Regarding the 1-Wire serial number / thermometer chip, the Maxim DS18B20U+:
The data sheet says that when operated from a 3.3V supply (2.5V is not possible), the minimum input high voltage level (Vih) is 2.2V.
On the other hand the Spartan 6 (that is used on the SPEC) data sheet says that its minimum output high voltage level (Voh) on a 2.5V powered bank is 2.1V. So it's close but not ideal.

• Verify above statement
• Decide if is acceptable or not.