
116
8111C–MCU Wireless–09/09
AT86RF231
9.6
Crystal Oscillator (XOSC)
The main crystal oscillator features are:
16 MHz amplitude controlled crystal oscillator
330 s typical settling time after leaving SLEEP state
Configurable trimming capacitance array
Configurable clock output (CLKM)
9.6.1
Overview
The crystal oscillator generates the reference frequency for the AT86RF231. All other internally
generated frequencies of the radio transceiver are derived from this unique frequency. There-
fore, the overall system performance is mainly determined by the accuracy of crystal reference
frequency. The external components of the crystal oscillator should be selected carefully and the
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
9.6.2
Integrated Oscillator Setup
Using the internal oscillator, the oscillation frequency depends on the load capacitance between
the crystal pins XTAL1 and XTAL2. The total load capacitance C
L 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.
the pin input capacitance, summarized to C
PAR.
Figure 9-7.
Simplified XOSC Schematic with External Components
Additional internal trimming capacitors C
TRIM are 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).
CX
16MHz
XTAL2
XTAL1
EVDD
CTRIM
CPAR
AT86RF231
PCB
XTAL_TRIM[3:0]
EVDD
VDD
XTAL_TRIM[3:0]