MPU9255: Unreliable AK8963 Communications
I know it's been quite old but, I come across the same situation when I use MPU9250 spi with long wire, and read Ak8963 by the MPU9250 Aux I2C. Oscilloscope on SPI SCLK and Aux SCL shows that when there is a SPI reading and I2C reading happen at the same time, the I2C would hang and halt, I can see crosstalk spikes on I2C signal whenever there is SPI CLK edges.
Strangely enough I change the AUX SCL probe attenuation ratio from 10x to 1x the spike reduces and all the transaction functions normally for hours. The input resistance of that probe is 10Mohm in 10x and 1Mohm in 1x so I think reduce the I2C load impedance would minimize the crosstalk spikes.
Have you tested the root cause of this, or better solution? I wrote a test sketch to simply manipulate each SPI io high and low, and only SCLK would have this effect on effecting AUX SCL. Pull up AUX SCL with 2k resistor does not work. This happens only when SCLK wire is long so I think there might be something wrong with the SCLK bus capacitance and for some magic it affect AUX SCL.