Hi Hui Shao,
According with the Getting Started document (page 32), the core clock for the TWR-K20D50M is 48MHz.
If you want to modify the clock configurations, I suggest you to follow the attached document.
I hope this helps, have a nice day!
Have a great day,
Sol
-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------