Flash 片内程序存储

// 存：校准值写到 App 区末尾 1 页
HAL_FLASH_Unlock();
FLASH_EraseInitTypeDef fe = {
  .TypeErase = FLASH_TYPEERASE_PAGES,
  .PageAddress = APP_END_PAGE_ADDR,   // 如 0x0803F800
  .NbPages = 1
};
uint32_t err = 0;
HAL_FLASHEx_Erase(&fe, &err);
HAL_FLASH_Program(FLASH_TYPEPROGRAM_WORD, APP_END_PAGE_ADDR, calib_value);
HAL_FLASH_Lock();

// 读：直接指针访问
uint32_t saved = *(volatile uint32_t*)APP_END_PAGE_ADDR;