--- Quote Start ---
VCCINT are power pins (Not I/O) and must be connected to 1.2V. You can leave unused I/O pins floating as you are doing for EP4CE15. Refer to Cyclone IV Device family pin Connection Guidelines.
--- Quote End ---
Thanks for the response, Danish, but I don't think you understood my question. Supporting migration between devices does in fact require tying 6 normal I/O pins on the EP4CE15 to +1.2V, because those pins become VCCINT pins on the larger EP4CE22 device. (Another 6 normal I/O pins become GND on the larger device and must be tied to GND on the board.)
My concern is what happens when the EP4CE15 is installed. The 3.3V pins that are tied to +1.2V will become tri-stated inputs with weak internal pullups after configuration, but the +1.2V will overcome the weak pullup so the input to the buffer will be +1.2V. With a normal 3.3V input buffer 1.2V is right in the middle of the transition zone where both the high and low transistors of the totem pole input are conducting, which would draw excessive current. The only way to prevent that would be to isolate the input buffer from the pin, but I don't think there is a switch in the Cyclone IV IOB that can do that.
Can anyone from Altera explain exactly how this works?
Honored Contributor
