Honored Contributor
Honored ContributorI have to agree with Dave's final sentiment. I would not recommend driving signals of 2.5V amplitude into an I/O bank powered at 1.8V. Generally, this would be considered bad practice.
However, Cyclone IV is a very resilient device family. As such is has been designed to withstand voltages on pins prior to the device being powered - hot-plugging. The device may well not have power to a particular I/O bank before active signals are presented to I/O pins in that bank. The handbook does state the device can handle over 4V on I/O pins. Having said that, I repeat, it is not good practice to over drive I/O pins on any device and I would recommend you condition the signal appropriately for the particular bank that the I/O pin resides in. Over driving the pin will result in the device dumping the excess power through its protection circuitry whenever the input voltage exceeds the voltage of the I/O bank. If you do wish to do this ensure you use a suitably high value in-line resistor (ensuring the signal's source impedance isn't too low) limiting the current the protection circuitry has to handle. If you don't wish to condition the signal you can always drive it into a different I/O bank whose voltage is better suited - e.g. 2.5V.
