Work with I2C on Raspberry Pi

March 08, 2020

Article is describing basic steps on how to start working with an I2C device. The end goal is a simple LED clock. Everything was done on the Raspbian “Buster” with using Adafruit 4-Digit 7-Segment Display.

Essential links:

Implementing I2C device drivers in userspace - basics on how to work with I2C in Linux
i2c-tools - user-space library and command line tools
Adafruit 1.2" 4-Digit 7-Segment Display w/I2C Backpack - information about used LED display

First, enable I2C:

$ sudo raspi-config nonint do_i2c 0

Then install needed use-space tools and library:

$ sudo apt install i2c-tools libi2c-dev

Run next command to check that LED display is connected (default address is 0x70):

$ sudo 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: 70 -- -- -- -- -- -- --

Simple communication with the display can be done with the help of i2cget and i2cset tools. Adafruit LED display is using HT16K33 as a driver and addresses and values of its internal registers can be found in the datasheet. In addition, source code of the i2c-tools can be used as an example on how to read and write data. For example to turn display on:

$ sudo i2cset -y 1 0x70 0x81

Below is the resulting source code of the program which reads system time and shows it on the display. To compile it:

$ g++ ledclock.cpp -li2c -o ledclock

Simple way to run it in background:

$ dtach -n /tmp/ledclock ./ledclock

Source code: