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Nordic Semiconductor technical article (Nordic Semiconductor
editorial contact: Steven Keeping, e-mail:
[email protected], Tel: +61 (0)403 810827) TITLE: Not a
standard wireless solution STANDFIRST: Some would claim low-cost
wireless is wrapped-up by Bluetooth and ZigBee, but look deeper and
youll find proprietary RF alternatives that could be better suited
for Japanese manufacturers products. By John Leonard, Nordic
Semiconductor, Oslo, Norway TEXT: You could be forgiven for
thinking that low-cost wireless means IEEE 802 in its Bluetooth
(IEEE 802.15) and ZigBee (IEEE 802.15.4) guises. Undoubtedly they
get most of the publicity both are backed by aggressive Special
Interest Groups comprising a whos who of electronics industry
heavyweights but they arent the only wireless games in town.
Bluetooth is ideal for widely compatible communications on a
personal area network (PAN) comprising PDA, headset, mobile phone
and laptop PC, for example, where adhering to the standard does
indeed eliminate much of your design challenge. You can be sure
that your design will communicate with another built to the same
standard and will have the desired range and data transfer rate.
And the recently-ratified ZigBee standard excels for products used
on networks comprising scores of nodes where infrequent, yet
reliable communications are needed, and the batteries have to last
for years. However, adhering to these standards does come at a
price: the silicon is relatively expensive, and there is
significant data packet overhead simply to ensure compatibility,
which increases data transfer time and consumes power. Much of the
design effort and testing for 802.15 solutions is needed to ensure
compliance with the standards. This makes sense when ensuring
interoperability between mobiles, laptops or wireless sensors from
many manufacturers, but if the application is destined for a
one-to-one dedicated link such as wireless mouse to keyboard, it
becomes an unnecessary expense. These low-cost, low power
consumption applications are increasingly important as
manufacturers seek to develop innovative products for the export
market. This article seeks to illustrate the benefits of an
integrated proprietary RF chip manufactured by the authors company
(the nRF24xx series), for these types of applications. We will
compare the design of a wireless mouse using Bluetooth, ZigBee and
this IC to demonstrate how this alternative wireless technique
fares. The basic elements of these designs remain essentially
unchanged for other simple applications such as gaming controllers
and intelligent sports equipment. RF compared The Bluetooth
protocol allows data to be transferred between 1 master and up to 7
slaves (in a PAN or piconet) at rates of up to 723 kbit/s. However,
the actual data payload is usually reduced due to the overhead of a
communications protocol
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defining the type of each unit with address and other header
information to ensure compatibility with other Bluetooth devices.
The standard employs a GFSK (Gaussian Frequency Shift Keying)
modulation scheme using 83, 1 Mbit/s channels within the 2.4 GHz
band. GFSK applies Gaussian filtering to the modulated baseband
signal before it is applied to the carrier. This results in a
dampened or gentler frequency swing between the high (1) and low
(0) levels. The result is a narrower and cleaner spectrum for the
transmitted signal compared with the straightforward approach of
FSK (Frequency Shift Keying). Because Bluetooth operates on the
same licence-free ISM band as other wireless technologies (for
example Wi-Fi) interference can compromise data rates because
corrupted packets need to be re-transmitted. Version 1.2, however,
addresses this problem by incorporating Adaptive Frequency Hopping
(AFH). This allows two communicating Bluetooth devices to
constantly change their mutual frequency across the band to avoid a
clash with other RF devices in the vicinity. Bluetooth is available
in 3 basic power levels: Class 1 (100 m line of sight range), Class
2 (10 m), and Class 3 (2-3 m). Most contemporary consumer devices
are Class 2. The devices in a Bluetooth piconet each have a unique
48-bit identity number. The first device identified (usually within
2 seconds) becomes the master, and sets the 1600 frequencies to be
used each second across the band. All other devices in the piconet
lock or synchronise to this sequence. The master transmits in even
slots, the slave responds in odd slots. Active slave devices in the
piconet are assigned an address, and listen for slots addressed to
themselves. Slaves may also go into lower power sniff, hold or park
modes. In sniff mode a device listens only periodically, during
specific sniff slots, but does retain the synchronisation. In hold,
a device listens only to determine if it should become active. In
park, a device gives up its address. Although hold and park modes
extend battery life, it does mean the device loses synchronisation
for at least 1600 hops and has to wait for a new link to be set up.
