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2014-07-02 - Gecko Family - d0001_Rev1.30
101
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Switching clock source for LFACLK and LFBCLK is done by setting the LFA and LFB fields in
CMU_LFCLKSEL. To ensure no stalls in the Low Energy Peripherals, the clock source should be ready
before switching to it.
Note
To save energy, remember to turn off all oscillators not in use.
11.3.3 Oscillator Configuration
11.3.3.1 HFXO and LFXO
The crystal oscillators are by default configured to ensure safe startup and operation of the most common
crystals. In order to optimize startup margin, startup time and power consumption for a given crystal, it is
possible to adjust the gain in the oscillator. HFXO gain can be increased by setting HFXOBOOST field in
CMU_CTRL, LFXO gain can be increased by setting LFXOBOOST field in CMU_CTRL. It is important
that the boost settings, along with the crystal load capacitors are matched to the crystals in use. Correct
values for these parameters can be found using the energyAware Designer.
The HFXO crystal is connected to the HFXTAL_N/HFXTAL_P pins as shown in Figure 11.4 (p. 101)
Figure 11.4. HFXO Pin Connection
Similarly, the LFXO crystal is connected to the LFXTAL_N/LFXTAL_P pins as shown in Figure 11.5 (p.
101)
Figure 11.5. LFXO Pin Connection
EFM32
LFXTAL_N
LFXTAL_P
32.768kHz
C
L1
C
L2
It is possible to connect an external clock source to HFXTAL_N/LFXTAL_N pin of the HFXO or LFXO
oscillator. By configuring the HFXOMODE/LFXOMODE fields in CMU_CTRL, the HFXO/LFXO can be
bypassed.
11.3.3.2 HFRCO, LFRCO and AUXHFRCO
It is possible to calibrate the HFRCO, LFRCO and AUXHFRCO to achieve higher accuracy (see the
device datasheets for details on accuracy). The frequency is adjusted by changing the TUNING fields
in CMU_HFRCOCTRL/CMU_LFRCOCTRL/CMU_AUXHFRCOCTRL. Changing to a higher value will
result in a higher frequency. Please refer to the datasheet for stepsize details.
The HFRCO can be set to one of several different frequency bands from 1 MHz to 28 MHz by setting the
BAND field in CMU_HFRCOCTRL.The HFRCO frequency bands are calibrated during production test,
and the production tested calibration values can be read from the Device Information (DI) page. The DI
page contains a separate tuning value for each frequency band. During reset HFRCO tuning value is
set to the production calibrated value for the 14 MHz band, which is the default frequency band. When
changing to a different HFRCO band, make sure to also update the tuning value.
The LFRCO and AUXHFRCO are also calibrated in production and their TUNING value is set to the
correct value during reset.
Summary of Contents for EFM32G
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