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Designing a BLE advertise-only beacon

Posted: 17 Jun 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Bluetooth Low Energy? BLE? beacons? sensors? personal area networks?

Single-mode Bluetooth Low Energy (BLE) devices may operate in an advertise-only, "connectionless" mode where devices only transmit advertising packets. This mode requires a greatly simplified software stack, further reduces power consumption and reduces the overall bill of materials. This mode of operation is optimal for applications that do not require bi-direction communication, such as beacons and sensors.

I have described BLE's advertise-only mode in "Taking advantage of Bluetooth LE advertising mode". In this article, I and my co-authorMark Jakusovszky give you examples of advertise-only mode in action: two reference designs for BLE sensor beacons that can be powered by a single 1.5V AAA or coin cell battery. These designs use EM Microelectronic's BLE controller, and the beacon device is EM's design. (Full disclosure: I was hired as a consultant by EM Microelectronic to work on the design. Mark works for EM.)

Bluetooth LE in advertising mode
Bluetooth Low Energy is the most recent extension of the well-known wireless protocol for personal area networks (PAN) introduced in the Bluetooth 4.0 specification.1BLE is optimised to enable wireless communication between a smart phone and a low duty cycle, coin cell battery operated, potentially disposable device located in close vicinity.

The cost and power advantages of BLE are the result of an asymmetrical network topology, with more complex dual-mode BLE devices, which support BLE as well as classic Bluetooth BR/EDR communications, and single-mode BLE devices, which support only BLE protocol. This topology takes advantage of the simplicity of a single-mode stack for sensor-like devices, but allows for the versatility of a dual-mode stack for more complex multi-functional devices such as mobile phones.

Another distinguishing characteristic of BLE is that a device may operate in different roles depending on required functionality. At any one time, a BLE device may assume the role of a master or a slave; that is, as a scanner, or an advertiser, respectively. As a slave, the BLE device periodically transmits advertising information, waiting for a connection request from an interested master device. As a master, the device listens for advertising information transmitted by other devices and chooses whether or not it wishes to connect to those slave devices. Therefore, in a conventional BLE network, both masters and slaves transmit and receive data, with the intention of "pairing" to an appropriate partner device.

However, Bluetooth Low Energy also allows for an advertise-only, connectionless mode, where slave devices only transmit advertising packets while master devices only use their receive functionality to listen for these packets.

This advertise-only mode greatly simplifies the requirements of the BLE software stack by excluding stack sections normally required for discovery and connection (or pairing) and allowing the use of a lower-cost microcontroller with a much smaller flash/RAM memory footprint. Advertise-only mode also allows data to be simultaneously communicated from a single "peripheral" slave node to multiple master devices.

Designing a BLE advertising beacon
To implement a BLE advertise-only beacon, the device has to transmit a non-connectable unidirectional advertising package as defined in the Bluetooth specification.1 This type of advertising packet signals to scanning (master) devices that this device does not support a complete BLE data connection but can only broadcast information. The advertising packet contains the MAC address (6 octets using IEEE format) of the device sending the information and a data payload of up to 31 octets. This broadcasting information may contain device ID, sensor readings, TX power level, battery level, etc. To ensure compatibility between all BLE devices and support proper interpretation of the received data, the BLE specification defines the required advertising data format, which includes a tag for manufacturer-specific information that allows embedding proprietary data into a standardised advertising format.

The EM9301 BLE Controller from EM Microelectronic,1 integrates a 2.4GHz radio and a BLE controller that handles specification-compliant packet assembly, transmission timing, and RF channel management. As a fully functional BLE controller, EM9301 supports all modes of operation for both master and slave types of devices and communicates with any number of host microcontrollers, which contain the upper layers of the BLE stack, via a standard peripheral interface (SPI) bus using standardised Host Controller Interface (HCI) commands.

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