I2C bus

In this tutorial we are going to introduce the I2C bus and how to use it with Toit.


We assume that you have set up your development environment as described in the IDE tutorial.

We also assume that you have flashed your device with Jaguar and that you are familiar with running Toit programs on it. If not, have a look at the Hello world tutorial.

While not necessary, we recommend to have a look at the LED tutorial to get familiar with the basics of using the GPIO pins.


I2C is a serial communication bus that is widely used in electronics. It consists of a data line (SDA) and a serial clock line (SCL). Multiple devices (sensors, or more complex devices like OLED displays) can be connected to the same bus.

Each device reacts to a hardware device ID. This ID is typically hard-wired and can not be changed dynamically. Many sensors have a way to physically modify the hardware to select a different "alt" ID. For example, the BME280 sensor has a solder jumper that can be used to select between two different addresses.

The I2C's clock runs at a certain frequency. Originally, this frequency was capped at 100kHz, but was then extended to 400kHz in "fast" mode. Not all hardware supports the fast mode.

The I2C bus is a controller-target bus. This means that there is one controller device that controls the bus. This device is typically a microcontroller, but can also be a more powerful device like a Raspberry Pi. The controller initiates all communication on the bus. The target devices can not initiate communication on their own.

On the ESP32 the default pins for the I2C bus are pin 21 for SDA and pin 22 for SCL, but these can be rerouted to other pins. In our example we will use pin 25 for SCL and pin 26 for SDA.


Without knowing which devices are connected to the I2C bus, it is not possible to communicate with them. There is not standardized protocol to interface with I2C devices. However, it is possible to scan for devices on the bus, and to list their addresses.

Write an i2c.toit file and watch it with Jaguar:

import gpio
import i2c

  scl := gpio.Pin 25
  sda := gpio.Pin 26
  frequency := 100_000

  bus := i2c.Bus --sda=sda --scl=scl --frequency=frequency

  devices := bus.scan
  devices.do: print it

This program will scan the I2C bus and print the addresses of all devices that are found.

You can look through Adafruit's i2c-addresses list to find out what the addresses mean.

Since I2C addresses are typically written in hexadecimal format, you might want to change the last line of the program to the following:

  devices.do: print "0x$(%02x it)"

This will use a string interpolation with the 2-digit hexadecimal format to print the addresses.