for Cynthion and similar SAMD11 and SAMD21 boards

Based on opendime/DAFU/ and t2-firmware/boot/.

Compatible with DFU Utils and pyfwup.

Saturn-V is the Device Firmware Upgrade (DFU) bootloader for Cynthion. It is used to load Apollo firmware over USB onto Cynthion's on-board debugger. Alternatively it may be used as a bootloader for other devices featuring a SAMD11 or SAMD21 microcontroller.

Saturn-V uses only 2 KiB of flash memory, leaving plenty of space for application firmware. Space optimization in Saturn-V was achieved using some of the tricks in SAMDx1-USB-DFU-Bootloader which is even smaller at 1 KiB but lacks features such as Microsoft-compatible descriptors.

To invoke Saturn-V on Cynthion, hold down the PROGRAM button while connecting power or while pressing and releasing the RESET button. LED C will blink, indicating that Saturn-V is running. In this mode you can update Apollo with Apollo's make dfu command or use dfu-util or another DFU utility.

Normally Saturn-V needs to be installed only once on a newly assembled board.

To compile for the latest Cynthion hardware revision, type:

$ cd saturn-v
$ make

Alternatively, use variables to specify the board and/or hardware revision:

$ cd saturn-v
$ make BOARD=cynthion BOARD_REVISION_MAJOR=1 BOARD_REVISION_MINOR=3

If you're not using the arm-none-eabi- toolchain, you'll need to specify your compiler prefix using the CROSS variable.

Once the bootloader has been built, use an SWD programmer to load the bootloader.elf file, or program bootloader.bin to the start of flash (address 0x00000000). If you're using the Black Magic Probe, this might look like:

$ arm-none-eabi-gdb -nx --batch \
    -ex 'target extended-remote /dev/ttyACM0' \
    -ex 'monitor swdp_scan' \
    -ex 'attach 1' \
    -ex 'monitor unlock_bootprot' \
    -ex 'load' \
    -ex 'monitor lock_bootprot 4' \
    -ex 'kill' \
    bootloader.elf