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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 a...
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  • 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!

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.