SPI 8-Bit frame

[LuLeWi]

Hi, 

I hope this is the right place to ask. I'm currently working on interfacing an SD Card with a nucleo f767. I used the DS-Bits in CR2 to set the data frame to 8 Bit (what also should be the reset value). Despite that, MOSI always sends the whole data register, which is 16 Bits. So if I use the function below to send 0xFF, the hardware will send 0x00FF. Does anybody know this issue? 

uint8_t spi_txrx(uint8_t b) {

  while (!(SPI3->SR & SPI_SR_TXE));

  SPI3->DR = b;

  while (!(SPI3->SR & SPI_SR_RXNE));

  return SPI3->DR;

}

[TDK]

Write 8 bits to the register instead of 16:

*(volatile uint8_t*)&SPI3->DR = b;

Look up data packing in the Reference Manual.

[LuLeWi]

Thank you for the quick response. I already tried this cast, but it leads into a system crash. With the following version, the cmd0 command works. Is it save to use?
void spi_send(uint8_t b) {
while (!(SPI3->SR & SPI_SR_TXE));
__asm__ volatile("strb %0, [%1]"
:
: "r"(b), "r"(&SPI3->DR)
: "memory");
while (!(SPI3->SR & SPI_SR_TXE));
}

[TDK]

Maybe this instead:

*((volatile uint8_t*)&(SPI3->DR)) = b;

stm32f7xx-hal-driver/Inc/stm32f7xx_ll_spi.h at 417ddbbea8a8e3f1d7cfe07aeefae9b409b8a30d · STMicroelectronics/stm32f7xx-hal-driver