docs(serial-protocol): add images and flowchart

docs/en/advanced-topics/diag/command_packet_format.diag

@@ -0,0 +1,13 @@
+packetdiag command_packet_format{
+    colwidth = 16
+    node_width = 50
+    node_height = 50
+    default_fontsize = 16
+
+    0: "0x00";
+    1: "Cmd";
+    2-3: "Size";
+    4-7: "Checksum";
+    8-15: "Data" [color = lightgrey];
+    16-31: "..." [color = lightgrey];
+}

docs/en/advanced-topics/diag/download_procedure_chart.diag

@@ -0,0 +1,39 @@
+blockdiag download_procedure_diagram {
+    node_height = 40;
+    node_width = 150;
+    span_width = 40;
+    span_height = 45;
+    default_fontsize = 12
+    orientation = portrait;
+    edge_layout = flowchart;
+    default_group_color = none;
+
+    // nodes
+    start [label = "Start", shape = flowchart.terminator];
+    sync [label = "Synchronization", shape = box];
+    success_cond [label = "Success?", shape = flowchart.condition];
+    erase_data [label = "Erase data", shape = box];
+    transmit_data [label = "Transmit data", shape = box];
+    finish_cond [label = "Finish?", shape = flowchart.condition];
+    transmit_finish [label = "Transmit finish frame", shape = box];
+    finish [label = "Finish", shape = flowchart.terminator];
+    // fake nodes to adjust shape and edge label position
+    succ_fin [shape = none];
+    fincon_fin [shape = none];
+
+    // edges
+    start -> sync -> success_cond;
+    success_cond -> erase_data [label = "Yes"];
+    erase_data -> transmit_data;
+    transmit_data -> finish_cond;
+    success_cond -- succ_fin [label = "Timeout"];
+    finish_cond -> transmit_finish [label = "Yes"];
+    finish_cond -- fincon_fin [label = "Failure"];
+    succ_fin -- fincon_fin;
+    fincon_fin -> finish;
+    transmit_finish -> finish;
+
+    // group
+    group{transmit_finish, fincon_fin};
+    group{erase_data, succ_fin};
+}

docs/en/advanced-topics/diag/response_packet_format.diag

@@ -0,0 +1,13 @@
+packetdiag command_packet_format{
+    colwidth = 16
+    node_width = 50
+    node_height = 50
+    default_fontsize = 16
+
+    0: "0x01";
+    1: "Cmd";
+    2-3: "Size";
+    4-7: "Value";
+    8-15: "Data" [color = lightgrey];
+    16-31: "..." [color = lightgrey];
+}

docs/en/advanced-topics/serial-protocol.rst

@@ -27,7 +27,7 @@ The host computer sends a SLIP encoded command request to the ESP chip. The ESP
 Low Level Protocol
 ^^^^^^^^^^^^^^^^^^
 
-The bootloader protocol uses `SLIP <http://en.wikipedia.org/wiki/SLIP>`_ packet framing for data transmissions in both directions.
+The bootloader protocol uses `SLIP <https://en.wikipedia.org/wiki/Serial_Line_Internet_Protocol>`_ packet framing for data transmissions in both directions.
 
 Each SLIP packet begins and ends with ``0xC0``. Within the packet, all occurrences of ``0xC0`` and ``0xDB`` are replaced with ``0xDB 0xDC`` and ``0xDB 0xDD``, respectively. The replacing is to be done **after** the checksum and lengths are calculated, so the packet length may be longer than the ``size`` field below.
 
@@ -36,6 +36,11 @@ Command Packet
 
 Each command is a SLIP packet initiated by the host and results in a response packet. Inside the packet, the packet consists of a header and a variable-length body. All multi-byte fields are little-endian.
 
+.. packetdiag:: diag/command_packet_format.diag
+    :caption: Command packet format
+    :align: center
+
+
 +--------+-------------+--------------------------------------------------------------------------------------------------------------------+
 | Byte   | Name        | Comment                                                                                                            |
 +========+=============+====================================================================================================================+
@@ -55,6 +60,10 @@ Response Packet
 
 Each received command will result in a response SLIP packet sent from the ESP chip to the host. Contents of the response packet is:
 
+.. packetdiag:: diag/response_packet_format.diag
+    :caption: Command packet format
+    :align: center
+
 +--------+-------------+--------------------------------------------------------------------------------------------------------------+
 | Byte   | Name        | Comment                                                                                                      |
 +========+=============+==============================================================================================================+
@@ -270,7 +279,7 @@ ROM loaders will not recognise these commands.
 Checksum
 ^^^^^^^^
 
-The checksum field is ignored (can be zero) for all comands except for MEM_DATA, FLASH_DATA, and FLASH_DEFL_DATA.
+The checksum field is ignored (can be zero) for all commands except for MEM_DATA, FLASH_DATA, and FLASH_DEFL_DATA.
 
 Each of the ``_DATA`` command packets (like ``FLASH_DEFL_DATA``, ``MEM_DATA``) has the same "data payload" format:
 
@@ -297,6 +306,14 @@ To calculate checksum, start with seed value 0xEF and XOR each individual byte i
 Functional Description
 ----------------------
 
+.. blockdiag:: diag/download_procedure_chart.diag
+    :caption: Download procedure flow chart
+    :align: center
+
+
+.. note::
+    This flow chart is used to illustrate the download procedure (writing to flash), other commands have different flows.
+
 Initial Synchronisation
 ^^^^^^^^^^^^^^^^^^^^^^^
 .. list::