STM32G491CEU6 (G4) ChibiOS bootloader: USB Serial (SDU1) not created + linker “location counter backwards”

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Hi,

I’m trying to build an ArduPilot ChibiOS bootloader for a custom STM32G491CEU6 (G4) board (PCBMagNode).

Goal

  • Bootloader: USB serial only (CDC / SDU1), basic LEDs

  • Application: AP_Periph over CAN to a Pixhawk

  • Flashing: USB preferred, STLink V3 Mini available

Build command:

rm -rf build/PCBMagNode
Tools/scripts/build_bootloaders.py PCBMagNode

Environment

  • MCU: STM32G491CEU6 (512 KB flash, 128 KB RAM)

  • Host: macOS (Apple Silicon)

  • Toolchain: arm-none-eabi gcc 10.2.1 (2020-q4-major)

  • ArduPilot: ChibiOS (commit/branch can be provided)

Problem A — USB / SDU1 not generated

Depending on the USB syntax used in hwdef-bl.dat, I get one of the following:

A1 — USB peripheral not assigned

#error "USB driver activated but no USB peripheral assigned"

A2 — SDU1 missing

error: 'SDU1' was not declared in this scope
#define BOOTLOADER_DEV_LIST (BaseChannel *)&SDU1,(BaseChannel *)&SD1

So maybe either:

  • the USB peripheral is not bound correctly, or

  • USB compiles but SDU1 is never generated in hwdef.h.

Question A

For STM32G491 / STM32G4xx, what is the correct hwdef-bl.dat syntax to enable Serial-over-USB (CDC) so that SDU1 exists?

Specifically:

  • What is the correct USB peripheral name for G4 (USB vs USB1 vs OTG1)?

  • What are the correct pin function tokens for PA11 / PA12?

  • What is the correct usage of:

    • USB USB_SERIAL

    • SERIAL_ORDER …

    • any required define HAL_USE_SERIAL_USB or related flags?

Problem B — Linker error / flash layout

When I get past the USB compile stage, linking sometimes fails with:

ld: common.ld:202 cannot move location counter backwards
(from 0x080021e0 to 0x0800000)

In some configure runs I also see:

env set FLASH_TOTAL=0

Question B

What are the correct flash / reserve directives for an STM32G491 bootloader?

For example, for a typical 32 KB bootloader:

  • Which of these must be set in hwdef-bl.dat?

    • FLASH_SIZE_KB

    • FLASH_RESERVE_START_KB

    • FLASH_BOOTLOADER_LOAD_KB

  • What must match in the application hwdef.dat so AP_Periph starts after the bootloader region?

Current

hwdef-bl.dat

(attempt)

# hwdef-bl.dat — PCBMagNode Bootloader (STM32G491xx)
APJ_BOARD_ID 99

MCU STM32G4xx STM32G491xx

FLASH_SIZE_KB 512
FLASH_BOOTLOADER_LOAD_KB 0
FLASH_RESERVE_START_KB 0

OSCILLATOR_HZ 8000000

# USB attempt
PA11 USB_DM USB
PA12 USB_DP USB
USB USB_SERIAL

# Optional UART
PA9  USART1_TX USART1
PA10 USART1_RX USART1

SERIAL_ORDER USB USART1

USB_VENDOR 0x0483
USB_PRODUCT 0x5740
STRING_PRODUCT "MagNode BL"
define HAL_USE_EMPTY_STORAGE 1

# LEDs
PA0 LED1 OUTPUT LOW
PA1 LED2 OUTPUT LOW

# keep bootloader minimal
define AP_SCRIPTING_ENABLED 0
define AP_NETWORKING_ENABLED 0
define AP_FILESYSTEM_ENABLED 0
define HAL_LOGGING_ENABLED 0
define HAL_USE_ADC FALSE
define HAL_USE_I2C FALSE
define HAL_USE_SPI FALSE
define HAL_USE_CAN FALSE
define HAL_USE_PWM FALSE

What I already tried

  • OTG-style tokens:
PA11 OTG_FS_DM OTG1
PA12 OTG_FS_DP OTG1

  • → parser errors or “unknown pin function”

  • SERIAL_ORDER OTG1, USB1, etc.

  • Multiple combinations of USB naming

I haven’t yet found a configuration where:

  • the USB peripheral is assigned correctly,

  • SDU1 exists, and

  • the bootloader links cleanly.

Looking for

  • A known working hwdef-bl.dat example for STM32G4xx / G491

  • The correct USB token naming for this MCU in ArduPilot

  • The correct flash reserve setup for bootloader + matching app layout

Thanks in advance for any pointers!!