# Inter Integrated Circuit

> I²C (Inter-Integrated Circuit), pronounced I-squared-C, is a multi-master, multi-slave, single-ended, serial computer bus invented by Philips Semiconductor (now NXP Semiconductors). It is typically used for attaching lower-speed peripheral ICs to processors and microcontrollers. [Wikipedia](https://en.wikipedia.org/wiki/I²C)

* [Linux Journal Greg Kroah-Hartman I2C Drivers Part I](http://www.linuxjournal.com/article/7136)
* [Linux Journal Greg Kroah-Hartman I2C Drivers Part II](http://www.linuxjournal.com/article/7252)

### Required Hardware

One I2C sensor

## Kernel Integration

### Kernel Display Message

```bash
    root@edison:~# dmesg | grep -i i2c
    [    0.190675] I2C bus = 1, name =      pcal9555a-1, irq = 0x 0, addr = 0x20
    [    0.190711] I2C bus = 1, name =      pcal9555a-2, irq = 0x 0, addr = 0x21
    [    0.190743] I2C bus = 1, name =      pcal9555a-3, irq = 0x 0, addr = 0x22
    [    0.190774] I2C bus = 1, name =      pcal9555a-4, irq = 0x 0, addr = 0x23
    [    0.746686] i2c /dev entries driver
```

## Userspace Interfaces

### SysFs

```bash
    root@edison:~# ls /sys/class/i2c-dev/
    i2c-1  i2c-2  i2c-3  i2c-4  i2c-5  i2c-6  i2c-7
```

### DevFs

```bash
    root@edison:~# ls /dev/i2c-*
    /dev/i2c-1  /dev/i2c-2  /dev/i2c-3  /dev/i2c-4  /dev/i2c-5  /dev/i2c-6  /dev/i2c-7
```

### ProcFs

```bash
    root@edison:~# cat /proc/interrupts | grep i2c
      7:         44          0   IO-APIC-fasteoi   i2c-    designware-1
     10:          0          0   IO-APIC-fasteoi   i2c-designware-2
     12:          0          0   IO-APIC-fasteoi   watchdog, i2c-designware-3
     13:          0          0   IO-APIC-fasteoi   i2c-designware-4
     14:          0          0   IO-APIC-fasteoi   i2c-designware-5
     15:          0          0   IO-APIC-fasteoi   i2c-designware-6
     16:          0          0   IO-APIC-fasteoi   i2c-designware-7
```

## Applications / Libraries

### Setup

#### Opkg

```bash
    root@edison:~# opkg install i2c-tools
    Package i2c-tools (3.1.1-r0) installed in root is up to date.
    ...
```

### Programs

#### i2cdetect

```bash
    root@edison:~# i2cdetect -y -r 1
         0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
    00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
    10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
    20: UU UU UU UU -- -- -- -- -- -- -- -- -- -- -- -- 
    30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
    40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
    50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
    60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
    70: -- -- -- -- -- -- -- --
```


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