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Bluetooth headset design— not just about chips By Karen Parnell Product Marketing Manager Antenna CSR plc Module-specific PS keys: E-mail: firstname.lastname@example.org ∗ Unique Bluetooth address ∗ Crystal trim Bluetooth headsets have now External RF ∗ Power settings reached mass-market appeal due to their practical use, espe- components Set during production. Different cially when driving in countries for each individual headset. where legislation is tough on using mobile phones while on Bluetooth HMI configuration PS keys: wheels. Bluetooth is useful to headset EEPROM car drivers because they do not chip ∗ Ringtone ∗ PIOs used have to wrestle with wires and ∗ LED flashing pattern purchase costly hands-free kits Speaker ∗ Power management that have to be installed every Set once during development. time they change their phones. Identical for every headset Bluetooth-based headsets of the same type. Microphone allow true phone indepen- dence, so users can take advan- tage of the latest cellphone fea- tures, such as built-in cameras and PDA functions, from diffe- Figure 2: The Bluetooth stack and headset profile are stored in ROM inside the headset software. The chip rent manufacturers without reference parameter PS keys are stored in a small external rewritable memory. having to change the headset each time. Low-end mono headsets that sets in this case become more rers bringing out headsets that With mass-market appeal are cheap and easy to use are like miniature phones. are specially designed for wo- comes greater end-user seg- still popular. These are cost- Audio quality and voice cla- men to complement accessories mentation. We have seen the effective and can bundle with rity are becoming important to such as jewelry. Such products headset market divide into low-, new mobile phones; mono branded headset manufactur- look like a small earpiece that mid- and high-end models. This Bluetooth headsets provide ers. To improve audio quality, fits discreetly into the ear and lets manufacturers target spe- that out-of-the-box experience midrange chips now have on- can be removed after each call. cific market segments, allowing to consumers. Midrange head- chip DSPs that allow echo and These newer headsets are them to either offer more fea- sets appeal more to Bluetooth noise-cancellation software driving increased device-level tures and differentiate products veterans who want more func- such as the Clear Voice Capture integration while needing extra from the competition’s or tionality like noise cancella- (cVc) software to run. features such as a DSP for im- choose to take on the low-cost, tion, LCD screen, call vibrate Further segmentation has proved audio quality through high-volume market. and voice recognition. Head- also occurred with manufactu- echo cancellation and noise reduction, on-chip batter y- charge circuits and a switch- mode power supply (SMPS). Speaker Interfaces Clock Baseband and logic Design considerations gen Antenna There are many challenges that Crystal Audio oscillator Flash codec MIC Bluetooth headset designers bias gen face today—not the least of Filter Balun RAM which are the size and weight of USB Microphone RF the final product, power con- Synchronous serial sumption, audio quality and DAC MMU interface interoperability. Other pres- UART sures include time-to-market, PHY layer Audio total cost and the eventual Power control HW engine Li-polymer/ion cell and regulation PCM Bluetooth Qualification Body interface Voltage User testing. Apart from these, the regulator pushbutton Microcontroller headsets themselves must not Programmable I/O Battery and LEDs charger Interrupt RISC only function brilliantly, but microcontroller controller also appeal to a wide audience Switch mode Event with practical, easy-to-use and regulator timer aesthetic designs. Integrated into the device Other considerations in- clude the Bluetooth chip, the Bluetooth stack and headset Figure 1: All blocks must be inside the device itself, but this would involve multiple dice in one package. profile software, the hardware/ software development kit, re- headset is the battery. A typical tensive interoperability tes- headset user interface is de- ference design, interoperability Li-ion battery designed to fit ting, it is no longer required to fined). They are then pro- testing and local technical into a headset is around 5mm x stay with flash in high-volume grammed into every headset. support. 