Create guide for using an ESP devboard with USB_UART bridge chip as serial adapter (#4483)

guides/devboard_as_flasher.rst

@@ -0,0 +1,50 @@
+Using an ESP devboard as a USB-UART bridge
+==========================================
+
+.. _devboard-as-flasher:
+
+ESP development boards usually have an onboard USB interface, either built into the chip (e.g. ESP32-S3) or via an onboard USB-UART bridge chip.
+However some ESP based devices not designed for development work don't bother with this,
+and only expose the UART0 pins (TX and RX) for flashing purposes.
+
+Normally you would use a dedicated USB-UART interface board for this but what if you don't have one?
+In this "emergency" situation it is possible to use a development board that does have a USB-UART bridge chip to flash another device.
+This is achieved by holding the ESP chip in reset so that it doesn't interfere with the bridge chip operation.
+
+It does NOT require any firmware to be flashed onto the development board
+and will not change anything already flashed onto it - it's purely a way to use the serial interface chip.
+
+We will refer to the devboard with functional USB_UART bridge chip as flasher board for this guide.
+
+Make sure you've read the :doc:`/guides/physical_device_connection` for properly understanding the functionality of your flasher devboard.
+
+.. figure:: /guides/images/devboard-as-flasher.png
+    :align: center
+    :width: 75.0%
+
+    Connection diagram for an ESP flash target
+
+You need to make the following electrical connections:
+
+.. note::
+
+    - Most ESP32 S and C series boards do *not* have a separate USB-UART chip - they have it built into the ESP - so are not suitable for this application.
+    - The 5V connection on either board may be labelled either ``5V`` or ``VIN``. Some boards may not have a 5V connection and will require 3.3V only.
+    - Rather than powering the target board from the flasher board, it is also possible to use a separate power supply, just make sure all the ground pins are connected together.
+
+- connect both ``EN`` and ``GND`` together in the flasher devboard
+- ``+5.0V`` or ``3V3`` on the flasher devboard to ``VIN`` or ``3V3`` respectively of the target device
+- ``GND``, or ground of flasher devboard to ``GND`` of the target device
+- ``TX`` of flasher devboard to ``TX`` of the target device
+- ``RX`` of flasher devboard to ``RX`` of the target device
+
+Pulling down ``EN`` by connecting it to ``GND`` on the flasher board prevents
+the ESP chip on flasher module from booting and polluting the serial lines.
+
+.. note::
+
+    - If the board has not previously had ESPHome loaded, you may need to pull the ``IO0`` pin low (i.e. connected to ``GND``) to force the board into flash mode.
+      This must be done before power is applied.
+    - Do not connect 3V3 to VIN of the target devices with a 3V3 LDO as it may lead to brownouts.
+
+Once the connections are made, plug the flasher board into your computer via USB and proceed with flashing the target board via whichever means you intend to use.

guides/physical_device_connection.rst

@@ -3,7 +3,7 @@ Physically Connecting to your Device
 
 The most difficult part of setting up a new ESPHome device is the initial
 installation, which requires connecting your ESP device to a computer using a
-cable.
+data cable.
 
 **You only need to do this once per device.** Once you've flashed ESPHome on a
 device, you can use :doc:`/components/ota/index` to upload new
@@ -119,8 +119,9 @@ USB Port on Device
 
 Development boards often come with a USB port built in. This USB port is
 connected to a serial adapter, so you don't need a separate serial adapter. You
-can use just a :ref:`USB cable <usb-cable>` to connect it to your computer to
-program it.
+can use just a :ref:`USB data cable <usb-cable>` to connect it to your computer to
+program it. Additionally, a development board can also be used to flash other ESPs.
+:doc:`Read more here. </guides/devboard_as_flasher>`
 
 This isn't likely to be very useful without connecting additional sensors to it
 by either soldering or using a breadboard, but you do not need anything else to
@@ -241,7 +242,8 @@ require different parts and tools.
         .. _usb-cable:
     * - :ref:`USB to micro-USB/mini-USB/USB-C <usb-cable>`
       - If your target device has a USB port on it, you need the appropriate
-        cable to connect to it.
+        data cable to connect to it. A power only USB cable that usually
+        comes presupplied with powerbanks won't work.
       - $3 to $10
       - .. image:: /guides/images/usb-cable.jpg
             :alt: From https://www.stockvault.net/photo/271754/usb-cable
@@ -259,6 +261,9 @@ require different parts and tools.
 
         The `Tasmota website provides a good set of suggestions on what to buy
         <https://tasmota.github.io/docs/Getting-Started/#needed-hardware>`_.
+
+        Any ESP development board with functioning USB_UART bridge chip can also be used instead.
+        :doc:`Read instructions here. </guides/devboard_as_flasher>`
       - $3 to $10
       - .. image:: /guides/images/usb-serial-adapter.jpg
             :alt: From https://tasmota.github.io/docs/Getting-Started/