This can take several seconds and is a drawback when the user
requires a constant fast response. The Bluetooth standard includes
a range of profiles which you can select to target your
development. All Bluetooth applications must, however, be certified
for compliance with the standard and all users must be members of
the Bluetooth Special Interest Group (www.bluetooth.org). Because
of commercial pressures from members of the Bluetooth SIG most of
the profiles are suited to media and file transfer applications on
mobile phones. Consequently, development using Bluetooth profiles
is not trivial and can make the standard somewhat unwieldy for
simple applications. ZigBee is a more recent RF standard
specifically developed for low power, low data rate wireless
monitoring and control applications across a large number of
distributed nodes. The standard is defined by IEEE 802.15.4 (see
www.zigbee.com) and is a simple data protocol offering high
reliability. This includes acknowledgement of each transmission
burst and other techniques to maintain communications integrity.
ZigBee doesnt require Bluetooths synchronisation, decreasing power
requirements considerably. Like Bluetooth, ZigBee operates in the
ISM 2.4 GHz band (16 channels at 5 MHz spacing). The standard also
provides for versions operating in the European 868 MHz (single
channel) and US 915 MHz (10 channels spaced at 2 MHz) bands. It
promises a maximum data rate of 250 kbit/s.
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ZigBee relies on a DSSS (Direct Sequence Spread Spectrum) scheme
for data transmission of data. DSSS offers some immunity to
interference, but this comes at a cost of transmitting excessive
data packets, incurring bandwidth usage and current consumption
overheads. The standard has attempted to address the potential
weaknesses of Bluetooth in certain application environments,
typically low-latency and low data rate applications. However,
ZigBee applications at the RF physical layer still have to carry
the overhead needed to achieve the interoperability functions
required by the 802.15.4 specification. Complementary technologies
According to the Bluetooth and ZigBee organisations the standards
are complementary rather than competitive. ZigBee does allow for
many more nodes up to 4090 compared to Bluetooths 7 plus master.
The ZigBee protocol suits industrial and domestic monitoring and
control applications where extremely low activity and scaleable
network functionality is required over a high node network. Power
consumption is a major differentiator. ZigBee is designed for very
low duty cycle, ultra long life applications where battery life is
measured in years, whereas continuous Bluetooth communications
typically drain batteries in a matter of hours. And ZigBee chipsets
cost a fraction of a Bluetooth solution (although there are
variants of the Bluetooth protocol stack that offer less than the
full range of profiles for less expense). As a recently ratified
standard, however, ZigBee chipsets are somewhat limited at present.
The authors company (www.nordicsemi.no) manufactures a proprietary
wireless solution, dubbed the nRF24xx. It is a system-on-chip
device, comprising the RF transceiver, an 8051 microcontroller,
4-channel, 12 bit ADC and various standard interfaces, manufactured
using a 0.18 m CMOS process. The product uses a GFSK modulation
scheme (similar to Bluetooth). It offers a nominal data rate of 1
Mbit/s has been designed with minimal overhead to maximise RF and
minimise the power budget. The product introduces a hardware based
physical layer protocol processing which is transparent in normal
operation. (Figure 1 (a) and (b) compare a ZigBee protocol stack
with the proprietary solution.)
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Figure 1(a): Proprietary ZigBee protocol stack and (b) Nordic
nRF protocol stack The proprietary solution has been developed to
be familiar to a small-scale embedded systems developer. Such a
person using this silicon radio for a wireless project will be
comfortable with the SPI-based interfaces used by the device. A
120-bit register is used to set up communication links on the
device, covering the functionality aspects. The integrated
microcontroller is used to setup the parameters the first time
only. Thereafter it clocks the destination address and the actual
data. Significantly, because the design does not have to be
qualified to a standard, the time-to-market schedule is decreased.
Although the product must conform to the appropriate communications
authoritys regulations such as those of Europes ETSI or the USs
FCC, this is true of any RF communication whether it is designed to
a standard or not. Bluetooth, ZigBee and the proprietary solution
each use a unique packet structure (see sidebar Packet structure).
Using its packet structure, with a data packet of 32-bits, the
proprietary product can perform message transfer in 80 bits,
yielding an overhead of 48 bits, and giving a packet
data-efficiency of 40 percent. In comparison, Bluetooth requires
160 bits, with an overhead of 128 bits and an efficiency of 20
percent. To transmit exactly the same amount of data the ZigBee
device would take 152 bits, yielding an efficiency of 21 percent.
