Implementing STM32L4S5ZIT6 Bootloader with a Memory Marker

[ZAYN]

Hello,

I am working on the development of a bootloader for the STM32L4S5ZIT6, and I need help to implement a feature where the bootloader is triggered after receiving a user request in the main application (the main app saves a marker in memory).

Here is what I am trying to achieve:

1. The main application saves a marker (a magic number) in memory.
2. After jumping to the bootloader, the bootloader checks for the presence of this marker.
3. If the marker is found, the bootloader proceeds to execute (e.g., triggering a firmware update).
4. If the marker is not found, the bootloader jumps back to the main application.

My question

• Is SRAM backup (BKPSRAM) the best place to store the marker? If so, could anyone provide me with the exact address of BKPSRAM for the STM32L4S5ZIT6 (I couldn't find it in the user manual)?
• If BKPSRAM isn't suitable, can you propose another solution for saving the marker in a persistent memory region that survives resets?

I tried using SRAM to store a marker, then I perform a reset. What I don't understand is that the data is not cleared with the code below. Could you please clarify this for me

__attribute__((section(".backup_sram"))) volatile uint32_t marker;

int main(void)
{
  /* USER CODE BEGIN 1 */

  /* USER CODE END 1 */

  /* MCU Configuration--------------------------------------------------------*/

  /* Reset of all peripherals, Initializes the Flash interface and the Systick. */
  HAL_Init();

  /* USER CODE BEGIN Init */

  if(marker == 0x12345678U)
	  marker = 0x00000000;

  /* USER CODE END Init */

  /* Configure the system clock */
  SystemClock_Config();

  /* USER CODE BEGIN SysInit */

  /* USER CODE END SysInit */

  /* Initialize all configured peripherals */
  MX_GPIO_Init();

  /* Enable SRAM2 retention in Standby mode */
  //HAL_PWREx_DisableSRAM2ContentRetention();

  marker = 0x12345678U;

  HAL_NVIC_SystemReset();


  /* USER CODE BEGIN 2 */

  /* USER CODE END 2 */

  /* Infinite loop */
  /* USER CODE BEGIN WHILE */
  while (1)
  {
    /* USER CODE END WHILE */

    /* USER CODE BEGIN 3 */
  }
  /* USER CODE END 3 */
}

/****** Linker script ************/

MEMORY
{
  RAM    		(xrw)    : ORIGIN = 0x20000000,   LENGTH = 640K
  RAM2    		(xrw)    : ORIGIN = 0x10000000,   LENGTH = 64K
  RAM3    		(xrw)    : ORIGIN = 0x20040000,   LENGTH = 384K
  SHARED_MEMORY (xrw) 	 : ORIGIN = 0x2003FF9C,   LENGTH = 100 /* SRAM2 */
  FLASH    		(rx)     : ORIGIN = 0x8000000,   LENGTH = 2048K
}

/* Sections */
SECTIONS
{
  .backup_sram :
  {
      . = ALIGN(4);
      *(.backup_sram)
      . = ALIGN(4);
  } > SHARED_MEMORY

I would greatly appreciate any help or suggestions.

Thank you very much!