Paul request; Separate Vin/VUSB Teensy 3.2

G

gwideman

Well-known member
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:
TeensyPDN_PCBLayoutRequest.png

... 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)
TeensyPDN_ThePlan.png

Implemented by cutting the Vin/VUSB trace (as suggested in the pinout diagram), and adding a Schottky diode, as shown here.
TeensyPDN_Implementation.png

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).

TeensyPDN_MyJumpers.png

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!
 
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.

Teensy32_PCB_VUSB_VIN_annotated.png

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.

Teensy32_Sch_VUSB_VIN_cuts.png

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
 
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.
 
PaulStoffregen said:
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.
Click to expand...

Hi. Thanks for the info. I'd like to install an on/off switch to the Teensy 3.2 so, I'm thinking of cutting there and using the two pads for the switch, is that a correct approach? The idea is to continue using +5V from the micro USB connector.

https://1drv.ms/u/s!Anq8XYv0zgb2hfVp8CPxjlHCTLhPMQ?e=BiEcHW
 
Cadorna said:
Hi. Thanks for the info. I'd like to install an on/off switch to the Teensy 3.2 so, I'm thinking of cutting there and using the two pads for the switch, is that a correct approach? The idea is to continue using +5V from the micro USB connector.

https://1drv.ms/u/s!Anq8XYv0zgb2hfVp8CPxjlHCTLhPMQ?e=BiEcHW
Click to expand...

You could also just solder wires to the VIN and VUSB holes in the PCB. You do have to cut the jumper.

On some of my Teensys, have put stacking headers on the Teensy. On the right side with VIN/GND/3.3v, I will sometimes add another pin before VIN, and connect VUSB to it. Then I can put the Teensy in a breadboard and attach the switch between those two pins. On others, with the stacking headers, I will solder either a 5 pin female or 1 pin female non-stacking header on the inside, and I just use a solid wire to attach VIN to VUSB.
 
Thanks. And how about installing a second activity led? Could I add one with a 470 ohms resistor to pin #13 and leave both working without any risk?
 
Cadorna said:
Thanks. And how about installing a second activity led? Could I add one with a 470 ohms resistor to pin #13 and leave both working without any risk?
Click to expand...

You should be able to do it. I've done it several times. Note, pin 13 is also used for the Teensy 3.x audio board and for normal SPI traffic, so if you use either of those, you will see the LEDs blinking more as they are used. If you have a Teensy 4.x, the audio board does not use pin 13, but SPI does.

The reason why pin 13 overlaps with the SPI pins, is that is how it was done with some of the original Arduino boards like the UNO (with the AVR 328p processor).
 
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