As far as I understand; SD card are backwards compatable with MMC cards (open
source), if you use the SPI mode (1-bit mode).
The faster 4-bit mode of SD cards is not supported in MMC.
SD flash requires developers to license their documentation and patents. They
don't allow licensees to create open source drivers for the SD Card. Open
source drivers would reveal the SD Card's specs, and the SD Card Association
make money by licensing these. SD Card also have digital rights management
features that are rarely used. A lot of the secrecy of SD seems to be because
of these features.
It is legal to make an open source SD Card drivers if you do it by reverse
engineering without the spec sheet for the SD card. And it appears that is
what has happend for the SD Card driver included in the Linux 2.6 kernel. As
long as you don't violate any patents this should be fine.
Since it violates the GPL to link anything with the Linux kernel that is not
GPLed, closed sourced drivers must be loaded after the kernel starts. This is
dynamic linking, not static linking so it is OK! But it makes booting a Linux
a little messy.
Now the MMC association has come up with a new standard called MMC Plus. This
standard supports 4-bit, 8-bit and the original 1-bit. It appears the 4-bit
mode is backwards compatible with the 4-bit mode of SD Flash!
So technically we should be able to make a linux driver statically linked into
the kernel that uses the MMC 4-bit mode and they will work with SD Cards.
Now Technologic Systems uses a CPLD to control these cards. Their must be
registers in the CPLD to access the SD Card, if they publish these registers
we can build an SD driver.
If Technologic doesn't publish the CPLD registers we can reverse engineer them
from the boot loader of the TS-7300.
A TS-7260 would not need NAND flash if we booted directly from an SD Card.
Perhaps TS could sell this as an option!
So my question is this:
Will TS provide us with the CPLD SD Flash registers?
SD/MMC Links:
SD Flash PCB for TS-7200 and TS-7250 users:
http://www.sparkfun.com/shop/index.php?itemid=488&cat=1&shop=1&;
SD and MMC Specs:
http://www.sandisk.com/Oem/Manuals/
SD Wikipedia
http://en.wikipedia.org/wiki/Secure_Digital_Card
MMC Association:
http://www.mmca.org/home
MMC open source page
http://mmc.drzeus.cx/wiki/Welcome
MMC/SD adding to a WRT54G router, with driver
http://wiki.openwrt.org/OpenWrtDocs/Customizing#head-00b294c0c885db1d544fbfcd48e9367d20b38b5a
http://www.allaboutjake.com/network/linksys/wrt54g/hack/
MMC/SD PIC 1-Bit simple example
http://www.microchipc.com/sourcecode/#mmc
-Curtis.
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