Overview

VCON is a framework for remote (OTA) firmware updates and remote serial monitor, for a wide range of microcontrollers. VCON framework consists of two parts:

• A pre-built firmware for the ESP32 / ESP32C3 device
• A device management cloud (public at https://dash.vcon.io or private).

Any ESP32 / ESP32C3 device or module can be used. Flashing VCON pre-built firmware turns that device into a remote programmer and serial monitor. It can be registered on the management cloud, and be controlled remotely via GUI or the REST API. Once ESP32 is wired to the external device, that external device can be re-programmed over the secure REST API. An external, target device is called a "Host".

In addition to the OTA wiring, VCON module can be wired to the Host's UART. In this case, VCON module could be configured to act as a network bridge - either in transparent, or non-transparent mode:

• In trasparent mode, everything that Host MCU prints to the UART, VCON forwards to the server of your choice, MQTT or Websocket. Everything that VCON receives from the server, VCON writes to the MCU's UART. This enables bi-directional communication, where Host MCU "thinks" it talks to the UART where in fact it talks to the remote server of your choice - e.g. AWS IoT, or Azure, or your private MQTT server.
• In non-transparent mode, Host MCU can exchange JSON-RPC commands with the VCON shield and access a wide range of functionality - call external REST services, manage files, etc. Also, it can export RPC services itself, and VCON shield bridges those services to the REST API. This way, it is easy to create custom management interface to communicate with your MCU via familiar, secure REST API.

Supported targets

The following list shows target devices that can be reflashed by the current version of VCON firmware. Please contact us if you'd like to support your architecture of choice:

• ST Microelectronics STM32 G0, G4, L0, L1, L4, F0, F1, F2, F3, F4, F7, H7
• Microchip SAMD21, SAMD5x, SAME5x
• Mictochip ATMEGA328, ATMEGA128
• Raspberry PI RP2040
• Texas Instruments TM4C

NOTE: UART bridging works on all architectures, not just on those listed above.

Recommended hardware

Any ESP32 / ESP32C3 device or module can be used as a VCON module. Some of our customers integrate ESP32 modules into their own PCB. Some use existing development boards available on the market. The choice is yours. Below we list only some of the options we think are worth noting, with our subjective opinion on the best applicable use case.

Name	Image	Pros	Cons	Buy
ESP32C3 XIAO		Very small form factor Reset button Inexpensive (about 5$) Surface-mountable	No onboard antenna No status LED	Digikey
Beetle ESP32C3		Small form factor Status LED	No reset button Not SMT	DFRobot
ESP32C3-Mini		Cheap (< $2) Onboard antenna	Cannot be used standalone	Digikey
Adafruit AirLift		Compact Surface mountable Status LED	No USB power No reset button	Adafruit

Downloads

• esputil open source ESP32 flashing / configuration utility (github repository)
  • esputil_macos - MacOS binary. Right click, "Save as" esputil, then chmod 0755 esputil
  • esputil_linux - Linux binary. Right click, "Save as" esputil, then chmod 0755 esputil
  • esputil.exe - Windows binary. Right click, "Save as" esputil.exe
• "Full" Firmware for initial flashing
  • esp32c3.hex - for ESP32C3. Right click, "Save as" vcon.hex
  • esp32.hex - for ESP32. Right click, "Save as" vcon.hex
• "App" Firmware update
  • esp32c3.bin - for ESP32C3. Right click, "Save as" vcon.bin
  • esp32.bin - for ESP32. Right click, "Save as" vcon.bin
• version.json - the latest firmware version
• CHANGELOG - firmware change log

NOTE: The difference between the "full" and "update" firmwares is that the "full" firmware contains the bootloader, the partition table, and the VCON app. The "update" firmware contains only VCON app. To update a production VCON module, use the "update" firmware.

