HT46R62/HT46C62
The RTC oscillator circuit can be controlled to oscillate
quickly by setting the ²QOSC² bit (bit 4 of RTCC). It is
recommended to turn on the quick oscillating function
upon power on, and then turn it off after 2 seconds.
two types of software instructions; ²CLR WDT² and the
other set - ²CLR WDT1² and ²CLR WDT2². Of these
two types of instruction, only one type of instruction can
be active at a time depending on the options - ²CLR
WDT² times selection option. If the ²CLR WDT² is se-
lected (i.e., CLR WDT times equal one), any execution
of the ²CLR WDT² instruction clears the WDT. In the
case that ²CLR WDT1² and ²CLR WDT2² are chosen
(i.e., CLR WDT times equal two), these two instructions
have to be executed to clear the WDT; otherwise, the
WDT may reset the chip due to time-out.
The WDT oscillator is a free running on-chip RC oscilla-
tor, and no external components are required. Although
the system enters the power down mode, the system
clock stops, and the WDT oscillator still works with a pe-
riod of approximately 65ms@5V. The WDT oscillator can
be disabled by options to conserve power.
Watchdog Timer - WDT
Multi-function Timer
The WDT clock source is implemented by a dedicated
RC oscillator (WDT oscillator) or an instruction clock
(system clock/4) or a real time clock oscillator (RTC os-
cillator). The timer is designed to prevent a software
malfunction or sequence from jumping to an unknown
location with unpredictable results. The WDT can be
disabled by options. But if the WDT is disabled, all exe-
cutions related to the WDT lead to no operation.
The HT46R62/HT46C62 provides a multi-function timer
for the WDT, time base and RTC but with different
time-out periods. The multi-function timer consists of an
8-stage divider and a 7-bit prescaler, with the clock
source coming from the WDT OSC or RTC OSC or the
instruction clock (i.e., system clock divided by 4). The
multi-function timer also provides a selectable fre-
quency signal (ranges from fS/22 to fS/28) for LCD driver
circuits, and a selectable frequency signal (ranging from
fS/22 to fS/29) for the buzzer output by options. It is rec-
ommended to select a nearly 4kHz signal for the LCD
driver circuits to have proper display.
Once an internal WDT oscillator (RC oscillator with pe-
riod 65ms@5V normally) is selected, it is divided by
212~215 (by option to get the WDT time-out period). The
minimum period of WDT time-out period is about
300ms~600ms. This time-out period may vary with tem-
perature, VDD and process variations. By selection the
WDT option, longer time-out periods can be realized. If
the WDT time-out is selected 215, the maximum time-out
period is divided by 215~216about 2.1s~4.3s. If the WDT
oscillator is disabled, the WDT clock may still come from
the instruction clock and operate in the same manner
except that in the halt state the WDT may stop counting
and lose its protecting purpose. In this situation the logic
can only be restarted by external logic. If the device op-
erates in a noisy environment, using the on-chip RC os-
cillator (WDT OSC) is strongly recommended, since the
HALT will stop the system clock.
Time Base
The time base offers a periodic time-out period to gener-
ate a regular internal interrupt. Its time-out period
ranges from 212/fS to 215fS selected by options. If time
base time-out occurs, the related interrupt request flag
(TBF; bit 5 of INTC1) is set. But if the interrupt is en-
abled, and the stack is not full, a subroutine call to loca-
tion 14H occurs.
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The WDT overflow under normal operation initializes a
²chip reset² and sets the status bit ²TO². In the HALT
mode, the overflow initializes a ²warm reset², and only
the program counter and SP are reset to zero. To clear
the contents of the WDT, there are three methods to be
adopted, i.e., external reset (a low level to RES), soft-
ware instruction, and a ²HALT² instruction. There are
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Watchdog Timer
Rev. 1.60
13
July 14, 2005
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
