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Si4455
3.5. Crystal Oscillator
The Si4455 includes an integrated crystal oscillator with a fast start-up time of less than 250 μs. The design is
differential with the required crystal load capacitance integrated on-chip to minimize the number of external
components. By default, all that is required off-chip is the crystal. The default crystal is 30 MHz, but the circuit is
designed to handle any XTAL from 25 to 32 MHz, set in the EZConfig setup. The crystal load capacitance can be
digitally programmed to accommodate crystals with various load capacitance and to adjust the frequency of the
crystal oscillator. The tuning of the crystal load capacitance is programmed through the GLOBAL_XO_TUNE API
property. The total internal capacitance is 11 pF and is adjustable in 127 steps (70 fF/step). The crystal frequency
adjustment can be used to compensate for crystal production tolerances. The frequency offset characteristics of
the capacitor bank are demonstrated in Figure 4.
Figure 4. Capacitor Bank Frequency Offset Characteristics
An external signal source can easily be used in lieu of a conventional XTAL and should be connected to the XIN
pin. The incoming clock signal is recommended to be ac-coupled to the XIN pin since the dc bias is controlled by
the internal crystal oscillator buffering circuitry. The input swing range should be between 600 mV–1.8 V
peak-to-peak. If external drive is desired, the incoming signal amplitude should not go below 0 V or exceed 1.8 V.
The best dc bias should be approximately 0.7 V. However, if the signal swing exceeds 1.4 Vpp, the dc bias can be
set to 1/2 the peak-to-peak voltage swing. The XO capacitor bank should be set to 0 whenever an external drive is
used on the XIN pin. In addition, the POWER_UP command should be invoked with the TCXO option whenever
external drive is used.
3.6. Battery Voltage and Auxiliary ADC
The Si4455 contains an integrated auxiliary 11-bit ADC used for the internal battery voltage detector or an external
component via GPIO. The Effective Number of Bits (ENOB) is 9 bits. When measuring external components, the
input voltage range is 1 V, and the conversion rate is between 300 Hz to 2.44 kHz. The ADC value is read by first
sending the GET_ADC_READING command and enabling the desired inputs. When the conversion is finished and
all the data is ready, CTS will go high, and the data can be read out. For details on this command and the formulas
needed to interpret the results, refer to the EZRadio API documentation zip file available from www.silabs.com .
Rev 1.1
17
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