This project is a C implementation of the LEGO Powered Up Bluetooth Low-Energy (BLE) protocol which can be used to control LEGO Powered Up Hubs. The project has minimal dependencies (only libbluetooth), and the communication is easy to follow with a debugger since message processing is synchronous. Furthermore comments are added to explain the tricky parts so the project is useful to learn about the basics of BLE communication.
The LEGO Hub uses the Bluetooth Attribute Protocol (ATT) for controlling the Hub. This protocol defines how attributes can be enumerated, read, or written, and how clients can receive notifications about attribute value changes. Normally attributes are used for accessing server data and notifications are used to inform the client about the new value of an attribute. As for LEGO Hubs, attribute writing is used for issuing commands, and notifications are used for receiving various messages from the Hub. The reason of using attribute protocol for communication is that it has a reserved channel id (its value is 4) and its message format is simple so the protocol overhead is acceptable (around three bytes are wasted for each message).
The detailed LEGO Generic Attribute Protocol (GATT) can be found here:
https://lego.github.io/lego-ble-wireless-protocol-docs/
Due to the synchronous nature of the library, it is easy to define the necessary steps (stages) to control a given LEGO model. It is recommended to enable only those notifications which are required by the model (i.e. when built-in tilt sensor is not needed, it is better to keep it disabled). Processing these notifications can be postponed and unwanted notifications can be discarded easily.
The project is in its early phases. Any contributions are welcome.