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stm32f207 use nss pin as gpio when using SPI

Question asked by ericsson.ola on Sep 3, 2013

Hi everyone,
I have a problem with a stm32f207 device.
I use the NSS pin of the SPI2 as a GPO. (The chip select to my SPI2 devices comes from an external I2C IO expander).
NSS pin (I4) seems to be working ok as a GPO until I enable SPI2 interface.

If I have the I4 (NSS) high when I enable the SPI2 I can see that it goes to approx. 2v.
This means as far as I understand that the NSS pin goes low, which it should when I enable the SPI, but I have not assigned it to its alternative function.

Can anyone spot what I have missed?

I think this is the relevant init code…

  //Configure PI.4 as motor control */
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_OUT;
  GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_4;
  GPIO_Init(GPIOI, &GPIO_InitStructure);
  // Connect PI.1 to SD_SPI pins: SCK
  GPIO_PinAFConfig(GPIOI, GPIO_PinSource1, GPIO_AF_SPI2);
  //Configure PI.2 as SD_SPI pins: MISO
  GPIO_PinAFConfig(GPIOI, GPIO_PinSource2, GPIO_AF_SPI2);
  //Configure PI.3 as SD_SPI pins: MOSI
  GPIO_PinAFConfig(GPIOI, GPIO_PinSource3, GPIO_AF_SPI2);

  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
  GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
  GPIO_InitStructure.GPIO_PuPd  = GPIO_PuPd_UP;
  // Configure CLK, MISO, MOSI pins
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_1 | GPIO_Pin_2 | GPIO_Pin_3;
  GPIO_Init(GPIOI, &GPIO_InitStructure);
  // SPI configuration
  SPI_InitStructure.SPI_Direction = SPI_Direction_2Lines_FullDuplex;
  SPI_InitStructure.SPI_Mode = SPI_Mode_Master;
  SPI_InitStructure.SPI_DataSize = SPI_DataSize_8b;
  SPI_InitStructure.SPI_CPOL = SPI_CPOL_High;
  SPI_InitStructure.SPI_CPHA = SPI_CPHA_2Edge;
  SPI_InitStructure.SPI_NSS = SPI_NSS_Soft;
  SPI_InitStructure.SPI_BaudRatePrescaler = SPI_BaudRatePrescaler_4;

  SPI_InitStructure.SPI_FirstBit = SPI_FirstBit_MSB;
  SPI_InitStructure.SPI_CRCPolynomial = 7;
  SPI_Init(SPI2, &SPI_InitStructure);

  /*!< Enable the SPI  */