ZigBee are considered by most design engineers to be just about the only two choices for low-power solutionsdepending, of course, on the application.
Bluetooth is ideal for widely compatible communications on a personal area network (PAN) comprising PDA, headset, mobile phone and laptop PC, for example, where the standard eliminates much of the design challenge.
ZigBee 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 comes 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 Bluetooth (IEEE 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 in China, where designers have targeted innovative products for the export market.
This article first describes the design challenges in a generic way and later shows the benefits of an integrated proprietary RF chip manufactured by the author's company (the nRF24xx series), for these types of applications.
A wireless mouse will be used as a case study that allows the comparison of Bluetooth, ZigBee and a proprietary IC.
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 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 synchronization.