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Altera_Forum's avatar
Altera_Forum
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18 years ago

Open drain usage with internal pullups

Is it possible to use the programmable internal pullup for open drain operation, without adding an external pullup?

I guess it should work without problems, besides performance. If so, what would be the estimated maximum frequency on Cyclone II devices? I can't find the resistor value on the datasheet. IO standard will be LVTLL.

3 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    18 years ago

    For any strange reason, the datasheets strictly avoid to tell this simple and obvious fact. However, there's a knowledge base entry, that confirms it for Cylone.

    --- Quote Start ---

    The value of the programmable pull-up resistor for Cyclone® series devices in user mode can be found by referring to the Rconf specification in the "DC and Switching Characteristics" chapter of the respective device handbook.

    --- Quote End ---

  • Altera_Forum's avatar
    Altera_Forum
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    18 years ago

    Thanks,

    I didn't know it was the same pullup used before user mode.
  • Altera_Forum's avatar
    Altera_Forum
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    18 years ago

    As far as I know, the weak pull-up value is identical to value of i/o pin pull-up resistor before and during configuration documented in table 5-3 of DC characteristics, that is 10 to 50 k for LVTTL. I use it e. g. for static inputs as configuration switches, but slow dynamic operation would be also possible. Susceptibility to capacitive coupled interferences should be considered, howevewer.