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Thread: Teensy 4.1: I think I made a mistake on my carrier board

  1. #1
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    Teensy 4.1: I think I made a mistake on my carrier board

    I think I was being dumb when designing my circuit board that would be the carrier for the Teensy 4.1. Since all my extra circuitry would be running off 3.3V and I was worried that the 250mA of the onboard voltage regulator wouldn't be enough, I instead was just going to use a 3.3V offboard supply coming in to a connector to power my external circuitry and the Teensy. However, looking at the schematic for the Teensy 4.1 (https://www.pjrc.com/teensy/schematic.html) which my idiot self is kicking myself for not having done more scrupulously before having the boards made, it appears that I **have to** have 5V either from USB or external.

    Is this correct?

    If so, I think I may be able to salvage this board (I am only populating 4 of them for a consulting job). So, I think I could run in 5V on the power pin instead of 3.3V, cut the trace from the connector pin to the Teensy 4.1 3.3V pin, then cut the 5V USB pad-to-pad trace, and direct solder from the connector pin or positive lead of the throughhole decoupling cap to the one side of the 5V USB pad-to-pad pads or 5V pin. I think that in the end I will be under the 250mA total (by about half).

    Does this sound like reasonable as a fix?

    Click image for larger version. 

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  2. #2
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    Well, I think someone answered my question, though it took a bit of searching to find it.

    https://forum.pjrc.com/threads/58874...l=1#post226095

    This makes me think that I can connect directly to the 3.3V pin with my external supply and then the VUSB will only be used to supply the various USB pins and devices for when programming and debugging over serial.

  3. #3
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    Double post. not sure how that happened...

  4. #4
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    I'm a little confused about your situation. The above board image shows 3.3V connected to 5V on the USB connector but that is different from your question. The Teensy 4.0 and 4.1 (and I think all teensys) can be powered externally by 3.3V. That's one reason why there is a VUSB jumper you can cut (upper left on the schematic). I don't know all the implications of external 3.3V without cutting the VUSB jumper but they aren't good if you are going to connnect a PC to the Teensy's microUSB. There is a VUSB pin that is powered regardless of the cut jumper so you can get 5V for off-teensy use.

  5. #5
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    Quote Originally Posted by PhilB View Post
    I'm a little confused about your situation. The above board image shows 3.3V connected to 5V on the USB connector but that is different from your question. The Teensy 4.0 and 4.1 (and I think all teensys) can be powered externally by 3.3V. That's one reason why there is a VUSB jumper you can cut (upper left on the schematic). I don't know all the implications of external 3.3V without cutting the VUSB jumper but they aren't good if you are going to connnect a PC to the Teensy's microUSB. There is a VUSB pin that is powered regardless of the cut jumper so you can get 5V for off-teensy use.
    Am I completely misreading the Teensy 4.1 pinout document? I think that upper right pin in my schematic is ground, the lower right pin (without a connection) is the 5V pin, and the lower third-from-the-right pin is the 3.3V pin (which is connected to an external supply via my Molex Micro-fit connector).

    I also was under the impression that the reason for the VIN-VUSB jumper was so that you could cut it and power the VIN pin from an external supply that had to be above 3.6V because of the drop in the regulator. I am pretty sure that if you apply 3.3V to the VIN pin with the VIN-VUSB jumper cut that the drop in the regulator will make the voltage too low to power the Teensy 4.0/4.1.

    My question was whether it is OK to cut the VIN-VUSB jumper and then apply 3.3V directly to the 3.3V pin (which is what I think I have designed my board to do). I think that the link I gave above confirms that this is OK, as long as you understand that none of the USB circuitry (and any other 5V dependent parts) will be functional unless the USB cable provides that 5V (which should be the case every time you are plugged into a computer for programming and serial monitor).

  6. #6
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    Yes, I think you are correct. You can do either - apply 3.3V to the 3.3V pin and apply V above the dropout voltage of the VReg to Vin. Both work though I only use 5V Vin on my boards which then satisfies the 5V VBUS requirement of the chip. There is some question of how well USB works in that scenario in the thread you linked to, btw.

  7. #7
    Senior Member PaulStoffregen's Avatar
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    Teensy 4.0 and 4.1 are meant to be powered by VUSB or VIN. The IMXRT processor has a complex power sequencing requirement which is only fully met when you power by VUSB or VIN, or when you have a coin cell or other 3V power on VBAT *before* you apple 3.3V power.

    If you try to power only by 3.3V and the board doesn't boot up (whether it starts will depend on the rate of voltage change from 0 to 3.3V - faster is better), the solution is to use a coin cell or other "always on" power on VBAT.

  8. #8
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    I think I will probably make the board mod I mentioned above, where I cut the trace from the micro-fit connector to the 3.3V pin, jumper from the micro-fit connector to the VIN pin, and change my microfit cable supply from a 5V supply instead of 3.3V.

    Just as a follow on question. On the Teensy 4.1 pinout it says that the 3.3V pin can supply 250mA max. Do I need to subtract the 100mA used by the Teensy 4.1 at 600MHz, or does this 250mA already have that 100mA factored in and is completely available for external circuitry?

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