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#832 Using I²C on the Raspberry Pi

All about configuring the I²C on the Raspberry Pi. Demonstrates using the bus with BME280 and BMP280 environmental sensors attached, with examples of probing the bus with i2c-tools, and reading the sensors with python, using the smbus library.

Build

Notes

Circuit Design

Designed with Fritzing: see i2c.fzz.

bb

schematic

bb_build

Configuring I²C

First, need to ensure ARM I2C support is enabled. Run sudo raspi-config and enable ARM I2C support under “3. Interface Options”

setup01

This change can be checked in config.txt:

pi@raspi1:~ $ grep -i i2c /boot/firmware/config.txt
dtparam=i2c_arm=on

Checking that the required kernel modules are loaded:

pi@raspi1:~ $ lsmod | grep i2c
i2c_bcm2835            12288  1
i2c_dev                12288  2

If i2c_dev is missing, user-space access won’t work.

Finally, check for I²C devices:

pi@raspi1:~ $ ls -1 /dev/i2c-*
/dev/i2c-1
/dev/i2c-2

The /dev/i2c-1 device is the tain header I²C on GPIO2/3.

I am using a Raspberry Pi B+. On this device, the second device /dev/i2c-2 (BSC0) is reserved for camera/display/HAT EEPROM.

Probing the I²C bus with i2c-tools

The i2c-tools are useful for diagnosing issues and testing devices on the I²C bus, without code. First install tools if needed:

sudo apt install -y i2c-tools

Detecting buses:

pi@raspi1:~ $ i2cdetect -l
i2c-1 i2c        bcm2835 (i2c@7e804000)           I2C adapter
i2c-2 i2c        bcm2835 (i2c@7e805000)           I2C adapter

Checking devices on i2c-1 … perfect, it is seeing the two devices on address 0x76 and 0x77, as expected:

pi@raspi1:~ $ i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- 76 77

Checking devices on i2c-2 … nothing, as expected:

pi@raspi1:~ $ i2cdetect -y 2
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Raw Query: BME280 (0x76)

Using i2cget to probe the BME280 at address 0x76

pi@raspi1:~ $ i2cget -y 1 0x76 0xD0
0x60
pi@raspi1:~ $ i2cget -y 1 0x76 0xFA
0x80
pi@raspi1:~ $ i2cdump -y 1 0x76
No size specified (using byte-data access)
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f    0123456789abcdef
00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
80: 7e 70 89 0f 81 55 69 06 91 6e e3 66 32 00 41 90    ~p???Ui??n?f2.A?
90: 9b d5 d0 0b 68 16 57 00 f9 ff b4 2d e8 d1 88 13    ????h?W.?.?-????
a0: 00 4b 35 00 00 00 00 00 00 00 00 00 33 00 00 c0    .K5.........3..?
b0: 00 54 00 00 00 00 60 02 00 01 ff 7f 1f 60 03 00    .T....`?.?.??`?.
c0: 00 00 00 ff 00 00 00 00 00 00 00 00 00 00 00 00    ................
d0: 60 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    `...............
e0: 00 64 01 00 14 2f 03 1e 9d 41 ff ff ff 7f ff 7f    .d?.?/???A...?.?
f0: ff 00 00 00 00 00 00 80 00 00 80 00 00 80 00 80    .......?..?..?.?

Raw Query: BMP180 (0x77)

pi@raspi1:~ $ i2cget -y 1 0x77 0xD0
0x58
pi@raspi1:~ $ i2cget -y 1 0x77 0xFA
0x80
pi@raspi1:~ $ i2cdump -y 1 0x77
No size specified (using byte-data access)
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f    0123456789abcdef
00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
80: 91 6e 90 4b 30 4f ee 00 15 6a 17 66 18 fc 34 94    ?n?K0O?.?j?f??4?
90: 85 d5 d0 0b 73 07 ec 00 f9 ff 8c 3c f8 c6 70 17    ????s??.?.?<??p?
a0: 00 00 be 00 00 00 00 00 00 00 00 00 33 00 00 c0    ..?.........3..?
b0: 00 54 00 00 00 00 60 02 00 01 ff be 13 60 03 00    .T....`?.?.??`?.
c0: 00 00 00 ff 00 00 00 00 00 00 00 00 00 00 00 00    ................
d0: 58 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    X...............
e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
f0: 00 00 00 00 00 00 00 80 00 00 80 00 00 00 00 00    .......?..?.....

Using Python to query the BME280 and BMP280 Devices

The two devices I have on the bus are similar environmental sensors.

These examples use the Python smbus library, and do the bare minimum to read from the sensors.

Although these are presented as separate scripts, the sensors operate very similarly, so it would be quite easy to refactor a script that could handle both sensors.

BME280 (0x76) with Python

See the source in simple_bme280_logger.py.

Test run:

pi@raspi1:~ $ ./simple_bme280_logger.py 76
| Temperature | Pressure    | Humidity |
|-------------|-------------|----------|
|    26.04 °C | 1006.77 hPa |  33.15 % |
|    26.04 °C | 1006.80 hPa |  33.08 % |
|    26.05 °C | 1006.72 hPa |  33.11 % |
|    26.04 °C | 1006.71 hPa |  33.14 % |
|    26.04 °C | 1006.79 hPa |  33.10 % |
^C
pi@raspi1:~ $ ./simple_bme280_logger.py 77
Error: Invalid BME280 chip ID: 0x58
pi@raspi1:~ $

BMP280 (0x77) with Python

See the source in simple_bmp280_logger.py.

Test run:

pi@raspi1:~ $ ./simple_bmp280_logger.py 77
| Temperature | Pressure    |
|-------------|-------------|
|    27.33 °C | 1006.82 hPa |
|    27.33 °C | 1006.79 hPa |
|    27.33 °C | 1006.84 hPa |
|    27.34 °C | 1006.78 hPa |
^C
pi@raspi1:~ $ ./simple_bmp280_logger.py 76
Error: Invalid BMP280 chip ID: 0x60
pi@raspi1:~ $

Credits and References

About LEAP#832 Raspberry PiI2CSensorsBME280BMP280Python

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

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