I previously used an older Virtex device (0.25 um 5-layer metal process) and the unused incremental VCCINT pins on the smaller packages where NCs (no connects) just as documented in the 1999 data sheet. I recently received new devices with the newer device (0.22um/0.18 um 6-layer metal hybrid process), and now these NC pins are driving High to 2.5V.
Why are these pins driving High? Has the pin-out of the Virtex packages been changed?
In summary, the VCCINT pins on the 0.25 micron package version were assigned one die pad to one package ball. The one-to-one mapping left the smaller devices in the same package with extra designated VCCINT balls that were not physically connected on the die since the die had fewer VCCINT pads than the VCCINT balls on the package.
The newer packages for the 0.22/0.18um die were designed so that all the VCCINT pads on the die are connected together to an internal package plane. This plane is supplied by all the designated VCCINT balls on that package, so that regardless of the device size, the VCCINT ball count remains the same. The change was done to improve power distribution of the VCCINT in the packages and avoid the inadvertent cases in the previous setup where a skipped required VCCINT ball left sections of the circuit without adequate power support.
For more information, please see Customer Notification PCN2004-12 - Clarification on Handling Unconnected VCCINT Pins for Virtex Devices located at: