I am using a Spartan-3 device in a TQ144 package. Using a DCI output signal in Bank 2 and VRN/VRP as a LVCMOS33 input in Bank 3, PAR errors out:
ERROR:Place:899 - The following IOBs use the Digitally Controlled Impedance feature (DCI) and have been locked (LOC constraint) to the I/O bank 3. This feature requires the VRN and VRP pins within the same I/O bank to be connected to reference resistors. The following VR pins are currently locked and can't be used to supply the necessary reference.
IO Standard: Name = LVDCI_33, VREF = NR, VCCO = 3.30, TERM = DRIVER
List of locked IOB's: my_dci_output
List of occupied VR Sites:VR site PAD62 is occupied by comp my_vrn_used_as_IO
The problem is related to a specific feature of Spartan-3 with a tq144 package. These devices have so-called shared banks when two banks share the same VCCO. In the case of locked components, the available bank capacity is treated incorrectly. This issue will be fixed in ISE version 9.2i. Meanwhile, a work-around is to NOT use VRN/VRP as IO in bank 3 if DCI is used in Bank 2.