![Silicon Laboratories EFR32xG14 Wireless Gecko Reference Manual Download Page 298](http://html1.mh-extra.com/html/silicon-laboratories/efr32xg14-wireless-gecko/efr32xg14-wireless-gecko_reference-manual_1271728298.webp)
11.3.2.5 LFXO Configuration
The Low Frequency Crystal Oscillator (LFXO) is default configured to ensure safe startup for all crystals. In order to optimize startup
time and power consumption for a given crystal, it is possible to adjust the startup gain in the oscillator by programming the GAIN field
in CMU_LFXOCTRL. Recommendations for the GAIN setting are as follows:
1. C0 must be < 2 pF
2. For 12.5 pF < CL < 18 pF, GAIN = 3
3. For 8 pF < CL < 12.5 pF, GAIN = 2
4. For 6 pF < CL < 8 pF, GAIN = 1
5. For CL = 6 pF, GAIN = 0
Refer to the device data sheet and application notes for guidelines in selecting correct components and crystals as well as for configu-
ration trade-offs.
The LFXO can be retained on in EM4 Hibernate/Shutoff. In that case its required configuration is latched/retained throughout EM4 even
though the CMU_LFXOCTRL register itself will be reset. Upon EM4 exit, the CMU_LFXOCTRL register therefore needs to be reconfig-
ured to its original settings and the LFXO needs to be restarted via CMU_OSCENCMD, before optionally unlatching the retained LFXO
configuration by writing 1 to EM4UNLATCH in the EMU_CMD register. The LFXO startup time is configured via the TIMEOUT bitfield of
the CMU_LFXOCTRL register. If the LFXO has been retained throughout EM4 Hibernate/Shutoff, it can be quickly restarted for use as
LFACLK, LFBCLK or LFECLK by using its minimum TIMEOUT setting. While retained, the LFXO can be used down to EM4 Hibernate
as source for LFECLK and down to EM4 Shutoff as source for RFSENSECLK and CRYOCLK.
The LFXO crystal is connected to the LFXTAL_N/LFXTAL_P pins as shown in
Figure 11.11 LFXO Pin Connection on page 298
LFXTAL_N
LFXTAL_P
32.768kHz
C
L1
C
TUNING
C
TUNING
Gecko Device
C
L2
Figure 11.11. LFXO Pin Connection
By configuring the MODE field in CMU_LFXOCTRL, the LFXO can be bypassed, and an external clock source can be connected to the
LFXTAL_N pin of the LFXO oscillator. If MODE is set to BUFEXTCLK, an external active sine source can be used as clock source. If
MODE is set to DIGEXTCLK, an external active CMOS source can be used as clock source.
The LFXO includes on-chip tunable capacitance, which can replace external load capacitors. The TUNING bitfield of the
CMU_LFXOCTRL register is used to tune the internal load capacitance connected between LFXTAL_P and ground and LFXTAL_N and
ground symmetrically. The capacitance range and step size information is available in the device data sheets. Use the formula below to
calculate the TUNING bitfield:
TUNING = ((desiredTotalLoadCap * 2 - Min(C
LFXO_T
)) / C
LFXO_TS
)
Figure 11.12. CMU LFXO Tuning Capacitance Equation
These tunable capacitors can also be used to compensate for temperature drift of the XTAL in software. Crystals normally have a tem-
perature dependency which is given by a parabolic function. The crystal has highest frequency at its turnover temperature, normally
25C. The frequency is reduced following a parabola for higher and lower temperatures. The LFXO offers a mechanism to internally add
capacitance on the LFXTAL_N and LFXTAL_P pins (in parallel to an optional external load capacitance). The variation in frequency as
a function of temperature can therefore be compensated by adjusting the load capacitance. When the temperature compensation
scheme is used, the maximum internal capacitance should be used to obtain good frequency matching at the turnover temperature. For
higher and lower temperatures software then has the maximum range available to adjust the tuning. The external load capacitance
must then of course be reduced accordingly. Note that the ADC0 (
26. ADC - Analog to Digital Converter
) includes an embedded tem-
perature sensor and that the EMU (
10. EMU - Energy Management Unit
) offers a temperature management interface, both of which can
be used in combination with this LFXO temperature compensation scheme.
Reference Manual
CMU - Clock Management Unit
silabs.com
| Building a more connected world.
Rev. 1.1 | 298