User Guide

Module flashing

1. Connect your ESP32 device hardware to the workstation. It should be accessible via a serial port COMPORT. On Windows, it is COMn, like COM3, COM4, ... - open Windows' device manager to double-check. On Linux, it is usually /dev/ttyUSBn or /dev/ttyACMn. On Mac, it is usually /dev/cu.usbXXXX
2. Download an open-source esputil tool for your OS:
   • esputil_macos for MacOS. Right click, "Save as" esputil, then chmod 0755 esputil
   • esputil_linux for Linux. Right click, "Save as" esputil, then chmod 0755 esputil
   • esputil.exe for Windows. Right click, "Save as" esputil.exe
3. Download VCON firmware
   • esp32c3.hex - for ESP32C3. Right click, "Save as" vcon.hex
   • esp32.hex - for ESP32. Right click, "Save as" vcon.hex
4. Start command prompt (or terminal on Mac/Linux). Run the following:

   cd DIRECTORY_WITH_DOWNLOADED_FILES
   ./esputil -p COMPORT flash vcon.hex
   

   Note: for ESP32C3 boards, you might need to specify an extra argument for flash parameters, -fp 0x22f. The most common parameters are -fp 0x220. For more options, see https://github.com/cpq/esputil#flash-parameters.

Module registration

This section describes how to configure network on a recently flashed ESP32 device, and how to register it on the https://dash.vcon.io cloud. There are two methods - one is using CLI (command line interface) over the USB connection, and another is using a Web UI app which works over Bluetooth.

CLI method

Run the following command to see module's debug logs:

esputil -p COMPORT monitor

You should see debug logs appearing. Press "enter" once to trigger command-line mode. You should see this:

Entering CLI mode, log output suspended. CLI commands:
  set PATH VALUE	Set config PATH to VALUE
  call NAME ARGS	Call RPC function NAME
  cat FILE		Show file contents
  reset			Reset to factory defaults
  reboot		Reboot device
  <enter>		Exit CLI mode, resume logs

Enter the following commands, line by line, terminating by single "enter":

set $.wifi.sta.ssid "YOUR_WIFI_NAME"
set $.wifi.sta.pass "YOUR_WIFI_PASSWORD"
set $.dash.pass "DEVICE_CLOUD_PASSWORD"
reboot

Note the double quotes! To get a DEVICE_CLOUD_PASSWORD, login to https://dash.vcon.io. Click on "add device" to add a new device. Click on "action" button. Click on "Copy device password to clipboard:

Web UI method

Open https://vcon.io/app/ in any Bluetooth-enabled browser, like Google Chrome or Microsoft Edge. Click on "Choose device" button:

Select a device and click "Pair":

Login to https://dash.vcon.io. Each connected OTA shield must have an associated "cloud device" with unique access key (password). Click on "add device" to add a new device. Click on "action" button. Click on "Copy device password to clipboard:

Switch back to the configuration app. Fill in WiFi network name, WiFi password, device password and save settings:

Your device on a dashboard should become online:

After WiFi and cloud credentials are set and device is rebooted, your device should become online:

Congratulations! Now ESP32 module can be controlled remotely. The next step is to wire an external microcontroller to it.

Module-to-device wiring

In order to wire ESP32 module to your target (Host) device, both programming pins, and UART pins should be connected together.

Programming pins depend on the programming method. ARM-based targets use SWD programming method, which uses two pins: SWDIO and SWDCLK. Some targets, for example STM32, support reprogramming via a serial bootloader. That method requires four pins: UART RX, UART TX, RESET, and BOOT0. ATMEGA targets use SPI-based programming, which uses three pins: MISO, MOSI, and CLK.

To wire UART pins, select any two available ESP32 pins and wire them to the target's UART RX and TX pins.

After physical wiring us done, change ESP32 configuration to reflect the wiring. The configuration can be changed via the UI, or via the API. In the examples below, both methods are shown.

PICO-W5500-EVB

• VCON module: ESP32C3 XIAO
• Target (Host) board: W5500-EVB-PICO

In this setup, it is ESP32C3 that powers both boards over USB. After the setup is done, it could the vica versa: the RPI board can power both.

