I am using the K64F with KSDK 1.3.
By inserting the line:
#define CLOCK_SETUP 1u
into the system_MK64F12.h file I have configured the chip to run the pre-configured "Maximum achievable clock frequency configuration".
Unfortunately, this breaks the the functions that retrieve clock speeds, for example CLOCK_SYS_GetBusClockFreq(). It always returns zero. I debugged a little, and found that it was returning the value of g_xtal0ClkFreq which is also zero.
I can workaround by hard coding the what I know the bus frequency to be, but it's not a preferred solution. Is this an error in the SDK? The function documentation didn't say anything relevant to this issue.