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Thread: Teensy 3.5 behaving badly

  1. #1
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    Teensy 3.5 behaving badly

    I have a project which attaches a Teensy 3.5 to a vintage computer, supported by some 74LS logic chips. I've produced over 40 boards which all work great, but for the first time it seems like the Teensy is bad. I replaced the board it's plugged into, the logic chips, and all components, and while the Teensy does boot up (I can debug with it), several I/O lines are being pulled low inexplicably, but only while the computer is on. My guess is that a couple pins somewhere are being shorted and pulling these 2 lines low somehow, but I can't figure out how to test it. Is there some kind of test to find out if the Teensy I/O is working improperly, or should I just return this one Teensy?

  2. #2
    Senior Member+ defragster's Avatar
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    There is a sketch here to test for shorts: Possible-new-shields

    and this one that sets pins HIGH or LOW and then using a wire to PULL LOW or HIGH it will show the changes to Serial Monitor: Possible-new-shields

    Working on a sketch to combine them - but not ready.

  3. #3
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    I tried your tests.

    On the first one it didn't report anything for pull down, then for pull up:
    1:2 4:5 5:6 6:7 7:8 13:14

    The other test same thing, nothing for pull down, but pull up:
    02:30:03.422 -> d#=13 val=0
    02:30:03.422 -> d#=18 val=0

    Furthermore, I tried the physical ground test and it seems normal except pins 18 and 27 are unresponsive. This confirms my previous tests, where pin 27 (mode INPUT) was stuck high, and pin 18 (mode INPUT) was high when disconnected and low when connected to the output of a 74LS11, and was also pulling 2 of the inputs to the 74LS11 (which are connected to other pins). In other words, when the 74LS11 was powered, all 3 pins (the inputs and outputs of an AND gate) were all stuck low, and when it was not powered, all readings were high.

    I am trying to decipher, but maybe you can tell me what this may indicate?

  4. #4
    Senior Member+ defragster's Avatar
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    Those tests are generic showing suspicious behavior, pins to check or evaluate. They may be influenced by soldering itself or what it may be soldered to.

    Ideally a fresh Teensy would not find any issues and test well - and the same after soldering pins or other neutral things where components wouldn't add outside influence or feedback to pins.

    Going further would take DMM/Scope or other things to test suspect pins or connects of the device at hand AFAIK.

  5. #5
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    Thanks for your help defragster. I sent a support ticket to PJRC. I am not going to spend any more time on this because I already spent several hours. It's not worth it to diagnose this one Teensy. Above and beyond, the Teensy 3.5 works great, this just seems to be a defective one (1 of 50 ain't bad!). I don't think the problem is my fault since it is the Teensy Pins version (jumper pins pre-soldered) so it might just have been a bad soldering job to begin with.

    I have destroyed a few Teensy 3.5's in the past by ripping traces after re-soldering, but now I always use the pre-pinned version so I don't have that problem.

  6. #6
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    I have a hunch as to what happened. I have been using the battery pin with an extra jumper pin, and apparently the pin occasionally stuck high was right next to the battery pin. Furthermore, upon desoldering the pin, the trace lifted. So I suspect the trace from the battery pin was loosely touching the pin next to it.

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