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BWalker's avatar
BWalker
Icon for New Contributor rankNew Contributor
2 years ago
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Will a Cyclone IV be damaged if a a 3.3V LVTTL output is driven high, but connected to ground?

Will a Cyclone IV be damaged if a 3.3V LVTTL output is driven high, but connected to ground?

The pin has the current strength set to 8 mA.

I'm working on an old design where the original designer added a switch to ground on an output pin that may get set high by the FPGA logic. I'm trying to determine if this is a risk to the FPGA.

Generic Io
  • BWalker's avatar
    BWalker
    2 years ago

    Thank you for answering my question!

    However, I'm still misunderstanding something.

    I thought it would be safe, because this pin has the "CURRENT_STRENGTH_NEW 8MA" assignment set. I thought this meant there was an internal current limiting feature to limit the output current to 8 mA under any situation, including when shorted to ground.

    8 mA is well below the Absolute Maximum Rating of 40 mA for the DC output current per pin.

    Would you please confirm I misinterpreted how this works?

    Thank you!

3 Replies

  • FvM's avatar
    FvM
    Icon for Super Contributor rankSuper Contributor
    2 years ago
    Hi,
    according to output characteristics documented in Ibis files, shorted output current ttl33 8mA will probably exceed absolute maximum ratings for pin current. Damage possible.
    • BWalker's avatar
      BWalker
      Icon for New Contributor rankNew Contributor
      2 years ago

      Thank you for answering my question!

      However, I'm still misunderstanding something.

      I thought it would be safe, because this pin has the "CURRENT_STRENGTH_NEW 8MA" assignment set. I thought this meant there was an internal current limiting feature to limit the output current to 8 mA under any situation, including when shorted to ground.

      8 mA is well below the Absolute Maximum Rating of 40 mA for the DC output current per pin.

      Would you please confirm I misinterpreted how this works?

      Thank you!

      • FvM's avatar
        FvM
        Icon for Super Contributor rankSuper Contributor
        2 years ago

        Hi,
        you are misunderstanding the meaning of 8 mA spec. It tells that the output voltage drop is keeping the LVTTL limits, e.g. 0.4V. Short circuit current can be as high as 40 or 50 mA.
        As mentioned, Ibis files contain a full set of static output characteristics with min, typ and max current for all IO standards.

        Here's the respective 8 mA pull-up curve

        If I understand the datasheet correctly, the absolute maximum sink current per pin is 25 mA, already exceeded by typical GND short current.