void SPIBLOCKING_write(Spi *spid, const uint8_t *txbuf, uint16_t len)
{
  SPI_TypeDef* spi = spid->spi;

  while (len--) {
    spi->DR = *txbuf++;             // write a byte

    while (!(spi->SR & SPI_SR_RXNE));
    (void)spi->DR; // dummy read

    while (!(spi->SR & SPI_SR_TXE)); // make sure previous TX is done
  }
}

void SPIBLOCKING_read(Spi *spid, uint8_t *rxbuf, uint16_t len)
{
  SPI_TypeDef* spi = spid->spi;

  while (len--) {
    spid->spi->DR = 0xFF;             // write a dummy byte

    while(!(spi->SR & SPI_SR_RXNE));  // wait for data
    *rxbuf++ = spi->DR;               // read it

    while(!(spi->SR & SPI_SR_TXE));   // make sure previous TX is done
  }
}