I2C (Inter-Integrated Circuit) is a serial, bus based, hardware interface for connecting peripheral devices, especially those with lower speed requirements. SMBus (System Management Bus) is derived from I2C but for the Raspberry Pi the terms are often used interchangeably.
The Raspberry Pi has two I2C buses (0 and 1) but only the second one is commonly used. Theoretically up 127 devices can be attached to each bus because each attached device is addressed with a 7-bit number but in practice this number is much lower because of electrical constraints and because certain addresses are reserved.
I2C interfaces are disabled by default. To activate them add the following lines to /boot/config.txt and reboot:
| Bus | Config Line |
|---|---|
| 0 (uncommon) | dtparam=spi=onbcm2708.vc_i2c_override=1 |
| 1 | dtparam=i2c_arm=on |
You should see a subset of the following Linux devices if everything is configured properly.
| Linux device | bus pins |
|---|---|
| /dev/i2c-0 | I2C0 SDA, SCL |
| /dev/i2c-1 | I2C1 SDA, SCL |
The table below shows where to find the I2C bus interface on the Raspberry Pi’s header.
| Description | BCM | Pin | Pin | BCM | Description |
|---|---|---|---|---|---|
| 3.3VCC | 01 | 02 | 5VCC | ||
| I2C1 SDA | 02 | 03 | 04 | 5VCC | |
| I2C1 SDA | 02 | 03 | 04 | ||
| I2C1 SCL | 03 | 05 | 06 | ||
| 04 | 07 | 08 | 14 | ||
| GND | 09 | 10 | 15 | ||
| 17 | 11 | 12 | 18 | ||
| 27 | 13 | 14 | GND | ||
| 22 | 15 | 16 | 23 | ||
| 3.3VCC | 17 | 18 | 24 | ||
| 10 | 19 | 20 | GND | ||
| 09 | 21 | 22 | 25 | ||
| 11 | 23 | 24 | 08 | ||
| GND | 25 | 26 | 07 | ||
| I2C0 SDA | 00 | 27 | 28 | 01 | I2C0 SCL |
| 05 | 29 | 30 | GND | ||
| 06 | 31 | 32 | 12 | ||
| 13 | 33 | 34 | GND | ||
| 19 | 35 | 36 | 16 | ||
| 26 | 37 | 38 | 20 | ||
| 39 | 40 | 21 | |||
| GND |
Note that only two wires are necessary for carrying the I2C signals.
A handy diagnostic tool for I2C is i2cdetect1.
This invocation will show all devices listening on the bus 1 which is useful for obtaining the 7-bit device addresses needed for communication.
$ i2cdetect -y 1
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- 24 -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- 51 -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
I this case we have two devices listening on the bus with addresses 0x34 and 0x51 respectively.
In Python you get access to I2C via the smbus library2
Sample code for sending data to a device looks like this:
import smbus
BUS_NO= 1
DEVICE_ADDR = 0x51
bus = smbus.SMBus(BUS_NO)
bus.write_byte_data(DEVICE_ADDR, 0x00, 0x01)