STM32F303 SPI alternative use of the NSS pin problem

Don't define it as an AF pin?

I've had the exact same issue!

I used the STM32CubeMX application to specify SPI2 with software controlled nSS and set the nSS pin (Port B Pin 12) as a GPIO output.  The auto-generated intialisation code looks fine (extracts below):

main()

...

MX_GPIO_Init()

...

  GPIO_InitStruct.Pin = GPIO_PIN_0|GPIO_PIN_12|GPIO_PIN_4;

  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;

  GPIO_InitStruct.Pull = GPIO_NOPULL;

  GPIO_InitStruct.Speed = GPIO_SPEED_LOW;

  HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

...

MX_SPI2_Init

{

  hspi2.Instance = SPI2;

  hspi2.Init.Mode = SPI_MODE_MASTER;

  hspi2.Init.Direction = SPI_DIRECTION_2LINES;

  hspi2.Init.DataSize = SPI_DATASIZE_8BIT;

  hspi2.Init.CLKPolarity = SPI_POLARITY_LOW;

  hspi2.Init.CLKPhase = SPI_PHASE_1EDGE;

  hspi2.Init.NSS = SPI_NSS_SOFT;

  hspi2.Init.BaudRatePrescaler = SPI_BAUDRATEPRESCALER_8;

  hspi2.Init.FirstBit = SPI_FIRSTBIT_MSB;

  hspi2.Init.TIMode = SPI_TIMODE_DISABLED;

  hspi2.Init.CRCCalculation = SPI_CRCCALCULATION_DISABLED;

  hspi2.Init.NSSPMode = SPI_NSS_PULSE_ENABLED;

  HAL_SPI_Init(&hspi2);

}

What I noticed is that after MX_GPIO_Init() the mode set for Port B pin 12 is “GPIO Ouptut�?, but after the call to MX_SPI2_Init() the mode is “Alternate Function�?.  This seems to be as a result of setting the SPI2 configuration registers (not any direct access to the Port’s MODER register), so the peripheral is taking control of the pin even though we specified a software driven nSS.  This doesn’t appear to happen to the nSS pin for SPI1.

The way I fixed it was to manually restore Port B Pin 12 to the correct mode after all the other initialisation:

#define STM32_PIN_MODE_BITS_PER_PIN  (2)     /**< Number of bits used to specify each pin's mode */

#define STM32_PIN_MODE_BITS_MASK     (3) /**< Selects all bits relative to the GPIO modes */

/** Mode that each GPIO pin can have.  Each pin can be one of these modes */

typedef enum

{

    STM32_PIN_MODE_GPIO_INPUT = 0,

    STM32_PIN_MODE_GPIO_OUTPUT = 1,

    STM32_PIN_MODE_ALTERNATE_FUNCTION = 2,

    STM32_PIN_MODE_ANALOG = 3,

}

Stm32GpioPinMode;

/** Function used to explicitly set the mode of any GPIO pin. Note that the pin number

* is an integer note a pin bit map, so use ''4'' not ''GPIO_PIN_4'' */

void stm32_set_pin_mode(GPIO_TypeDef * pGpio, uint8_t pinNumber, Stm32GpioPinMode mode)

{

    pGpio->MODER &= ~(STM32_PIN_MODE_BITS_MASK<<(pinNumber*STM32_PIN_MODE_BITS_PER_PIN));

    pGpio->MODER |= (mode<<(pinNumber*STM32_PIN_MODE_BITS_PER_PIN));

}

main()

...

   * pin, which means the GPIO is claimed by the SPI peripheral as a dedicated nSS

   * pin, even though we request a software driven nSS.  This work around resets the

   * pin as an output.  This doesn't seem to happen with SPI1 */

    stm32_set_pin_mode(GPIOB, 12, STM32_PIN_MODE_GPIO_OUTPUT);

}

SPI_NSS_PULSE_DISABLED