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Figure 11.5. LFXO Pin Connection
LFXTAL_N
LFXTAL_P
32.768kHz
C L1
C L2
EFM32
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.
The CMU has built-in HW support to efficiently calibrate the RC oscillators at run-time, see Figure 11.6 (p.
100) The concept is to select a reference and compare the RC frequency with the reference frequency.
When the calibration circuit is started, one down-counter running on HFCLK and one up-counter running
on a selectable reference clock are started simultaneously. When the down-counter has reached 0, both
counters are stopped and software can read out the reference counter value (CALCLK counter) and
compare with the start value of the down-counter. Then it is easy to find the ratio between the reference
and the oscillator subject to the calibration. With this HW support, it is simple to write efficient calibration
algorithms in software.
2011-04-12 - d0001_Rev1.10
99
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