Add notes for ADC on RP2040 (#2446)

components/sensor/adc.rst

@@ -6,9 +6,12 @@ Analog To Digital Sensor
     :image: flash.svg
 
 The Analog To Digital (``adc``) Sensor allows you to use the built-in
-ADC in your device to measure a voltage on certain pins. On the ESP8266
-only pin A0 (GPIO17) supports this. On the ESP32 pins GPIO32 through
-GPIO39 can be used.
+ADC in your device to measure a voltage on certain pins.
+
+- ESP8266: Only pin A0 (GPIO17) can be used.
+- ESP32: GPIO32 through GPIO39 can be used.
+- RP2040: GPIO26 through GPIO29 can be used.
+
 
 .. figure:: images/adc-ui.png
     :align: center
@@ -105,7 +108,7 @@ To measure the VCC voltage, set ``pin:`` to ``VCC`` and make sure nothing is con
 .. note::
 
     To avoid confusion: It measures the voltage at the chip, and not at the VCC pin of the board. It should usually be around 3.3V.
-    
+
 .. code-block:: yaml
 
     sensor:
@@ -113,6 +116,22 @@ To measure the VCC voltage, set ``pin:`` to ``VCC`` and make sure nothing is con
         pin: VCC
         name: "VCC Voltage"
 
+RP2040 Internal Core Temperature
+--------------------------------
+
+The RP2040 has an internal temperature sensor that can be used to measure the core temperature. This sensor is not available on the GPIO pins, but is available on the internal ADC.
+The below code is how you can access the temperature and expose as a sensor. The filter values are taken from the RP2040 datasheet to calculate Voltage to Celcius.
+
+.. code-block:: yaml
+
+    sensor:
+      - platform: adc
+        pin: TEMPERATURE
+        name: "Core Temperature"
+        unit_of_measurement: "°C"
+        filters:
+          - lambda: return 27 - (x - 0.706f) / 0.001721f;
+
 Multiple ADC Sensors
 ---------------------