TODO: 

1. Only certain frequencies are currently supported by the driver. The frequency can be specified by the 3-byte register FRF for the sx127x chip. For details see the datasheet.

Example:
 freq = 868.1E6
 temp = int(freq * 2**19 / f_xosc)
 lsb = temp & 0xFF
 mid = temp >> 8 & 0xFF
 msb = temp >> 16 & 0xFF 

2. Spreading factors 11 and 12 do not work correctly with LoRaWAN. Possible reason: TTN mandates the use of 'Low datarate optimization' at these spreading factors. Maybe we can enable this in the driver?

3. PBC for RFM95 module
https://github.com/attexx/rfm9x_breakout_board
