The VCCAUX constraint is used to define the voltage value of the VCCAUX pin for the device. The CONFIG VCCAUX constraint is set in the UCF or the NCF file. The valid value for this attribute is 2.5 (default) or 3.3. This attribute affects the banking rules for I/O placement within the automated placer, as well as in the Pace pin assignments tool. It also affects the end-generated bitstream for the device.

If the CONFIG VCCAUX constraint is not set and a user chooses to change VCCAUX voltage to 3.3V instead of the default 2.5V, there could be problems with the threshold levels on the inputs. This should not cause any damage to the device, but the inputs to the device might not meet the higher threshold requirements, so there might be logic failures in your design.

UCF and NCF Syntax Example:
Placed on output or bi-directional port:
CONFIG VCCAUX="value";
Where value is 2.5 or 3.3
Example:
CONFIG VCCAUX=3.3;

IMPORTANT NOTE: For the Spartan-3AN devices, VCCAUX MUST be set to 3.3V. The software setting default is 3.3V, and this should not be changed.

For more information on the CONFIG VCCAUX constraint, please see the Constraints Guide in the Xilinx Software Manuals:
http://toolbox.xilinx.com/docsan/xilinx10/books/docs/cgd/cgd.pdf