After initial bring-up, if a link resynchronization is requested 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 resynchronize (the assumption is that LMBC counters continue to free-run and remain valid).
- A receive core will re-acquire multiblock lock and output received data on the next LMBC boundary.
- A transmit core will continue to transmit multiblock data.
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.