Firmware Hex File

When MicroPython is built, the compiler produces an Intel Hex file containing the MicroPython firmware. Additional data can then be added to this file to contain information about the MicroPython version, or the Python code to execute on start-up.

The general memory layout used is:

  • 0x00000000: Start of MicroPython firmware - up to 248 KBs
  • 0x0003e000: Start of appended script (optional) - up to 8 Kbs
  • 0x100010c0: UICR customer[16] register, start of MicroPython information - 28 bytes


If you append any data or modify the Intel Hex file, please ensure the addresses of the data stored progress in incremental order. If there is an address jump backwards DAPLink will fail to flash the file.

Appended script format

MicroPython checks the first 2 bytes at address 0x0003e000 for a magic string to indicate if there is an appended script. If the magic string is found, it will automatically execute the Python code stored there, unless there is a file stored in the MicroPython filesystem.

  • 0x0003e000: 2 bytes “MP”
  • 0x0003e002: 2 bytes, little endian integer for the length (in bytes) of the appended script (not counting this 4 byte header)
  • 0x0003e004: Script stored as bytes, for MicroPython to decode using utf-8.

UICR format

The User Information Configuration Registers (UICR) is a region of Non-Volatile Memory available to store user-specific settings. The first 128 Bytes are reserved, but we can use the other 128 Bytes to store any arbitrary data.

MicroPython stores the following information, in little endian, starting from the UICR customer[16] register:

  • 0x100010c0: 4-byte integer with magic value 0x17eeb07c
  • 0x100010c4: 4-byte integer with value 0xffffffff
  • 0x100010c8: 4-byte integer with value 0x0000000a (log base 2 of the flash page size, being 1024 bytes)
  • 0x100010ca: 2-byte integer with value 0x0000 (start page of the firmware)
  • 0x100010cc: 2-byte integer storing number of pages used by the firmware
  • 0x100010d0: 4-byte integer with value 0xffffffff
  • 0x100010d4: 4-byte integer with the address in the firmware of the version string
  • 0x100010d8: 4-byte integer with value 0x00000000

Steps to create the firmware.hex file

The yotta tool is used to build MicroPython, but before that takes place additional files have to be generated by the Makefile in preparation for the build, and additional data is added to the hex file after.

Running the make all command executes the following steps:

  • The tools/ script creates the microbitversion.h file with macros containing build information
  • Yotta builds the source and creates a bare hex file with just the firmware
  • The tools/ script adds the UICR to the bare hex
  • The final hex file is placed in build/firmware.hex
  • The user can optionally append a script using tools/ (or other tools)