16 bit register addressing through i2c

Question

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 <<= 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 & _BV(TWINT)) );
 // check if the start condition was successfully transmitted
 if((TWSR & 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 & _BV(TWINT)) );
 // check if the device has acknowledged the READ / WRITE mode
 uint8_t twst = TW_STATUS & 0xF8;
 if ( (twst != TW_MT_SLA_ACK) && (twst != TW_MR_SLA_ACK) ) return 1;
 return 0;
}

• Ignacio Vazquez-Abrams: How does Wire not handle this already?
• tsf144: I should add, i'm not using a full arduino, just the atmega168 chip. And i'm using twi.h library from atmel studio.
• Ignacio Vazquez-Abrams: So... then I don't understand why you asked this here.
• tsf144: Arduino and the atmega168 are essentially the same core. If the arduino can address 16-bit word over I2C, the atmega can do it too.
• Ignacio Vazquez-Abrams: But why ask in the community where they use Wire, when your needs would be better served by a wider audience?

Answer 1

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 <<= 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;
}