STM32WB55 Running BLE Stack not resettable using hardware reset

[DWalk.3]

I have a custom board running the BLE (full) stack. The application code is written using the Zephyr RTOS. The power source is a CR2032 coin cell battery. After operating for a while (the actual times vary), some units (not all as far as I know) get into a state where I cannot get the device back by asserting the reset signal on the processor. Only a POR will recover the device.

Some notes:

• I've measured the current consumption in this state and it is a consistent 400-500uA. The current consumption increases when I assert reset so I know the reset signal is getting asserted
• It does not seem to be going into the bootloader as it doesn't come up as a DFU device over USB.
• The ST-LINK debugger cannot connect to the target using any mode (Hot-plug, normal, or under reset).

How can the chip get into such a state where power cycling is necessary to effectively restart it? Has anyone seen something like this before?

[Remi QUINTIN]

Do you use the SMPS?

Can you get access to the device and download the option bytes (OB)?

I assume you connect to your board via the SWD port. Get the OB just after the POR.

[DWalk.3]

Yes, I am using the SMPS. When the processor is in this state, I cannot access it at all over SWD. Once I do a BOR, I can access it. The only option byte I modify is the "User configuration" bytes where nBOOT0, nBOOT1, and nSWBOOT0 are all set, The other bytes are all at their default values. Initially, the radio stack is working. After a time, it would stop and end up in this state where a BOR is required to continue.