Implementing STM32 DFU bootloader firmware upgrade in Marlin using M997

Marlin implements the M997 command which is intended to switch the mainboard into a firmware upgrade mode.

All STM32 variants have an integrated (hard-coded – so no need to flash it yourself) bootloader that is notoriously difficult to active.

However, Marlin implements M997 on the STM32 as just a reboot:

void flashFirmware(const int16_t) { HAL_reboot(); }

This only works if you are using a custom bootloader on your board – however, it does not use the STM32 integrated bootloader.

The following code was tested on the STM32F446 (BigTreeTech Octopus V1) but should work on any STM32 variant. It is based on previous work by Dave Hyland on StackOverflow. Replace the default flashFirmware() function in Marlin/src/HAL/STM32/HAL.cpp

void flashFirmware(const int16_t) {


    // arm-none-eabi-gcc 4.9.0 does not correctly inline this
    // MSP function, so we write it out explicitly here.
    //__set_MSP(*((uint32_t*) 0x00000000));
    __ASM volatile ("movs r3, #0\nldr r3, [r3, #0]\nMSR msp, r3\n" : : : "r3", "sp");

    ((void (*)(void)) *((uint32_t*) 0x00000004))();

    // This will never be executed

Note that we left a HAL_reboot() call as a safeguard at the end, just in case the previous calls fail.

On my BigTreeTech Octopus V1 (STM32F446), by using this code, you can successfully enter the integrated DFU bootloader.

Also see our previous posts on how to use the STM32 in DFU bootloader mode: