Category: Specifications
Tools: Quartus® Prime / Quartus II
Device: Stratix® V


Stratix V devices support MultiVolt I/O, allowing 3.3V input even when VCCIO is 2.5V.
Although sufficient confirmation by IBIS or SPICE simulation is required, Altera recommends using an external clamp diode.

For details, refer to the document below.
https://www.altera.com/en_US/pdfs/literature/hb/stratix-v/stx5_core.pdf
(Search for MultiVolt I/O Interface in Stratix V Devices or clamping diode.)