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AR# 31375

Spartan-6 and Spartan-3A Extended Generation FPGA - CONFIG VCCAUX constraint must be used when changing VCCAUX voltage


In the Spartan-6 and Spartan-3A extended families (which include Spartan-3A, Spartan-3AN, and Spartan-3A DSP FPGA), the VCCAUX voltage can be set to either 2.5V or 3.3V.

Is there anything that needs to be set in software to make this change?


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:


Where value is 2.5 or 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 ISE 10.1 Design Suite Software Manuals and Help - PDF Collection" found at:

AR# 31375
Date 12/15/2012
Status Active
Type General Article
  • Spartan-3
  • Spartan-3A
  • Spartan-3A DSP
  • More
  • Spartan-3AN
  • Spartan-6 LXT
  • Spartan-6 LX
  • Spartan-6Q
  • Less