AT86RF230_09 ATMEL [ATMEL Corporation], AT86RF230_09 Datasheet - Page 67

• Manufacturer Part Number: AT86RF230_09
• Description: Low Power 2.4 GHz Transceiver for ZigBee, IEEE 802.15.4, 6LoWPAN, RF4CE and ISM
• Manufacturer: ATMEL [ATMEL Corporation]

9.6.1 Overview
9.6.2 Integrated Oscillator Setup
5131E-MCU Wireless-02/09
The crystal oscillator generates the reference frequency for the AT86RF230. All other
internally generated frequencies of the radio transceiver are derived from this unique
frequency. Therefore, the overall system performance is mainly based on the accuracy
of this reference frequency. The external components of the crystal oscillator should be
selected carefully and the related board layout should be done meticulously (refer to
section 5).
The register 0x12 (XOSC_CTRL) provides access to the control signals of the
oscillator. Two operating modes are supported. It is recommended to use the integrated
oscillator setup as described in Figure 9-4, nevertheless a reference frequency can be
fed to the internal circuitry by using an external clock reference as shown in Figure 9-5.
Using the internal oscillator, the oscillation frequency strongly depends on the load
capacitance between the crystal pins XTAL1 and XTAL2. The total load capacitance CL
must be equal to the specified load capacitance of the crystal itself. It consists of the
external capacitors CX and parasitic capacitances connected to the XTAL nodes. In
Figure 9-4, all parasitic capacitances, such as PCB stray capacitances and the pin input
capacitance, are summarized to C
available. Any value in the range from 0 pF to 4.5 pF with a 0.3 pF resolution is
selectable using XTAL_TRIM of register 0x12 (XOSC_CTRL). To calculate the total
load capacitance, the following formula can be used
The trimming capacitors provide the possibility of reducing frequency deviations caused
by production process variations or by external components tolerances. Note that the
oscillation frequency can be reduced only by increasing the trimming capacitance. The
frequency deviation caused by one step of C
capacitor values.
A magnitude control circuit is included to ensure stable operation under different
operating conditions and for different crystal types. A high current during the amplitude
build-up phase guarantees a low start-up time. At stable operation, the current is
reduced to the amount necessary for a robust operation. This also keeps the drive level
of the crystal low.
Generally, crystals with a higher load capacitance are less sensitive to parasitic pulling
effects caused by external component variations or by variations of board and circuit
parasitics. On the other hand, a larger crystal load capacitance results in a longer start-
up time and a higher steady state current consumption.
CL = 0.5•(CX+C
TRIM
+C
PAR
).
PAR
. Additional internal trimming capacitors C
TRIM
decreases with increasing crystal load
AT86RF230
TRIM
are
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