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USB Mass Storage bootloader (based on UF2) for SAMD21 and SAMD51

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UF2 Bootloader

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).

Build Status


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

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.


  • 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
    target to extract the logs using
  • double-tap reset to stay in the bootloader mode
  • automatic reset after UF2 file is written

Board identification

Configuration files for board

are in
. You can build it with
make BOARD=foo
. You can also create
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

, 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

file. Presence of this file can be tested to see if the board supports
flashing, while contest, particularly
field, can be used for feature detection.

The current flash contents of the board is exposed as

file. This file includes the bootloader address space. The last word of bootloader space points to the string holding the
file, so it can be parsed by a programming environment to determine which board does the
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.

Bootloader update

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

with an equivalent Arduino sketch. You can copy&paste it into Arduino IDE and upload it to the device.



The SAMD21 supports a

fuse, which write-protects the flash area of the bootloader. Changes to this fuse only take effect after device reset.

OpenOCD exposes

at91samd bootloader
command to set this fuse. This command is buggy. It seems to reset both fuse words to
, which prevents the device from operating correctly (it seems to reboot very frequently). In
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

file and the Arduino sketch) clear the
(i.e., set it to
) before trying to flash anything. After flashing is done, they set
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



  • make
    and an Unix environment
  • node
    .js in path (optional)
  • arm-none-eabi-gcc
    in the path (the one coming with Yotta will do just fine). You can get the latest version from ARM:
  • 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

compatible debugger. IBDAP is cheap and seems to work just fine. Another option is to use Raspberry Pi and native bit-banging.

will flash 16k, meaning that on SAMD21 the beginning of user program (if any) will be overwritten with
. This also means that after fresh flashing of bootloader no double-tap reset is necessary, as the bootloader will not try to start application at

Build commands

The default board is

. You can build a different one using:
make BOARD=metro_m0

If you're working on different board, it's best to create

with say
to change the default. The names
refer to subdirectories of

There are various targets: *

- just compile the board *
- compile and deploy to the board using openocd *
- shows logs *
- burn, wait, and show logs

Typically, you will do:

make r


There is a number of configuration parameters at the top of

file. Adjust them to your liking.

By default, you cannot enable all the features, as the bootloader would exceed the 8k(SAMD21)/16k(SAMD51) allocated to it by Arduino etc. It will assert on startup that it's not bigger than 8k(SAMD21)/16k(SAMD51). 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
    disabled); 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 (SAMD21) or 0x00004000 (SAMD51).

Code of Conduct

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact [email protected] with any additional questions or comments.


See THIRD-PARTY-NOTICES.txt for the original SAM-BA bootloader license from Atmel.

The new code is licensed under MIT.

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