SPI CPHA affects NSS

[sdianoff]

I use MCU STM32L462ET6TR. When I set CPHA=1, the NSS signal is always at low. When CPHA=0, NSS signal works normally. There is my initialization code:

void MX_SPI1_Init(void)
{
  hspi1.Instance = SPI1;
  hspi1.Init.Mode = SPI_MODE_MASTER;
  hspi1.Init.Direction = SPI_DIRECTION_2LINES;
  hspi1.Init.DataSize = SPI_DATASIZE_8BIT;
  hspi1.Init.CLKPolarity = SPI_POLARITY_LOW;
  hspi1.Init.CLKPhase = SPI_PHASE_1EDGE; // NSS SIGNAL WORKS NORMALLY
//  hspi1.Init.CLKPhase = SPI_PHASE_2EDGE; // NSS SIGNAL DOESN'T WORK
  hspi1.Init.NSS = SPI_NSS_HARD_OUTPUT;
  hspi1.Init.BaudRatePrescaler = SPI_BAUDRATEPRESCALER_16;
  hspi1.Init.FirstBit = SPI_FIRSTBIT_MSB;
  hspi1.Init.TIMode = SPI_TIMODE_DISABLE;
  hspi1.Init.CRCCalculation = SPI_CRCCALCULATION_DISABLE;
  hspi1.Init.CRCPolynomial = 7;
  hspi1.Init.CRCLength = SPI_CRC_LENGTH_DATASIZE;
  hspi1.Init.NSSPMode = SPI_NSS_PULSE_ENABLE;
  if (HAL_SPI_Init(&hspi1) != HAL_OK)
  {
    Error_Handler();
  }
}

Why CPHA bit affects the NSS signal?

[waclawek.jan · Accepted solution]

This is how ST decided to implement the NSS Pulsed feature, see NSS pulse mode subchapter in RM:

This mode is activated by the NSSP bit in the SPIx_CR2 register and it takes effect only if 

the SPI interface is configured as Motorola SPI master (FRF=0) with capture on the first 

edge (SPIx_CR1 CPHA = 0, CPOL setting is ignored).

JW

[sdianoff]

Yes, thank you.

Functions HAL_SPI_Transmit and HAL_SPI_Receive doesn't set SPI disable, so that NSS doesn't go up to high. This is my mistake.

[waclawek.jan]

Note, that SPI disable does not pull NSS high; rather, it stops controlling the pin from SPI module and NSS pin goes effectively threestate. If you want it to go high, you have to switch on a pullup, or re-enable SPI immediately after disabling it.

JW