Configuration using GUI

Once wiring is done, go to https://dash.vcon.io and select "action" → "Manage device" to configure the Host. This loads device management console with several different sections. Now we're interested in "Configuration" section. Change values to make it look like this and hit "save":

Configuration using API

The following console command performs the same setup as the GUI method above:

curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/rpc/config.set \
  -d '{"config":{"host":{"arch":400,"mode":3,"uart":{"rx":6,"tx":7},"swd":{"io":8,"clk":9}}},"save":true,"reboot":500}'

In the command above, API_KEY is a cloud authentication key. It could be copied from the UI dashboard. ID is the actual ID you're working with. It can be taken from the UI dashboard.

See Configuration Reference for a detailed description of all configurable parameters.

STM32 BluePill

Configuration using GUI

Once wiring is done, go to https://dash.vcon.io and select "action" → "Manage device" to configure the Host. This loads device management console with several different sections. Now we're interested in "Configuration" section. Change values to make it look like this and hit "save":

Configuration using API

The following console command performs the same setup as the GUI method above:

curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/rpc/config.set \
  -d '{"config":{"host":{"arch":200,"mode":3,"uart":{"rx":33,"tx":35},"swd":{"io":13,"clk":14}}},"save":true,"reboot":500}'

In the command above, API_KEY is a cloud authentication key. It could be copied from the UI dashboard. ID is the actual ID you're working with. It can be taken from the UI dashboard.

See Configuration Reference for a detailed description of all configurable parameters.

Adafruit ItsyBitsy M4

Configuration using GUI

Once wiring is done, go to https://dash.vcon.io and select "action" → "Manage device" to configure the Host. This loads device management console with several different sections. Now we're interested in "Configuration" section. Change values to make it look like this and hit "save":

Configuration using API

The following console command performs the same setup as the GUI method above:

curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/rpc/config.set \
  -d '{"config":{"host":{"arch":100,"mode":3,"uart":{"rx":33,"tx":35},"swd":{"io":13,"clk":14}}},"save":true,"reboot":500}'

In the command above, API_KEY is a cloud authentication key. It could be copied from the UI dashboard. ID is the actual ID you're working with. It can be taken from the UI dashboard.

See Configuration Reference for a detailed description of all configurable parameters.

Arduino Nano

Configuration using GUI

Once wiring is done, go to https://dash.vcon.io and select "action" → "Manage device" to configure the Host. This loads device management console with several different sections. Now we're interested in "Configuration" section. Change values to make it look like this and hit "save":

Configuration using API

The following console command performs the same setup as the GUI method above:

curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/rpc/config.set \
  -d '{"config":{"host":{"arch":300,"mode":3,"uart":{"rx":33,"tx":35},"swd":{"io":13,"clk":14}}},"save":true,"reboot":500}'

In the command above, API_KEY is a cloud authentication key. It could be copied from the UI dashboard. ID is the actual ID you're working with. It can be taken from the UI dashboard.

See Configuration Reference for a detailed description of all configurable parameters.

Firmware update

A device firmware can be programmed by a simple HTTP API call:

curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/ota --data-binary @firmware.bin

In the command above, API_KEY is a cloud authentication key. It could be copied from the UI dashboard. ID is the actual ID you're working with. It can be taken from the UI dashboard.

That command pushes new device firmware firmware.bin to the VCON module attached to your device, and VCON module reprograms your device over the programming pins. Note that the whole communication is authenticated, and secured by the industry-standard TLS connection:

VCON module can also update itself, using the same cloud API. Just add an extra ?target=vcon query string to the ota endpoint, and send a VCON firmware, not your MCU firmware:

curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/ota?target=vcon --data-binary @vcon.bin

Serial monitor

A device's UART can be monitored by a simple HTTP API call:

curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/tx?t=5

In the command above, API_KEY is a cloud authentication key. It could be copied from the UI dashboard. ID is the actual ID you're working with. It can be taken from the UI dashboard.

