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av4103
Visitor II
April 30, 2026
Question

Boot Mode Error in Cube Programmer.

  • April 30, 2026
  • 1 reply
  • 102 views

I am working with the STM32L4S5VIT6 and currently facing an issue while trying to communicate with the system bootloader over USART. I am using the B-L4S5I-IOT01A Discovery board, on which I am attempting this setup. I have configured BOOT0 correctly and verified that the MCU enters boot mode (the application stops running as expected). I connected USART2 on PA2 (TX) and PA3 (RX), tried swapping TX/RX lines, and also interfaced through a MAX3223 level shifter to a MOXA device, but the STM32CubeProgrammer still reports a “KO – device not responding” error. I have confirmed that the MCU runs user code correctly (LED blink test), so basic functionality is intact. At this point, I suspect either a configuration misunderstanding from AN2606, incorrect peripheral mapping for this specific MCU, or a hardware-level issue with the UART path. Has anyone faced a similar issue with STM32L4 series bootloader communication, or can help clarify the exact USART instances/pins supported in boot mode for this device?

 

Regards,

~av

1 reply

Shirley.Ye
Shirley.Ye
ST Employee
May 2, 2026

ShirleyYe_0-1777731207315.png

according to AN2606, you need to check the boot0 pin, the nBoot1 and nBoot0_SW option byte

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