Custom I²C Device
=================

Lots of devices communicate using the I²C protocol. If you want to integrate
a device into ESPHome that uses this protocol you can pretty much use almost
all Arduino-based code because the ``Wire`` library is also available in ESPHome.

See the other custom component guides for how to register components and make
them publish values.

.. code-block:: cpp

    #include "esphome.h"

    class MyCustomComponent : public Component {
     public:
      void setup() override {
        // Initialize the device here. Usually Wire.begin() will be called in here,
        // though that call is unnecessary if you have an 'i2c:' entry in your config

        Wire.begin();
      }
      void loop() override {
        // Example: write the value 0x42 to register 0x78 of device with address 0x21
        Wire.beginTransmission(0x21);
        Wire.write(0x78);
        Wire.write(0x42);
        Wire.endTransmission();
      }
    };


I²C Write
---------
It may be useful to write to a register via I²C using a numerical input. For example, the following yaml code snippet captures a user-supplied numerical input in the range 1--255 from the dashboard:

.. code-block:: yaml

    number:
        - platform: template
        name: "Input 1"
        optimistic: true
        min_value: 1
        max_value: 255
        initial_value: 20
        step: 1
        mode: box
        id: input_1
        icon: "mdi:counter"
        
We want to write this number to a ``REGISTER_ADDRESS`` on the slave device via I²C. The Arduino-based looping code shown above is modified following the guidance in :doc:`Custom Sensor Component </components/sensor/custom>`.
 
.. code-block:: cpp

    #include "esphome.h"
 
    const uint16_t I2C_ADDRESS = 0x21;
    const uint16_t REGISTER_ADDRESS = 0x78; 
    const uint16_t POLLING_PERIOD = 15000; //milliseconds
    char temp = 20; //Initial value of the register

    class MyCustomComponent : public PollingComponent {
     public:
      MyCustomComponent() : PollingComponent(POLLING_PERIOD) {}
      float get_setup_priority() const override { return esphome::setup_priority::BUS; } //Access I2C bus

      void setup() override {
        //Add code here as needed
        Wire.begin();
        }
  
      void update() override {  
      char register_value = id(input_1).state; //Read the number set on the dashboard
      //Did the user change the input?
      if(register_value != temp){
            Wire.beginTransmission(I2C_ADDRESS);
            Wire.write(REGISTER_ADDRESS);
            Wire.write(register_value);
            Wire.endTransmission();
            temp = register_value; //Swap in the new value
            }
        }
    };
        
The ``Component`` class has been replaced with ``PollingComponent`` and the free-running ``loop()`` is changed to the  ``update()`` method with period set by ``POLLING_PERIOD``. The numerical value from the dashboard is accessed with its ``id`` tag and its state is set to the byte variable that we call ``register_value``.  To prevent an I²C write on every iteration, the contents of the register are stored in ``temp`` and checked for a change. Configuring the hardware with ``get_setup_priority()`` is explained in :doc:`Step 1 </components/sensor/custom>`.




See Also
--------

- :ghedit:`Edit`