The t=5 parameter tells the number of seconds to monitor. When that time is over, the command exits. If you want a longer monitor time, use some larger value, for example t=9999.

VCON module can not only read, but also write to the device's UART:

curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/rpc/serial.write -d '{"data": "hello"}'
curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/rpc/serial.write -d '{"hex": "610d0a"}'

NOTE: a serial console is provided by the https://dash.vcon.io GUI.

Using remote serial read and write, it is easy to implement remote device control and automated device testing.

Management Cloud

Overview

VCON cloud provides management API access to your devices, and an easy to use Web UI with device dashboard:

Authentication

Cloud API access can be authenticated via Basic authentication, where username is set to the empty string, and password is set to the API key string. The API key string can be copied from the GUI dashboard. The following table provides a summary:

Test account

The https://dash.vcon.io provides a test account with the following credentials:

It is open to everyone for demonstration purposes. Anyone can login to dash.vcon.io using test/test as an email/password. And below is an example of an API usage:

Get all devices registered under a test account:

curl -su :test https://dash.vcon.io/api/v3/devices

List all available functions of device 1:

curl -su :test https://dash.vcon.io/api/v3/devices/1/rpc/rpc.list

Call device 1, function sys.reboot:

curl -su :test https://dash.vcon.io/api/v3/devices/1/rpc/sys.reboot

Notifications

VCON cloud provides a special secure WebSocket endpoint wss://dash.vcon.io/api/v3/notify. This is a read-only notifications endpoint. Each notification is a JSON object with three keys:

• name - a notification name, e.g. online, offline, and so on
• did - an ID of a device that generated the event
• data - an optional notification-specific data.

Below is the list of the events:

• {"name":"online","did":ID} - generated by the cloud when a device becomes online
• {"name":"offline","did":ID} - generated by the cloud when a device becomes offline
• {"name":"updated","did":ID,"data":{...}} - generated by the cloud when a device changes its state attribute
• {"name":"ble.adv","did":ID,"data":{...}} - generaged by the VCON module in the BLE bridge mode, when ble.mode=2. The data contains BLE beacon data.
• {"name":"CUSTOM","did":ID,"data":CUSTOM} - a custom event that could be generated by a Host microcontroller in the RPC bridge mode. A Host micro must call the cloud.CUSTOM RPC function with any params you want. For example, this is how a Host microcontroller can send some sensor data:

jsonrpc_printf(fn, NULL, "{%Q:%Q,%Q:{%Q:%g}}",
               "method", "cloud.temperature",
               "params", "value", 123.456)

This call generates {"name":"temperature","did":ID,"data":{"value":123.456}}

NOTE: the cloud UI uses notification endpoint to catch device changes, like online/offline, and dispays that respectively. You can create your custom dashboard using cloud API and notifications.

You can catch all notifications sent by the cloud. Below is a simple NodeJS application that catches all incoming notifications and prints them on a console. Create file catcher.js, copy/paste the folowing content:

const Websocket = require('ws');  // npm install -g ws
const addr = 'wss://dash.vcon.io/api/v3/notify?access_token=API_KEY';
const reconnect = function() {
  const ws = new Websocket(addr, {origin: addr});
  ws.on('error', msg => console.log('Got error:', msg.toString()));
  ws.on('message', msg => console.log('Got message:', msg.toString()));
  ws.on('close', () => setTimeout(reconnect, 1000));
};
reconnect();

Change API_KEY to your API key which could be copied from the "Account" tab in the cloud UI. Run this file and see how arriving notifications are printed:

$ node catcher.js
Got message: {"name":"online","did":1}

NOTE: by bringing up a custom notification catcher, you can integrate with any other 3rd party service - for example, store data into a Google/AWS/Azure, dump data in your custom dashboard, send SMS, run analytics, et cetera.

