@RepRap_Pro @Adrian_Bowyer thanks for the reply and tests. Yes, seems plausible that it is an electrical problem since I didn’t change the software. So here is what I did:
- disassembled the board from the printer and checked all soldering joints for bent connections or obvious shorts -> found nothing
- looked if the chip is inserted correctly -> looked fine
- carefully removed the chip again and checked for bent pins -> all as straight as new
- inserted the chip again, messed up and bent a pin -> now I know what you where looking for
- straightened the pin and reinserted the chip carefully and correctly
- reassembled everything
The problem unfortunately still persists. Everything is fine except for the extruder spinning endlessly until reset. (BTW: it might be useful to know that it will only start spinning when the correct gcode was sent, not on it’s own when powering the board).
To make absolutely sure there is nothing wrong with any of the other electronics, I removed the 1284p again and reinserted the old chip. I checked all axes and the extruder and everything works exactly as it should so the electronics are fine.
So any other ideas what could cause the problem? From what I understand the firmware is checksumed when transferred to the chip, so we can be sure their wasn’t a problem on the transfer, right?
Could the chip itself be defect?