Are all of the VCCINT, VCCAUX , and VCCO pins in the Spartan-II/-IIE/-3/-3E devices connected internally?
VCCINT inputs feed a common metalization inside the chip, so they are connected internally. However, all VCCINT balls should be soldered for best decoupling capacitor performance, and to minimize IR drop and noise on the power rails.
The distribution of VCCINT (or ICCINT) is not even with incomplete connections. Although your design might work, it is exposed to uncontrolled internal VCC drop and VCC noise. Xilinx does not recommend this approach.
NOTE: VCCAUX pins exist in Spartan-3 and Spartan-3E only.
All VCCAUX bumps on the die go to a common metal interconnect and to a common connection on the laminate package.
As with VCCINT, you should NOT leave any VCCAUX pins unconnected, as it increases the inductance to the bypass caps and will slightly increase noise and jitter. VCCAUX pins must be well-bypassed.
Please see (Xilinx Answer 11182) for more information on VCCAUX.
VCCO pins are only tied together for pins in the same bank.
Design rules for connecting package pins is detailed in the data sheets.
Spartan-3 Data Sheet in the Pin-out Tables document at:
Spartan-3E Data Sheet in the Pin-out Tables document at:
For additional information on Spartan-3 3.3V Configuration, please see XAPP453:
Other relevant references include the Spartan-3 Generation FPGA User Guide (UG331):
and the Spartan-3 Generation Configuration User Guide (UG332):