Cloud API Reference

NOTE: URI in a table below must be prefixed with https://dash.vcon.io/api/v3. Example API call that lists all devices registered by user:

curl -su :API_KEY https://dash.vcon.io/api/v3/devices

RPC API Reference

The following table summarises built-in RPC functions that are exposed by a VCON module. These functions can be called remotely via REST, or locally via UART (for example, Host MCU can call any of the VCONs RPC function).

Example call that lists files on a module (module must be online):

curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/rpc/fs.list

Some API calls require parameters, a JSON string in HTTP POST body:

$ curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/rpc/fs.read -d "{\"file\":\"config.json\"}"
{"id":2,"result":{"file":"config.json","offset":0,"left":0,"data":"ewogICJ3aWZp..."}}

{"config":{...},"save": false}

{"pin": 4,"val": 0}

{"pin": 4}

{"file": "NAME","offset": 0}

{ "file": "NAME","data": "BASE64","append": true}

{"req": "rst rd0"}

{"size": 128,"addr": 0}

{"data": "aabb","addr": 0}

{"data":"...","hex":"...","base64":"..."}

{"cmd":"COMMANDS"}

{"method": "POST","url": "https://foo.com/bar","headers":"A:B\r\nC:D\r\n","body":"a=1&b=2"}

{"data": "hello there"}

VCON module

Configuration Reference

VCON module configuration is a JSON string. It is stored on the root filesystem in "config.json" file. It could be viewed by calling config.get RPC function:

curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/rpc/config.get
{"wifi":{"sta":{"ssid":"MyWiFiNetwork","pass":"mypassword"}},"dash":{...},...}

In order to change one or more configuration option, call config.set RPC function and pass a JSON object with 2 parameters: required config with your changes, and optional boolean save to save your changes to "config.json". For example, this command changes an architecture of Host microcontroller to 200 (STM32 with 1k flash pages), and saves:

curl -su :API_KEY https://dash.vcon.io/api/v3/devices/ID/rpc/config.set -d "{\"config\":{\"host\":{\"arch\":200}},\"save\":true}"

NOTE: in order to restore the default value for any option, set it to null.

The following table describes all configuration options. Every option name is specified by its JSON path.

ESP32 Pinout Reference

• R column (reset state)
  • 0: input disabled
  • 1: input enabled
  • 2: input enabled, pull down
  • 3: input enabled, pull up
• Strapping pins:
  • 0: high - boot from flash, low - enter boot loader mode
  • 2: low when pin 0 is low
  • 12: if high, boot fail
  • 15: if low, boot messages are not printed
• N column (special notes): R: RTC/Analog, I: Input Only
• Colors: strapping, internal flash, input only

BLE gateway

In the BLE bridge mode, VCON module receives BLE beacon advertisements from nearby BLE beacons, and forwards them to the cloud as device notifications. You can catch those notifications by using a notification catcher, and do whatever you want with the data - store in a database, do real-time processing, etc.

Also VCON provides a BLE.Advertise RPC function, which you can call from anywhere via the cloud and trigger VCON module to send a BLE advertisement with configurable payload.

To configure BLE bridge, see ble section in the Configuration Reference.

Troubleshooting

If any quick start guide step fails, or the VCON module behaves in an unexpected way, please follow these steps:

• Attach a serial console to the ESP32 (ERX and ETX pins)
• Physically power cycle the device
• Post a message using a contact form with the following:
  • Which hardware you're using
  • Which guide you are following
  • Which exactly step (or command) fails, include the command and an output
  • The full console log, including all boot messages

Recipes

Live debug with GDB

If a target microcontroller is ARM, and SWD pins are wired to the VCON module, it is possible to debug it remotely using gdb. The trick is to run a simple Node.JS application which implements a GDB server, and translates GDB commands to the SWD RPC commands to the target device connected to dash.vcon.io. Here is the sequence of actions:

Step 2. Install required NPM packages. Open terminal/command prompt and execute:

$ npm i -g ws

Step 2. Download vcon-gdb-server.js. Open terminal/command prompt and execute (take API_KEY from the https://dash.vcon.io. DEVICE_ID is the device number you want to debug):

