Very Long Reset with NVIC_SystemReset();

>, when i do so using NVIC_SystemReset(), the spi communication can't be turned on before at least 15-20

So if you do another kind of reset it takes less time?Maybe 20ms is the time it takes for your app to reset.

What if you initialice SPI sooner in your programm?

int main(void)
{
 /* USER CODE BEGIN 1 */
	uint8_t ReturnSPI;
 /* USER CODE END 1 */
 
 /* MCU Configuration--------------------------------------------------------*/
 
 /* Reset of all peripherals, Initializes the Flash interface and the Systick. */
 HAL_Init();
 
 /* USER CODE BEGIN Init */
 
 /* 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();
 MX_CRC_Init();
 MX_SPI1_Init();
 MX_TIM3_Init();
 MX_RTC_Init();
 /* USER CODE BEGIN 2 */
 MX_MEMS_Init();
 initCalib = 0;
 /* USER CODE END 2 */
 /* Infinite loop */
 /* USER CODE BEGIN WHILE */
 while (1)
 {
	 // MX_MEMS_Process(); // Here not working
 
	 ReturnSPI = HAL_SPI_Receive(&hspi1, &input, 1, 11);
 
 //Here, if it receives an SPI request to calibrate again, i call a function to do so
 if (command)...
 clearCalib();
 
 MX_MEMS_Process();
 /* USER CODE END WHILE */
 
 /* USER CODE BEGIN 3 */
 }
 /* USER CODE END 3 */
}
 
 
 
 
void clearCalib()
{
	HAL_FLASH_Unlock();
 
	__HAL_FLASH_CLEAR_FLAG(FLASH_FLAG_OPTVERR);
	/* Get the 1st page to erase */
	FirstPage = GetPage(ADDR_Calib_1);
	/* Get the number of pages to erase from 1st page */
	NbOfPages = GetPage(ADDR_Calib_4+8) - FirstPage + 1;
	/* Get the bank */
	BankNumber = GetBank(0x0801F800);
	/* Fill EraseInit structure*/
	EraseInitStruct.TypeErase = FLASH_TYPEERASE_PAGES;
	EraseInitStruct.Banks = BankNumber;
	EraseInitStruct.Page = FirstPage;
	EraseInitStruct.NbPages = NbOfPages;
 
	if (HAL_FLASHEx_Erase(&EraseInitStruct, &PAGEError) == HAL_OK)
	{
 
	}
 
	else
	{
		testWriteFlash = 11;
	}
	HAL_FLASH_Lock();
	NVIC_SystemReset();
	//////////////////////////////////////////////////////////////////
}

This part has nothing to do with motion FX as it is not used before rebooting once the spi message is received.

The main issue is i don't know if it's the SPI that doesn't reboot or if the MCU itself did'nt boot, as my only access is from SPI connection.

Is there another way to reboot? Instead of NVIC_SystemReset function i mean?

I don't think resetting via other methods will help. See what is taking so long in your code to reach the SPI calls. Instrument it or run a debugger and hit pause during those 15-20 seconds to see where it is.

The NVIC_SystemReset is istantaneous, you're missing something here.

First of all try to power off the board and check the SPI delay from poweron to working state, it must be the same 15-20 seconds, than search for the cause of this delay in your code and so try to reduce timings.

When you have free IO pin, you can define it as output with open colecter, set begin level to Hi, and wired it to reset pin. Then you can do HW reset by set this pin to Low. You can see, if this kind of reset behave the same way as reset core or differently.

The NVIC_SystemReset is very short, you can toggle a gpio to check the delay before NVIC_SystemReset and at the first line of main().