C8051F970-A-GM Datasheet, PDF (256/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
25.2. SmaRTClock Clocking Sources
The SmaRTClock peripheral is clocked from its own timebase, independent of the system clock. The SmaRTClock
timebase can be derived from the internal LFO or the SmaRTClock oscillator circuit, which has two modes of
operation: Crystal Mode, and Self-Oscillate Mode. The oscillation frequency is 32.768 kHz in Crystal Mode and can
be programmed in the range of 10 kHz to 40 kHz in Self-Oscillate Mode. The internal LFO frequency is
16.4 kHz ±20%. The frequency of the SmaRTClock oscillator can be measured with respect to another oscillator
using an on-chip timer. See Section “32. Timers (Timer0, Timer1, Timer2, and Timer3)” on page 390 for more
information on how this can be accomplished.
Note: The SmaRTClock timebase can be selected as the system clock and routed to a port pin. See Section “24. Clocking
Sources” on page 242 for information on selecting the system clock source and Section “26. Port I/O (Port 0, Port 1, Port
2, Port 3, Port 4, Port 5, Port 6, Crossbar, and Port Match)” on page 278 for information on how to route the system clock
to a port pin.
25.2.1. Using the SmaRTClock Oscillator with a Crystal
When using Crystal Mode, a 32.768 kHz crystal should be connected between XTAL3 and XTAL4. No other
external components are required. The following steps show how to start the SmaRTClock crystal oscillator in
software:
1. Configure the XTAL3 and XTAL4 pins for Analog I/O.
2. Set SmaRTClock to Crystal Mode (XMODE = 1).
3. Disable Automatic Gain Control (AGCINT) and enable Bias Doubling (BIASX2) for fast crystal startup.
4. Set the desired loading capacitance (RTC0XCF).
5. Enable power to the SmaRTClock oscillator circuit (RTC0INT = 1).
6. Wait 20 ms.
7. Poll the SmaRTClock Clock Valid Bit (CLKVLD) until the crystal oscillator stabilizes.
8. Poll the SmaRTClock Load Capacitance Ready Bit (LOADRDY) until the load capacitance reaches its
programmed value.
9. Enable Automatic Gain Control (AGCINT) and disable Bias Doubling (BIASX2) for maximum power
savings.
10. Enable the SmaRTClock missing clock detector.
11. Wait 2 ms.
12. Clear the PMU0CF wake-up source flags.
Rev 1.0
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