How to address i2c slaves

[GoogleCodeExporter]

 It's not really a problem, but I'm rather perplexed as to how to talk to specific I2C slaves. I only saw the i2ceeprom example, which somehow doesn't seem like what I need.

 I basically have an I2C slave on address 0x48 (it's an A->D converter, but that doesn't really matter). Later on I'll also add another I2C slave on the same line, so I really need to specify the proper addresses to talk to them.

 So how could I do something like:

#define ADS1015_REG_POINTER_CONFIG      (0x01) 
#define ADS1015_REG_POINTER_CONVERT     (0x00)

 int Address=0x48;
 ioctl(i2cFile, I2C_SLAVE, Address);
 i2c_smbus_write_word_data(i2cFile,ADS1015_REG_POINTER_CONFIG,0x1122);
 i2c_smbus_read_word_data(i2cFile,ADS1015_REG_POINTER_CONVERT);

 with libmpsse?

Original issue reported on code.google.com by Rezodl...@gmail.com on 10 Mar 2014 at 8:51

[GoogleCodeExporter]

 Posting a solution if anyone else gets perplexed like me ...

 Basically, you have to start writing to I2C with the address of slave SHIFTED 1 bit to the left (last bit 0 for writes, or 1 for reads) so the above code would be something like:

//writing part
 char Data={"\0x90\0x01\x11\x22"};
 Start(context);
 Write (Context,Data,4);  //equals ioctl and i2c_smbus_write_word_data above
 Stop(context);

//reading part
 Start(Context);
 Data[1]=\0x00;           // ADS1015_REG_POINTER_CONVERT=0x00
 Write (Context,Data,2);  //set the register for read
 Stop(context);
 Start(Context);
 Data[0]=\0x91;
 Write (Context,Data,1);  //State you want to read from address 0x48 (0x48<<1)|1
 Read (Context,2);        //read the word (2 bytes) of data
 Stop(Context);

Original comment by Rezodl...@gmail.com on 19 Mar 2014 at 5:24

[dereks]

This was useful to me. Thanks for taking the time to post the solution.

In the i2ceeprom.c example, it writes each of these values as the start of each transaction:

#define WCMD    "\xA0\x00\x00"  // Write start address command
#define RCMD    "\xA1"      // Read command

The Write Command (WCMD) is this:

0xA0: The I2C header: Slave Address of the eeprom device shifted left one bit, with the Read Flag (bit0) set to zero. 0x0000: The eeprom's 32-bit start address to write to, in this example, it starts at zero (0x0000).

The Read Command (RCMD) is just the I2C Header: the Slave Address byte again (0xA0), but this time with the Read Flag set to one (0x1). So (0xA0 | 0x1) == 0xA1, and the eeprom responds with its saved bytes (starting at 0x0000).