The correct way to do sensor calibration is as following:
1. power on the BNO055 sensor
2. set to the NDoF mode
3. keep board in still for few mini-seconds
4. move the boards along to positive and negative position of each axis
5. then trying to keep reading the CALIB_STAT register, then you will get 0xFF value in the end
It will take a few seconds to get all calibration done. but normally, you just make sure system calibration, gyro calibration and magnetic calibration goes to 3, then your output is good enough. acc calibration will go to 3 during your usage.
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