Add info for multiplexing to sml.rst

Add info on how multiple meters can be read using a single HardwareSerial UART.

Author: johnzielke

components/sml.rst

@@ -224,6 +224,63 @@ These meters can also measure the instantaneous power usage.
         device_class: power
         state_class: measurement
 
+Reading multiple meters
+---------------------------------
+If you are reading data from more meters than your controller has UARTs available (e.g. more than 2 for an ESP32), you can use multiplexing to switch between reading data from different meters.
+
+In order to do this, after each SML update, the used UART can be set to listen to a different pin.
+An example on how to do this is this:
+
+.. code-block:: yaml
+
+    uart:    
+     - baud_rate: 9600
+       data_bits: 8
+       rx_pin: 
+        number: 17 # Set to the first of the GPIO pins
+        id: uart_multiplex_rx_pin
+       stop_bits: 1
+       rx_buffer_size: 512
+       id: uart_multiplexed
+
+    sml:
+    - id: sml_multiplexed
+      uart_id: uart_multiplexed
+      on_data:
+        - lambda: |-
+            std::vector<gpio_num_t> multiplex_pins = {::GPIO_NUM_17,::GPIO_NUM_19};
+            static size_t current_index = 0;
+            current_index = (current_index + 1) % multiplex_pins.size();
+            gpio_num_t new_rx_pin = multiplex_pins[current_index];
+            ESP_LOGD("uart_multiplex", "Switching to RX pin: %d (%d)", new_rx_pin, current_index);
+            id(uart_multiplex_rx_pin).set_pin(new_rx_pin);
+            id(uart_multiplexed).load_settings(true);
+
+    sensor:
+    - platform: sml
+      name: "Solar Roof"
+      sml_id: sml_multiplexed
+      server_id: "12345ab" # IMPORTANT! Set the correct server id
+      obis_code: "1-0:2.8.0"
+      unit_of_measurement: kWh
+      accuracy_decimals: 2
+      device_class: energy
+      state_class: total_increasing
+      filters:
+        - multiply: 0.0001
+    - platform: sml
+      name: "Solar Carport"
+      sml_id: sml_multiplexed
+      server_id: "67890cd" # IMPORTANT! Set the correct server id
+      obis_code: "1-0:2.8.0"
+      unit_of_measurement: kWh
+      accuracy_decimals: 2
+      device_class: energy
+      state_class: total_increasing
+      filters:
+        - multiply: 0.0001
+
+
 See Also
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