I've made a self documenting project (C source code) for reading/writing
on MMC/SD cards in spi mode and with a hello world LED as output.
The code works with many MMC/SD cards but not all; it is beta.
The reasons are that some waiting times are not limited in the
standard and that the cards are a little outside the standard
For a release version, which does not cost much and which has a mapping layer for reading/writing 1..512 Bytes starting at any address, you can contact me.
I published the code under a beerware license:
This driver is without an explicit file system; it's for low level access. I'm using it for storing data in a file which fills the whole card (usually behind the FAT) and has an identification at the file beginning. The MSP simple writes/updates his data structure (one relative address for the begin of data, one address for the end of data and data, maybe more) behind the identification. This is the SDS (simple data storage) format, embedded in a file. Here are two pictures of two cardreaders i made with this software and SDS:
In october 2005 Ti released an application note with a revised edition of the driver, because it was done as a cooperation with Ti: