
Rev. 2.50
54
��ne 22� 20��
Rev. 2.50
55
��ne 22� 20��
HT66F20/HT66F30/HT66F40/HT66F50/HT66F60
HT66FU30/HT66FU40/HT66FU50/HT66FU60
A/D Flash MCU with EEPROM
HT66F20/HT66F30/HT66F40/HT66F50/HT66F60
HT66FU30/HT66FU40/HT66FU50/HT66FU60
A/D Flash MCU with EEPROM
External 32.768kHz Crystal Oscillator – LXT
The External 32.768kHz Crystal System Oscillator is one of the low frequency oscillator choices,
which is selected via configuration option. This clock source has a fixed frequency of 32.768kHz
and requires a 32.768kHz crystal to be connected between pins XT1 and XT2. The external resistor
and capacitor components connected to the 32.768kHz crystal are necessary to provide oscillation.
For applications where precise frequencies are essential, these components may be required to
provide frequency compensation due to different crystal manufacturing tolerances. During power-up
there is a time delay associated with the LXT oscillator waiting for it to start-up.
When the microcontroller enters the SLEEP or IDLE Mode, the system clock is switched off to stop
microcontroller activity and to conserve power. However, in many microcontroller applications
it may be necessary to keep the internal timers operational even when the microcontroller is in
the SLEEP or IDLE Mode. To do this, another clock, independent of the system clock, must be
provided.
However, for some crystals, to ensure oscillation and accurate frequency generation, it is necessary
to add two small value external capacitors, C1 and C2. The exact values of C1 and C2 should be
selected in consultation with the crystal or resonator manufacturers specification. The external
parallel feedback resistor, Rp, is required.
Some configuration options determine if the XT1/XT2 pins are used for the LXT oscillator or as I/O
pins.
• If the LXT oscillator is not used for any clock source, the XT1/XT2 pins can be used as normal
I/O pins.
• If the LXT oscillator is used for any clock source, the 32.768kHz crystal should be connected to
the XT1/XT2 pins.
For oscillator stability and to minimise the effects of noise and crosstalk, it is important to ensure
thatthe crystal and any associated resistors andcapacitors along with interconnectinglines are all
located as close to the MCU as possible.
External LXT Oscillator
LXT Oscillator C1 and C2 Values
Crystal Frequency
C1
C2
32.�68kHz
�0pF
�0pF
Note:
�. C� and C2 val�es are for g�idance only.
2. R
P
=5MΩ~10MΩ is recommended.
32.768kHz Crystal Recommended Capacitor Values