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I'm programming a custom ESP32 board using Arduino where GPIO25 and GPIO32 are connected to pogo pins via short 5mm traces and to nothing else. The goal is to poll for a short between them as part of an alignment procedure - pretty straightforward.

The issue is, pinMode(X, INPUT_PULLUP) does not actaully turn on the pullup for either pin; the ADC readings are always floating. Whichever pin I'm using still successfully detects a short when intended, but because It is floating and never pulled high, it causes a lot of false flags. Code below:

    int foo=9999;
    pinMode(25, OUTPUT);
    pinMode(32,INPUT_PULLUP);
    while(foo>0){
        foo=analogRead(32);
        Serial.println(foo);
        //do other stuff
    }
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  • \$\begingroup\$ I typed the code in here directly, that was a typo. Fixed now. I'm reading the correct ADC pin when the described problem happens \$\endgroup\$ Sep 19 '20 at 17:29
  • \$\begingroup\$ This Reddit thread has that IDF library direct approach that may work. I'm about to try it out myself. Reddit Thread on ADC w/ internal pullup \$\endgroup\$
    – Rudy Moore
    Jul 18 at 19:38
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Hi all I got to the bottom of this for anyone googling:

Using analogRead attaches the GPIO pin you're using to the ADC channel and disconnects it from the pull-up. Calling pinMode(X, INPUT_PULLUP) after every analogRead will reinstate the bias.

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  • \$\begingroup\$ Would that actually cause the pullup to be active during the ADC sampling? Or does this just mean you end up relying on stray capacitance to hold the value? \$\endgroup\$ Sep 21 '20 at 22:30
  • \$\begingroup\$ The latter it seems, which is good enough for me as a workaround. I no longer get any false positives with this method. I'd bet there's a way to achieve both using IDF library functions directly. \$\endgroup\$ Sep 22 '20 at 2:14
  • \$\begingroup\$ If it's really capacitance making it happen, I'd be tempted to add my own capacitors. Or external resistors, unless that causes a power consumption issue... \$\endgroup\$ Sep 22 '20 at 2:39

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