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Power consumption for microSD cards ?

Discussion in 'Electronic Design' started by Rodo, Mar 30, 2007.

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  1. Rodo

    Rodo Guest

    Hi all,

    I'm thinking of using a microSD card to store data in my next project at
    work (Microchip PIC18 or PIC24). I was looking at the specs from a mfr (I
    think it was Sandisk) for the regular SD card and it says that standby power
    was about 250uA. This is good. But the read and write cycle take a max of
    100mA. This is a killer. I'll be running off a 3.6 C cell with about 7Ah
    storage. This sounds like a lot but the device has to work for several years
    on that battery and the 100mA will just kill it. I realize that it won't
    draw 100mA all the time but if I write every minute about 25 bytes to it...
    well... it will kill my battery way sooner than sales/marketing like.

    Does anyone know the real power consuption for writting and reading a
    microSD card ?

    Thanks
     
  2. Guest

     
  3. Bob

    Bob Guest

    How long is a write cycle and how often do you need to repeat it?
    Perhaps a large low leakage capacitor would supply enough current.

    Guessing some numbers. Lets say 100mA for 0.01s with 0.2V voltage
    drop being acceptable.

    i=C(dv/dt)
    works out to 50,000uF
    An electrolytic that big can have 100uA leakage current, though
    it sounds like your device will have power all the time which helps.

    If the write cycle is less than 0.01s or you can get a way with
    a bit more voltage drop you may be able to use a smaller cap.

    I used 0.01s as that is standard for little EEPROM's.
    Flash may well be much shorter.

    If you need multiple writes wihtout time for the battery to charge
    the cap you may need more.

    I havn't looked up the leakage of current supercaps. Last
    time I looked I was amazed to see they available up to thousands
    of farads. I'm told the current kilofarad caps don't contain
    horribly toxic stuff like the early ones.

    Bob
     
  4. linnix

    linnix Guest

    SD card has a controller, probably more powerful than a PIC.
    So, you have to pay for it in power.
    If you don't mind a few more pins (8 to 10),
    you can interface to NAND chip directly.
    They are 10 to 20 mA on average.
     
  5. Donald

    Donald Guest

    There are a bunch of question I have for you.

    But where to start.

    OK

    Lets make some assumptions, for info you did not give.

    An SD card has 512 byte sectors. So writing 25 bytes at a time would be
    writing the same sector 20 times in 20 minutes.

    Most larger micros, let alone systems, have at least 512 bytes of
    battery backed RAM.

    So writting a 512 byte sector every 20 minutes would extend your battery
    by 20 times, by your calculations.

    You would write 3 times an hour, instead of 60 time an hour.

    I hope this help you with your battery analysis.

    donald

    PS: All this goes out the window if you want FAT16/FAT32 compatability !
    But using a battery backed RAM buffer and writing to the SD when
    enough data have been collected will extended your battery life by
    years.

    PPS: Good Luck
     
  6. A write cycle is a few 10us to a few ms for a sector.
    This current is easily supplied by a ceramic 10uF
    capacitor close to the pins. You should do some
    measurements on how the current averages under the
    assumed temporal writing density

    Rene
     
  7. Rodo

    Rodo Guest

    Could you tell me why "All this goes out the window if you want FAT16/FAT32
    compatability !
    " ? I do need the data to be readable by inserting the card into a pc.

    Thanks and thanks to everyone for the replies.
     
  8. Donald

    Donald Guest

    Oh boy, now I've done it.

    Well, this is something you are going to have to learn on your own.

    There are lots of discussions on the web about FAT16/32 directories,
    tables, Boot Sectors, and other things related to "DOS" formats.

    Google will find tons out there, so spend the weekend studing and reading.

    By Monday you will either understand it or you won't.


    Hmmm,

    In a nut shell, for every 512 byte data block you write , you will also
    have to write at least 2 FAT tables, read blocks of the directory
    structure from the begining to find an empty location. Read blocks from
    the FAT table from the begining till you find an empty location. Worst
    case is reading all the directdory/FAT table blocks ( as few as 128
    blocks to 1024 blocks),( reading a block off an SD card is almost as
    much current as writing a block )

    So, please do yourself a favor and truly understand what you are
    getting this project into. And you won't get this done in a months time,
    without really understanding all the little details of FAT file systems.

    Good luck, its not magic, but it is work.

    donald
     
  9. jasen

    jasen Guest

    because then everytime you extend the file you should update the
    directory entry and possibly the FAT too.
    It's OK to write the FAT and directory only immediately before the
    card is removed, you'll have to provide a way to detect that.

    Bye.
    Jasen
     
  10. jasen

    jasen Guest

    no, for every cluster, which can be upto 128 blocks, it's determined when
    the media is formatted.
    you can cache these writes and only write each FAT sector when it's full,
    also, there's no requirement to have two FATs,
    easiest way is to cheat and reformat each new card so that only a subset of
    FAT-FS needs to be implemented.

    Bye.
    Jasen
     
  11. Donald

    Donald Guest

    Thank you Jason for extending my off-the-cuff description.

    There are all kind of tricks to cheat FAT.

    My point and yours is, understand the details of FAT before starting.

    FAT is not as simple as so would think.

    donald

    PS: or google for FFS.
     

  12. Don't you need a license and a health certificate for that job?


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  13. Donald

    Donald Guest

    Depends what you do on your weekends. ;-)

    donald
     

  14. "The Old Engineer's Stud Farm", where we work day and night to
    produce out next generation of Engineers! ;-)


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
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