void spi_init(void) {
    /* Wait for synchronization */
    while(BOOT_SPI_MODULE->SPI.SYNCBUSY.bit.ENABLE);

    /* Disable the SERCOM SPI module */
    BOOT_SPI_MODULE->SPI.CTRLA.bit.ENABLE = 0;

    /* Wait for synchronization */
    while(BOOT_SPI_MODULE->SPI.SYNCBUSY.bit.SWRST);

    /* Perform a software reset */
    BOOT_SPI_MODULE->SPI.CTRLA.bit.SWRST = 1;

    /* Wait for synchronization */
    while(BOOT_SPI_MODULE->SPI.CTRLA.bit.SWRST);

    /* Wait for synchronization */
    while(BOOT_SPI_MODULE->SPI.SYNCBUSY.bit.SWRST || BOOT_SPI_MODULE->SPI.SYNCBUSY.bit.ENABLE);


    BOOT_SPI_MODULE->SPI.CTRLA.reg = SERCOM_SPI_CTRLA_MODE_SPI_MASTER | SERCOM_SPI_CTRLA_DIPO(BOARD_SERCOM_DIPO_PORT) | SERCOM_SPI_CTRLA_DOPO(BOARD_SERCOM_DOPO_PORT) |  SERCOM_SPI_CTRLA_DORD;


    // mux the pins
    // we use PMUXE for the PMUXO pins as the .bit.PMUXO already gives us the offset
    #if BOARD_MOSI_PIN % 2
    PORT->Group[BOARD_MOSI_PORT].PMUX[BOARD_MOSI_PIN / 2].bit.PMUXO = PORT_PMUX_PMUXE_D;
    #else
    PORT->Group[BOARD_MOSI_PORT].PMUX[BOARD_MOSI_PIN / 2].bit.PMUXE = PORT_PMUX_PMUXE_D;
    #endif
    #if BOARD_MISO_PORT % 2
    PORT->Group[BOARD_MISO_PORT].PMUX[BOARD_MISO_PIN / 2].bit.PMUXO = PORT_PMUX_PMUXE_D;
    #else
    PORT->Group[BOARD_MISO_PORT].PMUX[BOARD_MISO_PIN / 2].bit.PMUXE = PORT_PMUX_PMUXE_D;
    #endif
    #if BOARD_SCK_PIN % 2
    PORT->Group[BOARD_SCK_PORT].PMUX[BOARD_SCK_PIN / 2].bit.PMUXO = PORT_PMUX_PMUXE_D;
    #else
    PORT->Group[BOARD_SCK_PORT].PMUX[BOARD_SCK_PIN / 2].bit.PMUXE = PORT_PMUX_PMUXE_D;
    #endif

    PORT->Group[BOARD_MOSI_PORT].PINCFG[BOARD_MOSI_PIN].bit.PMUXEN = 1;
    PORT->Group[BOARD_MISO_PORT].PINCFG[BOARD_MISO_PIN].bit.PMUXEN = 1;
    PORT->Group[BOARD_SCK_PORT].PINCFG[BOARD_SCK_PIN].bit.PMUXEN = 1;

    /* Enable transmit and receive and set data size to 8 bits */
    BOOT_SPI_MODULE->SPI.CTRLB.reg = SERCOM_SPI_CTRLB_RXEN | SERCOM_SPI_CTRLB_CHSIZE(0);


    while(BOOT_SPI_MODULE->SPI.SYNCBUSY.bit.CTRLB);

    /* Load the baud value */
    GCLK->CLKCTRL.reg = GCLK_CLKCTRL_GEN_GCLK0 | GCLK_CLKCTRL_CLKEN | BOARD_GCLK_ID;
    while(GCLK->STATUS.bit.SYNCBUSY); //Wait for synchronisation
    BOOT_SPI_MODULE->SPI.BAUD.reg = SERCOM_SPI_BAUD_BAUD(60);

    /* Wait for synchronization */
    while(BOOT_SPI_MODULE->SPI.SYNCBUSY.bit.ENABLE);
    /* Enable SERCOM UART */
    BOOT_SPI_MODULE->SPI.CTRLA.bit.ENABLE = 1;

    while(BOOT_SPI_MODULE->SPI.SYNCBUSY.bit.ENABLE);
}

void spi_transfer(uint8_t* send_buff, unsigned int bc, uint8_t* receive_buff) {
    do {
        while(!BOOT_SPI_MODULE->SPI.INTFLAG.bit.DRE);
        if (send_buff == NULL) {
            BOOT_SPI_MODULE->SPI.DATA.reg = 0xFF;
        } else {
            BOOT_SPI_MODULE->SPI.DATA.reg = *send_buff++;
        }

        while(!BOOT_SPI_MODULE->SPI.INTFLAG.bit.TXC);
        while(!BOOT_SPI_MODULE->SPI.INTFLAG.bit.RXC);

        uint8_t b = BOOT_SPI_MODULE->SPI.DATA.reg;
        if ( receive_buff != NULL ) {
            *receive_buff++ = b;
        }
    } while (--bc);
}