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initialize jtag chain error.

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ilikebbs

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I can not initialize jtag chain. I use xc2c128 from Xilinx.
I have two boards. One is OK, the other has problems.

I measure each jtag pin resistance to ground of two board. And i find that resistance of TMS pin to ground is only 250 ohm in the abnormal board.
The resistance of TMS pin to ground is 10k ohm in the normal board.
All jtag pins have been pulled up with 10k ohm resistor.
So can I get the conclusion that the TMS pins has been destroyed?
Do you meet the similar situation before?
Thanks.
I do not have the usb download cable. My parallel download cable can be used to downlad jed file in other board. It is ok.
 

TMS is an input only signal to the FPGA. The 250 ohm reading suggests that chip is blown. I would remove the 10K pulldown and recheck the resistance. If the value goes up then perhaps the pulldown is the wrong value. Otherwise, I suspect the input protection diode in the FPGA is defective. Nothing you can do other than replace the chip, or configure this device with EEPROM only.

Damaging devices like this is not unheard of using a parallel port cable. I think the problem is making connection to the JTAG pins, but having the ground open or floating. Does your download cable have series resistors in the JTAG lines? This is often done to limit the current on input lines due to misconnections.
 

Thanks,
I also think that the TMS pins is destroyed when I connect the JTAG pins. I will replace the CPLD.
I do not know if the download cable has series resistor or not.
 

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