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A new wireless connectivity standard, IEEE
802.15.4, defines suitable media access
control (MAC) and PHY layers to enable
wireless control and sensing networks for
low-data-rate applications. Opportunities
include networked sensors in industrial,
commercial, and health care applications,
as well as low-cost toys and games.
The IEEE 802.15.4 standard combines
well with the new ZigBee™ network and
application support layers. When these
standards became available in 2003,
designers demanded a suitable development
platform almost immediately. Such a
platform had to remain flexible and have a
rapid design cycle.
CompXs introduced Blencathra, the
first certified IEEE 802.15.4-compliant
development system, in November 2003
(Figure 1). Blencathra leverages the capacity
of Xilinx Virtex-II™ FPGAs to provide
extensive stack monitoring and debug facilities
within the FPGA. This helps development
and compliance testing and allows
our customers to observe the operation of
the stack in real time.
CompXs also offers a MAC/PHY module
built using the low-power, low-cost
Spartan™-3 family. The module does not
include the stack monitoring features of
Blencathra, but is ideal for customers
wishing to deploy their applications cost-effectively.
It also includes an integrated
2.4 GHz radio.
Wireless Networking for Control Applications
ZigBee defines network and application
support layers for wireless networks based
on the MAC and PHY layers of IEEE
802.15.4. The IEEE standard uses the
global 2.4 GHz ISM (industrial, scientific,
and medical) band, as well as the
American 915 MHz and equivalent
European 868 MHz unlicensed bands.
The maximum data rate in each band is
250 Kbps, 40 Kbps, and 20 Kbps, respectively.
The range is typically 30 meters
but can extend to 100 meters in optimal
conditions.
The 802.15.4 physical layer uses
direct sequence spread spectrum to
spread the information over a range of
frequencies. For devices that transmit
infrequently, this allows for greater
power conservation than Bluetooth’s™
frequency-hopping scheme. At the
MAC level, another advantage of
802.15.4 is that it has only two power
modes: active or sleep. This greatly simplifies
power management.
All devices have 64-bit IEEE
addresses, allowing virtually unlimited
devices in a network. This allows
for massive sensor arrays and control
networks, but the option also exists to
allocate 16-bit addresses to reduce
packet size.
IEEE 802.15.4 is well suited for periodic
data (such as sensor outputs generated at
a rate defined by the application), intermittent
data generated externally by a switch,
or repetitive low latency data allocated to a
specific time slot (such as mouse data).
The ZigBee Alliance has defined the
upper layers of the protocol stack to use the
IEEE 802.15.4 MAC and PHY. ZigBee
includes the parts of the protocol from the
network layer to the application layer,
including application profiles. The first
profiles were published in mid-2003.
Blencathra
The Blencathra development platform
allows developers to build and analyze
ZigBee/802.15.4 designs quickly and at little
design risk. Blencathra implements the
entire 802.15.4 MAC and PHY in hardware
using a Xilinx XC2V1500 Virtex-II FPGA.
Within the FPGA, CompXs’ IP implements
the MAC and PHY state machines,
with shared MAC and PHY RAM. Timing,
encryption, and modem functional blocks
are also implemented in the device.
The 17,280 logic cells of the
XC2V1500 FPGA provide vastly more
capacity than needed to implement the
802.15.4 MAC and PHY, which are
designed to have a very small footprint.
The remainder of the device, more than
75% in fact, is used to implement compliance
verification logic.
By using the 864 KB of on-chip block
RAM, pipelining the event log to implement
a high-speed port on board the FPGA is easy.
Through this port, you can inspect activity
all the way up and down the 802.15.4 stacks
in real time. This is an extremely valuable
capability, because it shows very clearly how
changes at the ZigBee layers affect behavior
throughout the design.
Bowfell
CompXs has also created Bowfell, an
802.15.4 MAC/PHY module that combines
easily with ZigBee software and
includes an integrated 2.4 GHz radio. After
proving the design using the Blencathra
development system, you can use these
cost-effective modules to quickly configure
networks with many ZigBee nodes. As
there is no need for on-board verification
logic, the modules are built using the lowpower,
low-cost Xilinx Spartan-3 FPGA.
