I am using the MPC5777C EVB in connection with the MPC5777C-416DS. Using the FEC in RMII mode I want to send some data to the onboard PHY (DP83848C) and receive the same data back on the MPC5777C. I have therefore configured the PHY registers for RMII and loopback.
If the FEC is configured to 10Mbps operation then the setup works as supposed, the MPC5777C receives the exact same data back that it sends. If the FEC is configured to 100Mbps then only about 20% of the received data is correct, the rest of the data is corrupted (several bits change their values).
Using the FEC in MII mode there are no issues with 100Mbps operation.
Should RMII in 100Mbps operation be working with this setup? Do you have an idea on the cause of this issue or any other hint?