Only VREF1 should be used for all DDR1, DDR2 and DDR3 memory controller applications. This is because only the VREF1 pad includes a dedicated circuit to detect preamble stages on the DQS signal coming from the memory device. It is important to know that you must not connect VREF1 to VREF2 together. It is because the detector circuit characteristics can be affected by the connected VREF2 pad when the VREF2 pad's pull-up resister is turned on. Note that you can use VREF2 as a general I/O in DDR memory interface applications.