The SYREF Delay bits in the CTRL_SYSREF register can be used to add delay to the
SYSREF
signal after it is captured (see Table 15). This allows the effective phase of
the local multiframe clock (LMFC)/ local extended multiblock clock (LEMC) to be
adjusted. The value programmed into the SYSREF delay register equates to the number of
core clock cycles that SYSREF will be delayed by.
For 8B10B linecoding, the deterministic latency mechanism, as defined in the JESD204C standard, requires that the multiframe size be larger than the maximum possible delay across the link. In practice, this can be difficult to achieve, particularly with small frame sizes. However, as long as the multiframe size is greater than the maximum variation between lanes in delay across the link, then deterministic latency can be achieved. A potential issue occurs when the maximum lane delay variation causes the overall latency to straddle the boundary between two adjacent LMFC periods. In such a case, latency variations of exactly one LMFC period can be observed between system restarts. In this case, the SYSREF might be delayed to adjust the LMFC boundary position to alleviate the problem.
For 64B66B linecoding, the deterministic latency mechanism defined in the JESD204C standard requires that the maximum variation between lanes in delay across the link be less than the Extended Multiblock size. However, the alignment buffer in the 64B66B RX IP core is fixed at a size of two multiblocks. Care should be taken when designing systems where the number of Multiblocks in an Extended Multiblock (MB_IN_EMB) is greater than two because this can cause the buffers to overflow if more than two Multiblocks of data need to be stored before the next LEMC buffer release point. The SYSREF Delay register can be used to offset the phase of the LEMC clock from SYSREF such that even for large values of MB_IN_EMB the RX IP core LEMC can be adjusted such that the required amount of data to be buffered is less than two Multiblocks. The TX IP core also has this register to aid system latency optimization.