Hi
The K66 reference manual shows Ethernet module clock generation:

So customer's code can refer below code:
SIM->SOPT2 &= ~SIM_SOPT2_RMIISRC_MASK;
SIM->SOPT2 |= SIM_SOPT2_TIMESRC(2);
Wish it helps.
Have a great day,
Ma Hui
-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------