$ node vcon-gdb-server.js API_KEY DEVICE_ID
Started GDB server at port 2424

Step 3. Start gdb. Open terminal/command prompt and execute (change firmware.elf to the actual file name of your firmware):

$ arm-none-eabi-gdb -q -ex 'set confirm off' \
                       -ex 'set remotetimeout 10' \
                       -ex 'target remote localhost:2424' \
                       firmware.elf
Reading symbols from firmware.elf...
Remote debugging using localhost:2424
Reset_Handler () at cmsis_h7/Source/Templates/gcc/startup_stm32h743xx.s:61
61	  ldr   sp, =_estack      /* set stack pointer */
(gdb) mon reset halt
(gdb) b main
Breakpoint 1 at 0x8000418: file hal.h, line 94.
(gdb) c
Continuing.

Breakpoint 1, main () at hal.h:94
94	  gpio_init(pin, GPIO_MODE_OUTPUT, GPIO_OTYPE_PUSH_PULL, GPIO_SPEED_HIGH,
(gdb)

Send data to MQTT

The commumication chip can be configured to read the serial output of the Host MCU and forward all data to the MQTT server of your choice. That is called a "UART-MQTT bridge mode". In order to enable that,

Step 1. Login to https://dash.vcon.io, make sure your device is online, choose "action" → "Manage device"

Step 2. In the Configuration section, enable "expert view". Edit the configuration JSON, "mqtt" section:

  ...
  "mqtt": {
    "rx": "vcon/rx",
    "tx": "vcon/tx",
    "url": "mqtt://broker.hivemq.com:1883"
  },
  ...

Step 3. In the Configuration section, set $.host.mode to 2:

  ...
  "host": {
    "mode": 2,
  ...

Step 4. Click on "save" button, wait until device reboots

That's it! Now all data that is printed to the serial, gets forwarded to the topic vcon/tx of the MQTT server broker.hivemq.com:1883. Feel free to choose a different topic or server address.

Note there is an rx topic, too. You've guessed it: everything that is sent to that MQTT topic, a communication chip catches and sends to the Host MCU serial line, so MQTT/Serial communication is bi-directional.

NOTE: on Arduino framework, Serial on VCON-328 and Serial1 on VCON-D51.

Connect to AWS IoT

Below is a detailed step-by-step guide on how to enable VCON module to connect to AWS IoT and forward UART data to/from the specified MQTT topics.

1. Create a permissive IoT policy:

• Login to AWS IoT console
• On the left bar, click on "Policies"
• On the right pane, click on "Create"
• Fill in fields in the following way and click "Create":
• Name: Policy1, Action: iot:*, Resource ARN: *, Effect: allow

2. Register AWS IoT thing

• On the left bar, click on "Manage" → "Things"
• On the right pane, click on Create things → Create single thing → Next
• Enter thing name, for example "thing1", click "Next"
• Choose "Auto-generate new certificate", click "Next"
• Chooae policy "Policy1", click "Create thing"

3. Download certificate files#

• In the dialog box that appears, download all three generated certificates:
• Rename xxx-certificate.pem.crt to cert.pem
• Rename xxx-private.pem.key to key.pem

4. Upload cert.pem and key.pem to VCON

• Login to VCON dashboard, select your device → Manage
• In the File Manager section, click on "upload" button
• Upload two files, cert.pem and key.pem

5. Modify MQTT configuration

• In the Configuration section, click on "expert view", search for "mqtt" section
• Fill MQTT section as follows (note, to find out YOUR_AWS_IOT_ENDPOINT_URL, click on "Settings" link on the AWS IoT console left bar and copy the "Endpoint" URL on the right pane):

"mqtt": {
  "url": "mqtts://YOUR_AWS_IOT_ENDPOINT_URL:8883",
  "rx": "vcon/rx",
  "tx": "vcon/tx",
  "cert": "/spiffs/cert.pem",
  "key": "/spiffs/key.pem",
  "hexdump": 0
},

• Click save. Wait until your device reboots and becomes online

6. Configure UART / MQTT bridge

• Refresh your device manager view. The "expert" view should become unchecked now
• In the UART bridge, select "UART <-> MQTT"
• Fill in UART rx and tx pins of your Host MCU

Send data to anything

What if you want to send data to something that is not MQTT - for example, to a custom server, or to a database like Influx? That is also possible. When a communication chip reads Host MCU serial, it generates a notification on the management cloud. It is possible to intercept that notification, and have a custom code to forward data to any destination.

