NUCLEO-WB09KE / STM32WB09 with Zephyr: enabling CONFIG_BT=y may cause intermittent boot failure before main()

[Eyck-Huang]

Board / SoC

• Board: NUCLEO-WB09KE
• SoC: STM32WB09

Software

• Zephyr version: v4.4.0-rc2-41-g149c8b1758a8
• Toolchain / build system: West + Zephyr standard build flow

Issue summary
When building a very small Zephyr application for NUCLEO-WB09KE, simply enabling:

CONFIG_BT=y

can cause intermittent boot failure.

The failure happens very early in boot:

• no UART output
• no boot banner
• no application printk()
• main() appears to never run in failed boots

This happens even when the application does not call any Bluetooth API.

Minimal application
main.c

#include <zephyr/kernel.h>
#include <zephyr/sys/printk.h>

int main(void)
{
 while (1) {
 printk("M\n");
 k_busy_wait(100000);
 }
 return 0;
}

Minimal project configuration
prj.conf

CONFIG_BT=y

CONFIG_CONSOLE=y
CONFIG_UART_CONSOLE=y
CONFIG_EARLY_CONSOLE=y
CONFIG_PRINTK=y

CONFIG_ASSERT=y
CONFIG_ASSERT_VERBOSE=y

CONFIG_LOG=n
CONFIG_PM=n

Observed behavior

1. With Bluetooth disabled, the board boots and repeatedly prints:

   M
   M
   M
   ...

2. With CONFIG_BT=y enabled, boot becomes intermittent:
   • successful boots: system starts normally
   • failed boots: absolutely no UART output at all
3. In failed cases:
   • no printk("M\n")
   • no Zephyr boot banner
   • no log output
   • this strongly suggests failure occurs before main()

[Imen.D]

Hello @Eyck-Huang ,

This may be caused by hardware bootloader activation.

Did you check the jumper settings and hardware reset sequence to ensure the bootloader is running and UART is enabled?