Cyclone IV nCONFIG pin is not stable

Thank you Carlhermann!

I observed all power supply voltages before and after the nCONFIG problem. All voltages are stable. I hope to find some synchronized changing between nCONFIG and any other signals, but have found nothing.

Your suggestion about the undefined pins is definitely very helpful. During the testing, we did let many pins undefined. During the testing process and more functions are added, more and more pins are defined. This could be a source of the problem because the nCONFIG signal suddenly gets correctly stable since the second debugging day after the re-heating, without any special actions of us.

Based on this consideration, we generated an empty project with only a defined clock pin and one output pin. All other pins are undefined. We loaded this project down to the FPGA and also to the EPCS chip. We let the system run this empty project for a very long time, but the nCONFIG pin voltage always keeps high and stable.

Till now, two whole days have past and the nCONFIG voltage always keeps stable. No problem appears any more. This is good so that we can process our test. However, it is bad that we still don't know the cause of the problem. We are afraid that this problem will be present again in our new system versions :-(