Nordic Semiconductor technical article (Nordic Semiconductor editorial contact: Steven Keeping, e-mail: steven.keeping@nordicsemi.no, 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

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.

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.)

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.)

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.

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.

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.)

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.

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

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