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Thread: Anything special about Pin 28 on a Teensy 3.5?

  1. #1
    Senior Member
    Join Date
    Jun 2017
    Posts
    192

    Anything special about Pin 28 on a Teensy 3.5?

    I needed a digital I/O pin on my current T3.5 project, and more or less randomly selected Pin 28 from among the unassigned pins available. However, when I tried to use it to detect when a normally grounded circuit was broken (power plug mating with power jack), I couldn't get it to work. So, after tearing what's left of my hair out for a while, I discovered that on this particular Teensy 3.5, Pin 28 doesn't respond the same as adjacent (i.e. pins 29-32) ones.

    Here's a very short Teensy 3.5 program to demonstrate the issue:

    Code:
    /*
        Name:       T35_PIN28_DEMO.ino
        Created:	11/23/2021 4:08:28 PM
        Author:     FRANKNEWXPS15\Frank
    */
    
    
    void setup()
    {
    
      pinMode(32, INPUT_PULLUP);  //goes HIGH when chg cable connected
      pinMode(31, INPUT_PULLUP);  //goes HIGH when chg cable connected
      pinMode(30, INPUT_PULLUP);  //goes HIGH when chg cable connected
      pinMode(29, INPUT_PULLUP);  //goes HIGH when chg cable connected
      pinMode(28, INPUT_PULLUP);  //goes HIGH when chg cable connected
    
    
    }
    
    void loop()
    {
      Serial.printf("%d\t%d\t%d\t%d\t%d\n", digitalRead(32), digitalRead(31), digitalRead(30), digitalRead(29), digitalRead(28));
      delay(200);
    }
    And here is some output:

    Code:
    Opening port
    Port open
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	0	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	0	1	1
    1	1	0	1	1
    1	1	0	1	1
    1	1	0	1	1
    1	1	0	1	1
    1	1	0	1	1
    1	1	0	1	1
    1	1	0	1	1
    1	1	0	1	1
    1	1	0	1	1
    1	1	0	1	1
    1	1	0	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	0	1	1	1
    1	1	1	1	1
    1	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    0	1	1	1	1
    
    Port closed
    I tested each pin by connecting it to ground via a jumper - otherwise each pin was left open-circuited. So, out of the five pins I tested, the only one that DOESN'T behave is the one I chose for my project - go figure

    So, do I have a blown port on this Teensy, or is there something else going on?

    TIA,

    Frank

  2. #2
    Senior Member+ KurtE's Avatar
    Join Date
    Jan 2014
    Posts
    9,755
    I ran it on one of my t3.5 and pin 28 worked like pins 29-32...

    So maybe something with your board... you might try touching up the solder on pin 28 and see if that makes a difference... I have been there before, where I had a bad solder joint

  3. #3
    Senior Member
    Join Date
    Jun 2017
    Posts
    192
    Kurt,

    Thanks for the quick response. I did take a look at the solder joint and retouched it (even though it looked OK to begin with). Unfortunately this did not change the (mis)behavior of pin 28. Guess I have a blown port - oops! No big deal, as I still have plenty of spares, so it was just my bad luck to randomly pick the ONE (at least I hope its only one!) dead port.

    Frank

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