Alright... I guess I have identified the root cause: Our power-supplies have a ground lift in the range of 100VAC (Including the one that comes with the Cyclonve 5 SX dev kit, you can actually feel it if you touch it)... So when you unplug the USB-blaster, and the ground pins are by chance the first pins that get disconnected, the Board is on somewhat high potential compared to the computer's USB, which is referenced to the mains earth.
So how do we go about this? We have a few ideas:
- Always ground the board (will get forgotten here)
- High Ohmic grounding of the power supplies will most likely get rid of the (capacitively coupled?) ground lift (impractical modification for all supplies)
- Put a buffer AMP or one to one level shifter in the JTAG path of future boards. Hopefully these are less susceptible to overvoltage or just die first (Most likely what we will do)
- Put a series resistance in the signal path (Anyone tried that? Will it help?)
- Take ESD diodes with lower voltage rating (Does 5V already kill the JTAG? I am not going to try this out on a 5CGXFC7C7U19C8N again!, but probably someone can comment)
Any suggestions or other ideas are welcome.
Cheers, Peter
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