First explain that the chip I used is S32R274,the IDE is S32 Design Studio for Power 2017 R1.
I set the system clock at 120MHz,and try two ways,one is open the PLL1 fraction function,another is closed.At two situation,I use the I/O toggle to test,find the speed of toggling is equal.And then I set the system clock to be 60MHz,the speed of toggling is a half.
To get the real number of the system clock,I try to read the CMU register,because it has a function for metering the frequence,but just for the IRC.
So!!! How can I get the real system clock?Read the register or use the oscilloscope.