AR# 47564: Zynq-7000 SoC, DDR - I/O Buffers do not Support External VREF
Zynq-7000 SoC, DDR - I/O Buffers do not Support External VREF
DDR I/O buffers (DDRIOB) use differential input receivers for which one input to the receiver is connected to the data input and the other is connected to a voltage reference pin called VREF (set to half of the DDR VCC I/O voltage).
VREF needs to be supplied from the internal voltage source because the external VREF feature for DDRIOB is not supported.
Minor. Use the internal VREF reference.
Use the internal VREF reference, see the Solution section for more information.