I²C (Integer-Integrated Circuit) also known as I2C, IIC, I-squared-C is a multimaster serial single-ended computer bus which originates from Philips semiconductor division. It is commonly used to attach low-speed peripherals to motherboards.

If you happen to have I²C controller which is recognized by the kernel you can run i2cdetect to enumerate slaves sitting on the bus 1. You can easily see if your kernel has recognized any I²C controllers just by checking whether any /dev/i2c-* block devices are present. The only required argument is the number of the bus, where zero translates to /dev/i2c-0.

i2cdetect 0

I am not sure what' going on with the slave UU on the first I²C of Cubietruck, but as far as I know I am currently running a kernel that does not actually power up that particual I²C bus:

WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-0.
I will probe address range 0x03-0x77.
Continue? [Y/n] y
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- UU -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

The second bus gives me some output:

WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-1.
I will probe address range 0x03-0x77.
Continue? [Y/n] y
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- 37 -- -- 3a -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: 50 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Interestingly all the slaves dissapear when I unplug the HDMI cable from my Cubietruck, that's because the second I²C bus is connected to HDMI port.

You can use i2c-tools -l to list I²C controllers:

i2c-tools -l

i2c-0       i2c             sunxi-i2c.0                             I2C adapter
i2c-1       i2c             sunxi-hdmi-i2c                          I2C adapter

You probably have to install i2c-tools in order to use that command:

sudo apt-get install i2c-tools picocom

In this example we'll connect an I²C based accelerometer to a computer's USB port via Buspirate. Frist off we need tools to talk to the Buspirate and I²C devices:

sudo apt-get install i2c-tools picocom

Buspirate is a nifty tool for converting USB port to various other ports: SPI, I²C.

picocom -b 115200 -p n -d 8 /dev/ttyUSB0

This should open up Buspirate prompt:

Bus Pirate v3b
Firmware v5.10 (r559)  Bootloader v4.4
DEVID:0x0447 REVID:0x3043 (24FJ64GA002 B5)
http://dangerousprototypes.com
HiZ>

Typing m and pressing Enter outputs the list of wire protocols this particular Buspirate can emulate:

HiZ>m
1. HiZ
2. 1-WIRE
3. UART
4. I²C
5. SPI
6. 2WIRE
7. 3WIRE
8. LCD
9. DIO
x. exit(without change)

To enter I²C mode press 4 and Enter. You'll be greeted with I²C prompt which actually accepts some Buspirate commands like enabling power on 3.3V and 5V pins:

I²C>W
Power supplies ON

You can use I²C command (1) to enumerate slaves connected to the bus. This is what I get with GY-50 accelerometer attached:

I²C>(1)
Searching I²C address space. Found devices at:
0xD2(0x69 W) 0xD3(0x69 R)

With DS1307 I get following:

I²C>(1)
Searching I²C address space. Found devices at:
0xA0(0x50 W) 0xA1(0x50 R)

DS1307 does not respond properly with 3.3V volt power supply.

1

Scanning a I²C bus for available slave devices