12mm x 45mm, which is much production. Most high-volume In a typical mono headset larger than an 8mm x 8mm de- designs migrate to ROM to SW/HW development systems design (Figure 1), there are vice package. lower costs and reduce risks As with most complex chips, many on-chip functions that by allowing tweaking of the manufacturers will provide can reduce bill-of-material cost Bluetooth software EEPROM-stored code, should good-quality development sys- and size and weight of the de- Bluetooth lower-layer hardware the need arise. tems to shorten time-to-market sign. Ideally, we would like to and firmware are essentially The Bluetooth stack and for new products. A typical pack in more blocks inside the fixed by the Bluetooth specifica- headset profile are stored in hardware development system device itself, but in reality, this tion and are very similar among ROM inside the headset soft- will include a development would be costly, since it would target end-applications, ran- ware. The chip reference para- board showing a well-designed, involve multiple dice in one ging from headsets to computer meters are called persistent fully-functional application de- package. For example, stacked- keyboards and mice. The main store (PS) keys for module- sign with the headset software pre-loaded onto the Bluetooth device. It should also contain the appropriate software needed to change PS key values, application notes, datasheets and user setup guide. For most Bluetooth headset designs, a standard off-the-shelf solution is perfect and allows user con- figuration in PS keys to satisfy end-user requirements. This methodology is suitable for quickly producing headsets that are fully interoperability-tested and relatively risk-free. To change deeply embedded functions, software develop- ment kits, such as the CSR BlueLab SDK, are available. These development kits maxi- mize headset customization. Bluetooth antenna and RF design are still a complex part of the overall headset design Figure 3: The end-to-end design process from board layout, software, choosing the right chip through to and should never be taken for reference designs, development and product testing needs to be considered well. granted. A good reference de- sign is worth its weight in gold die techniques can make ana- difference among suppliers is specific setup information. The as it defines the actual PCB lay- log, digital and multiple vol- the level and extensiveness of PS keys are stored in a small out that can ensure that none of tages live in the same package. their interoperability testing. external rewritable memor y the board components cause There are such devices avail- Some device-level features men- (EEPROM). interference or EMI issues. able today, but they are for high- tioned are specific to headsets, Figure 2 shows the software Bluetooth device manufactur- volume, low-cost designs and such as battery-charge circuits system architecture for head- ers should be able to provide are generally prohibitive. A and SMPS. However, a lot of the set-specific software running everything required to take a more practical solution would headset-specific personality is on a headset-specific Bluetooth reference design and build to be to integrate the RF, memory, written and defined by software. ROM chip. print. This includes fully- interfaces, microcontroller, There are generally two In addition to radio calibra- costed bill-of-materials, layout battery-charge circuit, clock types of headset devices—one is tion, the Bluetooth standard schematics and Gerber files for generator, DAC and SMPS into flash-based for low-volume pro- requires for each device to be the PCB layout to ensure that one device. This leaves only the duction or pre-production, in allocated a unique identifier— the design works. balun, filter, speaker, micro- which the headset software is the Bluetooth address. This is Bluetooth headset designs phone, battery, user pushbut- contained in flash memory; the set using the module-specific are not just about chips, but the tons and LEDs external to the other is ROM-based that has the PS key that configures para- total solutions available from device. headset software stored in on- meters that are specific to an in- wireless chip companies. The The package type should be chip ROM with user-configu- dividual module. These para- end-to-end design process from as small as possible, lead-free to rable keys being stored in off- meters are mainly specific to board layout, software, choos- meet the global green legisla- chip EEPROM. the module’s radio perfor- ing the right chip through to tion and easy to handle in pro- In the ROM version, the mance. Bluetooth companies reference designs, develop- duction to keep manufacturing headset personality is loaded on provide a recommended test ment and product testing (in- costs to a minimum. Packages power-up into the Bluetooth plan, test software and support cluding interoperability test- such as an 8mm x 8mm, 1mm device from EEPROM. The for setting up production test ing) needs to be considered well pin-pitch TFBGA strike a good flash device has the advantage systems for Bluetooth headsets. (Figure 3). High-quality local balance between manufac- of allowing software upgrades Human-machine interface technical support is also essen- turability and size. The main to accommodate new phones. configuration keys are set once tial to ensure the success of any factor in overall size of the But today, with the advent of ex- during development (when the end-product.
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