// application note

AN0002 Programming STM32 microcontrollers over I2C with the Binho Pulsar

The system bootloader programmed into the ROM of STM32 microcontrollers offers an I2C interface on most families, allowing internal flash memory to be erased, programmed and verified over two wires with no debug probe attached. This application note describes that interface, the connections required between a Binho Pulsar USB host adapter and the target, and the procedure for programming a device. The same procedure and the same utility work with a Binho Supernova. A complete reference utility written in Python accompanies the document. Measured results are given for an STM32H503 on a NUCLEO-H503RB board.

Products: Binho Supernova Binho Pulsar
Protocols: I2C
Topics: STM32 Bootloader Flashing
Revision: Rev 1.1 · September 3, 2026

// outcomes

What you'll walk away with

This document describes:

the structure of the I2C system bootloader protocol, including the mechanism used to report command status and the mechanism used to signal that the target is busy
the connections required between a Binho Pulsar and an STM32 target
the procedure for identifying, erasing, programming and verifying a device
a method for confirming from outside the programming tool that the target is running the newly programmed image
the accompanying reference utility, which drives either a Pulsar or a Supernova

It does not cover programming over UART, USB, SPI, CAN or I3C; option-byte programming; readout protection, which the ROM bootloader on the tested part does not offer over this interface; or production programming fixtures. For I3C on the STM32H5 series, see AN0001.

// background

Background

The system bootloader programmed into the ROM of every STM32 microcontroller provides a means of programming internal flash memory over a serial interface, without a debug probe and without any user code present on the device. The interfaces offered by the bootloader vary by device and are listed per part number in AN2606.

I2C appears on that list for most STM32 families, which makes it the broadest of the in-system programming options. Where a board already routes an I2C bus to reach sensors, EEPROMs or a power-management device, the same two wires can carry a firmware update, and no dedicated programming header is required. Where a product is already manufactured in volume, an I2C bus is usually present somewhere on the board whether or not it was planned for programming.

This note is the I2C counterpart to AN0001, which covers the I3C interface on the STM32H5 series. The command set is nearly the same on both. What differs is how the target reports status and how it says it is busy, and those differences shape the host code more than the command list does.

// applicable devices

Applicable devices

The I2C system bootloader is not specific to one family. The pins it uses and the address it answers on differ per part, and both are given in AN2606 for every device that offers the interface. Neither is guessable, and neither is consistent across families.

Family	Example part	Bootloader I2C interface	Basis
STM32H5	STM32H503	I2C2, `PB3`/`PB4`, address `0x67`	Tested for this document
STM32H5	STM32H563	Per AN2606	AN2606, not tested
STM32G0, G4	none	Per AN2606	AN2606, not tested
STM32L4, L5	none	Per AN2606	AN2606, not tested
STM32U5	none	Per AN2606	AN2606, not tested

Only the STM32H503 row was exercised on hardware. The other rows record that AN2606 lists an I2C interface for those families; the pins, the address and the supported command list must be read from AN2606 for the part in hand before any of this document's commands are issued.

// what's covered