After initial bring-up, if a link resynchronization is requested (by the deassertion of SYNC by the receiving device), the desired core behavior relative to SYSREF can be controlled using the SYSREF Required control bit in the CTRL_SYSREF Handling register.
When SYSREF Required is set to 0, no SYSREF event is required for the link to re-synchronize (the assumption is that LMFC counters continue to free-run and remain valid).
- A receive core asserts SYNC on the next LMFC boundary after code group sync.
- A transmit core transmits the ILA sequence on the next LMFC boundary after SYNC is asserted.
When SYSREF Required is set to 1, a SYSREF event is required for the link to reestablish SYNC following a resync request. In this case, the behavior is the same as the initial link bring up detailed earlier.
This setting is particularly important in systems where a One-Shot SYSREF is used, or where SYSREF is periodic, but SYSREF Always is set to 0.