Read the Notifications section for exact details.

Remote control - simple

An easy remote control via MQTT could be implemented by the "Send data to MQTT" recipe. Configure your RX and TX topics, MQTT server, then send data to the TX topic.

Use a simple Arduino sketch to read Serial to turn an LED on or off:

// IMPORTANT! On VCON-D51, use Serial1 instead of Serial
void setup() {
  Serial.begin(115200);
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  if (Serial.available() > 0) {
    int character = Serial.read();
    if (character == '0') digitalWrite(LED_BUILTIN, LOW);  // '0' switches LED off
    if (character == '1') digitalWrite(LED_BUILTIN, HIGH); // '1' switches LED on
  }
}

Of course, the serial message format in this simple example is trivial. It is done for demonstration purposes. You could implement any format you wish, test it on a local serial monitor, then it is going to work the same way when you send commands over MQTT.

Keep it simple, though.

Remote control - advanced

When a communication chip is in Serial/MQTT bridge mode, a Host MCU is not aware it is connected to the Internet. It "thinks" that it is controlled via Serial, but in fact, serial data comes from the internets. A communication chip is, so to say, transparent, invisible to the Host MCU.

It is possible to put communication chip into an RPC mode. In this mode, a communication chip expects data to be in a JSON-RPC format: each chunk of data must be a JSON-RPC frame, separated by a newline character. This way, a communication chip is not transparent to the Host MCU: Host MCU knows it is there, it could send JSON-RPC requests to it and receive responses.

In RPC mode, a Host MCU can register any number of custom handler functions, like GetStatus, SwitchOnMotor, or whatever is required. A communication chip can bridge these to the management cloud, and any function could be called via a familiar REST API.

Read RPC section below for a detailed explanation and an example sketch.

Complete dashboard

This recipe demonstrates how to build a complete custom monitoring dashboard using VCON platform. The implementation is very compact, well commented, and is designed to be a reference implementation of the production system. It consists of the following parts:

1. A device, which

• can be controlled remotely by switching LED on or off
• sends data to the cloud that simulates sensor readings

2. A custom dashboard, which

• is authenticated, requires users to log in
• stores sensor data in a local database (to a JSON file for simplicity)
• could be modified, and hosted anywhere - on a development laptop or on online hosting platform like AWS. The dashboard looks like this:

The overall architecture of such setup is as follows:

Step 1. Follow Quick Start Guide to register a VCON device on a dash.vcon.io management dashboard

Step 2. Clone or download the https://github.com/cesanta/vcon-app-example repository to your workstation

Step 3. Flash the firmware/firmware.ino sketch to your VCON device using remote OTA.

This sketch implements a simple LED control and simulates periodic sensor data upload:

void setup() {
  Serial.begin(115200);
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  // Handle serial input: switch LED on/off
  if (Serial.available() > 0) {
    int ch = Serial.read();
    if (ch == '0') digitalWrite(LED_BUILTIN, LOW);   // '0' switches LED off
    if (ch == '1') digitalWrite(LED_BUILTIN, HIGH);  // '1' switches LED on
  }

  // Print current status to serial every 5 seconds
  // A status message is a JSON string like this: {"led": 0, "sensor": 27}
  static unsigned long prev;
  unsigned long curr = millis();
  if (curr - prev > 5000) {
    char buf[100];
    snprintf(buf, sizeof(buf), "{\"led\": %d, \"sensor\": %d}",
             (int) digitalRead(LED_BUILTIN), (int) random(20, 30));
    Serial.println(buf);
    prev = curr;
  }
}

Step 4. Install Node.js, or skip this step if you have it installed

Step 5. Install ws node package for Websocket support:

$ npm i -g ws

Step 6. Edit backend/config.json file, change vcon_api_key value and save the file. You can get you API key by logging to https://dash.vcon.io dashboard on the Account page.

