Hi Rakesh,
In the reference manual is SCG_SOSCCFG[EREFS] - External Reference Select bit where can be selected between External reference clock selected or Internal crystal oscillator of OSC selected.
You can look at hello_clocks_S32k144 example code available in the S32 Design Studio, but there is option EREFS =1.
Also, I recommend you to see chapters:
23.4.1 Clock source
28.3.10 System Oscillator Configuration Register (SCG_SOSCCFG)
Table 27-1. Clock descriptions
Figure 27-1. Clocking diagram
I hope it helps.
Best regards,
Diana
