Understanding BLE

BLE Basics

To understand how we communicate between the MagicLight Bulb and the Circuit Playground Bluefruit (CPB), it's first important to get an overview of how Bluetooth Low Energy (BLE) works in general.

The nRF52840 chip on the CPB uses Bluetooth Low Energy, or BLE. BLE is a wireless communication protocol used by many devices, including mobile devices. You can communicate between your CPB and peripherals such as the Magic Light, mobile devices, and even other CPB boards!

There are a few terms and concepts commonly used in BLE with which you may want to familiarize yourself. This will help you understand what your code is doing when you're using CircuitPython and BLE.

Two major concepts to know about are the two modes of BLE devices:

  • Broadcasting mode (also called GAP for Generic Access Profile)
  • Connected device mode (also called GATT for Generic ATTribute Profile).

GAP mode deals with broadcasting peripheral advertisements, such as "I'm a device named LEDBlue-19592CBC", as well as advertising information necessary to establish a dedicated device connection if desired. The peripheral may also be advertising available services.

GATT mode deals with communications and attribute transfer between two devices once they are connected, such as between a heart monitor and a phone, or between your CPB and the Magic Light.

Bluetooth LE Terms

GAP Mode

Device Roles:

  • Peripheral - The low-power device that broadcasts advertisements. Examples of peripherals include: heart rate monitor, smart watch, fitness tracker, iBeacon, and the Magic Light. The CPB can also work as a peripheral.
  • Central - The host "computer" that observes advertisements being broadcast by the Peripherals. This is often a mobile device such as a phone, tablet, desktop or laptop, but the CPB can also act as a central (which it will in this project).

Terms:

  • Advertising - Information sent by the peripheral before a dedicated connection has been established. All nearby Central's can observe these advertisements. When a peripheral device advertises, it may be transmitting the name of the device, describing its capabilities, and/or some other piece of data. Central can look for advertising peripherals to connect to, and use that information to determine each peripheral's capabilities (or Services offered, more on that below).

GATT Mode

Device Roles:

  • Server - In connected mode, a device may take on a new role as a Server, providing a Service available to clients. It can now send and receive data packets as requested by the Client device to which it now has a connection.
  • Client - In connected mode, a device may also take on a new role as Client that can send requests to one or more of a Server's available Services to send and receive data packets.
NOTE: A device in GATT mode can take on the role of both Server and Client while connected to another device.

Terms:

  • Profile - A pre-defined collection of Services that a BLE device can provide. For example, the Heart Rate Profile, or the Cycling Sensor (bike computer) Profile. These Profiles are defined by the Bluetooth Special Interest Group (SIG). For devices that don't fit into one of the pre-defined Profiles, the manufacturer creates their own Profile. For example, there is not a "Smart Bulb" profile, so the Magic Light manufacturer has created their own unique one.
  • Service - A function the Server provides. For example, a heart rate monitor armband may have separate Services for Device Information, Battery Service, and Heart Rate itself. Each Service is comprised of collections of information called Characteristics. In the case of the Heart Rate Service, the two Characteristics are Heart Rate Measurement and Body Sensor Location. The peripheral advertises its services. 
  • Characteristic - A Characteristic is a container for the value, or attribute, of a piece of data along with any associated metadata, such as a human-readable name. A characteristic may be readable, writable, or both. For example, the Heart Rate Measurement Characteristic can be served up to the Client device and will report the heart rate measurement as a number, as well as the unit string "bpm" for beats-per-minute. The Magic Light Server has a Characteristic for the RGB value of the bulb which can be written to by the Central to change the color. Characteristics each have a Universal Unique Identifier (UUID) which is a 16-bit or 128-bit ID.
  • Packet - Data transmitted by a device. BLE devices and host computers transmit and receive data in small bursts called packets.

This guide is another good introduction to the concepts of BLE, including GAP, GATT, Profiles, Services, and Characteristics.

This guide was first published on Jan 15, 2020. It was last updated on Jan 15, 2020. This page (Understanding BLE ) was last updated on Feb 20, 2020.