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USB power and debug signals on micro USB connector

Discussion in 'Embedded' started by Stef, Dec 20, 2012.

  1. Stef

    Stef Guest

    (you may also find this post on comp.arch.fpga, where I posted by
    accident. I'll try to cancel it there.)

    For a handheld device, powered by a Li-ion cell, I need a charge power
    input. Most standard chips for charging and powering the device are
    meant for USB power or simular. Since the current standard charging
    connector for mobile phones is a micro USB connector, it seems best to
    use that. It means you can use a lot of standard chargers. Agreed?

    For the device, I also need a serial debug connection. It is OK to have
    the 3V3 UART connection on an internal header. This means the device
    must be opened for debugging and maybe a slot must be made in the
    enclosure for longer test sessions with enclosure.

    But is it possible and/or advisable to use the spare pins on the micro
    USB connector to bring out the 3V3 UART signals? It would save a header
    and make debugging in the enclosure a lot simpler. But it must not
    lead to damage to the device and or (windows/linux/mac) PC when the
    device is plugged in to a PC. There must not be any strange behaviour on
    the PC and preferably there must not be any detection by the OS of USB

    You could ofcourse use a real USB/serial connection, but that would
    mean adding an FTDI chip or a big software effort on the DSP (which has
    USB hardware). It would also mean every PC user has acces to the debug
    channel, I'm not sure we want that.

    So putting the 3V3 UART signals (TX/RX only) on the micro USB and use
    a special breakout box (possibly with FTDI chip) for debugging seems the
    most practical solution for now. Any arguments against it? Any
    experiences with such a setup? And which signal on which pin for least
    chance on damage and strange behaviour? I know some USB chargers have
    shorts or resistors on the datalines, so at least a few protection
    resistors on the device side are required.
    Stef, Dec 20, 2012
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  2. Stef

    me Guest

    You can always add a switch to enable debug. It could be as simple as a pin. Just insert the pin to connect one of the USB & Serial line. I don't think its too much to ask for having a pin to debug it.
    me, Dec 20, 2012
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  3. Stef

    Tim Wescott Guest

    Put a header on the board that provides power, USB signals, and your
    micro's serial pins. Then build a little mezzanine card with an FTDI
    chip that makes it all go when it's plugged in.

    Without the mezzanine card, the USB port is power-only. With the
    mezzanine card, you've got your USB-serial.

    My liberal friends think I'm a conservative kook.
    My conservative friends think I'm a liberal kook.
    Why am I not happy that they have found common ground?

    Tim Wescott, Communications, Control, Circuits & Software
    Tim Wescott, Dec 20, 2012
  4. Stef

    Stef Guest

    In comp.arch.embedded,
    That is a practical and risk free approach. And I can combine it with
    Linnix's switch option. Put a small dual row header with the signals
    on the board. If I plug a card with FTDI chip on, I have direct USB/
    serial out. If I put jumpers on it, I get the raw serial lines on
    the USB connector pins. And if I need that to be a bit fancier, I
    can put the pin-switch across the header instead of the jumpers.

    No thoughts about connecting the serial lines to the USB connector directly,
    always, in a safe manner? Would save me the above header.

    Anyone ever seen 1.27mm shorts (jumpers)? Smallest I can find are the 2mm
    ones. There are plenty of 1.27mm dual row headers, but so far I did not
    find the matching jumpers.
    Stef, Dec 21, 2012
  5. Simon Clubley, Dec 21, 2012
  6. Stef

    Stef Guest

    In comp.arch.embedded,
    Yes, that is exactly what I meant. Thanks for the link!

    Had been looking for them a while ago and did not find any. Strange,
    because Farnell is usually the first place I start looking for such parts.
    The have those in black/red/blue and with/without handle. Must have missed
    them somehow, or are they new?
    Stef, Dec 21, 2012
  7. You are welcome.

    I don't know if they are new because I don't use them; the smallest size
    I use is 2.54mm. I just did a search at Farnell for "1.27mm pitch jumper"
    and picked one result at random out of the 6 returned. However, since you
    have obviously done the same, I am assuming they are indeed new.

    Simon Clubley, Dec 21, 2012
  8. Stef

    tasbihmr Guest

    I think these jumpers which you have picked up are very well designe
    indeed. I have been looking for such jumpers for a while. What I used to d
    instead is take 1.27 mm female type sockets, cut them in pairs to look lik
    jumpers, then cut the pins from the back, and then solder the pair togethe
    to get a very small jumper for the 1.27 mm male type as the result.
    I hope this helps.
    tasbihmr, Dec 21, 2012
  9. the same part (black with handle) is available here from Digikey at
    $0.34 qty 1. Unless I'm off on my conversion rates, that's
    significantly less than the 0.32 pounds on the Farnell site.

    Digikey lists them as 1.27mm shunts. They must be fairly new as the
    quantity prices don't drop nearly quickly as those for 2mm shunts.
    You can get 2mm shunts at 100 for $7.43. A hundred of the
    1.27mm is $22.30.

    Mark Borgerson
    Mark Borgerson, Dec 21, 2012
  10. The current exchange rate is roughly about 1.60 USD to 1 UKP so yes the
    Digikey price is cheaper.
    I don't recall ever seeing them been called shunts before, but only

    Is this another US/UK divided by a common language thing ? :)

    Simon Clubley, Dec 22, 2012
  11. Could be. However, the Digikey search engine finds them with either
    descriptive word, so I think the term 'jumper' is widely used
    here also.

    Mark Borgerson
    Mark Borgerson, Dec 23, 2012
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