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Altera_Forum
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14 years ago

TMS stuck low on CYCLONE III

Yesterday we had been happily programming our cyclone III based prototype via JTAG, Quartus and a USB blaster fine until all of a sudden we could no longer detect the JTAG chain! I traced the ...
Altera_Forum's avatar
Altera_Forum
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13 years ago

Hi Dave,

I just had the same problem with the TDO pin on the JTAG. I have a cyclone IV chip that has been working just fine. All of a sudden the JTAG stopped working. The TDO was shorted to ground with 40 ohms. It was still working, but just a lower voltage(.5V swing).

Following your suggestion I applied 3.8V to the pin and the resistance went up to 200 ohm. Now it works.

Like you I would like to know what happened so I can keep it from happening again.

The JTAG is configured to work on 2.5V correctly and everything worked just fine up until it didn't. I only have protection diodes on the ASDO etc pins to the EPCS device as Altera suggested. They don't recommend anything if your just going to their programmer.

Dan