How do you connect to a LoRa device without it being constantly in receive mode?
While LoRa is low-power, a battery-powered device constantly in receive mode will run out of energy in days, making such an approach infeasible.
I will describe one method which uses CAD (channel activity detection) to do this. This handshake/downlink/wakeup protocol, whatever you want to call it is similar to LoRaWAN class B but conceptually simpler and more power efficient on the node side.
There is an interesting relationship between the stability of a crystal oscillator and the stability of its supply voltage and MCU drive capability. Here I've used the ABS07-32.768KHZ-1-T on an STM32L4 board.
For reference, this is the LSI output with an unstable voltage (voltage drifting between 2.7V - 3.0V) sampled by a logic analyzer. Despite the unstable voltage, the LSI is relatively stable with only a 100 Hz difference between the Fmin and Fmax. It has a bit of inacurracy with a 200 Hz difference from its 32 kHz specification.
The Semtech LLCC68 does not support all spreading factors with all bandwidths. Source: see the note under Table 6-1 of the LLCC68 datasheet.
- Not all SF are available for any bandwidth with the LLCC68
This seems to be the only mention of it in the entire datasheet.
I've attached a screenshot of what it looks like to attempt to set an invalid SF/BW in a logic analyzer.