The proprietary solution duplicates Bluetooths channel scheme. Both
utilise up to 83, 1-MHz channels between 2.400 and 2.483 MHz. (Or
more accurately 2.402 to 2.483 GHz broken into 75, 1-MHz channels,
with a 2-MHz lower guard band and a 3.5-MHz upper guard band.) This
compares with ZigBees 16 channels. (See Figure 2.) This offers
Bluetooth and the proprietary solution many more alternative
relocation frequencies should they encounter interference from
other devices operating in the crowded 2.4 GHz band. (See sidebar
Handling interference.)
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Figure 2: ZigBee and the Nordic solution both operate in the
license-free ISM 2.4-GHz band. ZigBee incorporates 16 channels
separated by 5 MHz, while the Nordic solution mirrors Bluetooths
83, 1 MHz channels A question of bandwidth An RF wireless mouse
operating in the license-free ISM 2.4 GHz band is a classic example
of the simple, low power, cost sensitive wireless application that
Chinese manufacturers are so good at producing both for the
domestic market and for export. Lets compare the design of a
product based on the proprietary chip, with one based on ZigBee and
Bluetooth solutions. The typical usage pattern for a wireless mouse
is 10 percent active and 90 percent in sleep mode, with a
communications cycle of transmission and reception every 8 ms of
operation when active. Consequently, the net data rate is 0.1 x
(125 x 80 bit/s) = 1 kbit/s. Compare this with ZigBee. Its net data
rate in this application is 0.1 x (125 x 152 bit/s) = 1.9 kbit/s.
This is nearly double the proprietary solution. In addition, ZigBee
runs at a maximum of 250 kbit/s compared with the proprietary
solutions 1 Mbit/s. Consequently, it can be seen that ZigBee has a
bandwidth requirement 8 times that of the proprietary solution to
do the same job. Because Bluetooth has to maintain synchronisation
to avoid re-linking delays it sends a 160-bit packet every 675 S
(1600 packets/s, or a net data rate of 256 kbit/s) to maintain the
link, whether the mouse is in use or not. As we saw above, while
the link could be maintained without synchronisation, this can
result in re-acquisition periods of up to 3 seconds, hardly
practical for the user. The typical mouse data packet is
illustrated in Figure 3.
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Figure 3: Typical wireless mouse transmit package Extending
battery life Figures 4(a) and (b) show the sequence diagrams for
wireless mouse-to-USB dongle communications for a proprietary and
ZigBee equipped product respectively.
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Figure 4(a): Sequence diagram for ZigBee wireless mouse
communications and (b) Nordic nRF solution The sequence diagram for
the proprietary solution shows that the device is active for 195 +
16 + 80 + 202 + 49 + 16 s = 558 s. For the typical 8 ms
communications cycle this gives an actual duty cycle of 1 : 14.3.
Because the active time during the 8 ms communications cycle is
relatively low, the average current consumption when in constant
use is 855 A. Assuming the proprietary solution is operating from a
single AA battery (with a capacity of 2000 mAh) it would be
possible to achieve around 2350 hours of continuous link operation.
This is around a years operation for an average user (including the
battery power required by the mouse optical sensor, which together
with the wireless link comprise 95 percent of the power budget. The
microcontroller consumes the other 5 percent). Now lets look at
ZigBee. From the sequence diagram it can be seen that the device is
active for 192 + 200 + 192 + 26 + 608 + 192 + 352 +10 s = 1.772 ms.
For the typical 8 ms demand cycle this gives an actual duty cycle
of 1 : 4.5. The duty cycle is much higher than the proprietary
solution (primarily due to the need to transmit for 8 times longer
to maintain a comparable performance to the proprietary solution.)
During this communications cycle ZigBees average current
consumption is 4 mA. This means the single AA battery will give 500
hours of continuous link operation, yielding around two-and-a-half
months operation for the average user. Although Bluetooth also has
an average current consumption of 4 mA when active, it continues to
run at 8 mA even in idle mode in order to maintain synchronisation.
(The equivalent idle figures for the proprietary solution are 10.2
A and for ZigBee 351 A. These figures are summarised in Table
1.)
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Table 1: Comparison of current consumption for nRF, ZigBee and
Bluetooth in various operational modes Consequently, a user may
expect a Bluetooth mouse battery to last no more than a month.
