Best practice for sending 9-bit SPI data with 8-bit transfer mode ?

[durna]

Hello,
I need to communicate with a component whose configuration register expects a 9-bit value. However, my current SPI setup sends data in 8-bit chunks.

In this case, what would be the correct approach? Should I send two bytes back-to-back? Or would it be better to make this configurable in main.c so that the behavior can be adjusted easily?

If you were in my place, how would you handle this?

static void MX_SPI3_Init(uint32_t bit_size)
{

  /* USER CODE BEGIN SPI3_Init 0 */

  /* USER CODE END SPI3_Init 0 */

  /* USER CODE BEGIN SPI3_Init 1 */

  /* USER CODE END SPI3_Init 1 */
  /* SPI3 parameter configuration*/
  hspi3.Instance = SPI3;
  hspi3.Init.Mode = SPI_MODE_MASTER;
  hspi3.Init.Direction = SPI_DIRECTION_2LINES;
  hspi3.Init.DataSize = bit_size;                 //  <-- here
  hspi3.Init.CLKPolarity = SPI_POLARITY_LOW;
  hspi3.Init.CLKPhase = SPI_PHASE_1EDGE;
  hspi3.Init.NSS = SPI_NSS_SOFT;
  hspi3.Init.BaudRatePrescaler = SPI_BAUDRATEPRESCALER_8;
  hspi3.Init.FirstBit = SPI_FIRSTBIT_MSB;
  hspi3.Init.TIMode = SPI_TIMODE_DISABLE;
  hspi3.Init.CRCCalculation = SPI_CRCCALCULATION_DISABLE;
  hspi3.Init.CRCPolynomial = 0x0;
  hspi3.Init.NSSPMode = SPI_NSS_PULSE_ENABLE;
  hspi3.Init.NSSPolarity = SPI_NSS_POLARITY_LOW;
  hspi3.Init.FifoThreshold = SPI_FIFO_THRESHOLD_01DATA;
  hspi3.Init.TxCRCInitializationPattern = SPI_CRC_INITIALIZATION_ALL_ZERO_PATTERN;
  hspi3.Init.RxCRCInitializationPattern = SPI_CRC_INITIALIZATION_ALL_ZERO_PATTERN;
  hspi3.Init.MasterSSIdleness = SPI_MASTER_SS_IDLENESS_00CYCLE;
  hspi3.Init.MasterInterDataIdleness = SPI_MASTER_INTERDATA_IDLENESS_00CYCLE;
  hspi3.Init.MasterReceiverAutoSusp = SPI_MASTER_RX_AUTOSUSP_DISABLE;
  hspi3.Init.MasterKeepIOState = SPI_MASTER_KEEP_IO_STATE_DISABLE;
  hspi3.Init.IOSwap = SPI_IO_SWAP_DISABLE;
  if (HAL_SPI_Init(&hspi3) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN SPI3_Init 2 */

  /* USER CODE END SPI3_Init 2 */

}

[Andrew Neil]

What you need to do depends on what the slave requires/allows.

You haven't given any details of the slave.

Have you contacted the slave's manufacturer for advice?

---

@durna wrote:

If you were in my place, how would you handle this?

---

For the purpose of experimentation, bit-bang it to find what works.

Once you've found what works, see if that can be implemented using the STM32's SPI hardware.

[durna]

I only need to send a 9-bit configuration value once during initialization. After that, all read/write operations will use standard 8-bit communication.

[durna · Accepted solution]

I've found the answer :)