Step 4. Run your backend

$ node backend/main.js

That's it! Now point your browser to https://localhost:8000 and login as test/test. Here are the links for the core pieces of functionality:

• frontend/js/app.js - the frontend code for this dashboard, written in Preact
• backend/main.js - the backend code, written in Node.js
• firmware/firmware.ino - the Arduino firmware code

Here are some possible choices for where you can host your custom node.js backend, once the customisation is complete:

Calling VCON RPC

A simplest way to use VCON is to set $.host.mode parameter to a MQTT or Websocket bridge (2 or 3 respectively). In this case, everything that Host MCU sends UART, VCON catches and sends over to a remote server. VCON does not interpret UART data in any way, so the bridge is transparent.

However, if $.host.mode is set to 1 (RPC bridge), then VCON interprets UART data that Host MCU sends as JSON-RPC commands. VCON module exports many RPC methods (see RPC reference below), therefore Host MCU can call them and utilise additional functionality VCON offers - like, reading/writing files on VCON module, send HTTP requests to remote servers, etc.

Note that not all available RPC methods are exported via UART by default. Available methods are controlled by the $.rpc.safe configuration setting, see configuration reference below. Set $.rpc.safe to * to allow all methods. Note: for production, tighten that up, because methods allowed by $.rpc.safe are available over UART and BLE.

So, in this recipe, Host MCU periodically calls rpc.list RPC function that lists methods available for Host MCU to call. The Arduino sketch is for VCON-D51, or any other hardware setup where Serial is wired to the serial console, and Serial1 to the VCON.

Step 1. Follow Quick Start Guide to register a VCON device on a dash.vcon.io management dashboard

Step 2. When you device is online on dash.vcon.io, choose action → Manage device. In the FileSystem section, choose config.json file. Add "rpc": {"safe": "*"}, right after the first opening { at the beginning of the file. Change $.host.mode from 2 to 1. Click Save. Click "reboot device" in the "General Information" section.

Step 3. Open Arduino IDE, Tools → Manage Libraries, search for msjon and install mjson library.

Step 4. Create a new sketch. If you use VCON-D51, choose Adafruit ItsyBitsy M4 as a board. Copy-paste the following code: using remote OTA.

// Example firmware that calls VCON RPC function
// Set $.host.mode to 1 in the VCON configuration
#include <mjson.h>

int fn(const char *buf, int len, void *userdata) {
  return Serial1.write(buf, len);
}

int response_callback(const char *buf, int len, void *userdata) {
  Serial.write("Got response: ");
  Serial.write(buf, len);
  Serial.println();
  return len;
}

void setup() {
  Serial1.begin(115200);
  Serial.begin(115200);
  jsonrpc_init(response_callback, NULL);
}

void loop() {
  char buf[800];
  if (Serial1.available() > 0) {
    int len = Serial1.readBytes(buf, sizeof(buf));
    jsonrpc_process(buf, len, fn, NULL, NULL);
  }

  // Call RPC function every 3 seconds
  static unsigned long old;
  unsigned long now = millis();
  if (old > now || old + 3000 < now) {
    old = now;
    mjson_printf(fn, NULL, "{%Q:%d,%Q:%Q,%Q:{}}\n", "id", 1, "method", "rpc.list", "params");
  }
}

Step 5. Update Host MCU remotely. Open Serial console. You should see messages printed periodically by the response_callback() function, that shows a list of available RPC functions.

Congratulations! Now you know how to call any VCON RPC method from a Host MCU. See RPC reference below for a detailed description of each method.