HT49R50 Holtek Semiconductor Inc, HT49R50 Datasheet - Page 17

• Manufacturer Part Number: HT49R50
• Description: 8-Bit Microcontroller
• Manufacturer: Holtek Semiconductor Inc

Watchdog timer - WDT
The WDT clock source is implemented by a ded-
icated RC oscillator (WDT oscillator) or an in-
struction clock (system clock/4) or a real time
clock oscillator (RTC oscillator). 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 mask option. But if the WDT is dis-
abled, all executions related to the WDT lead to
no operation.
After the WDT clock source is selected, the
time-out period is f
If the WDT clock source chooses the internal
WDT oscillator, the time-out period may vary
with temperature, VDD, and process variations.
On the other hand, if the clock source selects the
instruction clock and the ²halt² instruction is exe-
cuted, WDT may stop counting and lose its pro-
tecting purpose, and the logic can only be
restarted by an external logic.
When the device operates in a noisy environ-
ment, using the on-chip RC oscillator (WDT
OSC) is strongly recommended, since the HALT
can stop the system clock.
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 PC and
SP are reset to zero. To clear the contents of the
WDT, there are three methods to be adopted,
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~ f
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.
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i.e., external reset (a low level to RES), software
instruction, and a ²HALT² instruction. There
are 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 mask option - ²CLR
WDT times selection option². If the ²CLR WDT²
is selected (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 ex-
ecuted to clear the WDT; otherwise, the WDT
may reset the chip due to time-out.
Multi-function timer
The HT49C50 provides a multi-function timer for
the WDT, time base and RTC but with different
time-out periods. The multi-function timer con-
sists of a 7-stage divider and an 8-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 frequency signal (ranges
from f
selectable frequency signal (ranges from f
f
recommended to select a near 4kHz signal to
LCD driver circuits for proper display.
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) for the buzzer output by mask option. It is
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to f
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) for LCD driver circuits, and a
August 18, 1999
HT49C50
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to