SI1010 Datasheet, PDF (195/384 Pages) Silicon Laboratories – Ultra Low Power, 16/8 kB, 12/10-Bit ADC MCU with Integrated 240-960 MHz EZRadioPRO Transceiver

Si1010/1/2/3/4/5
3. Poll for CLKRDY > 1.
19.1. Programmable Precision Internal Oscillator
All Si1010/1/2/3/4/5 devices include a programmable precision internal oscillator that may be selected as
the system clock. OSCICL is factory calibrated to obtain a 24.5 MHz frequency. See Section “4. Electrical
Characteristics” on page 44 for complete oscillator specifications.
The precision oscillator supports a spread spectrum mode which modulates the output frequency in order
to reduce the EMI generated by the system. When enabled (SSE = 1), the oscillator output frequency is
modulated by a stepped triangle wave whose frequency is equal to the oscillator frequency divided by 384
(63.8 kHz using the factory calibration). The deviation from the nominal oscillator frequency is +0%, –1.6%,
and the step size is typically 0.26% of the nominal frequency. When using this mode, the typical average
oscillator frequency is lowered from 24.5 MHz to 24.3 MHz.
19.2. Low Power Internal Oscillator
All Si1010/1/2/3/4/5 devices include a low power internal oscillator that defaults as the system clock after a
system reset. The low power internal oscillator frequency is 20 MHz ± 10% and is automatically enabled
when selected as the system clock and disabled when not in use. See Section “4. Electrical Characteris-
tics” on page 44 for complete oscillator specifications.
19.3. External Oscillator Drive Circuit
All Si1010/1/2/3/4/5 devices include an external oscillator circuit that may drive an external crystal, ceramic
resonator, capacitor, or RC network. A CMOS clock may also provide a clock input. Figure 19.1 shows a
block diagram of the four external oscillator options. The external oscillator is enabled and configured
using the OSCXCN register.
The external oscillator output may be selected as the system clock or used to clock some of the digital
peripherals (e.g., Timers, PCA, etc.). See the data sheet chapters for each digital peripheral for details.
See Section “4. Electrical Characteristics” on page 44 for complete oscillator specifications.
19.3.1. External Crystal Mode
If a crystal or ceramic resonator is used as the external oscillator, the crystal/resonator and a 10 Mresis-
tor must be wired across the XTAL1 and XTAL2 pins as shown in Figure 19.1, Option 1. Appropriate load-
ing capacitors should be added to XTAL1 and XTAL2, and both pins should be configured for analog I/O
with the digital output drivers disabled.
Figure 19.2 shows the external oscillator circuit for a 20 MHz quartz crystal with a manufacturer recom-
mended load capacitance of 12.5 pF. Loading capacitors are "in series" as seen by the crystal and "in par-
allel" with the stray capacitance of the XTAL1 and XTAL2 pins. The total value of the each loading
capacitor and the stray capacitance of each XTAL pin should equal 12.5pF x 2 = 25 pF. With a stray capac-
itance of 10 pF per pin, the 15 pF capacitors yield an equivalent series capacitance of 12.5 pF across the
crystal.
Note: The recommended load capacitance depends upon the crystal and the manufacturer. Please refer to the crystal
data sheet when completing these calculations.
Rev. 1.0
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