66AK2G02: System Clock

Hi,

SYSOSC_IN/OUT pins are the external main crystal interface pins connected to internal oscillator which sources reference clock for all the PLLs.

SYSCLK_P/SYSCLK_N are optional pins, which source reference clock for the PLLs.

Section 5.9.3.1.1 OSC0 External Crystal & Section 5.9.3.1.3 External Differential Input Clock from 66AK2G02 answer your question: 

When you connect crystal to SYSOSC_IN/OUT => SYSCLK_P/SYSCLK_N are left unused. 
   "An external crystal is connected to the device pins (SYSOSC_IN and SYSOSC_OUT). In that case the SYSCLK_P and SYSCLK_N pins are left unused."

When you use SYSCLK_P/SYSCLK_N as source for reference clock for the PLLS => SYSOSC_IN/OUT are left unused. 

    "The optional LVDS-compatible clock input can be used instead of the internal oscillator to provide the system LVDS clock. The external connections to support this are shown in Figure 5-10. The SYSCLK_P and SYSCLK_N pins are connected to the LVDS-compatible clock source. The SYSOSC_IN and SYSOSC_OUT pins are left unconnected."

Best Regards, 
Yordan