Spartan-6 FPGA does not have the same POR Threshold Voltage as the Spartan-3 families do.
For Spartan-6 FPGA, when powering on the device the minimum operating condition levels found in the data sheet for VCCINT, VCCAUX and VCCO can be used as the minimum POR threshold.
If programming @ 1.8v then the minimum VCCO2 spec must be met which is 1.65V.
Once the Spartan-6 device is powered the voltage must NOT drop below the VDRINT and VDRAUX minimum levels as specific in the
Spartan-6 Data Sheet.
For queries on the Spartan-3 POR threshold levels please see
(Xilinx Answer 19816).