Originally Posted by knc1
I am learning more about that as time passes.
USB Downloader mode is running the $10 SoC part as if it was a (rather dumb) $1 micro-controller.
It can start up the off-chip ram, the off-chip rom, run the serial or usb port ...
That's it folks. (So don't expect it to turn on the battery management or anything else.)
It uses limited ($1 microcontroller) functionality that can fit into the on-chip RAM. But you can send code to it over USB Downloader mode (such as my patched u-boot images) that can enable SDRAM, and can load a full linux kernel (with loadable kernel module drivers on initrd). If the kernel contains eink drivers, then it could also talk to eink. You can essentially bootstrap a full OS over USB Downloader mode if you really want to, without even touching the mmc.