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Rev. 1.20
70
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Rev. 1.20
71
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HT69F30A/HT69F40A/HT69F50A
TinyPower
TM
I/O Flash 8-Bit MCU with LCD & EEPROM
HT69F30A/HT69F40A/HT69F50A
TinyPower
TM
I/O Flash 8-Bit MCU with LCD & EEPROM
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 a WDT reset, which means a certain value except 01010B and 10101B written into the
WE4~WE0 field, the second is using the Watchdog Timer software clear instruction and the third is
via a HALT instruction.
There is only one method of using software instruction to clear the Watchdog Timer. That is to use
the single “CLR WDT” instruction to clear the WDT contents.
The maximum time out period is when the 2
18
division ratio is selected. As an example, with a
32kHz LIRC oscillator as its source clock, this will give a maximum watchdog period of around 8
second for the 2
18
division ratio, and a minimum timeout of 7.8ms for the 2
8
division ration.
“
CLR WDT
”
Inst�u�tion
�-stage Divide�
WDT P�es�ale�
WE4~WE0 �its
WDTC Registe�
Reset MCU
LXT
f
S
f
SUB
f
S
/2
�
�-to-1 MUX
CLR
WS2~WS0
(f
S
/2
�
~ f
S
/2
1�
)
WDT Time-out
(2
�
/f
S
~ 2
1�
/f
S
)
LIRC
M
U
X
Low Speed �s�illato�
Configu�ation option
Watchdog Timer
Reset and Initialisation
A reset function is a fundamental part of any microcontroller ensuring that the device can be set
to some predetermined condition irrespective of outside parameters. The most important reset
condition is after power is first applied to the microcontroller. In this case, internal circuitry will
ensure that the microcontroller, after a short delay, will be in a well defined state and ready to
execute the first program instruction. After this power-on reset, certain important internal registers
will be set to defined states before the program commences. One of these registers is the Program
Counter, which will be reset to zero forcing the microcontroller to begin program execution from the
lowest Program Memory address.
In addition to the power-on reset, situations may arise where it is necessary to forcefully apply a
reset condition when the is running. One example of this is where after power has been applied
and the is already running, the RES line is forcefully pulled low. In such a case, known as a normal
operation reset, some of the registers remain unchanged allowing the to proceed with normal
operation after the reset line is allowed to return high.
Another type of reset is when the Watchdog Timer overflows and resets the . All types of reset
operations result in different register conditions being setup. Another reset exists in the form of a
Low Voltage Reset, LVR, where a full reset, similar to the RES reset is implemented in situations
where the power supply voltage falls below a certain threshold.