Due to a problem in the Quartus® II software version 9.1 SP1 and earlier, for Cyclone® IV GX devices the auto-generated core_clk_out SDC constraint is made incorrectly and the following warning will be generated during the Analysis and Sythesis stage.
Warning: Ignored assignment: create_clock -name {core_clk_out} -period 8.000 -waveform { 0.000 4.000 } [get_nets {*altpcie_hip_pipen1b_inst|core_clk_out~clkctrl}]
Warning: Argument <targets> is an empty collection
To work around this problem, change the core_clk_out SDC constraint in the <variation name>.sdc file to:
create_clock -name {core_clk_out} -period 8.000 [get_nets *altpcie_hip_pipen1b_inst|core_clk_out*]
This problem is not scheduled to be fixed in a future release of the Quartus II software.