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arunb



Joined: 08 Sep 2003
Posts: 492
Location: India

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RE:
PostPosted: Fri May 30, 2008 10:36 pm     Reply with quote
Hi,

Why would there be problems in >2 GB card ??

thanks
arunb
Ttelmah
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PostPosted: Sat May 31, 2008 3:22 am     Reply with quote
File system.
FAT32 needed, instead of FAT16 (Technically, FAT16, can support up to 4GB, treating one value as 'unsigned', that defaults to signed, but only a few OS's - NT for example, supported this), so once you go over 2GB, you need to add FAT32 support. This is made harder, because technically, MS, have never 'released' this into the public domain - you really need to get a license from them to access FAT32!, and also there are quite a few minor variations in the versions used by different OS's.... :-(

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ckielstra



Joined: 18 Mar 2004
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Location: The Netherlands

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PostPosted: Sat May 31, 2008 6:32 am     Reply with quote
This thread is a continuation of Modifying the MMC drivers for a 1 GB MMC card ??

For MMC cards FAT16 was the prescribed file system and as mentioned by Ttelmah the 2Gb then becomes a barrier.

Besides the FAT16 issue there are some internal modifications in the card to support 2Gb+ capacities. Here a quote from the MMC standards document:
Quote:
6.1 Higher than a density of 2GB
The maximum density possible to be implemented according to the versions up to v4.1 of this specification
was limited in practise to 2GB. This was due to the following reasons:
• • Existed 32bit byte-address argument in the command frame (max 4GB could be addressed)
• • Existed formula according to which to calculate the density of a card (max 4GB could be indicated)
• • Capability of the FAT16 File System to address up to 2GB of address space per one partition
The lowest common nominator, 2GB in this case, will set the limit. The implementation of a higher than
2GB of density of memory will not be backwards compatible with the lower densities. First of all the
address argument for higher than 2GB of density of memory is changed to be sector address (512B sectors)
instead of byte address. Secondly the density of the card is read from the EXT_CSD register instead of
CSD register. And finally the system implementation needs to include a File System capable of handling
sector type of addresses.
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