I'm in a need for the higher transfer speeds with SD card, I'm running the card at 25 MHz via SPI and it works well, but I still need higher transfer speed.
I would like to use the 4-bit (parallel) mode of the communication with the SD card, and I know that to do that one has to be the member of some SD association and that only SPI mode is free... But since this is just a hobby project for myself, I wouldn't mind breaking a small copyright for my project or experiments.
So what is the problem? Is the 4-bit mode documented on the web, but you can't use it legaly without paying the SD association, or there is no public documentation and you need to pay a fee to get that documentation?