USB Mass Storage bootloader (based on UF2) for SAMD21 and SAMD51
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.hfor 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.
logstarget to extract the logs using
Configuration files for board
board.mk. You can build it with
make BOARD=foo. You can also create
BOARD=footo 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.TXTfile. Presence of this file can be tested to see if the board supports
UF2flashing, while contest, particularly
Board-IDfield, can be used for feature detection.
The current flash contents of the board is exposed as
CURRENT.UF2file. This file includes the bootloader address space. The last word of bootloader space points to the string holding the
INFO_UF2.TXTfile, so it can be parsed by a programming environment to determine which board does the
.UF2file 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
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*.inowith an equivalent Arduino sketch. You can copy&paste it into Arduino IDE and upload it to the device.
The SAMD21 supports a
BOOTPROTfuse, which write-protects the flash area of the bootloader. Changes to this fuse only take effect after device reset.
at91samd bootloadercommand 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.tclthere 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
.uf2file and the Arduino sketch) clear the
BOOTPROT(i.e., set it to
0x7) before trying to flash anything. After flashing is done, they set
BOOTPROTto 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
openocdcompatible debugger. IBDAP is cheap and seems to work just fine. Another option is to use Raspberry Pi and native bit-banging.
openocdwill 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
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=metroto change the default. The names
metrorefer 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
uf2.hfile. 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:
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 (SAMD21) or 0x00004000 (SAMD51).
See THIRD-PARTY-NOTICES.txt for the original SAM-BA bootloader license from Atmel.
The new code is licensed under MIT.