For JESD204 systems, to achieve SYNC all lanes must have achieved code group sync (CGS).
Once CGS has been achieved, the SYNC pin can go high.
To achieve CGS, a lane must see 0xBC (with charisk set) and there must be no errors (no not in table nor disparity errors).
Once CGS is achieved, the core stays in CGS unless 4 successive invalid octets are received (unexpected K-character, not in table error, or disparity error).
Achieving SYNC does not depend on any link parameters, other than line rate and active lanes.
Once in SYNC, there are 3 main reasons a system may fall out of sync (requesting a resync):