Si4430/31/32-B1
5.8. Crystal Oscillator
The Si4430/31/32 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.
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. Additional information on
calculating Cext and crystal selection guidelines is provided in “AN417: Si4x3x Family Crystal Oscillator.”
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" on page 25.
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" on page 51.
The Si4430/31/32 may also be driven with an external 30 MHz clock signal through the XOUT pin. When driving
with an external reference or using a TCXO, the XTAL load capacitance register should be set to 0.
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 Si4430/31/32. With the exception of the digital regulator, all
regulators are designed to operate with only internal decoupling. The digital regulator requires an external 1 µF
decoupling capacitor. All regulators are designed to operate with an input supply voltage from +1.8 to +3.6 V. The
output stage of the of PA is not connected internally to a regulator and is connected directly to the battery voltage.
A supply voltage should only be connected to the VDD pins. No voltage should be forced on the digital regulator
output.
40
Rev 1.0
IBM [ IBM ]
