5.1. Constraining Incoming SYSREF Signal
The setup time is analyzed when you set the timing constraint for the SYSREF signal in the user .sdc file. When the setup time is met, the SYSREF signal detection by the IP is deterministic; the number of link clock cycles of SYSREF signal that arrives at the FPGA pin to the LMFC counter resets, is deterministic.
Apply the set_input_delay constraint on the SYSREF signal with respect to device clock in the user .sdc file:
set_input_delay -clock <device clock name at FPGA pin> <sysref IO delay in ns> [get_ports <sysref name at FPGA pin >]
The SYSREF IO delay is the board trace length mismatch between device clock and SYSREF. For example:
set_input_delay -clock device_clk 0.5 [get_ports sysref]
The above statement constrains the FPGA SYSREF signal (sysref), with respect to the FPGA device clock (device_clk) pin. The trace length mismatch resulted in 500 ps or 0.5 ns difference in time arrival at the FPGA pins between SYSREF and device clock.
In most cases, the register in the IP, which detects the SYSREF signal, is far away from the SYSREF I/O pin. The long interconnect routing delay results in timing violation. You are recommended to use multi-stages pipeline registers to close timing. Use the same clock domain as the JESD204B IP's rxlink_clk and txlink_clk to clock the multi-stages pipeline registers.
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