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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 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.


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

Board identification

Configuration files for board foo are in boards/foo/board_config.h and You can build it with make BOARD=foo. You can also create Makefile.user file with BOARD=foo 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 INFO_UF2.TXT file. Presence of this file can be tested to see if the board supports UF2 flashing, while contest, particularly Board-ID field, can be used for feature detection.

The current flash contents of the board is exposed as CURRENT.UF2 file. 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.

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 update-bootloader*.uf2. 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.

OpenOCD exposes 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). In 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) clear the 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, 0x2).


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



Atmel Studio is not supported.

You will need a board with openocd support.

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 openocd compatible 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 that on SAMD21 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 at 0xffffffff.

Build commands

The default board is zero. You can build a different one using:

make BOARD=metro_m0

If you're working on different board, it's best to create Makefile.user with say BOARD=metro to change the default. The names zero and metro refer to subdirectories of boards/.

There are various targets:

  • all - just compile the board
  • burn or b - compile and deploy to the board using openocd
  • logs or l - shows logs
  • run or r - burn, wait, and show logs

Typically, you will do:

make r


There is a number of configuration parameters at the top of uf2.h 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 index.htm 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).

Bootloader Hold Switch

A Board can be configured to sense the state of a GPIO and to hold the bootloader from running an application if it is in the appropriate state. To enable this, add the following to your board configuration header file.

// The Pin that will tell us to stay in the bootloader or not.
#define HOLD_PIN PIN_PA02

// Optional, define if a Pull up or pulldown is needed.

// What is the Hold state of the GPIO, 0 or 1.
#define HOLD_STATE 1

Set HOLD_PIN to the appropriate GPIO and HOLD_STATE to the logic level which will hold the bootloader from running the application.

The definition of BOTH HOLD_PIN and HOLD_STATE triggers the inclusion of this feature. If either of these is undefined, this feature is not enabled.

If an internal pullup/pulldown is required for the IO, it can be enabled with the OPTIONAL HOLD_PIN_PULLUP or HOLD_PIN_PULLDOWN macros. If neither are defined, then no pullup/pulldown will be enabled for the io pin.

This switch is NOT dynamic. Once the bootloader has sensed this pin and decided not to run the application, then a change in this IO will not, itself, then cause the Application to run, without also resetting the board.

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 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.