Hi @Libra ,
Thank you so much for your interest in our products and for using our community.
Please check RT685 user manual, each Flexcomm I²S interface can only configured for one direction at a time—either transmit (TX) or receive (RX).

If you want realize full-duplex, you need to use two Flexcomm, one configured as TX, and the other as RX.
Best Regards
MayLiu