Protobuf (short for protocol buffers) is a structured data serialization method developed by Google. This extensible format, which is both language- and platform-neutral, is used extensively within Google’s infrastructure, where it is considered the "lingua franca" for data.

Protobuf is gaining momentum among developers and is being used more widely within open source software projects.

Bluetera II is one of the first open source platforms to have adopted Protobuf as a medium for seamless information exchange between hardware and software.

The Problem With BLE-Based Applications

In many Bluetooth Low-Energy (BLE) applications, adding new functionality means either:

• Adding new BLE Characteristics, or
• Changing a proprietary binary structure using some service along the lines of UART-over-BLE.

Based on our experience, both options face significant limitations.

Adding BLE Characteristics is usually a cumbersome, error-prone procedure, whereas binary structures are unreadable and hard to maintain. Furthermore, both methods often require changes to the entire SDK (firmware, mobile software, cloud infrastructure, etc.), which breaks backward-compatibility and complicates future development.

Protobuf to the Rescue!

Bluetera uses Protobuf to streamline messages between the BLE module and the BLE SDK. This has some nice benefits:

• Language-neutrality - You simply define the schema file. Protobuf scripts then auto-generate boilerplate code for your language of choice.
• Platform-neutrality - No more messing about with little/big/medium endianness, no more worrying about how to serialize and deserialize a 'double' properly, etc. The framework takes care of all that.
• Compatibility - All changes are backward- and forward-compatible. You can add new fields without changing anything in legacy code.
• Self-delimiting - You no longer have to worry about delimiting consecutive messages (or even knowing the length of the structure). The serialization layer takes care of that.
• Efficient - While not a binary format, Protobuf goes to great length to encode data efficiently.

Protobuf "Hello World" - Control Bluetera II’s LEDs

This simple example presents a Desktop WPF application to turn Bluetera’s LEDs on and off. Notice how simple it is to add the new messages. There is very little boilerplate, and most of the code is pure business logic.

The stages are:

1. Add a new LED-command to the schema file
2. Add a button and a handler to the PC application to send the new command
3. Add a handler to Bluetera's firmware to actually do the LED on/off work

You can find the sources for this example in the led-demo branch of our GitHub repository. To build and run this example, you will need to Download and install Protobuf.

Step 1 - Modify bluetera_messages.proto

• Clone the Bluetera firmware repository

• Locate bluetera_messages.proto and add the following messages:

// define a new LED command
message LedCommand {
    uint32 ledId = 1;
    bool isOn = 2;
}

// update UplinkMessage to include the new command
message UplinkMessage
{
    oneof payload {
        ImuCommand imu = 1;
        EchoPayload echo = 17;
        LedCommand led = 18;
    }
}

Step 2 - Add a Button to the PC Application

• Clone the Bluetera Windows repository

• Copy the bluetera_messages.proto file to the protobuf folder and generate the C# classes by running source/gen-messages.bat. This will generate all necessary boilerplate code (such as the LedCommand class).

• Add a ‘ToggleLed’ button to MainWindow.xaml

• Add the following code to MainWindow.xaml.cs

private async void ToggleLedButton_Click(object sender, RoutedEventArgs e)
{
  // toggle LED state
  isLedOn = !isLedOn;

  // create the Uplink message
  UplinkMessage msg = new UplinkMessage()
  {
    Led = new LedCommand()
    {
      LedId = 1,  // 1 - red, 2 - green, 3- blue
      IsOn = isLedOn
    }
  };

  // send message
  await _bluetera.SendMessage(msg);
}

Step 3 - Add the Firmware Handler

• Generate the boilerplate C code by running
  application/messages/gen-messages.bat. This will create the
  necessary types to handle the new message
  (BLUETERA_UPLINK_MESSAGE_LED_TAG, bluetera_led_command_t, etc.)

• Modify bluetera_uplink_message_handler() in main.c to handle the
  new command:

...
switch(msg->which_payload)
{        
  case BLUETERA_UPLINK_MESSAGE_LED_TAG:
  {
    const bluetera_led_command_t* cmd = (const bluetera_led_command_t*)&msg->payload.led;
    nrfx_gpiote_pin_t led_pin;

    switch(cmd->ledId)
    {
      case 1:
        led_pin = LED_RED_PIN;
        err = BLTR_SUCCESS;
        break;

      case 2:
        led_pin = LED_GREEN_PIN;
        err = BLTR_SUCCESS;
        break;
      case 3:
        led_pin = LED_BLUE_PIN;
        err = BLTR_SUCCESS;
        break;
    }

    if(err == BLTR_SUCCESS)
    {
      if(cmd->isOn)
        nrfx_gpiote_out_clear(led_pin);
      else
        nrfx_gpiote_out_set(led_pin);                    
    }
  }
  break;
  ...
}

• Build and update your Bluetera II!

Summary

Protobuf is Google’s way of streamlining data serialization. Most current applications of the framework are software-specific. Bluetera II is one of the first platforms to adopt this powerful technology as a way of handling interactions between hardware and software.

We hope the LED example above illustrates how Bluetera’s usage of Protobuf can help you:

• Save a great deal of time,
• Simplify your code,
• Lower the barrier to changes and enhancements, and
• Streamline development in a way that was not previously possible.

Meanwhile, please send us your questions, feedback, and suggestions!