14.4.7.2 Integrated Oscillator Setup

Using the internal oscillator, the oscillation frequency depends on the load capacitance between the crystal pin 26 (XTAL1) and pin 25 (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 figure below shows all parasitic capacitances, such as PCB stray capacitances and the pin input capacitance, summarized to C_PAR.

Additional internal trimming capacitors C_TRIM are available. Any value in the range from 0pF to 4.5pF with a 0.3pF resolution is selectable using the XTAL_TRIM bits in the XOSC_CTRL register (XOSC_CTRL.XTAL_TRIM). To calculate the total load capacitance, the following formula can be used

C_L[pF] = 0.5 x (CX[pF] + C_TRIM[pF] + C_PAR[pF]).

The AT86RF212B 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 only be reduced by increasing the trimming capacitance. The frequency deviation caused by one step of C_TRIM decreases with increasing crystal load capacitor values.

An amplitude control circuit is included to ensure stable operation under different operating conditions and for different crystal types. Enabling the crystal oscillator in P_ON state and after leaving SLEEP state causes a slightly higher current during the amplitude build-up phase to guarantee a short 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 parasitic. On the other hand, a larger crystal load capacitance results in a longer start-up time and a higher steady state current consumption.