[devLogs] Bluetooth Development

From my limited experiences in middleware development and research, writing programs to establish a Bluetooth(BT) connection between 2 programs is non-trivial.

Some learning points from 2-3 weeks of BT research:

• Not as simple as a client-server architecture, requires knowledge of Socket Programming, if building BT software at a much more granular level.

• The BT software stack has its own BT-specific communication protocols like L2CAP and RFCOMM, akin to how serial communication has serial communication protocols like SPI, I2C, UART.

• The nature of BT development is largely platform-specific - Linux has BlueZ has the official bluetooth stack for interactions involving linux devices (eg. RPi running raspbian)

  • This is the official API documentation for BlueZ - I have no idea how to read this, but after reading background information from textbooks and internet code examples, some words/functions are starting to make sense.

• There is a cross-platform library that is written in Python - pybluez. Online resources like Will investigate its effectiveness when used to write programs to establish connections with bluetooth devices, like the iPhone and iPad.

• The device ecosystem in a BT setting consists of central devices and peripheral devices. Central devices accept data from peripheral devices.

  • both central or peripheral devices can engage in the first phase of connection - device discovery - and broadcast their availability for connection and MAC address (BT mac address, different from internet MAC address)

  • central devices share a master-slave connection model with peripheral devices.

  • Peripheral devices send data to central devices.

  • The device which engages in device discovery will systematically broadcast its MAC address to devices nearby, and systematically query these nearby devices for its device name (in plain string) and/or MAC address.

• Profiles in BT are specifications which can be implemented as APIs. In the official, fully-verbose BT spec, there is a profile spec that is specifically for developers who want to develop HID (human input devices, eg. emulating a BT keyboard interface).

  • I just need to find a library/API which allows me to emulate keyboards.

Very useful help resources that leads to coding knowledge on BT:

• https://people.csail.mit.edu/albert/bluez-intro/index.html

• Getting Started with Bluetooth Low Energy

Interesting examples (repos)

• https://github.com/ruundii/bthidhub/tree/master

• https://github.com/quangthanh010290/keyboard_mouse_emulate_on_raspberry/blob/master/server/btk_server.py

• https://github.com/Heerkog/MicroPythonBLEHID/tree/master

• https://github.com/edrosten/libblepp

• (BLE) Most useful forum conversation (+ repo by petzval): https://forums.raspberrypi.com/viewtopic.php?t=305045

• (BT classic, audio streaming example) https://github.com/nkim-bitzap/bluetooth

• Official C BlueZ code https://github.com/luetzel/bluez/blob/master/doc/device-api.txt

ESP32-specific resources:

• https://docs.espressif.com/projects/esp-idf/en/stable/esp32s3/api-guides/bluetooth.html#applications

Google search term: ble server on raspberry pi which acts as a keyboard

Highest-level specs:

• https://www.bluetooth.com/specifications/specs/human-interface-device-profile-1-1-1/