1. Question
You have an application flashed on the M4 Core of the STM32WB. What happens to this application when you perform a firmware upgrade service (FUS) operation via STM32CubeProgrammer such as a FUS or wireless stack upgrade?
2. Answer
There are several connection interfaces available with STM32CubeProgrammer including SWD, USB DFU, and USART. The answer to this question depends primarily on which of these connection interfaces are used.
2.1 SWD
When using STM32CubeProgrammer to perform FUS operations via an SWD connection, an embedded firmware known as the FUS Operator is flashed onto the M4 Core of the STM32WB.
This FUS Operator is an embedded application used to handle communication between STM32CubeProgrammer and the M0+ Core of the STM32WB. It manages any FUS operations such as reading of the FUS state, upgrading the FUS or wireless stack, or deleting the existing wireless stack.
This means that any firmware present on the M4 Core is overwritten by the FUS operator and will need to be reflashed after completing FUS operations.
2.2 USB DFU and USART
When using the STM32CubeProgrammer to perform FUS operations via a USB DFU or USART connection, the system bootloader is used to communicate with the FUS on the M0+ Core of the STM32WB.
In this case, the application firmware on the M4 Core is preserved during any FUS operations performed.
3. Recommendation
Regardless of the communication interface used, it is good practice to always start the wireless stack and reflash your application firmware after performing FUS operations via STM32CubeProgrammer. This procedure assures a clean installation.
