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Micro SD power consumption

Discussion in 'Embedded' started by Mark Borgerson, May 23, 2009.

  1. I'm working on a low-power logger to continuously log
    data at fairly low rates (2-3KB/s) for months. I've successfully
    used SD and SD micro cards at lower rates, but I'm seeing
    a power dissipation problem with the KB rates. When the
    card is really idle, the current draw is well under 1mA.
    During and after writes it is about 20mA. (With a 2G
    Transcend micro SD).

    The simple approach is to buffer up a sector (512 bytes) and
    write it as needed. However, when I monitor the power,
    I find that it takes only a few milliseconds to write the
    data, but the power dissipation in the SD card stays high
    for ~250mS after the end of data transfer. This 250mS is,
    suspiciously, the same as the write timeout interval
    for SD cards. My working hypothesis is that the card
    is keeping the internal charge pump and other electronics
    turned on until the end of this interval.

    Is there a way to command the card to shut down the power
    hogs ASAP? I'm using the normal Idle command without
    result (or at least a long-delayed result).

    I've gotten the average current under 5mA by buffering
    up about 16KB before writing---but that requires an
    expensive SPI FRAM buffer to supplement my MSP430
    controller. It's either that or switch to an ARM
    variant with 32KB or more of RAM---and face a whole
    different set of power and cost issues.

    Mark Borgerson
    Mark Borgerson, May 23, 2009
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  2. Mark Borgerson

    -jg Guest

    Cheaper than a FRAM, might be direct power removal aka a 1.5c
    transistor :)

    If you know when the device has finished, then simply 'throw the big
    switch', and you know the current
    you will get then! ;)

    -jg, May 23, 2009
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  3. Mark Borgerson

    whygee Guest

    that is a practical possibility but success depends on
    more than that :
    - how can one be sure when to cut the power ?
    - if SD init takes 250ms then it's not worth it.
    - these parameters will change from model to model
    the only way to know is to try :-/
    whygee, May 23, 2009
  4. There is some uncertainty about when the device is finished.

    The other problem is that a cold start on and SD can require
    up to a second. The process for waking up the card and
    converting it to SPI mode is something that can take
    quite a while.

    Mark Borgerson
    Mark Borgerson, May 24, 2009
  5. Mark Borgerson


    Nov 15, 2009
    Likes Received:
    A possiblity exists in selecting a brand of card that supports an automatic low power mode

    The following from San Disk tells of an auto sleep feature.

    Quote "Automatic Sleep Mode
    A unique feature of cards in the SanDisk SD Card Product Family is automatic entrance and exit from sleep mode. Upon completion of an operation, cards enter sleep mode to conserve power if no further commands are received in less than 5 milliseconds (ms). The host does not have to take any action for this to occur. However, in order to achieve the lowest sleep current, the host needs to shut down its clock to the card. In most systems, cards are in sleep mode except when accessed by the host, thus conserving power.When the host is ready to access a card in sleep mode, any command issued to it will cause it to exit sleep, and respond."

    Hope this helps
    Moa, Nov 15, 2009
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