16 bit register addressing through i2c Note: I'm using just the atmega chip and atmel studio. Not exactly a full arduino.

I have been using the atmega168 with a MAX17040 fuel gauge in a project. The I2C interface between the atmel and the fuel gauge is working fine and I am able to read data from the fuel gauge.

Due to the power requirement of the project, the fuel gauge requires its compensation value to be adjusted. This is a 16 bit register, so according to the manufacturer I have to write 0x1234 across address 0x0C and 0X0D in one go. Otherwise the values are discarded.

All the datasheet and online forums talk about 8-bit I2C addressing (which I have working), but instruction on addressing 16 bit register is limited. I was wondering if anyone can help me on this.

My basic code is pasted below. The I2C_start takes care of TWCR, TWDR, etc. It could be something to do with Repated_Start, but not sure.

uint8_t I2C_write_register(uint8_t device, uint8_t address, uint8_t value)
{
    device &lt;&lt;= 1;

    uint8_t ret = I2C_start(device | TW_WRITE);
    if (ret)
        return 1;

    if (!ret)
        ret = I2C_write(address);
    if (!ret)
        ret = I2C_write(value);
    I2C_stop();

    return ret;
}

uint8_t I2C_start(uint8_t address){
    // reset TWI control register
    TWCR = 0;
    // transmit START condition
    TWCR = _BV(TWINT) | _BV(TWSTA) | _BV(TWEN);
    // wait for end of transmission
    while( !(TWCR &amp; _BV(TWINT)) );

    // check if the start condition was successfully transmitted
    if((TWSR &amp; 0xF8) != TW_START){ return 1; }

    // load slave address into data register
    TWDR = address;
    // start transmission of address
    TWCR = _BV(TWINT) | _BV(TWEN)|_BV(TWEA); //|_BV(TWEA) added
    // wait for end of transmission
    while( !(TWCR &amp; _BV(TWINT)) );

    // check if the device has acknowledged the READ / WRITE mode
    uint8_t twst = TW_STATUS &amp; 0xF8;
    if ( (twst != TW_MT_SLA_ACK) &amp;&amp; (twst != TW_MR_SLA_ACK) ) return 1;

    return 0;
}
Ahh, was just able to solve it. Essentially, do not use TW_REPEATED_START. Just keep sending data 8 bit at a time. 1 bit at the beginning of each 8 bit will be lost, so make sure you shift them first.

Wire library uses a very similar concept for use with arduino.

uint8_t I2C_write_register(uint8_t device, uint8_t address, uint8_t value, uint8_t value)
{
    device &lt;&lt;= 1;

    uint8_t ret = I2C_start(device | TW_WRITE);
    if (ret)
        return 1;

    if (!ret)
        ret = I2C_write(address);
    if (!ret)
        ret = I2C_write(value);
    if (!ret)
        ret = I2C_write(value2);
    I2C_stop();

    return ret;
}