Question

I need to write an I2C program which can write data to 32 bit and 16bit register address using the I2C_RDWR ioctl.

From what i know,the device address and data are together passed into the device using the same buffer buf in structure i2c_msg defined in the linux i2c driver :

struct i2c_msg {
        __u16 addr;      slave address
        __u16 flags;
#define I2C_M_TEN       0x10     we have a ten bit chip address
#define I2C_M_RD        0x01
#define I2C_M_NOSTART   0x4000
#define I2C_M_REV_DIR_ADDR      0x2000
#define I2C_M_IGNORE_NAK        0x1000
#define I2C_M_NO_RD_ACK         0x0800
        __u16 len;               msg length
        __u8 *buf;               pointer to msg data
};

Currently i am passing 32 bit register address as below :

    buf[3] = reg_addr & 0xff;
    buf[2] = (reg_addr >> 8) & 0xff;
    buf[1] = (reg_addr >> 16) & 0xff;
    buf[0] = (reg_addr >>24) & 0xff;
 //buf[4,5...] = data;
    msg.buf = buf;

If the register address is 16 bit,the following code is executed :

    buf[1] = (reg_addr & 0xff);
    buf[0] = (reg_addr >> 8);
    buf[2] = data;
 // buf[2,3...] = data;

How do i let the device know how many buffer arrays contain the address?

Also,is this code correct?

Unfortunately i dont have any I2C device at present to test.

Any help is welcome.

Thanx.

Answer 1

The I2C command will send a stream of bytes to the device selected by the I2C address. It is entirely up to the device how it interprets these, and you need to read the device datasheet to find out.

For example, consider Atmel I2C EEPROMs. Devices 512kBit or less use a fixed 16 bit address, and the 1MBit AT24C1024B uses a 17 bit address. 16 bits of this appear in the I2C command stream, and the MSB is encoded into the I2C device address.