USB Flashing Format (UF2)

UF2 (USB Flashing Format) is a name of a file format, that is particularly suitable for flashing devices over MSC (Mass Storage Class most commonly used by various pendrives) The file consists of 512 byte blocks, each of which is self-contained and independent of others.

Each 512 byte block consist of (see uf2.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.

In uf2conv.c you can find a small converter from .bin to .uf2.

Board identification

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.

License

MIT