External Loader buffer size

[macar]

Hello ST Community,

I’m developing a custom external loader for STM32CubeProgrammer where the STM32 acts as a bridge to program an external MCU (not external flash). The STM32 uses UART5 as a debug port and also as the communication interface to the external MCU.

In testing, I see CubeProgrammer calling Write() repeatedly with non‑sector‑aligned sizes and addresses that don’t start at sector boundaries. Sample output from my loader:

Starting...
Address: 0x90000000
Size: 0x0003C700
Size is not sector aligned:0x0003C700

Address: 0x9003C700
Size: 0x0003C700
Size is not sector aligned:0x0003C700
Address is not start of sector:0x9003C700
...
Address: 0x901E3800
Size: 0x00018E00
Size is not sector aligned:0x00018E00
Address is not start of sector:0x901E3800

Address: 0x901FC600
Size: 0x00018E60
Size is not sector aligned:0x00018E60
Address is not start of sector:0x901FC600

This looks like CubeProgrammer is splitting the image into chunks (e.g., 0x3C700, then smaller fragments near the end) and incrementing the Address, but not guaranteeing sector alignment. I understand this is expected for external flash loaders (since the loader handles alignment/erase internally), but in my case the target is an external MCU.

The external MCU requires sector-aligned writes because:

• It erases flash by sector before writing.
• It writes data sequentially based on a sector index. If CubeProgrammer sends a chunk that overlaps or partially rewrites a sector, the flashing process will fail.

How can I enforce sector alignment in CubeProgrammer?
Is there a way to configure CubeProgrammer so that all Write() calls are sector-aligned (both Address and Size)?

int Init(void)
{
    *(uint32_t *)0xE000EDF0 = 0xA05F0000; // enable interrupts in debug
    SystemInit();
    SCB->VTOR = 0x20000000 | 0x200;
    HAL_Init();
    SystemClock_Config();
    MX_GPIO_Init();

    // For debug output
    __HAL_RCC_UART5_FORCE_RESET();
    __HAL_RCC_UART5_RELEASE_RESET();
    MX_UART5_Init();

    printf("Starting...\n");
    /*
     * Initialization function
     *
     */

    return LOADER_OK;
}

int Write(uint32_t Address, uint32_t Size, uint8_t *buffer)
{
    printf("Address: 0x%08lX\n", Address);
    printf("Size: 0x%08lX\n", Size);

    // For external MCU programming, sector alignment required.
    // Check that the size is sector aligned
    if (Size % MEMORY_SECTOR_SIZE)
    {
        printf("Size is not sector aligned:0x%08lX\n", Size);
        // return LOADER_FAIL;
    }

    // Check that the address is the start of a sector
    if (Address % MEMORY_SECTOR_SIZE)
    {
        printf("Address is not start of sector:0x%08lX\n", Address);
        // return LOADER_FAIL;
    }
    // Forward 'buffer' as-is to external MCU.
    /*
     * writing code
     */

    return LOADER_OK;
}