title: SPI Slave setup on STM32F4 board
body: <p>I am trying to communicate between two STM32F4 discovery boards via SPI in Master &amp; Slave configuration. I already have the code for the master but I am a bit confused on the changes I need to make on the SPI initialisation for the slave. </p>
  
  <p>I would also like to implement an interrupt whenever the master is sending data, rather than having the slave poll the RXNE register all the time. However, I am unsure of the exact configurations for the NVIC for the SPI.</p>
  
  <p>Below is the master's configuration code</p>
  
  <pre><code>void init_SPI1(void)
  {
      GPIO_InitTypeDef GPIO_InitStruct;
      SPI_InitTypeDef SPI_InitStruct;
  
      // enable clock for used IO pins
      RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);
  
      /* configure pins used by SPI1
       * PA5 = SCK
       * PA6 = MISO
       * PA7 = MOSI
       */
      GPIO_InitStruct.GPIO_Pin = GPIO_Pin_7 | GPIO_Pin_6 | GPIO_Pin_5;
      GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF;
      GPIO_InitStruct.GPIO_OType = GPIO_OType_PP;
      GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;
      GPIO_InitStruct.GPIO_PuPd = GPIO_PuPd_NOPULL;
      GPIO_Init(GPIOA, &amp;GPIO_InitStruct);
  
      // connect SPI1 pins to SPI alternate function
      GPIO_PinAFConfig(GPIOA, GPIO_PinSource5, GPIO_AF_SPI1);
      GPIO_PinAFConfig(GPIOA, GPIO_PinSource6, GPIO_AF_SPI1);
      GPIO_PinAFConfig(GPIOA, GPIO_PinSource7, GPIO_AF_SPI1);
  
      // enable clock for used IO pins
      RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOE, ENABLE);
  
      /* Configure the chip select pin
         in this case we will use PE7 */
      GPIO_InitStruct.GPIO_Pin = GPIO_Pin_7;
      GPIO_InitStruct.GPIO_Mode = GPIO_Mode_OUT;
      GPIO_InitStruct.GPIO_OType = GPIO_OType_PP;
      GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;
      GPIO_InitStruct.GPIO_PuPd = GPIO_PuPd_UP;
      GPIO_Init(GPIOE, &amp;GPIO_InitStruct);
  
      GPIOE-&gt;BSRRL |= GPIO_Pin_7; // set PE7 high
  
      // enable peripheral clock
      RCC_APB2PeriphClockCmd(RCC_APB2Periph_SPI1, ENABLE);
  
      /* configure SPI1 in Mode 0 
       * CPOL = 0 --&gt; clock is low when idle
       * CPHA = 0 --&gt; data is sampled at the first edge
       */
      SPI_InitStruct.SPI_Direction = SPI_Direction_2Lines_FullDuplex; // set to full duplex mode, seperate MOSI and MISO lines
      SPI_InitStruct.SPI_Mode = SPI_Mode_Master;     // transmit in master mode, NSS pin has to be always high
      SPI_InitStruct.SPI_DataSize = SPI_DataSize_8b; // one packet of data is 8 bits wide
      SPI_InitStruct.SPI_CPOL = SPI_CPOL_Low;        // clock is low when idle
      SPI_InitStruct.SPI_CPHA = SPI_CPHA_1Edge;      // data sampled at first edge
      SPI_InitStruct.SPI_NSS = SPI_NSS_Soft | SPI_NSSInternalSoft_Set; // set the NSS management to internal and pull internal NSS high
      SPI_InitStruct.SPI_BaudRatePrescaler = SPI_BaudRatePrescaler_4; // SPI frequency is APB2 frequency / 4
      SPI_InitStruct.SPI_FirstBit = SPI_FirstBit_MSB;// data is transmitted MSB first
      SPI_Init(SPI1, &amp;SPI_InitStruct); 
  
      SPI_Cmd(SPI1, ENABLE); // enable SPI1
  }
  </code></pre>
  
  <p>The reference manual states that I need the CPOL and CPHA bits to be configured the same as the master, this is also true for MSB/LSB first frame format. Apart from this I am unsure on how to configure the rest.</p>