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Bluetooth headset design— not ju


									Bluetooth headset design—
not just about chips
By Karen Parnell
Product Marketing Manager                                          Antenna
CSR plc                                                                                                                                             Module-specific PS keys:
                                                                                                                                                    ∗ 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.
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).
                                               Clock                         Baseband
                                                                              and logic                                                                    Design considerations
   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-
                                                                                                                                                           sumption, audio quality and
                                              DAC                                MMU               interface                                               interoperability. Other pres-
                                                                                                      UART                                                 sures include time-to-market,
                                                                             PHY layer
                                                                                                                                                           total cost and the eventual
                                               Power control                 HW engine
                      Li-polymer/ion cell      and regulation                                           PCM                                                Bluetooth Qualification Body
                                                  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
                                                                    controller                                                                             also appeal to a wide audience
                                                                                                                                                           with practical, easy-to-use and
                                                                     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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