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CP2403-GM Datasheet(PDF) 74 Page - Silicon Laboratories |
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CP2403-GM Datasheet(HTML) 74 Page - Silicon Laboratories |
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74 / 110 page  CP2400/1/2/3 74 Rev. 1.0 11.2. SmaRTClock Clocking Sources The SmaRTClock peripheral is clocked from its own timebase, independent of the system clock. The SmaRTClock timebase is derived from 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 sub 20 kHz to above 40 kHz in self-oscillate mode. In crystal mode, XTAL1 and XTAL2 may be overdriven by an external CMOS clock. 11.2.1. Using the SmaRTClock Oscillator with a Crystal or External CMOS Clock 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. Set SmaRTClock to Crystal Mode (XMODE = 1). 2. Optional. Enable/Disable Automatic Gain Control (AGCEN) and Bias Doubling (BIASX2). See Section 11.2.4 for recommendations on using these oscillator features. 3. Set the desired loading capacitance (RTC0XCF). 4. Enable power to the SmaRTClock oscillator circuit (RTC0EN = 1). 5. Wait 2 ms. 6. Poll the SmaRTClock Clock Valid Bit (CLKVLD) until the crystal oscillator stabilizes. 7. Poll the SmaRTClock Load Capacitance Ready Bit (LOADRDY) until the load capacitance reaches its programmed value. 8. Enable the SmaRTClock missing clock detector. 9. Wait 2 ms. 10.Clear the PMU0CF wake-up source flags. In crystal mode, the SmaRTClock oscillator may be driven by an external CMOS clock. The CMOS clock should be applied to both XTAL1 and XTAL2. The input low voltage (VIL) and input high voltage (VIH) for these pins when used with an external CMOS clock are 0.1 and 0.8 V, respectively. The SmaRTClock oscillator should be configured to its lowest bias setting with AGC disabled. The CLKVLD bit is indeterminate when using a CMOS clock, however, the OSCFAIL bit may be checked 2 ms after SmaRTClock oscillator is powered on to ensure that there is a valid clock. 11.2.2. Using the SmaRTClock Oscillator in Self-Oscillate Mode The following steps show how to configure SmaRTClock for use in self-oscillate mode: 1. Set SmaRTClock to Self-Oscillate Mode (XMODE = 0). 2. Set the desired oscillation frequency: For oscillation at about 20 kHz, set BIASX2 = 0. For oscillation at about 40 kHz, set BIASX2 = 1. 3. The oscillator starts oscillating instantaneously. 4. Fine tune the oscillation frequency by adjusting the load capacitance (RTC0XCF). |
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