Notes from AS3935 datasheet:
Register Map: Page 11, detailed version on Page 12

Max clock freq for SPI = 2MHz

SPI command Structure: Page14
To activate this SPI, the CS has to be set to low. A SPI command consists of two bytes serial command and the data are sampled on the falling
edge of SCLK (CPHA=1). The next table shows command structure, from the MSB (B15) to LSB (B0). The command stream has to be sent to
the SPI from the MSB (B15) to the LSB (B0).

