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SanDisk Ultra II SD plus USB

Discussion in 'Embedded' started by Mark Borgerson, Dec 1, 2007.

  1. I just bought a SanDisk 1GB Ultra II SD + USB at Staples for about
    $27.


    I have several projects that use SD cards in SPI mode for data storage.
    This SD card seems to work fine after a limited amount of testing. My
    logger can read and write to the card in SPI mode. The files it writes
    can be read when the card is plugged into a USB port on my PC.

    Here's the question:

    Can I run 4 wires from the USB port on this card and get an almost free
    High-speed USB port for my data logger? Could it be as simple as not
    trying to write via the SPI port while the USB port is connected? Heck,
    I'd even be willing to tri-state the SPI data lines and clock if
    necessary. The logger uses an AT91Sam7S256, and I can tri-state the SPI
    port lines if necessary, but I can't now disconnect the +3.3V to the SDI
    card socket.

    I sent this question to SanDisk support---but I don't know if
    they work weekends. ;-)

    I nobody has any clues for me before tomorrow afternoon, the soldering
    iron comes out. I'll let you know what happens.


    Mark Borgerson
     
    Mark Borgerson, Dec 1, 2007
    #1
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  2. I got an email back from SanDisk on this one. No surprise, they won't
    offer any help because it is not a designed application for the SD
    Ultra II Plus cards.


    Here's what I've discovered:

    I soldered 4 #32 wires to the pads on the SD + card. I found that
    the gold facing on the USB connectors comes loose very easily at
    solder temperatures, but I was able to solder to the pad underneath.

    I connected the USB wires to a nearby USB-mini connector at the pads
    where I removed the 10-ohm resistors that used to connect the
    USB connector to the AT91Sam7S.


    A. With the USB cable connected, and the logger powered down, the
    PC could read and write files on the SD card. I have no
    data on the read or write speeds at this point.

    B. With the USB cable disconnected, the logger could read and write
    the SD card in SPI mode.


    C. With the USB cable connected, and the logger power off, the
    V+ line on the SD card interface was pulled up to about 3.5V. This
    is a bit higher than the normal 3.3V on the power supply. The
    logger 'sort of' started up, but did not respond to commands.
    I think a Schottky diode on the power supply might solve part of
    the problem, but I don't know if there were other leakage problems
    through the SPI clock and data lines.

    D. With logger power on and the usb cable connected, the logger
    seemed to work OK, but it could not read and write the SD card.


    My initial conclusion is that these cards offer some promise for
    an easy high-speed connection to the data storage part of a
    data logger. However, I will need further experiments to see
    if it is practical and cost-effective. I suppose that my first
    step would be to interpose 5V-tolerant buffers between the SD
    card and the logger and some way to disconnect the 3.3V power supply
    from the SD card.


    Mark Borgerson
     
    Mark Borgerson, Dec 3, 2007
    #2
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