Spartan-3 devices support low power
consumption, low cost, and fast time to
market – the IEEE standards were published
in October 2003, and by November
the development system and turnkey
modules were fully implemented using
Xilinx devices.
An ASIC would likely have provided
greater power savings, but the design cycle
is far longer. Choosing the FPGA route
enabled fully developed products to reach
the market very soon after the standards
were first published. Many of the details of
this standard continue to change
and evolve at this early stage.
Therefore, the extra flexibility to
reconfigure the hardware is valuable
both to customer developers
and to CompXs.
Application Development
You can use the Blencathra development
platform on its own to
develop a pure 802.15.4 wireless
communication channel for links
that require no network processing.
A wireless keyboard or mouse,
for example, requires no additional
layer to handle network tasks
such as routing. All you need is a
radio block (CompXs has a suitable
radio for development purposes), and
you can set up a representative point-topoint
link on the bench. The radio has been
designed to ease development headaches by
delivering strong performance.
For more complex applications requiring
network processing capability, the
ZigBee protocols add a network layer to
the 802.15.4 system. A CompXs daughterboard
plugs directly into Blencathra to
facilitate this. The board, named
Bannerdale, hosts network layer processing
for a simple ZigBee-compliant network.
On board is an eight-bit Flash microcontroller
with EEPROM.
Note that the microcontroller has just
one timer and serial peripheral interface
(SPI), and only 8K of ROM for the network
coordinator. This can easily support the network
layer, demonstrating that you need
only minimal microcontroller resources to
implement ZigBee. The microcontroller may also be able to host the application if
processing requirements allow.
Overall, ZigBee typically requires
between 4 KB and 30 KB of RAM and
ROM, depending on the complexity of the
application. This compares with the 250
KB required by Bluetooth, for example. So
ZigBee/802.15.4 not only simplifies the
process of embedding wireless communications
into products, but also makes for a
considerably lower bill of materials in the
final product.
Note that IEEE 802.15.4 is not dedicated
to ZigBee as a network layer. If the network
processing requirements are very
simple and can be implemented quickly
using very low memory resources, you can
define your own network layer if you prefer.
Easing the Design Challenge
Off-the-shelf ZigBee software libraries will
provide the fastest and easiest solution as
they become more widely available.
Current CompXs libraries include proprietary
network layers as well as ZigBee version
0.7-compliant network and
application support layers. These are ready
to be integrated with IEEE 802.15.4 on
Spartan-3 FPGA-based modules, or as part
of a system-on-chip.
The network layer implemented on the
Bannerdale daughter board for development
purposes is also available as a linkable
library to run on your target processor,
or as source code. In fact, CompXs
offers a complete set of development platforms,
network modules, and tools.
Available tools include an 802.15.4 platform
stack analyzer hat that displays and
logs activity to microsecond accuracy and
a passive 802.15.4/ZigBee packet sniffer
and analyzer (Figure 2).
The Steeple packet sniffer is based on
the IP contained within the FPGA on
Blencathra. Steeple will “sniff ” all of the
transmissions on a ZigBee/802.15.4 network
and then display those transmissions
in a convenient form on a PC
(Figure 3). It recognizes valid and invalid
transmissions and breaks down the packets
of data, displaying them in an easily
understood manner.
You can also quickly integrate proprietary
application software with the ZigBee
stacks via standard ZigBee APIs.
Conclusion
When a new networking standard emerges,
developers first look for the easiest way to
get a standard-compliant network up and
running. A reconfigurable development
platform is important, as well as large numbers
of low-cost modules that can implement
the standard-compliant elements
with a good RF stage.
In the case of IEEE 802.15.4 and
ZigBee, Xilinx FPGAs allowed for easy and
rapid designs of suitable development
tools. These tools will enable many new
applications to benefit from low-cost wireless
networking.
You can find more information on the
products described here at the CompXs website,
www.compxs.com. For details about the
ZigBee organization and the standards it
promotes, visit www.zigbee.org. And to learn
how 802.15.4 and ZigBee can be used in
your products, consider taking a training
course. For information about introductory
and in-depth/hands-on courses, visit www.zigbeetraining.com.
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