Example interface register write routine for Analog Devices ADAU1701 DSP in C.

ADAU1701_InterfaceWrite.c

/* Volume levels */
const int dspVolume[] = {

    0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x01,
    0x00, 0x00, 0x00, 0x02,
    0x00, 0x00, 0x00, 0x03,
    0x00, 0x00, 0x00, 0x04,
    0x00, 0x00, 0x00, 0x05,
    0x00, 0x00, 0x00, 0x06,
    0x00, 0x00, 0x00, 0x07,
    0x00, 0x00, 0x00, 0x08,
    0x00, 0x00, 0x00, 0x09,
    0x00, 0x00, 0x00, 0x0A,
    0x00, 0x00, 0x00, 0x0B,
    0x00, 0x00, 0x00, 0x0C,
    0x00, 0x00, 0x00, 0x0D,
    0x00, 0x00, 0x00, 0x0E,
    0x00, 0x00, 0x00, 0x0F,
    0x00, 0x00, 0x00, 0x10,
    0x00, 0x00, 0x00, 0x11,
    0x00, 0x00, 0x00, 0x12,
    0x00, 0x00, 0x00, 0x13,
    0x00, 0x00, 0x00, 0x14,
    0x00, 0x00, 0x00, 0x15,
    0x00, 0x00, 0x00, 0x16,
    0x00, 0x00, 0x00, 0x17,
    0x00, 0x00, 0x00, 0x18,
    0x00, 0x00, 0x00, 0x19,
    0x00, 0x00, 0x00, 0x1A,
    0x00, 0x00, 0x00, 0x1B,
    0x00, 0x00, 0x00, 0x1C,
    0x00, 0x00, 0x00, 0x1D,
    0x00, 0x00, 0x00, 0x1E,
    0x00, 0x00, 0x00, 0x1F,
    0x00, 0x00, 0x00, 0x20,
    0x00, 0x00, 0x00, 0x21,
};

/* Function to write volume level to Interface Register 0 */
void UpdateVolume(int newVolumeIndex) {

    int kk = 0;

    AllowInterfaceWrite();

    I2CMasterStart();
    I2CMasterWriteByte(DSP_WRITE_ADDRESS); /* DSP address */
    I2CMasterWriteByte(0x08); /* Subaddress */
    I2CMasterWriteByte(0x00);
    while (kk < 4) {
        I2CMasterWriteByte(dspVolume[(newVolumeIndex * 4) + kk]); /* New volume data - 4 bytes */
        kk++;
    }
    I2CMasterStop();

    DisallowInterfaceWrite();
}

/* Unlock interface register */
void AllowInterfaceWrite() {
    I2CMasterStart();
    I2CMasterWriteByte(DSP_WRITE_ADDRESS);
    I2CMasterWriteByte(0x08); /* DSP Core Register */
    I2CMasterWriteByte(0x1C);
    I2CMasterWriteByte(0x00); /* Set IFCW bit to allow writing to interface registers */
    I2CMasterWriteByte(0x5C);
    I2CMasterStop();
}

/* Lock interface register */
void DisallowInterfaceWrite() {
    I2CMasterStart();
    I2CMasterWriteByte(DSP_WRITE_ADDRESS);
    I2CMasterWriteByte(0x08); /* DSP Core Register */
    I2CMasterWriteByte(0x1C);
    I2CMasterWriteByte(0x00); /* Clear IFCW bit to disallow writing to interface registers */
    I2CMasterWriteByte(0x1C);
    I2CMasterStop();
}