It's an option in the Cell Menu. I know the option isn't present in bios v1.2, but it is in v1.6. When it is on auto I just change to the next and previous setting to determine the auto setting in volts. I can see the direct changes tot Ref0 and Ref1 when I change vtt and the result is the same. Ref0 will be about 63% of VTT and Ref1 will be about 66,5% of VTT.