LCD Type MCU
Note
The external resistor and capacitor components connected to the 32768Hz crystal are not neces-
sary to provide oscillation. For applications where precise RTC frequencies are essential, these
components may be required to provide frequency compensation due to different crystal manu-
facturing tolerances. In some applications only capacitor C1 is required.
During power-up there is a time delay associated with the RTC oscillator waiting for it to start-up.
To minimize this time delay, bit 4 of the RTCC register, known as the QOSC bit, is provided to have
a quick start-up function. During a power-up condition, this bit will be cleared to
²0²
which will initi-
ate the RTC oscillator quick start-up function. However, as there is additional power consumption
associated with this quick start-up function, to reduce power consumption after start-up takes
place, it is recommended that the application program should set the QOSC bit to
²1²
for about 2
seconds after power-on. It should be noted that, no matter what condition the QOSC bit is set to,
the RTC oscillator will always function normally, only there is more power consumption associated
with the quick start-up function.
Watchdog Timer Oscillator
The Watchdog Timer oscillator is a fully self-contained free running on-chip RC oscillator with a typ-
ical period of 65ms at 5V requiring no external components. This oscillator can be selected via a
configuration option to be one of the sources for f
S
, the internal clock source. When the device en-
ters the Power Down Mode, the system clock will stop running, however, the WDT oscillator contin-
ues to free-run and to keep the f
S
internal clock source active. In this way, all internal functions
which use f
S
as their clock source, such as the Watchdog Timer, the RTC Interrupt, the Time Base,
the LCD clock and Buzzer remain active. It should be noted that as the Watchdog Timer oscillator
remains active when the microcontroller is in the Power Down Mode, there will be a slight increase
in power consumption. If not selected as a clock source for f
S
then its oscillation function will be dis-
abled to conserve power.
Internal Clock Source
An internal clock source, known as f
S
, is generated internally and is used as the clock source for
other internal functions such as the Watchdog Timer, the internal RTC Interrupt, the Time Base in-
terrupt, the LCD clock and the Buzzer function. The clock source that generates f
S
can originate
from three different sources, the RTC oscillator, Watchdog Timer oscillator or the System oscilla-
tor/4, the choice of which is determine by the f
S
clock source configuration option. It is important to
note that if the RTC oscillator is selected as the system clock, f
SYS
, then f
S
will also be sourced from
the RTC oscillator.
R T C O s c illa to r
f
S
Y S
/4
W D T O s c illa to r
C o n fig u r a tio n
O p tio n
S e le c t
f
S
f
S
W a tc h d o g T im e r
B u z z e r
L C D
C lo c k
P A 0 /B Z
P A 1 /B Z
C O M
S E G
T im e B a s e In te r r u p t
R T C In te rru p t
Internal Clock Structure
104
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