Allow setting the initial mode of HLW8012 sensors (#268)

* Allow setting the initial mode of HLW8012 sensors

* Changes as per code review

* Update hlw8012.rst

components/sensor/hlw8012.rst

@@ -49,14 +49,42 @@ Configuration variables:
   :ref:`Sensor <config-sensor>`.
 - **voltage** (*Optional*): Use the voltage value of the sensor in V (RMS).
   All options from :ref:`Sensor <config-sensor>`.
+- **update_interval** (*Optional*, :ref:`config-time`): The interval to check the sensor. Defaults to ``60s``.
+
+Advanced Options:
+
 - **current_resistor** (*Optional*, float): The value of the shunt resistor for current measurement.
   Defaults to the Sonoff POW's value ``0.001 ohm``.
 - **voltage_divider** (*Optional*, float): The value of the voltage divider on the board as ``(R_upstream + R_downstream) / R_downstream``.
   Defaults to the Sonoff POW's value ``2351``.
 - **change_mode_every** (*Optional*, int): After how many updates to cycle between the current/voltage measurement mode.
   Note that the first value after switching is discarded because it is often inaccurate. Defaults to ``8``.
-- **update_interval** (*Optional*, :ref:`config-time`): The interval to check the sensor. Defaults to ``60s``.
+- **initial_mode** (*Optional*, string): The initial measurement mode. Defaults to ``VOLTAGE``.
+  Possible initial measurement modes are ``VOLTAGE`` or ``CURRENT``.
 
+Permanent SEL Pin
+-----------------
+
+Some devices have the SEL pin permanently pulled high or low. If this is the case, you can configure
+the initial measurement mode to match whichever mode the device uses, and disable mode switching.
+
+.. code-block:: yaml
+
+    # Example configuration entry for device with fixed measurement mode
+    sensor:
+      - platform: hlw8012
+        sel_pin: 5
+        cf_pin: 14
+        cf1_pin: 13
+        current:
+          name: "HLW8012 Current"
+        voltage:
+          name: "HLW8012 Voltage"
+        power:
+          name: "HLW8012 Power"
+        update_interval: 60s
+        initial_mode: CURRENT
+        change_mode_every: 4294967295
 
 See Also
 --------