Can not dowload the code into my Altera

[Altera_Forum]

I met a weird problem: I can download the code into my Altera chip EPM240T100 comfortably at the first time. But, I can not download the code again. The errors are: 

 Error: Can't access JTAG chain Error: Operation failed.  I tried to do "Auto Detect", still an error window popped out as  Unable to scan device chain. Can't scan JTAG chain.  Why???  I appreciate your advice and tips!

[Altera_Forum]

Measure your power supply voltages and make sure they are what they should be. Could be an I/O pin is driving vcc and the pin is connected to gnd or the I/O pin is driving gnd and the pin is connected to vcc.  

 May not be the problem but worth ruling out.

[Altera_Forum]

--- Quote Start ---  I met a weird problem: I can download the code into my Altera chip EPM240T100 comfortably at the first time. But, I can not download the code again. The errors are:  Error: Can't access JTAG chain Error: Operation failed.  I tried to do "Auto Detect", still an error window popped out as  Unable to scan device chain. Can't scan JTAG chain.  Why???  I appreciate your advice and tips!  --- Quote End ---    What kind of FPGA board do use ?  Kind regards  GPK

[Altera_Forum]

Thanks for your reply. I will check it

[Altera_Forum]

It is a "home-made" board....

[Altera_Forum]

--- Quote Start ---  It is a "home-made" board....  --- Quote End ---     OK,  can you describe what is on the board ? BTW you should check the power supply when the  Altera device is loaded.  Kind regards  GPK

[Altera_Forum]

Thanks for your reply.  

 I found the problem.For some reasons, the TMS, TDI, TDO and TCk pins were accidentally connected to four unused I/O pins besides being connected to the JTAG head. I cut the connections to the 4 I/O pins, it works.   Also, I noticed that in Settings, there is "Unused Pins" tab. I changed the setting of "Reserve all unused pins" from "As Output driving ground" to "As input tri-stated with weak pull-up resistor" and it works! I can download the code without cutting the connections mentioned above.   However, I found one of my output pin which was supposed to drve a LED does not work after changing the setting. Such pin is an output pin and it is not an "unused pin". I do not know why my setting change can affect it....  any idea?  Thank you!

[Altera_Forum]

--- Quote Start ---  Thanks for your reply.   I found the problem.For some reasons, the TMS, TDI, TDO and TCk pins were accidentally connected to four unused I/O pins besides being connected to the JTAG head. I cut the connections to the 4 I/O pins, it works.   Also, I noticed that in Settings, there is "Unused Pins" tab. I changed the setting of "Reserve all unused pins" from "As Output driving ground" to "As input tri-stated with weak pull-up resistor" and it works! I can download the code without cutting the connections mentioned above.   However, I found one of my output pin which was supposed to drve a LED does not work after changing the setting. Such pin is an output pin and it is not an "unused pin". I do not know why my setting change can affect it....  any idea?  Thank you!  --- Quote End ---    Hi,  great to hear that your main problem is solved. Regarding your Pin problem, you should check your Pin Assignment together with the schematics of your board. Maybe the LED is connected to one of your connection which you have cut ?  Kind regards  GPK

[Altera_Forum]

The LED driving pin was set as output pin "driving unspecified signals". It is a used pin instead of "unused" pin. I tested in the board without cutting any connections while setting the "All unused reserved pins" as "Input tri-stated with weak pull-up resistor". Then, I could load the code into the board but LED could not be lit anymore....

[Altera_Forum]

--- Quote Start ---  The LED driving pin was set as output pin "driving unspecified signals". It is a used pin instead of "unused" pin. I tested in the board without cutting any connections while setting the "All unused reserved pins" as "Input tri-stated with weak pull-up resistor". Then, I could load the code into the board but LED could not be lit anymore....  --- Quote End ---    Hi,  is the pin driven by your design ?  Kind Regards  GPK