Sub-GHz radio (SUBGHZ) RM0453
160/1450 RM0453 Rev 5
5.4 Sub-GHz radio clocks
5.4.1 Internal oscillators
The following sub-GHz radio dedicated internal RC oscillators are available:
• 64 kHz RC oscillator
– optionally used during the sub-GHz radio Sleep mode to wake up the transceiver
when performing periodic or duty cycled operations
– used by the sub-GHz radio RTC for time based events
• 13 MHz RC oscillator, enabled for all sub-GHz radio SPI communication
The frequency of each sub-GHz radio internal oscillators is calibrated using the HSE32
clock at every sub-GHz radio transition from Deep-Sleep or Sleep-to-Standby, and after a
sub-GHz radio reset. The calibration can also be done on demand by the command
Calibrate().
5.4.2 HSE32 reference clock
The high-precision 32 MHz frequency needed for the sub-GHz radio transmission and
reception is taken from HSE32. The HSE32 clock can also be used by the MCU. The use of
an external crystal (XTAL) or a temperature compensated crystal oscillator (TCXO) are
supported. The used clock source is configured in the RCC (see Section 7.2.1: HSE32 clock
with trimming for more details).
When using the HSE32 with a XTAL, the load capacitors are provided by the integrated
capacitor banks that can be trimmed. The trimming is provided by SUBGHZ_HSEINTRIMR
and SUBGHZ_HSEOUTRIMR registers. The load capacitances on OSC_IN and OSC_OUT
can be trimmed separately. Software trimming must be applied after the sub-GHz radio
entered Standby with HSE32 mode.
The TCXO regulator, integrated in the sub-GHz radio, can be used to supply an external
temperature compensated crystal oscillator (TCXO). The regulated V
DDTCXO
supply level is
controlled through set_TcxoMode()command.
Table 29. LoRa mode intermediate frequencies
Setting name Bandwidth [kHz] f
if
[kHz]
LORA_BW_500 500 0
LORA_BW_250 250
250LORA_BW_125 125
LORA_BW_62 62.5
LORA_BW_41 41.67 167
LORA_BW_31 31.25 250
LORA_BW_20 20.83 167
LORA_BW_15 15.63 250
LORA_BW_10 10.42 167
LORA_BW_7 7.81 250
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