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Im am already using succesfully MMC cards in an embedded device. I like to
use SD cards too (bigger memory). I do not need any fancy functions, just
the MMC compatible SPI mode. Unfortunately I can't find enough information
on the SD card command set.
 How can I initialise the SD card to work is the MMC compatible mode? What
ACMD41 exactly is?

Please point me to the right direction...

Re: SD vs MMC
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The SDCard organization controls the specifications for SD cards.
There is some information on their website ( that may
help you but I would not count on it without being a member.  To sell
a device that supports an SD card you have to pay this group a yearly
fee and enter into a license agreement with them.  Membership fees are
on the website as well and this will give you access to the full

If you are supporting MMC SPI mode, SD will require only a connector
change to handle the slightly taller card size from a hardware
standpoint.  From a software standpoint I don't know all the
differences.  I purchased the SDDK-05 development kit from SanDisk to
handle the SPI driver and FAT16 file system.

Agfa NDT Inc.

Re: SD vs MMC
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AFAIK this is true only if you actually use the security features on the
disk. You don't need an agreement to use the SD disk in the same way as you
use an MMC disk.

Your SD disk supplier should be able to give the SD disk spec to you - minus
the documentation of the security features.

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