Chapter 1 Hardware Structure
Note
An internal capacitor together with the external resistor, R
OSC
, are the components which deter-
mine the frequency of the oscillator. The external capacitor shown on the diagram does not influ-
ence the frequency of oscillation. This external capacitor should be added to improve oscillator
stability if the open-drain OSC2 output is utilized in the application circuit.
RTC Oscillator
When microcontrollers enter a power down or HALT condition, the internal clocks are normally
switched off to stop microcontroller activity and to conserve power. However, in many
microcontroller applications it may be necessary to keep some internal functions, such as timers,
operational, even when the microcontroller is in the HALT state. To provide this feature, all of
Holtek¬Ęs LCD Type MCU series incorporate an RTC oscillator, which will remain active at all times,
even when the microcontroller is in the HALT state. This clock source has a fixed frequency of
32768Hz and requires a 32768Hz crystal to be connected between pins OSC3 and OSC4.
This RTC oscillator can be used in conjunction with a separate RC or crystal system oscillator, con-
nected to pins OSC1 and OSC2. However, the RTC oscillator can also be used as the sole oscilla-
tor for all microcontroller functions, including the system oscillator, in which case pins OSC1 and
OSC2 will remain unused. As the frequency of this oscillator is low, even though it still runs when in
the Power Down Mode, its overall power consumption will be correspondingly low as well. The
RTC oscillator functions are controlled via configuration options and via one bit in the RTCC regis-
ter known as the QOSC bit.
When the RTC oscillator is not used as the system oscillator, a separate RC or crystal system oscil-
lator must also be connected in the usual way to pins OSC1 and OSC2. In this case when the
microcontroller enters a HALT state, the higher frequency system clock will cease operation to re-
duce power consumption, but the RTC clock will keep running. In this case the main function of the
RTC oscillator will be to keep the other internal timing functions such as the Watchdog Timer, inter-
nal RTC timer and Time Base functions active. However, if the RTC oscillator is also selected via
configuration options to be used as the system oscillator, no other oscillator is required and pins
OSC1 and OSC2 are left unconnected. In this case, when the microcontroller enters the Power
Down Mode, although the RTC oscillator will keep running to maintain the function of the internal
timers, no instructions will be executed to conserve power. It is important to note that in this case
the system clock will be only 32768Hz which is much lower than the normally higher system clock
speeds if a separate system oscillator is used.
If configuration options select the RTC oscillator as the system clock, then the internal clock
source, known as f
S
, and consequently the Watchdog Timer, the internal RTC interrupt and Time
Base functions must also use the RTC Oscillator as their clock source.
3 2 7 6 8 H z
C 1
C 2
R 1
O S C 3
O S C 4
RTC Oscillator
103
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