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Hi, I am in the process of developing my own DDR2 controller as an exercise. Consequently, I'm trying to avoid using tools like MIG. Unfortunately I could not fully escape the clutches of automated tools, as in order to correctly configure the .xdc constraints file, I've had to have a peek at the following .prj file generated by the MIG in the official Digilent DDR2 Demo implementation: https://github.com/Digilent/Nexys-4-DDR-OOB/blob/master/src/ip/ddr/mig.prj I interpret the following line: "<InternalVref>1</InternalVref>" as setting the INTERNAL_VREF property to 1V. Therefore, I add the following line in my .xdc file: set_property INTERNAL_VREF 1 [get_iobanks 34] However, this configuration fails to implement for the Nexys4 DDR chip with the following error: [DRC 23-20] Rule violation (IVREF-2) INTERNAL_VREF - Bank 34 has INTERNAL_VREF set to an unsupported value (1.000V). Supported VREF values for this part are: 0.600, 0.675, 0.750, 0.900. Which value should I choose to use?