BOOT0 on STM32U083

[MagnusErler]

I have a custom board with an STM32U083KCU6 MCU - see schematics below

VBAT = +3.6V

I have 2 boards:
1. board: I flashed it via SWD but forgot to enable it in STM32CubeMX, which means that I'm not able to flash it again via SWD

2. Board: This time, I remembered to enable SWD inside STM32CubeMX and was able to flash it successfully. It worked fine for a long time, but now it suddenly seems like my ST-LINK V2 programmer is no longer able to flash it for some reason...

NRST is connected to RST on the ST-LINK V2 programmer.

I can use the same ST-LINK V2 programmer to flash a STM32U083 Nucleo board, so it does not seem like the ST-LINK V2 programmer is broken

STM32_Programmer_CLI -c port=SWD sn=223215051117303030303032 -d
 -------------------------------------------------------------------
 STM32CubeProgrammer v2.15.0 
 -------------------------------------------------------------------

libusb: warning [libusb_exit] device 4.76 still referenced
libusb: warning [libusb_exit] device 4.75 still referenced
libusb: warning [libusb_exit] device 4.74 still referenced
libusb: warning [libusb_exit] device 4.73 still referenced
libusb: warning [libusb_exit] device 4.72 still referenced
libusb: warning [libusb_exit] device 4.1 still referenced
libusb: warning [libusb_exit] device 3.1 still referenced
libusb: warning [libusb_exit] device 2.2 still referenced
libusb: warning [libusb_exit] device 2.1 still referenced
libusb: warning [libusb_exit] device 1.118 still referenced
libusb: warning [libusb_exit] device 1.5 still referenced
libusb: warning [libusb_exit] device 1.3 still referenced
libusb: warning [libusb_exit] device 1.124 still referenced
libusb: warning [libusb_exit] device 1.127 still referenced
libusb: warning [libusb_exit] device 1.13 still referenced
libusb: warning [libusb_exit] device 1.126 still referenced
libusb: warning [libusb_exit] device 1.125 still referenced
libusb: warning [libusb_exit] device 1.123 still referenced
libusb: warning [libusb_exit] device 1.121 still referenced
libusb: warning [libusb_exit] device 1.122 still referenced
libusb: warning [libusb_exit] device 1.120 still referenced
libusb: warning [libusb_exit] device 1.119 still referenced
libusb: warning [libusb_exit] device 1.117 still referenced
libusb: warning [libusb_exit] device 1.14 still referenced
libusb: warning [libusb_exit] device 1.1 still referenced
ST-LINK SN : 223215051117303030303032
ST-LINK FW : V2J45S7
Board : --
Voltage : 3,16V
Error: No STM32 target found! If your product embeds Debug Authentication, please perform a discovery using Debug Authentication

I have soldered a wire between BOOT0 and VBAT, but am still not able to connect to it via SWD.

Note that the UART is working on board 1, and I am to receive messages from the MCU, which means it has power

 

Is there anything I'm missing to be able to flash it again via SWD, or what are my options?

[MHoll.2]

Unfortunatly on this MCU (and other new MCU's from ST) the Boot0 Pin needs to be enabled via the Flash Option register (chapter 3.3.7 in Reference manual rm0503). So You are out of options if "Connect under reset" is not helping.

[MagnusErler]

I found that the configuration of the BOOT0 behaviour is controlled by the "NBOOT_SEL" bit in the user option bytes. By default, "NBOOT_SEL" is set to 1, which means the BOOT source is determined by the "nBOOT0 option bit", not the BOOT0 pin.

To enable boot selection via the BOOT0 pin, I need to clear NBOOT_SEL to 0. This will configure the MCU to use the BOOT0 pin state during startup instead of the fixed option bit.

Is that correctly understood?

[MHoll.2]

Yes, but to do this you need access via the SWD interface.

[Tesla DeLorean]

If you connect NRST to the ST-LINK, the "Connect Under Reset" becomes an option to connect/recover.

When you can connect with STM32 Cube Programmer (GUI) use the connection options, and when you connect Mass Erase to get the broken code off the device.

[MagnusErler]

When using the STM32CubeProgramemr and setting the mode to "Under reset". I receive the following error