Reference design: One volt Bluetooth Low Energy advertise-only beacon
I have described BLE’s advertise-only mode in "Make the most of Bluetooth LE advertising mode". In this article, I and my co-author Mark 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 my 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.1 BLE is optimized 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 multifunctional 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. More details on BLE protocol structure and stack portioning, see my earlier two-part article on EE Times (Galeev, ).2, 3
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 Blue 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 standardized advertising format. More information on the BLE advertising payload structure may be found in [4].
The EM9301 BLE Controller from EM Microelectronic,6 integrates a 2.4-GHz 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 standardized Host Controller Interface (HCI) commands.
In the simplest case of an advertise-only BLE device, the advertising mode can be fully implemented with just three HCI commands; setting the data for the advertising payload, configuring the time interval between advertising packets (up to 10 seconds), and requesting the controller to start a periodic advertising broadcast.
The critical selection criteria for a BLE beacon MCU is its ability to be operated by battery for extended periods of time. Hence it makes sense to use an 8-bit MCU with a large operation voltage range from 3.6v down to 0.9v. (EM6819, such an 8-bit MCU from EM Microelectronic, makes it a perfect match for BLE beacon applications.)7
A block diagram of a reference design for a BLE advertising bacon is shown in Figure 1. It includes a BLE controller, MCU and a few external components.
The EM6819 has a built-in high efficiency DC-DC converter, which allows a BLE beacon to operate from a single 1.5v battery by simply adding an external Ldcdc inductor. DC-DC provides a 2.5v power supply for the BLE controller and external sensors. If not required, the DC-DC circuitry could be easily bypassed to convert the design for operation from a 3v supply.
The BLE beacon software initializes and configures the BLE controller using a SPI interface, and also collects data from an internal temperature sensor and an external ambient light sensor as used in the reference design (the design’s flexibility allows integration of a variety of external sensors). After BLE advertising is enabled, its payload is periodically updated with the new sensor’s read out. Overall, the software has a very small footprint with less than 2K of flash memory used for implementation of BLE related functionality.
The MCU clock runs on a factory calibrated internal oscillator, so only one 26-MHz crystal is required for the BLE radio. EM9301 can support standard HCI commands via UART or SPI implementation. The reference design using a SPI bus allows for it to take advantage of a proprietary low-power mode of operation. With a 500-ms interval between advertising events and 0dBm RF output power during transmission, the average power consumption was measured at 300 ua @ 1.5 v.
The antenna port of the EM9301 has a 200-ohm differential impedance that allows the direct connection of a folded dipole antenna so no additional matching circuitry is required. Since for some applications a folded dipole is not practical due to its physical size (~ 45 mm @ 2.5 GHz), other antenna designs may be used and a proper impedance matching circuitry will be required.
Both approaches are demonstrated in two reference design implementations as shown in Photo 1. The "LOW COST" design uses a folded dipole antenna and has a minimum count of discreet components. The design size is 50mm x 30mm and is powered by one AAA battery. The "TINY" design is only 15 mm x 25 mm in size and is optimized for space constraint applications. It is powered by a coin cell battery and uses a space saving chip antenna with a discreet balun matching circuitry.
Click on image to enlarge.
Figure 1: One volt advertise-only BLE beacon block diagram
Click on image to enlarge.
Photo 1: "TINY" (left) and "LOW COST" (right) reference designs of 1V BLE Beacons.
Advertise-only BLE applications
But why would anyone want a Bluetooth transceiver that you can’t talk to or connect with?
In fact, broadcast-only devices have dominated consumer applications for over a century. Radio and television are the two biggest examples, GPS is another. For these systems to function, one party transmits information without any regard to whom or how many are listening, and for communications to occur, it only requires a properly oriented and capable receiver. For BLE advertise-only applications, the broadcaster is one (or many) low-cost, low-power BLE beacons, transmitting information (sensor data, location, etc.) without regard for who, or how many are listening. The receiver is any number of Bluetooth 4.0-capable devices, such as smartphones, tablets, laptops, or even dedicated-purpose BLE readers.
Advertise-only BLE Beacons that detect temperature and ambient light levels can be used as a "personal weather station" that communicates the temperature and cloud cover in your back yard to an application on your smartphone. As an added benefit, your spouse and kids can see the same information simultaneously on their smartphones as well. As an extension to this basic concept, household sensors could detect temperature, moisture, motion, smoke, and transmit this data to a dedicated home-based receiver device which could relay important changes to the homeowner through a wired internet connection, or even the cellular network.
Similar devices could be placed in fixed locations and transmit preprogrammed identification codes that could then be used by a guidance or location application to help orient visitors or consumers to their intended goal. Alternatively, these devices could be used as commercial advertisements, notifying interested consumers with an appropriate app, of deals on products of interest. The advertise-only nature of this communication also means that more than one consumer can be alerted simultaneously, and that the beacon would be less prone to malicious hackers than they would be, should they allow (or worse, require) a formal BLE connection.
One-way communicating with BLE
In this article, we describe a novel Bluetooth Low Energy architecture and associated applicable use cases. The BLE advertise-only architecture allows for less costly, less power-hungry and smaller BLE beacon devices. These devices are also potentially more secure, and provide for simultaneous communications with multiple users within reception range. Such BLE beacons are ideal for applications which only require one-way communication of static or dynamic information, such as environmental and safety sensors, location/guidance and wireless consumer advertisements.
Mikhail Galeev is a recognized expert in the area of wireless communication. With a primary interest in research and implementation of wireless protocols, he has over twelve years of experience and has authored multiple publications and patents related to applications of Bluetooth, ZigBee, Z-Wave and other wireless sensor networks. Mikhail holds a Master of Engineering Management degree from Northwestern University in addition to a MS EE degree from the University of South Alabama. Mikhail is the founder of Z-Focus Consulting, a company that develops software to enable BLE applications. You may reach him at mgaleev@z-focus.com
Mark Jakusovszky has spent over a quarter century defining, designing, developing and marketing advanced mixed-signal and low power wireless semiconductor solutions for companies like Atmel, National Semi, MicroLinear, Aeroflex and EM. In his current role at EM Microelectronic, he is empowering ultra-low power and energy harvesting sensor applications through EM’s Bluetooth Low Energy, High Datarate & Long Range 2.4GHz GFSK wireless, NFC and RFID solutions. Mark earned a MSEE at Santa Clara University and a BSEE at Rensselaer. You may reach him at markj@emmicro-us.com
Endnotes
[1] Specification of The Bluetooth System ver. 4.0 June 30 2010, www.bluetooth.org.
[2] Galeev, Mikhail. "Bluetooth 4.0: An introduction to Bluetooth Low Energy–Part I" published on EE Times.com’s Communications Designline, July 2010.
[3] Galeev, Mikhail. "Bluetooth 4.0: An introduction to Bluetooth Low Energy–Part I" published on EE Times.com’s Communications Designline, July 2010.
[5] Galeev, Mikhail. "Make the most of Bluetooth LE advertising mode" published on EE Times.com’s RF and Microwave Designline, April 2013.
[6] EM9301 Single-Cell Battery Bluetooth Low Energy Controller, EM Microelectronic, www.emmicroelectronic.com/products.asp?IdProduct=289
[7] EM6819 Sub-1V (0.6V) 8bit Flash MCU DC-DC Converter, EEPROM, EM Microelectronic, www.emmicroelectronic.com/Products.asp?IdProduct=272
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