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Is it possible to power Parallax propeller Protoboard USB from the USB port? — Parallax Forums

Is it possible to power Parallax propeller Protoboard USB from the USB port?

lyassalyassa Posts: 52
edited 2012-01-03 11:10 in Propeller 1
I have an application that requires little power. Basically just interfacing some switches to a computer. I will use the Propeller Protoboard USB for that.

http://www.parallax.com/StoreSearchResults/tabid/768/List/0/SortField/4/ProductID/509/Default.aspx?txtSearch=protoboard+usb

Can I power it from the USB port? Do you have suggestion to a better -smaller and cheaper- board to use? It needs to have enough space to fit about 20 resistors and 20 pin connector. I may need few of them in the near future.

Thanks in advance for your help.

Comments

  • localrogerlocalroger Posts: 3,452
    edited 2011-12-22 12:07
    If you solder a jumper between the USB 5V pin and the 5V regulator output / 3V3 regulator input, it should work. Applying power to the shut-down 5V regulator shouldn't do any harm.

    ON EDIT: After a quick look at the schematic and manual it appears that your biggest problem will be getting access to the USB 5V since it's not brought out to a convenient point and all the USB circuitry is SMT; you'll need to grab it at the connector itself, which is going to be kind of tricky, or from one end of R21 or C6. One of those is probably most practical.
  • S11D336BS11D336B Posts: 14
    edited 2011-12-22 14:03
    If you get one of these
    http://www.parallax.com/Store/Microcontrollers/PropellerDevelopmentBoards/tabid/514/CategoryID/73/List/0/SortField/0/Level/a/ProductID/748/Default.aspx
    it powers itself from USB by design.
    lyassa wrote: »
    I have an application that requires little power. Basically just interfacing some switches to a computer. I will use the Propeller Protoboard USB for that.

    http://www.parallax.com/StoreSearchResults/tabid/768/List/0/SortField/4/ProductID/509/Default.aspx?txtSearch=protoboard+usb

    Can I power it from the USB port? Do you have suggestion to a better -smaller and cheaper- board to use? It needs to have enough space to fit about 20 resistors and 20 pin connector. I may need few of them in the near future.

    Thanks in advance for your help.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-12-22 15:22
    Typically, just jumpering the USB's +5V supply to power a circuit is ill-advised, since the speciifed power-up sequencing and current-limiting are being ignored. If you do it anyway, though, be sure to remove the jumper that connects +5V to the servo headers. The reason is that this also connects a 1000uF cap to +5V. The inrush current required from the +5V supply to charge that cap will far exceed the USB's maximum limits. In any event, I would recommend going through a powered USB hub, rather than directly from your PC, in order to protect the PC's USB circuitry.

    -Phil
  • pedwardpedward Posts: 1,642
    edited 2011-12-22 15:30
    You can tap 3.3v off of the non-ground side of C8, since this is the 3.3v regulated output from the USB chip. I don't know off hand how much current the 3.3v output is good for.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-12-22 15:49
    pedward wrote:
    I don't know off hand how much current the 3.3v output is good for.
    'Probably something you'd want to check before recommending it. No, that output does not supply enough current to power the rest of the board.

    -Phil
  • Prophead100Prophead100 Posts: 192
    edited 2011-12-22 17:40
    Perhaps your design could be squeezed onto a QuickStart board which can run off USB. You may need to use a piggyback board but the QuickStart has a nice header and compact size.
  • lyassalyassa Posts: 52
    edited 2011-12-23 08:12
    Thanks for all the answers, quite insightful. I will steer away from powering the protoboard from USB. It seemed a good idea at first, but Phil's warning about the 1000uF capacitor very likely saved my computer usb port. The QuickStart board now seems to be the better way to go.

    I wish a board exists that is similar to the QuickStart board, but without the switches and LEDs.
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2011-12-23 08:34
    Before I read the USB specs about restricting the capacitance on the board to 10uF, I had/have been powering up a home made AVR dev board that has about 1000uF on it in various places. No harm has befallen any computer that I have used and that has included a couple of laptops a few main MBs and an unpowered hub.

    So in my blissful ignorance ....
  • localrogerlocalroger Posts: 3,452
    edited 2011-12-23 08:47
    Inrush current is very unlikely to damage a USB port; they are designed to limit current when more is drawn than the port design limit. What the capacitor might do is mess up the boot process as the voltage would be rising slowly as the capacitor is charged, or it might cause the USB port to turn its power output completely off.
  • localrogerlocalroger Posts: 3,452
    edited 2011-12-23 08:56
    Actually, the FTDI datasheet says the FTDI chip can supply 50 mA of external 3v3, which would be adequate for a lot of propeller applications. It would still probably be better to hook the USB 5V. Considering the existence of such USB accessories as lights and fans, I really think the risk of damaging a port with this mod would be minimal.
  • David CarrierDavid Carrier Posts: 294
    edited 2012-01-03 11:10
    lyassa,
    If you do not need the LEDs or buttons on the QuickStart, you can use their I/O pins for any other purpose. The LEDs are buffered and reflect the state of their respective I/O pins, but have a negligible effect on the I/Os themselves. The buttons similarly have an almost immeasurable effect. If you absolutely have to disable the LEDs to keep them from blinking while you are using the I/O pins, you can cut the trace that powers U5, the 74HC541 buffer on the right of the QuickStart board. The trace runs between the 'c' and the 'k' in the QuickStart logo on the right of the board.

    — David Carrier
    Parallax Inc.
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