Does the production silicon have support for internal reference?
The internal reference is an errata topic for the Virtex-6 FPGA production silicon.
However a fix has been implemented. Devices with a JTAG IDCODE of 2 or higher support the internal reference for the System Monitor.
You can obtain the JTAG Rev ID of a device from iMPACT with the "Get Device ID" operation.
The following is an example of the log file:
When using the internal reference, calibration must be enabled to met the accuracy specifications of the Data Sheet.
Devices with a JTAG of lower than 2 support internal VRef with the accuracy indicated in the Errata.