HT46R064B/065B/066B
crystals, to ensure oscillation and accurate frequency
generation, it is necessary to add two small value exter-
nal capacitors, C1 and C2. The exact values of C1 and
C2 should be selected in consultation with the crystal or
resonator manufacturer¢s specification. The external
parallel feedback resistor, Rp, is required. The devices
the LXT oscillator must be used together with the HIRC
oscillator.
It should be noted that, no matter what condition the
LXTLP bit is set to, the LXT oscillator will always func-
tion normally, the only difference is that it will take more
time to start up if in the Low-power mode.
Internal Low Speed Oscillator - LIRC
The LIRC is a fully self-contained free running on-chip
RC oscillator with a typical frequency of 13kHz at 5V re-
quiring no external components. When the device en-
ters the Idle/Sleep Mode, the system clock will stop
running but the WDT oscillator continues to free-run and
to keep the watchdog active. However, to preserve
power in certain applications the LIRC can be disabled
via a configuration option.
I
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Operating Modes
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By using the LXT low frequency oscillator in combina-
tion with a high frequency oscillator, the system can be
selected to operate in a number of different modes.
These Modes are Normal, Slow, Idle and Sleep.
2
.
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.
External LXT Oscillator - HXT
LXT Oscillator C1 and C2 Values
Mode Types and Selection
Crystal Frequency
C1
C2
10pF
The higher frequency oscillators provide higher perfor-
mance but carry with it the disadvantage of higher
power requirements, while the opposite is of course true
for the lower frequency oscillators. With the capability of
dynamically switching between fast and slow oscillators,
the device has the flexibility to optimise the perfor-
mance/power ratio, a feature especially important in
power sensitive portable applications.
32768Hz
8pF
Note: 1. C1 and C2 values are for guidance only.
2. RP=5M~10MW is recommended.
32768 Hz Crystal Recommended Capacitor Values
LXT Oscillator Low Power Function
The LXT oscillator can function in one of two modes, the
Quick Start Mode and the Low Power Mode. The mode
selection is executed using the LXTLP bit in the CTRL0
register.
For these devices, if the LXT oscillator is used then the
internal RC oscillator, HIRC, must be used as the high
frequency oscillator. If the HXT or the ERC oscillator is
chosen as the high frequency system clock then the
LXT oscillator cannot be used for sharing the same pins.
The CLKMOD bit in the CTRL0 register can be used to
switch the system clock from the high speed HIRC oscil-
lator to the low speed LXT oscillator. When the HALT in-
struction is executed and the device enters the
Idle/Sleep Mode the LXT oscillator will always continue
to run. For these devices the LXT crystal is connected to
the OSC1/OSC2 pins and LXT will always run (the
LXTEN bit is not used).
LXTLP Bit
LXT Mode
Quick Start
Low-power
0
1
After power on the LXTLP bit will be automatically
cleared to zero ensuring that the LXT oscillator is in the
Quick Start operating mode. In the Quick Start Mode the
LXT oscillator will power up and stabilise quickly. How-
ever, after the LXT oscillator has fully powered up it can
be placed into the Low-power mode by setting the
LXTLP bit high. The oscillator will continue to run but
with reduced current consumption, as the higher current
consumption is only required during the LXT oscillator
start-up. In power sensitive applications, such as battery
applications, where power consumption must be kept to
a minimum, it is therefore recommended that the appli-
cation program sets the LXTLP bit high about 2 seconds
after power-on.
Note that CLKMOD is only valid in HIRC+LXT oscillator
configuration.
Rev. 1.00
28
April 8, 2011
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
