Hi mgchoi,
When the BMM150 is connected to the BMI270, the auxiliary interface(I2C master) is used. BMI270 acts as an I2C master and BMM150 acts as an I2C slave. The I2C interface requires two pins SDA and SCL, corresponding to the 2 and 3 pins of BMI270.
For more detailed schematic examples, please refer to the BMI270 shuttle board: https://www.bosch-sensortec.com/media/boschsensortec/downloads/shuttle_board_flyer/application_board_3_1/bst-bmi270-sf000.pdf
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