Hi ADI,

The SD cards are apparently an upgrade to the MMC cards. They support an SPI mode. In this mode, they are very easy to interface to an MCU. 3 wire + chip select. In the SPI mode, you can transfer data at up to 20Mb/sec. To answer your question though... The SPI mode is documented somewhat and seems to be quite simple. The minimum would be to initialize the card 1) set block size (512 bytes) 2) disable crc16 (if you don't want crc), then you could just implement read and write block functions. Basically, you can transfer 512 byte blocks with a sector address. This makes them look pretty much just like a hard drive in lba mode. If you want to use all the fancy encryption stuff, you have to buy the documentation. I guess it is quite expensive. I also noticed last night that they are up to 2GB now! It seems almost too good to be true.... An extrememly high speed, easy-to-interface, 128MB for your microcontroller project for about $20
Here is THE reference:
http://www.sandisk.com/pdf/oem/manual-rs-mmcv1.0.pdf