RFM42B Datasheet, PDF (29/51 Pages) List of Unclassifed Manufacturers – ISM TRANSMITTER

RFM42B/43B
5.3. Crystal Oscillator
The RFM42B/43B includes an integrated 30 MHz crystal oscillator with a fast start-up time of less than 600 µs
A parallel resonant 30MHz crystal is used on the module. The design is differential with the required
crystal load capacitance integrated on-chip to minimize the number of external components.
The crystal load capacitance can be digitally programmed to accommodate crystals with various load capacitance
requirements and to adjust the frequency of the crystal oscillator. The tuning of the crystal load capacitance is
programmed through the xlc[6:0] field of "Register 09h. 30 MHz Crystal Oscillator Load Capacitance." The total
internal capacitance is 12.5 pF and is adjustable in approximately 127 steps (97fF/step). The xtalshift bit is a
coarse shift in frequency but is not binary with xlc[6:0].
The crystal frequency adjustment can be used to compensate for crystal production tolerances. Utilizing the on-
chip temperature sensor and suitable control software, the temperature dependency of the crystal can be
canceled.
The typical value of the total on-chip capacitance Cint can be calculated as follows:
Cint = 1.8 pF + 0.085 pF x xlc[6:0] + 3.7 pF x xtalshift
Note that the coarse shift bit xtalshift is not binary with xlc[6:0]. The total load capacitance Cload seen by the crystal
can be calculated by adding the sum of all external parasitic PCB capacitances Cext to Cint. If the maximum value
of Cint (16.3pF) is not sufficient, an external capacitor can be added for exact tuning.
The crystal oscillator frequency is divided down internally and may be output to the microcontroller through one of
the GPIO pins for use as the System Clock. In this fashion, only one crystal oscillator is required for the entire
system and the BOM cost is reduced. The available clock frequencies and GPIO configuration are discussed
further in "7.2. Microcontroller Clock" .
Add R/W Function/Description
09 R/W Crystal Oscillator Load
Capacitance
D7
xtalshift
D6
xlc[6]
D5
xlc[5]
D4
xlc[4]
D3
xlc[3]
D2
xlc[2]
D1
xlc[1]
D0 POR Def.
xlc[0] 40h
5.4. Regulators
There are a total of four regulators integrated onto the RFM42B/43B.With the exception of the digital regulator, all
regulators are designed to operate with only internal decoupling. _{All regulators are designed to operate with an}
input supply voltage from +1.8 to +3.6V. A supply voltage should only be connected to the VDD pins.
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