Tiny DFU Bootloader for AVR DU

• Directly read/write DU's flash, EEPROM and user row from PC using USB
• 1.5 KiB (3 pages) flash footprint, with read-only data stored in boot row

Usage

Flashing bootloader

avrdude -c <programmer> -p avr<flash>du<n_pin> -Ubootsize:w:0x03:m -Ubootrow:w:build/program.btr -Uflash:w:build/program.hex

A UPDI programmer is required to flash the bootloader itself. Following memories have to be programmed:

• FUSE.BOOTSIZE (bootloader size in 512-byte blocks of flash)
• Boot Row (used to store constants including descriptors)
• Flash (bootloader)

Downloading application

Build

• As the bootloader occupies the first three pages of flash, the application must have its start offset by 0x600 bytes. This can be achieved by adding -Wl,--section-start=.text=0x600 to the linker options. By default, the interrupt vector table is placed at the start of the application region, so offsetting the whole .text section works as long as FUSE.BOOTSIZE is properly configured.
• The images has to be in binary format. This can be done with avr-objcopy -I ihex -O binary <input_hex> <output_binary>.

Download

• Bootloader can be entered by pulling PC3 low on reset. The pin is internally pulled up during sampling, and while the bootloader is running.
• DFU interface has three alternate settings each corresponding to flash (0), EEPROM (1) and user row (2). The target partition can be specified using -a <alt_num> option if you are using dfu-util.
• All of these partitions support both writing/downloading -D and reading/uploading -U.
• First 1.5 KiB of flash, occupied by bootloader, does not appear in either downloaded or uploaded image. The binary file starts at 0x600. For the other two memories, the image starts at 0x0 and the entire region is accessible.
• Full command would look like dfu-util -a 0 -D firmware.bin for downloading firmware, or dfu-util -a 1 -U eeprom.bin for uploading EEPROM content.

Notes

• Various checks are skipped assuming the host will behave politely. Hardly compliant to USB/DFU specs. Don't use for anything remotely serious.
• That said, the bootloader was tested on AVR32DU32 and AVR16DU20 with dfu-util and seemed to be okay. Should work on 64 KiB parts too, though untested since I don't have one. As it involves dynamic FLMAP reconfiguration, it has more chance of something going wrong.