
87
42073A-MCU Wireless-02/13
ATmega2564/1284/644RFR2
oscillator should be selected carefully and the related board layout should be done with
The register XOSC_CTRL provides access to the control signals of the oscillator. Two
operating modes are supported. It is recommended to use the integrated oscillator
fed to the internal circuitry by using an external clock reference as shown in
Figure 9-279.6.5.2 Integrated Oscillator Setup
The output frequency of the internal oscillator 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.
The following figure shows all parasitic capacitances, such as PCB stray capacitances
and the pin input capacitance summarized to CPAR.
Figure 9-26. Simplified XOSC Schematic with External Components
CX
16MHz
XTAL2
XTAL1
EVDD
CTRIM
CPAR
IC internal
PCB
XTAL_TRIM[3:0]
EVDD
VEVDD
XTAL_TRIM[3:0]
Additional internal trimming capacitors CTRIM 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
XOSC_CTRL. To calculate the total load capacitance, the following formula can be
used
CL = 0.5 (CX + CTRIM + CPAR).
The trimming capacitors provide the possibility to reduce frequency deviations caused
by variations of the production process or by tolerances of external components. Note
that the oscillation frequency can only be reduced by increasing the trimming
capacitance. The frequency deviation caused by one step of CTRIM decreases with
increasing values of the crystal load capacitor.
An amplitude control circuit is included to ensure stable operation under different
operating conditions and for different crystal types. Enabling the crystal oscillator after
leaving SLEEP state causes a slightly higher current during the amplitude build-up
phase to guarantee a short start-up time. The current is reduced to the amount