The BMI160 protocol autodetection logic is as follows: 1) After reset, part is in i2c mode. 2) Upon rising edge of CSB pin, the part will switch to SPI mode until next reset. it is a good idea to included a dummy write to register 0x00 after each reset, to make sure the part is in SPI mode. In case you want to always use SPI, you can write the spi_en bit in Register 0x70 and back it up in the NVM, so that the part always start in SPI mode without dummy transaction. See datasheet section "Primary Interface I2C/SPI Protocol Selection" o_o
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