Step 1: Determine How Many LMFC Periods are Required (N) - 4.3 English - PG242

The delay between LMFC at TX and LMFC at RX (T) is an integer number (N) of LMFC periods adjusted by the internal delays from SYSREF to LMFC of TX and RX:

T = N*LMFC - T_TXLMFC + T_RXLMFC

To ensure the overall propagation delay is constant between system restarts, the maximum propagation delay must be less than T plus one LMFC period:

(T_WIRE(max) + T_RXIN(max) + T_TXOUT(max)) < T + LMFC

The minimum propagation delay must also be > T:

(T_WIRE(min) + T_RXIN(min) + T_TXOUT(min)) > T

Substituting (N*LMFC - T_TXLMFC + T_RXLMFC) for T gives:

(T_WIRE(max) + T_RXIN(max) + T_TXOUT(max)) < ((N+1)*LMFC - T_TXLMFC + T_RXLMFC)

(T_WIRE(min) + T_RXIN(min) + T_TXOUT(min)) > (N*LMFC - T_TXLMFC + T_RXLMFC)

It is possible that no valid value for N can be found if the received data arrives close to an RX LMFC boundary and the variation in the link causes it to fall just before or just after the boundary after resetting the system. This would be seen as a jump in latency of exactly one LMFC period between system restarts. If no valid value can be found for N then the SYSREF signals can be delayed relative to one another to move the RX LMFC boundary relative to the TX LMFC boundary. For each cycle of delay added to TX SYSREF add four to T_TXLMFC. Additional delay can be added to the SYSREF processing in the JESD204C core to accomplish this; see Table 15.