The user can freely set voltage VREF0 through pin 37 of B2B connector JM4. VREF0 is the reference voltage for setting the input switching threshold for certain I/O standards of FPGA bank 0. For more information on reference voltages, please consult Xilinx DS610: Spartan-3A DSP FPGA Family: Complete Data Sheet.
Recommended operating voltage for VREF0 is 0.75 to 1.5 V

Absolute maximum voltage range for VREF0 is -0.5 to VCCCIO+0.5 V.
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