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Running an SX on 3.3v for MMC/SD integration... — Parallax Forums

Running an SX on 3.3v for MMC/SD integration...

delboydelboy Posts: 6
edited 2005-04-14 16:26 in General Discussion
Hi guys - quick noob question for you,

I'm researching how to integrate an MMC / SD or even CompactFlash card with my SX28. From everything I've read about MMC/SD (from links in these forums) I'll have to power both the storage card itself at 3.3v and also communicate with it at 3.3v. I also seem to remember reading that if I drop the SX voltage to 3.3v that the SX-Key won't work with the SX chip (i.e. programming and debugging would be impossible).

I realize similar questions have been asked in the past, but is there an easy way to work with the SX28 / SX-Key at a lower voltage? Alternatively are there any MMC / SD / CF storage devices out there that can interoperate with the SX at it's "native" 5v? I'd really like to get "stuck-in" to the programming side of integrating an MMC but am not sure how best to get going on the hardware side.

Thanks for any tips or pointers!

D.

Comments

  • JonbJonb Posts: 146
    edited 2005-04-13 21:11
    Create a custom connector and feed 5v into the SX-key.

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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-04-13 21:23
    ·· Jonb has a point...On the Robolympics Medals we designed, they run from 4.5 Volts, so to program them a separate power supply is used.· Now with that supply it powers the SX-Key and the Medal, however, we used to have device called the SX-KeyRing which only powered the SX-Key, and that type of adapter may work for you.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • James NewtonJames Newton Posts: 329
    edited 2005-04-13 22:13
    As usual, the detailed answer at sxlist.com:

    http://www.sxlist.com/techref/parallax/sxkey.htm

    William K Borsum of Dascor says:
    The SX-Key must have five volts input.

    There is a DC-DC on the board but they couldn't/wouldn't tell me specifically what it was for.

    The outputs from the SX-Key (OSC1, 2) must NOT be clamped to anything. Apparently there is a higher voltage required on these pins to tell the target chip that it is in SX-Key/debug/program mode.

    The higher voltage on OSC1 and OSC2 will NOT appear on any other pins of the target chip.

    The five volt supply on the SX-key can be different from the target chip--i.e. the SXkey can run from a five volt supply (single 7805, caps and wall wart= SX-Key-Ring) while the target system runs from its own supply. Only common pins are GND, OSC1, OSC2. The power rails are NOT connected if a different voltage.

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    ---
    James Newton, Host of SXList.com
    james@sxlist.com 1-619-652-0593 fax:1-208-279-8767
    SX FAQ / Code / Tutorials / Documentation:
    http://www.sxlist.com Pick faster!



  • David BDavid B Posts: 592
    edited 2005-04-13 22:15
    I've gotten an SX to work with MMC and SD cards. I never tried CompactFlash.

    You're right - the Sx could work from 3.3V, but then you couldn't program it with the SX-Key. What I did was to make a separate 3.3V supply with zener diodes to power the memory card.

    My Sx drove 3 discrete transistors to drive the card inputs, where the transistor collector resistor went to the 3.3V supply, the collector went to the card, and the Sx drove the base resistor. I sent the card output directly to a 74HCT14 Schmidt buffer; the card output was sufficient to trigger the Schmidt which fed the Sx.

    So my hardware was just about as cheap and simple as possible but still worked.

    David
  • delboydelboy Posts: 6
    edited 2005-04-13 22:50
    Thanks for the great feedback guys!

    Chris - you mention the SX-Key-Ring used to be available - does that mean it's no longer available to order? Do you still have any information on it? I think from what James is saying the Key-Ring is basically just 7805 regulator and a couple of capacitors?

    DavidB - sounds like your approach might be simpler - from what I understand (and I'm very newbie here!) if I power the MMC seperately I should be fine (using the transistors you mention for input and a Schmidt buffer for the MMC output) - how would that kind of approach work in "production"? Once I've developed my application, would I simply drop the entire circuit down to 3.3v (and remove the transistors and Schmidt buffer) or would I require seperate batteries for the MMC's 3.3v power versus the SX?

    Thanks again guys,

    D.
  • David BDavid B Posts: 592
    edited 2005-04-13 23:12
    I've never tried what you're suggesting, but I don't see any reason your approach wouldn't work. Except for the voltage levels, the logic transition needs are the same. Same for power - I don't see any reason you couldn't use the same supply, as long as the voltage and current requirements of both devices are satisfied.

    A couple of the other posts suggest you could run the SX at a separate voltage from the SX-Key. Maybe you could drive the cards directly from the Sx, with both running from 3.3V, and power the SX-Key to 5V separately. I've never tried that, but that might be a simpler way to get your project started.
  • delboydelboy Posts: 6
    edited 2005-04-13 23:46
    Thanks David - when you mention in your first post that you made "a separate 3.3V supply with zener diodes to power the memory card" - does that mean you fed your MMC power from the regular SX 5v via the diodes? If you have time can you explain how that would work? Is that a better method than using something like a LP2980AIM5 3.3v regulator to power the MMC from the regular SX board 5v supply?

    D.
  • David BDavid B Posts: 592
    edited 2005-04-14 00:06
    No, I would have used a 3.3 volt regulator if I had one. I have a retired ham radio friend who gave me a whole bunch of parts he wasn't using, including hundreds of zeners of various values. So for me, it was free to string together zeners until I got the voltage I wanted with enough current. For anyone else, I'd recommend using a regulator!
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-04-14 07:49
    Delboy, it isn't nessesary to run the SX @ 3.3V to interface with SD/MMC, the output high is read as a high to an SX running at 5V (set pins to CMOS), and use a voltage divider for signals going from the SX to the card. Here is a schematic detailing it: http://www.captain.at/electronics/pic-mmc/
  • delboydelboy Posts: 6
    edited 2005-04-14 15:48
    Paul - superb - that's exactly what I was looking for!

    From what I can tell in that link, all I need is a 3.3v regulator to power the MMC and a couple of resistors for SX -> MMC. This is a really newb question, but can I "feed' the 3.3v regulator and the SX from the same 5v source on the SX board (or do I need a seperate wall-wart?)?

    Thanks for everyone's kind assistance!!

    D.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-04-14 15:55
    delboy said...(trimmed)
    Chris - you mention the SX-Key-Ring used to be available - does that mean it's no longer available to order? Do you still have any information on it? I think from what James is saying the Key-Ring is basically just 7805 regulator and a couple of capacitors?
    Correct, they're no longer available.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-04-14 16:26
    delboy said...
    Paul - superb - that's exactly what I was looking for!

    From what I can tell in that link, all I need is a 3.3v regulator to power the MMC and a couple of resistors for SX -> MMC. This is a really newb question, but can I "feed' the 3.3v regulator and the SX from the same 5v source on the SX board (or do I need a seperate wall-wart?)?

    Thanks for everyone's kind assistance!!

    D.
    Yes you need a 3.3V regulator, and if the current·consumed by the 3.3V regulator plus the·current of all devices running at 5V is less than the maximum·current·which can be handled by the 5V regulator then you can·use your 5V to supply the 3.3V regulator.·
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