HT46R23/HT46C23
Oscillator Configuration
(system clock divided by 4) decided by options. This
timer is designed to prevent a software malfunction or
sequence jumping to an unknown location with unpre-
dictable results. The watchdog timer can be disabled by
an option. If the watchdog timer is disabled, all the exe-
cutions related to the WDT result in no operation.
There are two oscillator circuits in the microcontroller.
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Once an internal WDT oscillator (RC oscillator with pe-
riod 65ms/@5V normally) is selected, it is divided by
212~215 (by options 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 options, longer time-out periods can be realized. If
the WDT time-out is selected 215, the maximum time-out
period is divided by 215~216 about 2.1s~4.3s.
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System Oscillator
Both are designed for system clocks, namely the RC os-
cillator and the Crystal oscillator, which are determined
by options. No matter what oscillator type is selected,
the signal provides the system clock. The HALT mode
stops the system oscillator and ignores an external sig-
nal to conserve power.
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 operates in a noisy environment, using the
on-chip RC oscillator (WDT OSC) is strongly recom-
mended, since the HALT will stop the system clock.
If an RC oscillator is used, an external resistor between
OSC1 and VSS is required and the resistance must
range from 30kW to 750kW. The system clock, divided by
4, is available on OSC2 with pull-high resistor, which can
be used to synchronize external logic. The RC oscillator
provides the most cost effective solution. However, the
frequency of oscillation may vary with VDD, tempera-
tures and the chip itself due to process variations. It is,
therefore, not suitable for timing sensitive operations
where an accurate oscillator frequency is desired.
The WDT overflow under normal operation will initialize
²chip reset² and set the status bit TO. Whereas in the halt
mode, the overflow will initialize a ²warm reset² only the
program counter and SP are reset to zero. To clear the
contents of WDT, three methods are adopted; external re-
set (a low level to RES), software instructions, or a HALT
instruction. The software instructions include CLR WDT
and the other set - CLR WDT1 and CLR WDT2. Of these
two types of instruction, only one can be active depending
on the options - ²CLR WDT times selection option². If the
²CLR WDT² is selected (i.e. CLRWDT times equal one),
any execution of the CLR WDT instruction will clear the
WDT. In case ²CLR WDT1² and ²CLR WDT2² are chosen
(i.e. CLRWDT times equal two), these two instructions
must be executed to clear the WDT; otherwise, the WDT
may reset the chip because of time-out.
If the Crystal oscillator is used, a crystal across OSC1
and OSC2 is needed to provide the feedback and phase
shift required for the oscillator, and no other external
components are required. Instead of a crystal, a resona-
tor can also be connected between OSC1 and OSC2 to
get a frequency reference, but two external capacitors in
OSC1 and OSC2 are required (If the oscillating fre-
quency is less than 1MHz).
The WDT oscillator is a free running on-chip RC oscilla-
tor, and no external components are required. Even if
the system enters the power down mode, the system
clock is stopped, but the WDT oscillator still works with a
period of approximately 65ms@5V. The WDT oscillator
can be disabled by options to conserve power.
If the WDT time-out period is selected fs/212 (by options),
the WDT time-out period ranges from fs/212~fs/213, since
the ²CLR WDT² or ²CLR WDT1² and ²CLR WDT2² in-
structions only clear the last two stages of the WDT.
Watchdog Timer - WDT
The clock source of the WDT is implemented by an dedi-
cated RC oscillator (WDT oscillator) or instruction clock
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Watchdog Timer
Rev. 1.90
11
July 13, 2005
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
