Если речь идет о Spartan 6 (телепатирую)...
При VAUX = 2.5V
нельзя программировать eFUSE. А без программирования он нафиг не нужен.
Spartan-6 FPGA Configuration User Guide p87
Цитата
VFS Pin
In Spartan-6 devices, the VFS pin is one of two pins dedicated to eFUSE operation. The VFS
pin should be treated as a power supply pin for testing purposes, such as power-up ramp
and ESD stress.
The voltage specification for the VFS pin during programming is 3.3V nominal. The supply
must be able to provide up to 40 mA of current during programming. For read mode, the
VFS pin only needs to be lower than the VCCAUX maximum operating condition. See
Table 5-21 for VFS bias conditions. For the full specification, see DS162, Spartan-6 FPGA
Data Sheet: DC and Switching Characteristics.
Цитата
VCCAUX Pin
The VCCAUX must be equal to or greater than VFS when programming the eFUSE. VCCAUX
can be any of the other recommended operating values allowed in DS162, Spartan-6 FPGA
Data Sheet: DC and Switching Characteristics, when reading or configuring from the eFUSE.
Spartan-6 FPGA Electrical Characteristics p3
Цитата
Table 3: eFUSE Programming Conditions
VFS External voltage supply 3.2 3.3 3.4 V