OpenBLT Bootloader

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====== ARM Cortex-M33 for STM32U5xx ======


===== Vector table location =====

The bootloader itself occupies the location where normally the user program's vector table is located. The vector table in the user program therefore needs to be relocated. The bootloader implementation is such that the user program's vector table must be located at address 0x08008000 instead of the standard 0x08000000. 

The vector table relocation can be achieved by changing the ROM start address in the linker descriptor file. Refer to the demo program for an example.

{{:manual:ports:stm32u5_linker_script_prog.png?|}}

In case you want to manually change the size of the bootloader, make sure the size is aligned to the sectors as outlined in the flashLayout[] array in file flash.c. For the STM32U5xx targets, the sectors are 0x2000 (8 kbyte) in size, which is also the smallest erase size of the flash memory. This means that the reserved space for the bootloader should always be a multiple of 8 kbyte.

Before the bootloader starts your firmware, it automatically reconfigured the start address of the interrupt vector table. Unfortunately, the default system initialization function SystemInit() from ST's driver library, overwrites this change. This function is implemented in source file system_stm32u5xx.c. To correct this you need to explicitly set the start address of the interrupt vector table as the first thing in function main(). Here is an example of how this can be done when using STM32CubeIDE as the IDE:

Add the following function to main.c:

<code c>
static void VectorBase_Config(void)
{
  /* The constant array with vectors of the vector table is declared externally in the
   * c-startup code.
   */
  extern const unsigned long g_pfnVectors[];

  /* Remap the vector table to where the vector table is located for this program. */
  SCB->VTOR = (unsigned long)&g_pfnVectors[0];
}
</code> 

And call this newly added function at the start of main():

<code c>
int main(void)
{
  /* Configure the vector table base address. */
  VectorBase_Config();

  ...

}
</code>

===== Checksum location =====

A 32-bit checksum value is programmed by the bootloader at the end of a programming session. Upon startup, the bootloader verifies the correctness of this checksum to determine if a valid user program is present and can be started.

The bootloader programs this value at the end of the user program's vector table. Its exact location is configured by macro BOOT_FLASH_VECTOR_TABLE_CS_OFFSET in flash.c. The vector table size can vary based on the microcontroller derivative you are using. Please verify that the default value of BOOT_FLASH_VECTOR_TABLE_CS_OFFSET is correct. If not, you can simply override the value by adding the macro with the correct value to the blt_conf.h configuration header file.

The user program must reserve space for this 32-bit checksum value. To reserve this space, simply enter one extra dummy entry into the interrupt vector table. This reserves space for the checksum at memory address: flash base address + flash reserved for bootloader  + FLASH_VECTOR_TABLE_CS_OFFSET. Refer to the demo program for an example.

{{:manual:ports:stm32u5_checksum_reservation.png?|}}

===== Stackpointer initialization =====

On the ARM Cortex-M33 the first entry into the vector table holds the initialization value for the stackpointer, which is automatically loaded into the CPU's stackpointer register upon reset.

After reset, the bootloader gets started first and, if a valid user program is present, it is the bootloader's responsibility to start the user program. This unfortunately means that the automatic initialization of the CPU's stackpointer register does not work for the user program. The user program therefore needs to explicitly set the initial value of the CPU's stackpointer register in the reset interrupt service routine. Refer to the demo program for an example.

{{:manual:ports:stm32u5_stackpointer_init.png?|}}
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