Expand custom I2C documentation (#2912)

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custom/i2c.rst

@@ -29,6 +29,64 @@ them publish values.
       }
     };
 
+
+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
 --------