Hi Bront,
Are you using our BMI270 API?
https://github.com/BoschSensortec/BMI270-Sensor-API/
If you are doing in register level programming, there are many things you have to check manually.
You just need to change communication part in common.c/h.
Therefore, I recommend to use our github API.
If not, for initial part, you have to check 4.4 POR and device initialization in our datasheet.
https://www.bosch-sensortec.com/media/boschsensortec/downloads/datasheets/bst-bmi270-ds000.pdf
Also, you need to check last part of 4.5 and 6.4 our SPI spec.
For example, you need to wait over 450us if you want to do write access in suspend mode, low power mode.
Thank you.
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