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Thread: Interchange Teensy T_LC and T_3.1/3.2 with VBat powered pin

  1. #1
    Senior Member+ defragster's Avatar
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    Interchange Teensy T_LC and T_3.1/3.2 with VBat powered pin

    I've put headers on my LC's and 3.1 and 3.2 units that are common so far. So I can plug one or the other onto the same PCB.

    PROBLEM/QUESTION: I made a board with a battery to support the T_3 RTC on VBat.

    What happens to the LC when it is plugged onto that T_3 RTC board with VBat battery power present on what is the T_LC pin 17 output pin?

    Doing some ILI9341 display work and Not realizing the effect/difference I dropped the LC most of the way on and saw the LED light and then yanked it off wondering what I did? I expect it was not USB powered during the changeover.

    The T_LC still seems to work - but I have never used pin 17. Seeing the LED light has me wondering it the RTC battery was back-feeding VIN and powering the teensy or if it was doing something else? Decided I should ask rather than just try it again.

  2. #2
    Senior Member+ defragster's Avatar
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    @Paul - what does putting 3.3v on LC pin 17 do?

  3. #3
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by defragster View Post
    what does putting 3.3v on LC pin 17 do?
    Are you asking about pin 17, which is between 16 and 18, or the buffered copy of pin 17 where VBAT is on Teensy 3.2, between pin 12 and 3.3V?

    Connecting 3.3V directly to the buffered output (next to pin 12) is bad. Don't do it. That buffer strongly drives the pin to GND to whatever voltage VIN happens to be. Don't short it to 3.3V.

  4. #4
    Senior Member+ defragster's Avatar
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    Per the OP I have a board socketed for a T3 that has a 3v cell for RTC .

    With common socket setup I put the LC on that, putting the vbat on pin 17

  5. #5
    Senior Member+ defragster's Avatar
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    From OP: PROBLEM/QUESTION: I made a board with a battery to support the T_3 RTC on VBat.

    What happens to the LC when it is plugged onto that T_3 RTC board with VBat battery power present on what is the T_LC pin 17 output pin?

    If I read post #3 right the answer is basically try to drive the battery current to ground? Not a good thing - would it back feed the VCC and power the LC - or just put power on the LED somehow.

    As noted I don't think it was plugged in USB - but I saw the LED light telling me something wasn't right. I didn't mean to do this - just an ad hoc test of LC on a display board - it is just not something I thought about before using the 'plug interchangeability' of the LC to go where a T_3 does.

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