Clock Sources; Rc Oscillators Clock References; High-Precision Clock References; Khz Crystal - Semtech LR1121 User Manual

6. Clock Sources

The LR1121 uses both low frequency (32kHz) and high frequency (32MHz) clock sources. For each frequency, the clock
signal can be obtained by either an RC oscillator, or a crystal oscillator. RC oscillators allow optimized power consumption
and faster switching times. Crystal oscillators provide a more precise frequency, in cases when frequency accuracy is
needed. RF operations require a 32MHz high precision clock reference, which can be provided by either an external crystal
oscillator or by a TCXO.
The clock requirements for LoRa Edge™ devices are summarized in AN1200.74 LoRa Edge Clock Requirements.

6.1 RC Oscillators Clock References

Two RC oscillators are available (refer to the LR1121 datasheet for the crystal/TCXO choice criteria):
• The 32.768kHz RC oscillator (RTC) can be used by the circuit in Sleep mode to wake-up the device when performing
periodic or duty cycled operations. Several commands make use of this RTC to generate time-based events.
• The 32MHz RC oscillator is enabled for all SPI communication to permit configuration of the device without the need
to start the 32MHz crystal oscillator.

6.2 High-Precision Clock References

This section provides relevant information about the reference clocks. For additional guidance on clock external clock
sources (crystal and TCXO) refer to AN1200.59 Selecting the Optimal Reference Clock.

6.2.1 32.768kHz Crystal

A 32.768kHz crystal oscillator can be used instead of the (default) 32.768kHz RC oscillator as a low frequency clock source
using command ConfigLfClock(...).

6.2.2 32MHz Crystal

A 32MHz crystal oscillator is the cheapest and lowest power consuming approach to provide the 32MHz clock reference to
the LR1121. The crystal loading capacitance are integrated, minimizing the overall BOM cost and optimizing the PCB space.
In case of crystal operation, the VTCXO pin should be left unconnected.
6.2.2.1 Frequency Drift During Packet Transmission and Thermal Insulation
The transmission of RF packets at high RF power by the LR1121 generates significant heat which may transfer to the 32MHz
crystal through the PCB. For long packet durations, this generates a frequency drift which may induce receive errors in the
peer device if no precautions are taken during PCB design. This effect is described in more detail in the SX1261/62
application note AN1200.37 Recommendations for Best Performance.
For example, in LoRa modulation when the Low Data Rate (LDRO) is used, a frequency drift rate of the transmitted packet
of 120Hz/s typically implies a 3dB sensitivity reduction in the LR1121 receiver for all SF and BW. This maximum drift rate is
110Hz/s when the LDRO is not used. Therefore, the frequency drift during packet transmission has to be kept below this
maximum value in order to ensure best packet reception. A design using a TCXO is not subject to frequency drift during the
packet transmission.
Implementing cuts in the PCB's ground plane layers reduces heat transfer between the LR1121 and the 32MHz crystal, as
shown in Figure 6-1: LR1121 Thermal Insulation on PCB Top
LR1121
User Manual Rev 1.1
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