spi_xfer:
        SPICSR = 0x0f ^ (1 << cn_id)

        for (i=0; i<=N; i++) {
                if (i != N) {
                        while (!(SPISR & 0x10));        /* wait TXRDY */
                        SPITXDR = data[i];
                }

                if (i != 0) {
                        while (!(SPISR & 0x08));        /* wait RXRDY */
                        data[i-1] = SPIRXDR;
                }
        }

        SPICSR = 0x0f