Here're a few things you can try to debug: make sure the JTAG connector is not loose, make sure you close Quartus II and then re-open to try again, make sure JTAG voltage is correct (either 3.3V or 2.5V according to datasheet), ohm out the JTAG signals TCK, TDI, TDO, TMS to see if they are shorted to ground. Also you can probe the JTAG signals to see whether they toggled and not stuck. It's very hard to kill JTAG but if you apply the wrong voltage, say, 3.3V rather than 2.5V, then it may work initially but then will die. If this is the case, you'll need to replace the FPGA.
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I cannot find my chip (ep3c40) by jtag today,but yestaday I download my jic file to eeprom by jtag,I gues that the jtag port of this fpga is burn,why that happen, Is there anybody give me some conslusions ,letting me make sure that tragedy will never happen again!,please!!!
the download device is usb-blaster,it is fine.
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