SPI slave on STM32U535

[lFurm.1]

Hi,

I'm trying to implement SPI slave on STM32U535 ( I have also NUCLEO-U575I). 

I need the SPI slave to have 4 bytes communication, 1st byte is a command byte, that has 2 options of command, the next 3 bytes the slave sends the content of one of 2 arrays (depends on the command byte). It uses hardware NSS.

No matter what I've tried, I have issues. Tried to implement it with inteupt, using LL libary for faster procceing the comand byte. I've set the FIFOThreshold to 1.

And still, at the output I recive the data delayed by 4 sends. I assume the TxFIFO is getting full first, and onlt then starting to send, with a delay, anyone can suggest how to overcome it and what am I missing. I'm attaching relevant code of the last configuration, but I've ttreied many others too.

Init:

static void MX_SPI2_Init(void)
{

  /* USER CODE BEGIN SPI2_Init 0 */

  /* USER CODE END SPI2_Init 0 */

  LL_SPI_InitTypeDef SPI_InitStruct = {0};

  LL_GPIO_InitTypeDef GPIO_InitStruct = {0};
  RCC_PeriphCLKInitTypeDef PeriphClkInit = {0};

  /** Initializes the peripherals clock
  */
  PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_SPI2;
  PeriphClkInit.Spi2ClockSelection = RCC_SPI2CLKSOURCE_SYSCLK;
  if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInit) != HAL_OK)
  {
    Error_Handler();
  }

  /* Peripheral clock enable */
  LL_APB1_GRP1_EnableClock(LL_APB1_GRP1_PERIPH_SPI2);

  LL_AHB2_GRP1_EnableClock(LL_AHB2_GRP1_PERIPH_GPIOB);
  LL_AHB2_GRP1_EnableClock(LL_AHB2_GRP1_PERIPH_GPIOA);
  /**SPI2 GPIO Configuration
  PB9   ------> SPI2_NSS
  PA9   ------> SPI2_SCK
  PB14   ------> SPI2_MISO
  PB15   ------> SPI2_MOSI
  */
  GPIO_InitStruct.Pin = LL_GPIO_PIN_9;
  GPIO_InitStruct.Mode = LL_GPIO_MODE_ALTERNATE;
  GPIO_InitStruct.Speed = LL_GPIO_SPEED_FREQ_MEDIUM;
  GPIO_InitStruct.OutputType = LL_GPIO_OUTPUT_PUSHPULL;
  GPIO_InitStruct.Pull = LL_GPIO_PULL_NO;
  GPIO_InitStruct.Alternate = LL_GPIO_AF_5;
  LL_GPIO_Init(GPIOB, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = LL_GPIO_PIN_9;
  GPIO_InitStruct.Mode = LL_GPIO_MODE_ALTERNATE;
  GPIO_InitStruct.Speed = LL_GPIO_SPEED_FREQ_HIGH;
  GPIO_InitStruct.OutputType = LL_GPIO_OUTPUT_PUSHPULL;
  GPIO_InitStruct.Pull = LL_GPIO_PULL_NO;
  GPIO_InitStruct.Alternate = LL_GPIO_AF_3;
  LL_GPIO_Init(GPIOA, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = LL_GPIO_PIN_14|LL_GPIO_PIN_15;
  GPIO_InitStruct.Mode = LL_GPIO_MODE_ALTERNATE;
  GPIO_InitStruct.Speed = LL_GPIO_SPEED_FREQ_HIGH;
  GPIO_InitStruct.OutputType = LL_GPIO_OUTPUT_PUSHPULL;
  GPIO_InitStruct.Pull = LL_GPIO_PULL_NO;
  GPIO_InitStruct.Alternate = LL_GPIO_AF_5;
  LL_GPIO_Init(GPIOB, &GPIO_InitStruct);

  /* SPI2 interrupt Init */
  NVIC_SetPriority(SPI2_IRQn, NVIC_EncodePriority(NVIC_GetPriorityGrouping(),0, 0));
  NVIC_EnableIRQ(SPI2_IRQn);

  /* USER CODE BEGIN SPI2_Init 1 */

  /* USER CODE END SPI2_Init 1 */
  /* SPI2 parameter configuration*/
  SPI_InitStruct.TransferDirection = LL_SPI_FULL_DUPLEX;
  SPI_InitStruct.Mode = LL_SPI_MODE_SLAVE;
  SPI_InitStruct.DataWidth = LL_SPI_DATAWIDTH_8BIT;
  SPI_InitStruct.ClockPolarity = LL_SPI_POLARITY_LOW;
  SPI_InitStruct.ClockPhase = LL_SPI_PHASE_2EDGE;
  SPI_InitStruct.NSS = LL_SPI_NSS_HARD_INPUT;
  SPI_InitStruct.BitOrder = LL_SPI_MSB_FIRST;
  SPI_InitStruct.CRCCalculation = LL_SPI_CRCCALCULATION_DISABLE;
  SPI_InitStruct.CRCPoly = 0x7;
  LL_SPI_Init(SPI2, &SPI_InitStruct);
  LL_SPI_SetStandard(SPI2, LL_SPI_PROTOCOL_MOTOROLA);
  LL_SPI_DisableNSSPulseMgt(SPI2);
  /* USER CODE BEGIN SPI2_Init 2 */

  /* USER CODE END SPI2_Init 2 */

}

in main:

	LL_SPI_EnableIT_RXP(SPI2);
	LL_SPI_EnableIT_TXP(SPI2);
	LL_SPI_EnableIT_OVR(SPI2);
	LL_SPI_SetTransferSize(SPI2, 0);
	LL_SPI_SetFIFOThreshold(SPI2, LL_SPI_FIFO_TH_01DATA);
	LL_SPI_TransmitData8(SPI2,0x00);
	LL_SPI_Enable(SPI2);

Interupt:

  */
void SPI2_IRQHandler(void)
{
  /* USER CODE BEGIN SPI2_IRQn 0 */
	
	if (LL_SPI_IsActiveFlag_RXP(SPI2)) 
	{
		received_byte = LL_SPI_ReceiveData8(SPI2);
		rxind++;
		if (rxind==4){
		rxind=0;}
		if (rxind == 1){
		coman=(received_byte & 0xC0) >>6;
			
					}
	}
	if (LL_SPI_IsActiveFlag_OVR(SPI2)) 
	{
		SPI_FlushRxFifo_Fast(SPI2);  
		LL_SPI_ClearFlag_OVR(SPI2);
	}
	if (LL_SPI_IsActiveFlag_TXP(SPI2)) 
  {
		if (coman==2){
	//		print_hex(spi_tx_data[txind]);
			LL_SPI_TransmitData8(SPI2, spi_tx_data[txind++]); }
		else if (coman==3){
		
	
			
			LL_SPI_TransmitData8(SPI2, spi_tx_temp[txind++]); 
		}
		else
			LL_SPI_TransmitData8(SPI2,0x00);
		
		if (txind==3) {txind=0; coman=8;}
	}
  if (LL_SPI_IsActiveFlag_UDR(SPI2)) 
  {
		LL_SPI_TransmitData8(SPI2, 0x00);   
		LL_SPI_ClearFlag_UDR(SPI2);
	}

  /* USER CODE END SPI2_IRQn 0 */
  /* USER CODE BEGIN SPI2_IRQn 1 */

  /* USER CODE END SPI2_IRQn 1 */
}

Thanks if anyone can help, or suggest how to implement what I need.