CC2520_11 Datasheet, PDF (87/133 Pages) Texas Instruments – 2.4 GHZ IEEE 802.15.4/ZIGBEE RF TRANSCEIVER

CC2520 DATASHEET
2.4 GHZ IEEE 802.15.4/ZIGBEE® RF TRANSCEIVER
SWRS068 – DECEMBER 2007
22 Crystal Oscillator
The internal crystal oscillator generates the main frequency reference. The reference frequency must be
32 MHz. Because the crystal frequency is used as reference for the data rate as well as other internal signal
processing functions, other frequencies cannot be used.
The crystal must be connected between the XOSC32M_Q1 and XOSC32M_Q2 pins. The oscillator is designed
for parallel mode operation of the crystal. In addition, loading capacitors (C121 and C131) for the crystal are
required. The loading capacitor values depend on the total load capacitance, C_L, specified for the crystal.
The total load capacitance seen between the crystal terminals should equal C_L for the crystal to oscillate at
the specified frequency. CC2520 has the ability to add more capacitance in order to tune the oscillator
frequency. The amount of extra capacitance is configurable with the FREQTUNE register.
C _L = C_tune + C _parasitic +
1
1
+
1
C₁₂₁ C₁₃₁
The parasitic capacitance is constituted by pin input capacitance and PCB stray capacitance. The total
parasitic capacitance is typically 2 pF - 5 pF.
Note that the default value for the FREQTUNE register means “no added capacitance”, which means that
only reduction of the frequency is possible. By reducing the external capacitors (C121 and C131), the
default frequency is increased. This way, the actual frequency tuning range can be moved so that both
positive and negative tuning around the target frequency is possible.
The crystal oscillator is amplitude regulated. This means that a high current is used to start up the
oscillations. When the amplitude builds up, the current is reduced to what is necessary to maintain a stable
oscillation. This ensures a fast start-up and keeps the drive level to a minimum. The ESR of the crystal must
be within the specification in order to ensure a reliable start-up.
See section 6 for crystal specific parameters (including tuning).
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