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Thread: Does the Teensy 3.2 gets from the computer the 250mA is it said to provide?

  1. #1

    Does the Teensy 3.2 gets from the computer the 250mA is it said to provide?

    Hello,

    I have just switched from a Teensy 3.1 to a 3.2 because I suspect the former cannot provide enough current to power 2 Sensel Morphs. The idea is to use the Teensy as a MIDI device concentrating the data from the Morphs connected over two serial ports.

    The Teensy 3.2 pinout diagram indicates that the 3.3V output can provide up to 250mA. As the developer's cable from Sensel needs 5V, I take the current from the USBV pin instead but I suppose I can get as much current there.

    However, I've run USBDeview on Windows to check how much current the Teensy is allowed to get, this software reports a value of 100mA. Is this indication from USBDeview not reliable? Or should I do something in order to provide the claimed 250mA to the Teensy?

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    Thank you in advance.

  2. #2
    Senior Member PaulStoffregen's Avatar
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    If you want to change this number, it's possible by editing usb_desc.c. To be technically correct, it should be edited when you connect circuitry to Teensy that will use more current. Also to be technically correct, that extra circuity isn't supposed to start using that current until after the USB host as completed enumeration. Or at least not until the set config step.

    But as a practical matter, those USB current numbers aren't very useful. USB hosts don't actually do anything with them, other than refuse to set the configuration when a high power device is connected to a port on an unpowered hub. The power delivery isn't changed in any way. Editing this number as no practical effect, other than the OS will warn you and disallow use with unpowered hubs... which might have worked anyway, if you're not using too much and the other ports are unused or lightly loaded.

    If you've concerned about following the USB spec to the letter, which is important for making a commercial product that you wish to have use the official USB trademarked logo (most products today don't bother), then this becomes important. If you use a lot of current and you sell to mass market customers, it can help avoid customer tech support & returns, because they'll understand the problem is their unpowered hub rather than your product. Otherwise, it's one of those little details of USB that you're technically supposed to follow, but in practice isn't very important and many devices just ignore it.

  3. #3
    Quote Originally Posted by PaulStoffregen View Post
    If you want to change this number, it's possible by editing usb_desc.c. To be technically correct, it should be edited when you connect circuitry to Teensy that will use more current. Also to be technically correct, that extra circuity isn't supposed to start using that current until after the USB host as completed enumeration. Or at least not until the set config step.

    But as a practical matter, those USB current numbers aren't very useful. USB hosts don't actually do anything with them, other than refuse to set the configuration when a high power device is connected to a port on an unpowered hub. The power delivery isn't changed in any way. Editing this number as no practical effect, other than the OS will warn you and disallow use with unpowered hubs... which might have worked anyway, if you're not using too much and the other ports are unused or lightly loaded.
    Thank you Paul for having quickly answered but I don’t get it. Do you mean that actually changing the number is probably useless? So what should I do to get 250mA?

  4. #4
    Senior Member+ Frank B's Avatar
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    Quote Originally Posted by amundsen View Post
    Thank you Paul for having quickly answered but I donít get it. Do you mean that actually changing the number is probably useless? So what should I do to get 250mA?
    Yes, it Is useless - you need to do nothing.

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