.UF2

Introduction

UF2 is a firmware format made by Microsoft for its MakeCode editor, for flashing microcontrollers without special software. A board’s bootloader appears on the computer as a USB drive; copying a .uf2 file onto it writes the firmware into flash, and the board restarts. The Raspberry Pi Pico, many Adafruit boards and other hobby boards work this way, and projects such as MicroPython and CircuitPython publish their firmware as UF2.

Structure

A UF2 file is a series of 512-byte blocks, each complete in itself. Every block carries its own target address, so the bootloader can write each one as it arrives, in any order, and ignore anything else the operating system writes to the drive. Each block starts with two magic numbers and ends with a third, all 32-bit little-endian, and holds up to 476 bytes of data, usually 256.

A flag can mark that the word at offset 28 is a family ID rather than the file size, which names the kind of chip the firmware is for, so a board can reject firmware meant for another.

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