Under normal program operation, a Watchdog Timer time-out will initialise a device reset and set the
status bit TO. However, if the system is in the SLEEP or IDLE Mode, when a Watchdog Timer
time-out occurs, the TO bit in the status register will be set and only the Program Counter and Stack
Pointer will be reset. Three methods can be adopted to clear the contents of the Watchdog Timer. The
first is an external hardware reset, which means a low level on the RES pin, the second is using the
Watchdog Timer software clear instructions and the third is via a HALT instruction.
There Watchdog Timer is cleared using two instructions, CLR WDT1 and CLR WDT2. These
instructions must be executed alternately to successfully clear the Watchdog Timer. Note that if CLR
WDT1 is used to clear the Watchdog Timer, successive executions of this instruction will have no
effect, only the execution of a CLR WDT2 instruction will clear the Watchdog Timer. Similarly after
the CLR WDT2 instruction has been executed, only a successive CLR WDT1 instruction can clear the
Watchdog Timer. For these devices the single CLR WDT instruction will have no effect so care must
be taken not to use this instruction.
The maximum time out period is when the 2
15
division ratio is selected. As an example, with the LIRC
oscillator as its source clock, this will give a maximum watchdog period of around 1 second for the 2
15
division ratio, and a minimum timeout of 7.8ms for the 2
8
division ration.
Rev. 1.50
46
April 28, 2020
BS83B08-3/B12-3/B16-3/B16G-3/C24-3
8-Bit Touch Key Flash MCU
8 - s t a g e D i v i d e r
W D T T i m e - o u t
( 2
8
/ f
L I R C
~ 2
1 5
/ f
L I R C
)
f
L I R C
/ 2
8
W D T P r e s c a l e r
8 - t o - 1 M U X
C l e a r W D T 1 / W D T 2
I n s t r u c t i o n
C L R
W S 2 ~ W S 0
L I R C O s c i l l a t o r
Watchdog Timer