RFM22B/23B
5.8. Crystal Oscillator
The RFM22B/23B includes an integrated 30 MHz crystal oscillator with a fast start-up time of less than 600 s.
A parallel resonant 30MHz crystal is used on the module. The design is differential with the required
capacitance integrated on-chip to minimize the number of external components.
crystal load
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 provides 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 (16.3 pF) is not sufficient, an external capacitor can be added for exact tuning.
If AFC is disabled then the synthesizer frequency may be further adjusted by programming the Frequency Offset
field fo[9:0]in "Register 73h. Frequency Offset 1" and "Register 74h. Frequency Offset 2", as discussed in "3.5.
Frequency Control" .
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 "8.2. Microcontroller Clock" .
D7
D6
D5
D4
D3
D2
D1
D0
POR Def.
Add R/W Function/Description
09 R/W Crystal Oscillator Load
Capacitance
xtalshift
xlc[6]
xlc[5]
xlc[4]
xlc[3]
xlc[2]
xlc[1]
xlc[0]
7Fh
5.9. Regulators
There are a total of six regulators integrated onto the RFM22B/23B . With theexception of the digital regulator, all
regulators are designed to operate with only internal decoupling.
All regulators are designed to operate with an
A supply voltage should only be connected to the VDD pins.
input supply voltage from +1.8 to +3.6V.
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