STM32F4 SPI issue - c

Question

I'am using STM32F407vg and i'am trying to write data in SPI data register, the following code shows the configuration function
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
* PA4 = NSS
* PA5 = SCK
* PA6 = MISO
* PA7 = MOSI
*/
GPIO_InitStruct.GPIO_Pin = GPIO_Pin_7 | GPIO_Pin_6 | GPIO_Pin_5|GPIO_Pin_4;
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, &GPIO_InitStruct);
// connect SPI1 pins to SPI alternate function
GPIO_PinAFConfig(GPIOA, GPIO_PinSource4, GPIO_AF_SPI1);
GPIO_PinAFConfig(GPIOA, GPIO_PinSource5, GPIO_AF_SPI1);
GPIO_PinAFConfig(GPIOA, GPIO_PinSource6, GPIO_AF_SPI1);
GPIO_PinAFConfig(GPIOA, GPIO_PinSource7, GPIO_AF_SPI1);
//Set chip select high
GPIOA->BSRRL |= GPIO_Pin_4; // set PE4 high
// enable peripheral clock
RCC_APB2PeriphClockCmd(RCC_APB2Periph_SPI1, ENABLE);
/* configure SPI1 in Mode 0
* CPOL = 0 --> clock is low when idle
* CPHA = 0 --> data is sampled at the first edge
*/
SPI_StructInit(&SPI_InitStruct); // set default config
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 ; // 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, &SPI_InitStruct);
SPI_Cmd(SPI1, ENABLE); // enable SPI1
}
and this is the function that send data
uint8_t SPI1_send(uint8_t data){
SPI1->DR = data; // write data to be transmitted to the SPI data register
while( !(SPI1->SR & SPI_I2S_FLAG_TXE) ); // wait until transmit complete
while( !(SPI1->SR & SPI_I2S_FLAG_RXNE) ); // wait until receive complete
while( SPI1->SR & SPI_I2S_FLAG_BSY ); // wait until SPI is not busy anymore
return SPI1->DR; // return received data from SPI data register
}
I think that i have set the good configuration (the choice of pins is not wrong in my opinion and also the configuration of the bus used ) the following pictures are extracted from the STM32F407 DataSheet
The problem is that the data in the DR registry is different than the data passed as argument .I don't know why this happen .Could anyone guide me to the right point

Answer 1

The SPI DR register is not a normal memory location where writing and reading access the same storage.
Instead, writing loads the output shift register, while reading reads the received input. Depending on device details, reading may also "claim" the input, clearing it from the register until another word is received.
For these reasons, trying to watch the SPI DR with a debugger is not only not going to give you the information you seek, it may even be damaging to the data you would otherwise receive.

Answer 2

After reading the SPI section from this book my problem is solved, i just connect SPI MOSI to SPI MISO pin (PA6 and PA7) then used the following function :
int spiReadWrite(SPI_TypeDef* SPIx, uint8_t *rbuf,
const uint8_t *tbuf, int cnt, enum spiSpeed speed)
{
int i;
SPIx->CR1 = (SPIx->CR1 & ~SPI_BaudRatePrescaler_256) |
speeds[speed];
for (i = 0; i < cnt; i++){
if (tbuf) {
SPI_I2S_SendData(SPIx, *tbuf++);
} else {
SPI_I2S_SendData(SPIx, 0xff);
}
while (SPI_I2S_GetFlagStatus(SPIx, SPI_I2S_FLAG_RXNE) == RESET);
if (rbuf) {
*rbuf++ = SPI_I2S_ReceiveData(SPIx);
} else {
SPI_I2S_ReceiveData(SPIx);
}
}
return i;
}
by using a debugger i can see that the data sent (from tbuf) are succesfully received (in rbuf)

Answer 3

note that SCLK_freq = APB2_freq / (BaudRatePrescaler + 1)