This repository contains a bootloader, derived from Atmel's SAM-BA, which in addition to the USB CDC (serial) protocol, also supports the USB MSC (mass storage).
UF2 (USB Flashing Format) is a name of a file format, developed by Microsoft, that is particularly suitable for flashing devices over MSC devices. The file consists of 512 byte blocks, each of which is self-contained and independent of others.
Each 512 byte block consist of (see
uf2format.h for details):
- magic numbers at the beginning and at the end
- address where the data should be flashed
- size of data
- data (up to 476 bytes; for SAMD it's 256 bytes so it's easy to flash in one go)
Thus, it's really easy for the microcontroller to recognize a block of a UF2 file is written and immediately write it to flash.
- UF2 specification repo: https://github.com/Microsoft/uf2
- #DeskOfLadyada UF24U ! LIVE @adafruit #adafruit #programming
- USB CDC (Serial emulation) monitor mode compatible with Arduino (including XYZ commands) and BOSSA flashing tool
- USB MSC interface for writing UF2 files
- reading of the contests of the flash as an UF2 file via USB MSC
- UART Serial (real serial wire) monitor mode (typically disabled due to space constraints)
- In-memory logging for debugging - use the
logstarget to extract the logs using
- double-tap reset to stay in the bootloader mode
- automatic reset after UF2 file is written
Configuration files for board
foo are in
board.mk. You can
build it with
make BOARD=foo. You can also create
Makefile.user file with
to change the default.
The board configuration specifies the USB vendor/product name and ID, as well as the volume label (main thing that the operating systems show).
There is also
BOARD_ID, which is meant to be machine-readable and specific
to a given version of board hardware. The programming environment might use
this to suggest packages to be imported (i.e., a package for a particular
external flash chip, SD card etc.).
These configuration values can be read from
Presence of this file can be tested to see if the board supports
while contest, particularly
Board-ID field, can be used for feature detection.
The current flash contents of the board is exposed as
This file includes the bootloader address space. The last word of bootloader
space points to the string holding the
INFO_UF2.TXT file, so it can be parsed
by a programming environment to determine which board does the
.UF2 file comes from.
When the user space application implements the USB MSC protocol, it's possible to handover execution to the bootloader in the middle of MSC file transfer, when the application detects that a UF2 block is written.
Details are being finalized.
The bootloader will never write to its own flash area directly. However, the user code can write there. Thus, to update the bootloader, one can ship a user-space program, that contains the new version of the bootloader and copies it to the appropriate place in flash.
Such a program is generated during build in files
If you're already running UF2 bootloader, the easiest way to update
it, is to just copy this file to the exposed MSD drive.
The build also generates
update-bootloader*.ino with an equivalent Arduino
sketch. You can copy&paste it into Arduino IDE and upload it to the device.
The SAMD21 supports a
BOOTPROT fuse, which write-protects the flash area of
the bootloader. Changes to this fuse only take effect after device reset.
at91samd bootloader command to set this fuse. This command is buggy.
It seems to reset both fuse words to
0xffffffff, which prevents the device
from operating correctly (it seems to reboot very frequently).
scripts/fuses.tcl there is an OpenOCD script
which correctly sets the fuse. It's invoked by
dbgtool.js fuses. It can be also
used to reset the fuses to sane values - just look at the comment at the top.
The bootloader update programs (both the
.uf2 file and the Arduino sketch)
BOOTPROT (i.e., set it to
0x7) before trying to flash anything.
After flashing is done, they set
BOOTPROT to 8 kilobyte bootloader size (i.e,
The SAMD51s bootloader protection can be temporarily disabled through an NVM command rather than a full erase and write of the AUX page. The boot protection will be checked and set by the self updaters.
So, if you've used self-updaters but want to load it directly, then you'll need to temporarily turn off the protection. In gdb the command is:
set ((Nvmctrl *)0x41004000UL)->CTRLB.reg = (0xA5 << 8) | 0x1a
makeand an Unix environment
node.js in path (optional)
arm-none-eabi-gccin the path (the one coming with Yotta will do just fine). You can get the latest version from ARM: https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads
openocd- you can use the one coming with Arduino (after your install the M0 board support)
Atmel Studio is not supported.
You will need a board with
Arduino Zero (or M0 Pro) will work just fine as it has an integrated USB EDBG port. You need to connect both USB ports to your machine to debug - one is for flashing and getting logs, the other is for the exposed MSC interface.
Otherwise, you can use other SAMD21 board and an external
debugger. IBDAP is cheap and seems to work just fine. Another option is to use
Raspberry Pi and native bit-banging.
openocd will flash 16k, meaning the beginning of user program (if any) will
be overwritten with
0xff. This also means that after fresh flashing of bootloader
no double-tap reset is necessary, as the bootloader will not try to start application
The default board is
zero. You can build a different one using:
If you're working on different board, it's best to create
BOARD=metro to change the default.
metro refer to subdirectories of
There are various targets:
all- just compile the board
b- compile and deploy to the board using openocd
l- shows logs
r- burn, wait, and show logs
Typically, you will do:
There is a number of configuration parameters at the top of
Adjust them to your liking.
By default, you cannot enable all the features, as the bootloader would exceed the 8k allocated to it by Arduino etc. It will assert on startup that it's not bigger than 8k. Also, the linker script will not allow it.
Three typical configurations are:
- HID, WebUSB, MSC, plus flash reading via FAT; UART and CDC disabled; logging optional; recommended
- USB CDC and MSC, plus flash reading via FAT; UART disabled; logging optional; this may have Windows driver problems
- USB CDC and MSC, no flash reading via FAT (or at least
index.htmdisabled); UART enabled; logging disabled; no handover; no HID; only this one if you need the UART support in bootloader for whatever reason
CDC and MSC together will work on Linux and Mac with no drivers. On Windows, if you have drivers installed for the USB ID chosen, then CDC might work and MSC will not work; otherwise, if you have no drivers, MSC will work, and CDC will work on Windows 10 only. Thus, it's best to set the USB ID to one for which there are no drivers.
The bootloader sits at 0x00000000, and the application starts at 0x00002000.
Code of Conduct
See THIRD-PARTY-NOTICES.txt for the original SAM-BA bootloader license from Atmel.
The new code is licensed under MIT.