compile error with ddr2 sdram controller

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hi everyone ,

I am using SOPC to control DDR2 SDRAM,when i set pin assignment to the SOPC system ,following error occurs:

Error: Illegal connection found on I/O input buffer primitive image:inst|ddr2_sdram:the_ddr2_sdram|ddr2_sdram_controller_phy:ddr2_sdram_controller_phy_inst|ddr2_sdram_phy:ddr2_sdram_phy_inst|ddr2_sdram_phy_alt_mem_phy:ddr2_sdram_phy_alt_mem_phy_inst|ddr2_sdram_phy_alt_mem_phy_clk_reset:clk|gen_mimic_diff_ibuf.fb_clk_ibuf.

SourceIO

image:inst|ddr2_sdram:the_ddr2_sdram|ddr2_sdram_controller_phy:ddr2_sdram_controller_phy_inst|ddr2_sdram_phy:ddr2_sdram_phy_inst|ddr2_sdram_phy_alt_mem_phy:ddr2_sdram_phy_alt_mem_phy_inst|ddr2_sdram_phy_alt_mem_phy_clk_reset:clk|DDR_CLK_OUT[0].mem_clk_obuf also drives out to other destination than the buffer.

when i have not set pin assignment ,and there is no error,is anyone konw the reason?thank you.

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I met the same questions.When I add some singles into the SignalTap,it happens.