RFM23
111
+13 dBm
5.8. Crystal Oscillator For RF23 IC
The RF23 includes an integrated 30 MHz crystal oscillator with a fast start-up time of less than 600 μs when a
suitable parallel resonant crystal is used. 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
30 MHz crystal blank.
The crystal load capacitance can be digitally programmed to accommodate crystals with various load capacitance
requirements and to slightly 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
course shift in frequency but is not binary with xlc[6:0].
The crystal load capacitance can be digitally programmed to accommodate crystals with various load capacitance
requirements and to slightly adjust the frequency of the crystal oscillator. This latter function can be used to
compensate for crystal production tolerances. Utilizing the on-chip temperature sensor and suitable control
software even the temperature dependency of the crystal can be canceled.
The crystal load capacitance is programmed using register 09h. The typical value of the total on-chip (internal)
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 course 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. See more on this,
calculating Cext and crystal selection guidelines in "11. Application Notes".
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 "3.6.
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 (i.e., internal division ratios) and the GPIO
configuration are discussed further in "8.2. Microcontroller Clock".
The RF23 may also be driven with an external 30 MHz clock signal through the XIN pin.
POR
Def.
Function/Descripti
on
D7
D6
D5
D4
D3
D2
D1
D0
Add R/W
36
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