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MMC Specification now available for free

Wow, it’s been a long time since I’ve written anything, but I figure this is a good enough reason to do so. The SD specification has been available for quite a while now, but up until now, the MMC specification has had a $5000 price tag stuck to it. This has kept it out of most people’s hands, although Nokia were kind enough to buy a copy for Pierre. However, they have now made the latest version (4.3) available without charge. I believe this change of heart stems from the standardisation of eMMC through JEDEC (They still charge for their other specs). Despite the focus on eMMC, it is the full specification (minus the section on using MMC over SPI which they have declared obsolete; this isn’t a big deal as there’s enough documentation around explaining how MMC over SPI works).

What does this mean? Not that much in terms of Linux kernel support for MMC; Pierre already used his copy of the 4.2 spec to fix any problems and 4.3 doesn’t really add anything new that affects us. eMMC (Embedded MMC) is mostly just a new form-factor and doesn’t appear differently from a regular card. The one substantive new feature is the introduction of a special ‘boot partition’ which are accessed in a simplified way – presumably this was added to make it easy for a bootloader to load an OS off an eMMC. I don’t really see much demand emerging for this, so we have no immediate plans to support it (and good luck finding a card with a boot partition!) but if the need arose, we’d be able to do it.

So, nothing’s changed in practical terms, but it’s still helpful for us that the spec is now freely available; however, it probably won’t do much to help MMC against the SD juggernaut. Anyone seen a high capacity MMC card despite them being ‘available’ for over two years now? Thought so.

{ 8 } Comments

  1. Ji-hun Kwak | 11th August 2008 at 00:57 | Permalink

    I would like to get MMC specification.

  2. Philip Langdale | 11th August 2008 at 06:50 | Permalink

    Just click on the link in the article…

  3. John Gilmore | 12th August 2008 at 14:17 | Permalink

    They released the spec “for free” but not as in freedom. You have to agree to a contract to get it, and the contract says you can’t share it with anyone else. What is the point here, guys? Either the spec is public or it ain’t, why are they trying to sit on the fence and “control it”???!

  4. Philip Langdale | 12th August 2008 at 21:08 | Permalink

    I don’t really get it – the SD spec is the same. They’ll give a copy to anyone who asks but they don’t want anyone to pass it on. I think old habits die hard.

  5. gds | 20th January 2009 at 15:34 | Permalink

    Been trying to find a “high capacity” mmc (or mmcPlus) and only found a Transcend 4GB mmcPlus card. The JEDEC spec says that if the device is >2GB it is “high capacity” and must be block addressed. Just wondering if you know if the Transcend device, ,
    is really HC?

  6. gds | 20th January 2009 at 16:09 | Permalink

    Looking at the eMMC spec 4.3, does it mean the devices don’t support SPI? (if there really are any eMMC devices). Why is it “obsolete”?

  7. Philip Langdale | 20th January 2009 at 17:40 | Permalink

    No, that’s an out-of-spec 4GB card. They can do it by switching to FAT32 and changing nothing else. The spec is conservative and doesn’t allow it, but it works in practice.

    As for eMMC, it’s hard to say what would happen in practice – the removal of SPI means that there’s no requirement to support it for conformance, but would manufacturers remove it? From actual cards, probably not, but eMMC is obviously fixed so the environment it works in is controlled, so the answer there is probably yes.

    The simple truth is that MMC is on its way out, so most of this is moot.

  8. slgi | 4th June 2010 at 03:17 | Permalink

    I would like to get MMC specification.
    Sorry! Wrong E-mail Address
    So Recomented!