HT46R74D-1 Holtek Semiconductor, HT46R74D-1 Datasheet - Page 12

• Manufacturer Part Number: HT46R74D-1
• Description: Dual Slope A/D Type MCU
• Manufacturer: Holtek Semiconductor

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Oscillator Configuration
The device provides three oscillator circuits for system
clocks, i.e., RC oscillator, crystal oscillator and a
32768Hz crystal oscillator, determined by configuration
options. The Power-down mode stops the system oscil-
lator, (RC and crystal oscillator only) and ignores exter-
nal signals in order to conserve power. The 32768Hz
crystal oscillator will continue running even when in the
Power-down mode. If the 32768Hz crystal oscillator is
selected as the system oscillator, the system oscillator is
not stopped; but instruction execution is stopped. Since
the 32768Hz oscillator is also designed for timing pur-
poses, the internal timing (RTC, time base, WDT) oper-
ation keeps running even if the system enters the
Power-down mode.
Of the three oscillators, if the RC oscillator is used, an
external resistor between OSC1 and VSS is required,
whose resistance should range from 30k
The system clock, divided by 4, is available on OSC2
with a pull-high resistor added, which can be used to
synchronise external logic. The RC oscillator provides
the most cost effective solution, however, the frequency
of the oscillation may vary with VDD, temperature, and
the device itself due to process variations. It is therefore,
not suitable for timing sensitive operations where accu-
rate oscillator frequency is desired.
If the crystal oscillator is selected, a crystal across
OSC1 and OSC2 is needed to provide the feedback and
phase shift required for the oscillator. No other external
components are required. A resonator may be con-
nected between OSC1 and OSC2 to replace the crystal
and to obtain a frequency reference, but two external
capacitors connected between OSC1, OSC2 and
ground are required.
Another oscillator circuit is supplied for the real time
clock. For this oscillator only a 32.768kHz crystal oscilla-
tor can be used, and should be connected between pins
OSC3 and OSC4.
The RTC oscillator circuit can be forced to start up
quickly by setting the QOSC bit, which is bit 4 in the
RTCC register. It is recommended to turn on the quick
oscillating function when power is first applied, and then
turn it off after 2 seconds.
The WDT oscillator is a free running on-chip RC oscilla-
tor, and no external components are required. Although
Rev. 1.10
to 750k .
System Oscillator
12
when the system enters the power down mode, the sys-
tem clock stops, the WDT oscillator keeps running with a
period of approximately 65 s at 5V. The WDT oscillator
can be disabled by a configuration option to conserve
power.
Watchdog Timer - WDT
The WDT clock is sourced from either its dedicated in-
ternal RC oscillator or from the instruction clock which is
the system clock/4. The WDT is provided to prevent
software malfunctions or a sequence from jumping to an
unknown location with unpredictable results. The watch-
dog timer can be disabled by a configuration option. If
the watchdog timer is disabled, the WDT timer will have
the same operation as if it were enabled except that the
timeout signal will not generate a device reset. So when
the watchdog timer is disabled, the WDT timer counter
can still be read out and can still be cleared. This func-
tion is used to permit the application program to access
the WDT frequency to obtain the temperature coefficient
for analog component adjustment. The WDT oscillator
needs to be disabled/enabled using its registers,
(WDTC :WDTOSC), to minimise power consumption.
There are 2 registers related to the WDT function,
WDTC and WDTD. The WDTC register controls the
WDT oscillator enable/disable function and the WDT
power source. The WDTD register is the WDT counter
readout register.
The WDTPWR bits can be used to choose the WDT
power source, the default source is VOCHP. The main
purpose of the regulator is to be used for the WDT Tem-
perature-coefficient adjustment. In this case, the appli-
cation program should enable the regulator before
switching to the regulator source. The WDTOSC bits
can be used to enable or disable the WDT OSC
(12kHz). If the application does not use the WDT OSC,
then it needs to disable it in order to reduce power
consumption.
If the internal RC oscillator, which has a nominal period
of 65 s, is selected, it is first divided by a value which
ranges from 2
mined by a configuration option, to obtain the actual
WDT time-out period. The minimum period of the WDT
time-out period is about 300ms~600ms. This time-out
period may vary with temperature, VDD and process
12
~2
15
the exact value of which is deter-
HT46R74D-1
January 11, 2007