nConfig drops during power up and stays low after on Cyclone3 EP3C5E144

[Altera_Forum]

Hi All, 

i have a design using Cyclone3 EP3CE144 device, and the JTag programmer (USB Blaster) can't scan or find the device. The symptoms are:  

1. The device can't be scanned by programmer (USB Blaster); 
2. nCONFIG pin drops to low during power-up, and then stays low (see the waveform attached); 
3. nSTATUS and CONFIG_DONE stay low all time. 

 All supplies(3.3V 2.5V 1.2V) are good and meet the POR spec. We have checked the nCONFIG pin on the cards and found there is no short to any third party (the pin connects to R33 (NO POP) and R250 (10k) ONLY). My understanding is: nCONFIG is input only, and if nCONFIG keeps low, the device keeps in reset state until nCONFIG is released. Because R33 is not pop'd, so only the pull-up R250 drives nCONFIG to high after powering up. But for some reason, it is driven to low during power up and stays low after, and I don't know who drives it and why? Does anybody know what is wrong? Any idea is appreciated. Thanks in advance.  Mike

[Altera_Forum]

This is what happens when the exposed "E-PAD" on the bottom of the E144 package is not connected to GND. This is a functioning ground pad and must be connected to ground for the device to work (not just for thermal dissipation).... if not device never exits POR and JTAG is not functional.... 

 I've seen this a hundred times.... you will need to connect the E-PAD to ground to get it to work whether by removing the part, attaching wire or contact and remounting (also make sure no shorting to exposed vias, etc) or very carefully drilling through PCB to bottom of package to attach wire... neither of which are quite trivial rework. Longer term, you will need to spin the board.  Good luck,  -Chriis

[Altera_Forum]

thanks, Chris. i have fixed the issue. how come nconfig is driven low if the Epad does not connected to gnd directly? maybe nconfig has I/O features? but datasheet says it is input only.