Getting the Best Bite from Bluetooth
The move towards application specific embedded processors.
By Ross Forman, SMART Modular Technologies
Bluetooth technology has started down the road to ubiquity. Within a year, Bluetooth wireless interconnectivity will be a standard feature in new PCs, printers, PDAs, pagers, mobile headsets, and more. Within the decade, Bluetooth will be pervasive in new vehicles and aircrafts; in houses, hospitals, nursing homes and factories; in the hands of meter readers and delivery personnel; in game sets and stereos in fact, any place where there's a need for devices to talk to each other over a short distance without a cumbersome clutter of wires.
Bluetooth market penetration is being driven by the availability of cost-effective Bluetooth embedded modules. These modules have evolved from early thin modules into today's fat modules containing highly functional software stacks. Currently, the trend is towards developing application-specific modules that are custom tailored to particular cost and application needs. This article offers suggestions on how designers and product developers can take advantage of the current development thrust and leverage the creative potential of Bluetooth for a vast range of unexplored applications.
Bluetooth enables devices to talk to each other by means of a low-cost transceiver chip that transmits and receives in the 2.45 GHz band. For example, a Bluetooth port embedded in a host computer would enable printers, laptops, bar code readers, and other peripherals with their own Bluetooth device to link to it wirelessly, without additional redesign. It is important to note that, with today's Bluetooth modules, there is no need to apply native processing resources to wireless communications. Communication processing can be done on the embedded Bluetooth module to keep from degrading the performance of the host device.
Thin to Fat
The first Bluetooth modules were thin and consisted only of an RF chip and antenna packaged with a baseband chip/host controller and interface software. The subsequent-generation of Bluetooth modules are fat in that a microprocessor and an upper layer Bluetooth software stack residing in ROM or Flash memory is added. The designers of these modules have to be very conscious of power requirements in their choice of a microprocessor. Fortunately, there is no need for a lot of processing power (MIPS) in most Bluetooth scenarios and designers can select relatively low-powered processors such as an inexpensive 16-bit or 32-bit RISC processor.
Basic Design Considerations
There are numerous decisions to be made when designing Bluetooth into a given application. The most basic consideration from an RF standpoint is the location and function of the device(s). Will it reside in a closed or relatively open space? For example, will it function in a truck, car or plane, or will it be in an office, home, store, shop floor, or even out in a field? How far physically will the remote device be from the printer, cash register, router, or other host device? How many registers, routers, and printers will there be in the environment? How will they be oriented?
Do you need a bonded or a promiscuous link? For example, is the printer controlled by one person or a community of users? Is the wireless application merely one of cable substitution, or is this a remote data access or sophisticated ad hoc networking application?
How critical is speed? Some applications don't necessarily need rapid transfers of information, but others do. The amount of data being transmitted and the nature of that data is a critical consideration. Finally there's the issue of power. Do you need Class 1, 2, or 3? Will the host device run off a car battery, a photocell, or an AC outlet?
To keep costs low and effectiveness high, Bluetooth modules have to be matched carefully to the opportunity.
Providing a Unique PersonalityWhen these issues are thoroughly worked through, a designer can begin evaluating Bluetooth profiles to see how they add functionality and reduce costs.
The Bluetooth specification defines different profiles that can be used to add functionality in order to help tailor a module to an application. Originally, 13 profiles were approved. This has now doubled and new profiles are being added all the time. Some of the more popular profiles include TCP/IP dialup networking, AT modem commands, object exchange (OBEX), Server Discovery Profile (SDP), fax, Hard Copy Report Profile (HCRP), image printing (BIP), voice processing, USB, and serial transfers between "dumb" devices.
It is possible to load a module up with a lot of these profiles, but there is a practical limit because the code takes up flash memory and ramps up the cost. Hence a module's specific personality is custom tailored to the job through a careful selection of profiles or the use of only a single profile.
Application-specific modules are embedded Bluetooth modules that take advantage of a unique application software stack, including specific Bluetooth profiles to present tailored personalities. They differ from fat modules in that they provide a more cost-effective and flexible hardware platform for leveraging the versatility inherent in the range of Bluetooth profiles. (See Figure 1)
Figure 1. This illustration shows the migration from Bluetooth's original and simplistic design to today's more sophisticated application specific modules.
Physically, an application-specific module is a fat module with just two chips. One chip contains the microprocessor along with the baseband and host controller functions. The second chip is the RF chip. This saves on indirect costs (e.g.: real estate) and also direct costs.