannot be used for rewriting by executing the
STP instruction.
When this bit is set to “0”, internal reset occurs by executing
the STP instruction.
When this bit is set to “1”, stop mode is entered by executing
the STP instruction.
<Notes on Watchdog Timer>
1. The watchdog timer is operating during the wait mode.
Write data to the watchdog timer control register to prevent
timer underflow.
2. The watchdog timer stops during the stop mode. However,
the watchdog timer is running during the oscillation stabi-
lizing time after the STP instruction is released. In order to
avoid the underflow of the watchdog timer, the watchdog
timer H count source selection bit (bit 7 of watchdog timer
control register (address 003916)) must be set to “0” just
before executing the STP instruction.
Fig. 33 Structure of watchdog timer control register
b7
b0
Watchdog timer H (read only for high-order 6-bit)
Watchdog timer H count source selection bit
0 : Watchdog timer L underflow
1 : f(XIN)/16
Watchdog timer control register
(WDTCON: address 003916, initial value: 3F16)
Disable (returns “0” when read)