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Thread: Paul request; Separate Vin/VUSB Teensy 3.2

  1. #1

    Paul request; Separate Vin/VUSB Teensy 3.2

    Hi Paul and all,

    The short request to Paul:

    I know that while the Teensy schematics are available, you generally keep the Teensy PCB layouts proprietary.

    However, I've recently hit a snag where I need to know the PCB layout in this region of Teensy 3.2:
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    ... because I'm trying to understand a recent failure, and avoid it in future. So I'm hoping you might be willing to post just that piece (all layers), as it concerns an area of Teensy mods that is anticipated by the board design and docs (the cuttable VIN/VUSB pads).

    Details

    I'm implementing a device that may be powered via Teensy's VIn (used as output), or may be powered by a separate 5V PSU (using Teensy's VIn as input).

    The strategy is shown in this diagram. (click to enlarge)
    Click image for larger version. 

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    Implemented by cutting the Vin/VUSB trace (as suggested in the pinout diagram), and adding a Schottky diode, as shown here.
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    Ultimately this worked fine, but not before the trouble described next...

    After making the the mods shown, the Teensy did not power up from VUSB or VIN, which I discovered was because continuity was lost between U3-6&1/C9 and the rest of the net joining F1 and VIN pin.

    I ended up adding the following jumper, and then things worked as intended (Teensy powers up from either USB or VIN, and there's no current path in the reverse direction through diode added across the cut pads).

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    This is on a fresh new Teensy that I had tested on USB before starting.

    My hypotheses are that either:
    • something mechanical happened while cutting the trace, or soldering the diode, though I didn't apply extraordinary force or heat, or...
    • an overcurrent occurred actually burning the trace, though I have not been able to see how that would occur


    Anyhow, at the moment I'm just looking for the PCB pattern of this neighborhood so I can think about this better, as the four items on that "VIN" net are not connected in the order shown in the schematic (nor would I necessarily expect that), and I' like to know the ground truth of what order they are actually connected in.

    Thanks!

  2. #2
    Senior Member
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    This thread may be worth checking out.

    https://forum.pjrc.com/threads/63347...in-the-process

    All the best.

  3. #3
    Quote Originally Posted by MatrixRat View Post
    This thread may be worth checking out.

    https://forum.pjrc.com/threads/63347...in-the-process

    All the best.
    Wow! MatixRat, that thread is a direct hit! Thanks! That indeed would explain what I saw.

    However, I would still like to understand the routing in that area. Ie: I'd appreciate if Paul was willing to share that section of PCB layout.

    Graham

  4. #4
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by gwideman View Post
    Anyhow, at the moment I'm just looking for the PCB pattern of this neighborhood so I can think about this better
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  5. #5
    Paul -- posting those PCB images on Christmas morning is really beyond the call of duty. Thanks!

    So here's my digested version.

    Here are the relevant layers of the PCB, with the "VIN" net indicated in orange.

    Click to enlarge.

    Click image for larger version. 

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    Note that as others have said on the other thread (Cutting Vin from Vusb and bricking Teensy 3.2 in the process), a section of the VIN net does indeed pass right under the trace that's to be cut between the two pads.

    For a clear idea of the effect created, here's the schematic redrawn with the various components and pads shown in the order they appear on the PCB.

    Click image for larger version. 

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    You can see that if you accidentally cut that inner trace, you'll want to wire in a jumper, which is most practically connected from the Vin pin to the end of C9.

    Again, many thanks Paul for the ridiculously quick response. Hopefully this doc will help out others who hit the same snag.

    Graham

  6. #6
    Senior Member PaulStoffregen's Avatar
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    I've updated the Teensy 3.2 page, under Power > VIN Pin, with a link to this thread. Hopefully if anyone else cuts the trace and needs to repair, it will help them find this info.

  7. #7
    Thank you! I couldn't figure out what I was doing wrong. I bricked 4 teensy 3.2 this way and only 1 of them came back when I bridged the cut with solder. On that one, the solder must have wicked down to layer 3. Time to revive a long abandoned project with a bit of green wire.

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