STM32F4 Serial Bootloader

STM32F4 serial bootloader using the libraries from https://github.com/ferenc-nemeth/stm32-bootloader.

This bootloader assumes that UART1 is being used and the MCU has 8MHz HSE (On NUCLEO boards this is provided by the integrated ST-Link).

SystemClock_Config() must be altered to use a different crystal.

uart.c and uart.h must be altered to use a different port.

How the bootloader works

To flash the firmware, the MCU must be reset.

When the MCU is reset, it sends a BEL character through UART1 and the host responds to this with an ACK character.

The MCU the sends its signature (ex. F411RE) for verification, telling the host what kind of MCU it is. The host responds an ACK character to pass verification.

The MCU then clears the memory reserved for the application and begins file transfer using the XMODEM protocol.

After the binary is transferred, the MCU jumps to the application code.

How to build for an MCU

On Arch Linux, this requires the following packages:

• arm-none-eabi-binutils
• arm-none-eabi-gcc
• arm-none-eabi-gdb
• arm-none-eabi-newlib

The makefile can build for the following MCUs:

• stm32f446retx
• stm32f411retx
• stm32f411rctx

To make a binary for the MCU, cd into the directory of the make file and run make mcu=<mcu type from the list above>.

For example, if I was building for an STM32F411RCT6, I would run:

make mcu=stm32f411rctx

Flash using st-flash

Erase the chip using:

st-flash erase

Then program the chip using:

st-flash write <path to binary> 0x8000000

Recommended circuitry

The DTR reset circuitry is the same as on the Arduino, but for reference:

This will force the MCU to reset whenever the DTR pin is changed.

Uploading binaries using the bootloader

The bootloader writes the application to address 0x8004000.

This means that, for each application to uploaded using this method, the linker script and VECT_TAB_OFFSET must be modified.

Linker script

The flash origin in the linker script should be changed to 0x8004000 and the length must be reduced by 16kB.

For example, the file STM32F411RETx_FLASH.ld has the memory layout as follows:

MEMORY
{
    RAM (xrw)      : ORIGIN = 0x20000000, LENGTH = 128K
    FLASH (rx)      : ORIGIN = 0x8000000, LENGTH = 512K
}

This should be modified to:

MEMORY
{
    RAM (xrw)      : ORIGIN = 0x20000000, LENGTH = 128K
    FLASH (rx)      : ORIGIN = 0x8000000 + 0x4000, LENGTH = 512K - 16K
}

VECT_TAB_OFFSET

For programs built using STM32CubeMX, the VECT_TAB_OFFSET is defined in system_stm32f4xx.c.

#define VECT_TAB_OFFSET  0x00

should be changed to:

#define VECT_TAB_OFFSET  0x4000

For PlatformIO

If you're using the STM32 platform in PlatformIO, these changes can be specified in the platformio.ini.

Linker script

The modified linker script can be placed into the project's root directory and the board_build.ldscript should then point to the linker script. For example, we can add the following line to the environment:

board_build.ldscript = "$PROJECT_DIR\modified_linker_script.ld"

VECT_TAB_OFFSET

The new vector table offset can be added to build flags. For example, in MarlinFirmware's platformio.ini my board's build_flags should be as follows:

build_flags = ${common_stm32.build_flags}
    -DVECT_TAB_OFFSET=0x4000