A question regard ddr sdram using niosii

Hi all,

I have a question with regard to ddr sdram using nios2 recently.

Why the write_clk of ddr sdram using SOPC builder in quartus2 is not the same with the sys_clk of ddr sdram,but using a-90 deg excusion with ddr sys_clk?

Thank you in advance for any advice.

Best Regards,

-Ivon