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In my current project I want to connect some logic to Altera DE2-115 using 40-pin exapnsion header (JP5). Unfortunately, I can't ensure that I won't exceed 3.3V on input port (i.e. I'll have ~4V). Is DE2-115 somehow protected against such situation? Are RN21 and RN23 such protection? Or do I need to put in some extra effort to provide stable 3.3V?

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    \$\begingroup\$ See the Cyclone IV Device Datasheet, section on Absolute Maximum Ratings, the following table on Maximum Allowed Overshoot or Undershoot Voltage, followed by Recommended Operating Conditions. The short answer is you'll likely break it if you do. The recommended maximum VI is 3.6 V. \$\endgroup\$ – user8352 Feb 9 '15 at 8:57
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The absolute maximum voltage permitted for Cyclone IV devices is 4.2V, but the recommended maximum operating input voltage is 3.6V. To ensure reliable operation you should reduce your signal voltages to 3.6V or less.

If you don't need high speed and/or the drivers are strong enough then you might get away with passive resistor dividers (eg. 33 Ohms in series followed by 150 Ohms to ground). Otherwise you should use level shifting buffers such as 74HC4050 or 74LVXC3245.

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