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Altera_Forum's avatar
Altera_Forum
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15 years ago

Cyclone 3 - JTAG Voltage

I am trying to figure out what voltage the JTAG pins can be powered from, in the Handbook it says that the signals are all powered from the VCCIO1 Bank (in my case its 3.3V)

The documentation goes on to say that teh maximum voltage per pin is 4.1V adn that the download cable should be powered off 2.5V with pullups to 2.5V.

Now why can't I run this at 3.3V? I currently have a 3.3V CPLD driving the JTAG pins on my board, do I need a buffer to 2.5V? Am I ok with 3.3V?

Can't I just run the programming pod at 3.3V also? Are you saying the overshoot of the pod when connected to 3.3V will go well over 4.1V?

Can I then just put my own clamping diodes on these nets and run completely at 3.3V?

Thanks

3 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    Thanks!

    Thats what I figured I just wanted to make sure I hadn't found some typo on something.
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    With suitable series diodes and protection diodes, you can effectively avoid overshoots. In so far, 3.3V can work without risk for Cyclone III and IV.

  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    I believe it's perfectly OK to run the cyclone III download cable from 3.3V. If memory serves me, I've done so in the past. The 2.5V is recommended to minimize the potential for overshoot on the JTAG pins. Diode clamping would be a good precaution.