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46
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Rev. 1.20
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BS82B12A-3/BS82C16A-3/BS82D20A-3
Touch Key 8-Bit Flash MCU with LED/LCD Driver
BS82B12A-3/BS82C16A-3/BS82D20A-3
Touch Key 8-Bit Flash MCU with LED/LCD Driver
External 32.768kHz Crystal Oscillator – LXT
For the BS82C16A-3 and BS82D20A-3 devices, 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 manufacturer specification. The external parallel
feedback resistor, Rp, is required. Note that the wire connected between the 32.768kHz crystal and
the XT1/XT2 pins should be kept as short as possible to minimize the stray noise interface.
The configuration option determines if the XT1/XT2 pins are used for the LXT oscillator or as I/O
pins or other pin-shared functions.
• If the LXT oscillator is not used for any clock source, the XT1/XT2 pins can be used as normal I/
O pins or other pin-shared functions.
• 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 minimize the effects of noise and crosstalk, it is important to ensure
that the crystal and any associated resistors and capacitors along with interconnecting lines are all
located as close to the MCU as possible.
External LXT Oscillator
LXT Oscillator C1 and C2 Values
Crystal Frequency
C1
C2
�2.768kHz
10pF
10pF
Note: 1. C1 and C2 val�es a�e fo� g�idance onl�.
2. R
P
=�M
Ω
~10
MΩ
is �ecommended.
32.768kHz Crystal Recommended Capacitor Values
Содержание BS82B12A-3
Страница 33: ...Rev 1 20 33 January 23 2015 BS82B12A 3 BS82C16A 3 BS82D20A 3 Touch Key 8 Bit Flash MCU with LED LCD Driver ...
Страница 34: ...Rev 1 20 34 January 23 2015 BS82B12A 3 BS82C16A 3 BS82D20A 3 Touch Key 8 Bit Flash MCU with LED LCD Driver ...
Страница 35: ...Rev 1 20 35 January 23 2015 BS82B12A 3 BS82C16A 3 BS82D20A 3 Touch Key 8 Bit Flash MCU with LED LCD Driver ...