Anything special about Pin 28 on a Teensy 3.5?

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paynterf

Well-known member
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 :p

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

TIA,

Frank
 
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
 
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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