The ADXL375’s addressable user registers manages access to all data and configuration parameters, through either a SPI or I2C interface, which many embedded processor platforms support with dedicate ports and a few lines of code. In both cases, the ADXL375 operates as a slave device. When the CS pin is tied high to VDD I/O, I2C mode is enabled. The CS pin must be tied high to VDD I/O or be driven by an external controller. If the CS pin is left unconnected, the user may not be able to communicate with the part. In SPI mode, the CS pin is controlled by the bus master. In both SPI and I2C modes of operation, ignore data transmitted from the ADXL375 to the master device during writes to the ADXL375.
For further details refer to "Serial Communications" section of the datasheet.