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Thread: Powering Teensy 3.6 with 3.3V?

  1. #1

    Powering Teensy 3.6 with 3.3V?

    I had posted about this issue in another thread, but figured it may deserve its own thread.

    I have been trying to power a T3.6 using the 3.3V pin and have been running into issues. The Teensy will run when powered off the 3.3V pin but it won't boot. In order to get it to boot, I need to plug into a USB port and press the reset button or upload a sketch (which I assume also resets the part). However, once it is running, I can disconnect the USB cable and it will continue to run on 3.3V. Looking at the T3.6 schematic, I can't see why this would be the case. Any ideas?

    For some context, we are using several T3.6 to run some small instruments for a scientific ballooning campaign. The T3.6's mount to carrier boards that contain a bunch of peripherals (ADCs, line drivers, sensors etc) that all run of 3.3V. As a result we would like to power the T3.6 off 3.3V as well so we don't need a 5V supply to run the Teensy, which is internally stepped down to 3.3V anyway.

  2. #2
    Senior Member KurtE's Avatar
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    Wonder what are you using to generate the 3.3v? Are you running directly off of 3.3v battery or are you using some external VR or DC to DC converter. If so which one? i.e. wondering if another flavor of some 5v converters don't work unless you reset the processor.

    Note: There is no reset button on the T3.6, but a program button. However there is Reset pin that if you connect to ground will reset the processor. Does this work when you are only connected up to your 3.3v supply? That is if you turn it on and it is not working, if you momentarily connect the reset pin to ground will it boot up?

  3. #3
    I have tried providing the 3.3V from both a high quality lab power supply and a board mounted 3.3V switching regulator (R-783.3-05) with appropriate filtering caps, so I don't think it is an issue of power quality.

    I was using the wrong terminology- I was pressing the program button, not pulling the reset line low. I will try pulling the reset line low while on the 3.3V supply and report back.

  4. #4
    Senior Member KurtE's Avatar
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    The reason was wondering about which converter is for example there were questions about the recom 5v converter in the thread: https://forum.pjrc.com/threads/37036...ighlight=RECOM

    Not sure if they resolved it or not. I have had an issue with one BEC, but have had luck with some Murata switching regulators. Over on Trossen robotics I have had recommendations for some of the pololu power boards, so I have two of them in the mail to me now including one of the: https://www.pololu.com/product/2851

  5. #5
    Senior Member Frank B's Avatar
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    Please try to connect 10nF between RESET and GND.

  6. #6
    I'll give it a try.

  7. #7
    Senior Member KurtE's Avatar
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    Quote Originally Posted by Frank B View Post
    Please try to connect 10nF between RESET and GND.
    FYI - I tried a 10uf Cap with my BEC setup and it worked... It was a pretty long delay before it booted, but it was the first through hole CAP I found in my stuff...

    Edit: it actually was a 220uf

  8. #8
    A capacitor between reset and ground fixed it. It can be as small as 0.01uF and it will boot OK. With such a small cap, it seems to boot almost instantly, but obviously it keeps it in reset just long enough.

    Thanks!

  9. #9
    Wo while the capacitor between reset and ground did fix the power problem, it has created a new issue - the part can no longer be programmed over USB. This is not a show stopper as I can remove the reset cap during development, and once the code is stable reinstall it for operational use. However it is worth mentioning. This also does not seem to be a dirty power supply issue. Even with a very high quality lab power supply, the issue persists both when turning on the power supply output while connected to the 3.3V line and turning the power supply non then connecting it to the 3.3V pin.

  10. #10
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    Did anyone figure out how to delay boot, *and* allow for USB programming? I know how to deploy the project, but I've got lots of development to do still, and for testing I'll frequently need the "real" power supply due to other voltage requirements on the board.

    I'm sure I can do this, but I'd like a more elegant solution.
    cheers,
    Doug

  11. #11
    I ended up soldering the necessary capacitor between ground and reset just on the reset side. I can easily bend the other leg of the cap to touch ground to run my widget autonomously or bend it not to touch ground for programming. Once the code is frozen I will solder the ground side as well. Not an elegant solution, but it works OK for now.

  12. #12
    Senior Member defragster's Avatar
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    Quote Originally Posted by DougMcK View Post
    Did anyone figure out how to delay boot, *and* allow for USB programming? I know how to deploy the project, but I've got lots of development to do still, and for testing I'll frequently need the "real" power supply due to other voltage requirements on the board.

    I'm sure I can do this, but I'd like a more elegant solution.
    cheers,
    Doug
    What happens if the cap is soldered and the VUSB trace it cut - then always use your external power? Does that allow programming or does it still block it on the programmer reset side?

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    Quote Originally Posted by defragster View Post
    What happens if the cap is soldered and the VUSB trace it cut - then always use your external power? Does that allow programming or does it still block it on the programmer reset side?
    Unfortunately, I can't test that easily. In a terrible fit of impatience I soldered the Teensy into my PCB with pins, rather than waiting for the parts to socket it... These pads are impossible to reach.
    Doug

  14. #14
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    Just a shot in the dark. Do you happen to have a serial available loop going? It really helps to see code in these instances.

  15. #15
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    I've primarily just been using "Blink" to test this.

  16. #16
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    Quote Originally Posted by Scientist View Post
    Wo while the capacitor between reset and ground did fix the power problem, it has created a new issue - the part can no longer be programmed over USB.
    What if you solder in a diode between the reset and the cap? That would still delay the startup when connected to battery, but would still allow the reset pin to be pulled low by the programmer. I have not tested anything similar to this, it was just an idea that struck me while reading the post.

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