Adding new STM32 target mcu

I have an STM32L4S7ZIT. The MCU is not found on a nucleo board. So I bought a Nucleo L4a6zg board and replaced the MCU with mine. I developed a simple code with the CubeIDE to try my modified nucleo board. The simple code works without any problem.

Now I want to develop a simple code for my modified nucleo using mbed-os. However, STM32L4S7ZIT is not among the supported MCUs in v6.16. I folowed below steps to add the MCU/custom board as a new target:

  1. Generated PinNames.h, PeripheralPins.c and CMakeLists.txt, folowing the instructions in the readme file in TARGET_STM folder. I generated these from the working *.ioc file
  2. Created TARGET_STM32L4S7xI folder. In this folder created NUCLEO_L4Custom and TOOLCHAIN_[ARM/GCC_ARM/IAR] folders. I downloaded the corresponding .S files from this repository. Copied the linker files form TARGET_STM32L4S5xI. I am compiling with GCC_ARM.
  3. Copied the generated files at the first step to my NUCLEO_L4Custom folder.
  4. From TARGET_STM32L4S5xI, copied CMakeLists.txt, cmsis_nvic.h and system_clock.c to my TARGET_STM32L4S7xI folder.
  5. Modified a bit the CMakeLists.txt in order to set the correct .S and .ld file paths.
  6. Added MCU_STM32L4S7xI and NUCLEO_L4Custom to targets.json. It simply it looks like this:
"MCU_STM32L4S7xI": {
        "inherits": [
            "MCU_STM32L4"
        ],
        "public": false,
        "components_add": [
            "FLASHIAP"
        ],
        "extra_labels_add": [
            "STM32L4S7xI"
        ],
        "macros_add": [
            "STM32L4S7xx",
            "MBEDTLS_CONFIG_HW_SUPPORT"
        ]
    },
    "NUCLEO_L4Custom": {
        "inherits": [
            "MCU_STM32L4S7xI"
        ],
        "supported_form_factors": [
            "ARDUINO_UNO"
        ],
        "device_name": "STM32L4S7ZITx",
        "detect_code": [
            "0776"
        ],
        "device_has_add": [
            "USBDEVICE"
        ]
    },

The code compiles but it does not run. I also added DISCO_F407VG as another target. When I compile for that board it works without any problem. However, TARGET_STM32F407xG is already available in the official release. I just had to add the discovery board as a new target.

I believe copying the files from TARGET_STM32L4S5xI (step 4) is correct because S5, 7, 9 MCUs are explained in the same datasheet and their core hw architecture is the same. I also compared those files with L4R5 and noticed they are identical except the .S and .ld file paths in cmake_lists.txt. So, using the same linker, system_clock.c, cmsis_nvic.h should be okay. I did no modifications in arm_pack_manager json files. I found this but it is quite old and not valid for the latest os version. Where is my mistake? Do I miss any steps or do I do a mistake while adding the new mcu? There is not a comprehensive documentation about how to add a new STM32 target and the above steps are what I understand by reading all the documentations I found and going through the file structure.

Okay it seems this is the solution. I found it after resolving the problem by myself.