Note: the battery life calculations are based on an average current
consumed in each mode as a proportion of the total period of 8 ms
(the communications cycle) shown in the sequence diagrams, for
constant usage (i.e. 125 packets/s every second the mouse is
switched on). As we have already seen when looking at the bandwidth
requirements above, a wireless mouse is never operated this way,
spending 90 percent of its time idle. The proprietary solution and
ZigBee would enter standby modes with A consumption during these
idle times while Bluetooth would continue to draw mA currents. The
critical factor here is that the Bluetooth device must maintain an
active state to ensure communication links are maintained compared
with the other wireless solutions.) Beyond the standard Bluetooth
and ZigBee demonstrate how the electronics community can
collaborate to create operating standards that ensure compatibility
across global markets. Both are excellent technologies that work
well in their defined sectors. You only have to attach a Bluetooth
headset to your mobile phone to experience this very practical RF
technique in action and to appreciate its benefits. Nonetheless,
technology based on standards does have its disadvantages. Firstly,
to employ the standard you have to meet the standard and that
commits you to costly NRE charges in initial design and testing for
compatibility. Secondly, by their very nature, standards have to be
a one-size-fits-all solution - as your competitors have their hands
on the same technology, it is difficult to differentiate your
product in an increasingly competitive global market. Finally,
standard solutions offer little opportunity for design flexibility;
for example, you are limited in how much you can reduce the power
consumption in your RF product. The wireless mouse product
described in this article illustrates how a proprietary solution
could be better than Bluetooth and ZigBee for a product that
demands long battery life, and reliable wireless communications
with low duty cycles. There are scores of other applications where
the same design criteria apply, for example, wireless games
controllers and wireless communication between a heartbeat sensor
and sports computer. And with the world becoming increasingly
wireless it could pay to look beyond the standard for your next
wireless communications link.
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About the Author. John Leonard gained his BEng(hons) Electronics
Engineering qualification from the University of Portsmouth, UK. He
is a field applications engineer for Nordic Semiconductor. His role
is to co-ordinate support resources toward project completion, and
on-site assistance to major customers around the globe. Project
support includes firmware development and software libraries for
customer use to speed up project development cycles. Layout issues
and antenna development are also supported together with a team of
five engineers based in Oslo and Trondheim, Norway. Sidebar 1
Packet structure Bluetooth
1. Access code 68 or 72-bit 2. Header 56-bit 3. Data payload
32-bit
ZigBee
1. Preamble - 32-bit 2. Frame de-limiter - 8-bit 3. Frame length
- 8-bit 4. Frame control - 16-bit 5. Data sequence number - 8-bit
6. Address ID - 32-bit 7. Data payload - 32-bit 8. Frame checksum -
16-bit
Proprietary
1. Preamble - 8-bit 2. Address 32-bit 3. Data payload 32-bit 4.
CRC 8-bit
Sidebar 2 Handling interference All three wireless topologies,
Bluetooth, ZigBee and the proprietary solution, have mechanisms to
reduce the effects of interference from other RF devices operating
in the same band. Bluetooth has a frequency-hopping spread spectrum
(FHSS) approach that ensures all 79, 1-MHz channels are covered
equally over time to avoid consistent channel interference. ZigBee
is geared more towards handling intermittent narrowband
interference with the use of DSSS across its 16 bands, and so in
the presence of other 802.11b/g devices is more prone to
interference and may have to wait for the other device to stop
transmitting. The proprietary device takes a more hybrid approach.
Because of its modest output power, interference is unlikely. To
minimise current consumption and complexity the proprietary
solution does not use a spread spectrum scheme simply transmitting
on a single frequency until a packet corruption threshold is
reached if there is interference. Channel relocation involves a
simple, single-byte SPI instruction to the device
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The availability of 79, 1-MHz channels allows ample option for
one-time relocation away from the other devices transmission
frequency for static applications. And even in locations such as
airport hotspots the necessity to re-locate in the spectrum will be
relatively infrequent, of the order of minutes or hours. In the
case of the wireless mouse, the co-channel rejection is typically 6
dBm. Consequently, as long as the distance from mouse (TX) to USB
dongle (RX) is half the distance from the interferer communication
will usually be uninterrupted. This is because 6 dB equates to a
doubling of distance in RF terms. (See Figure A.)
Figure A: Interference between co-located wireless mice is
limited because low RF output restricts signal strength at
co-located receiver NORDIC SEMICONDUCTOR, www.nordicsemi.no May be
reproduced with permission from Nordic Semiconductor