The world is turning wireless. The technology is already so pervasive that consumers hardly give a second thought when they switch on their laptop at an airport Wi-Fi hotspot to browse the Internet at broadband speeds. Or when they point their PDA at a printer and simply press a button to run-off documents without ever physically connecting to a network, courtesy of Bluetooth.
These wireless technologies use the 2.4 GHz frequency—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—because it is unlicensed and consequently needs no approvals for operation. What’s remarkable, considering how crowded the band has become, is that there is no interference from adjacent devices operating at the same time and on the same frequency, and yet, when two devices do want to talk, connection is practically instantaneous and completely seamless.
The reason why the technology works so well is in part because of standards. More often than not these are agreed protocols endorsed by the US-based IEEE—although there are other standards bodies such as Europe’s ISO. For example, 802.11a, b and g (Wi-Fi), 802.15 (Bluetooth) and the fledgling 802.15.4 personal area network standard that supports ZigBee. Engineers design the products according to strictly-defined guidelines that ensure their products will not interfere, and yet be compatible with, those from other manufacturers. However, such is the fickle nature of RF design, this does not necessarily make it a trouble-free procedure, and man-days can be spent analysing performance and modifying designs to ensure adherence to the standard.
In addition, the success of the standards has left some designers believing that they are the only wireless game in town. Yet while standard wireless protocols undoubtedly have their merits, their relative complexity can make them unsuitable for implementing industrial solutions where simplicity keeps costs down. Even ZigBee, touted as a “low cost” wireless solution targeted specifically at industrial applications, can be considered over-engineered for simple, point-to-point applications.
In reality there are many proprietary solutions that work perfectly well without adhering to a standard, for example, Wireless USB. Companies such as Cypress Semiconductor (distributed by Future Electronics , Arrow and Braemac ) have released single chip RF transceivers based on this technology that can be used to implement a link in hours.
Although developed for the desktop PC market, Wireless USB is nonetheless a convenient and effective implementation for a wide range of industrial applications. It uses a hub-and-spoke topology whereby several devices can connect to a single host. When coupled with a USB controller, the technology becomes an end-to-end transparent USB solution for cordless human interface devices (HIDs) such as mice, keyboards and joysticks, and requires no new device driver development.
The technology is a simple, low cost and robust wireless method designed for short range – from 30 to 50 m. A major attraction for a designer looking to quickly produce a working design based on an SPI interface and 8-bit microcontroller, is that it can be done in little more than a day, sometimes less. And, despite its name, Wireless USB doesn’t necessarily need a USB connection.
Wireless USB uses a Direct Sequence Spread Spectrum (DSSS) coding scheme, helping to prevent conflict with other 2.4-GHz protocols. This contrasts with, for example, Bluetooth’s Frequency Hopping Spread Spectrum (FHSS) scheme. The DSSS System increases range and reduces bit error rate by transmitting each bit as a pseudo-random noise (PN) code, with each element of the PN code known as a “chip”.
DSSS receivers use a data correlator to decode the incoming data stream. In the presence of interference the transmitted PN code can be corrupted. If the number of chip errors is less than the correlator error threshold, the data will be correctly received. Wireless USB offers either 32- or 64-chip codes, and the longer the chip code is, the greater the probability that the original data can be recovered, but the wider the bandwidth required.
In addition to separating transmissions by code, Wireless USB also separates transmissions by frequency. It can hop between the 75 1 MHz frequency channels. Consequently devices can transmit distinct signals by either using a unique PN code or a unique frequency. Two signals will not interfere unless they are using the same frequency channel and the same PN code. Theoretically, hundreds of Wireless USB devices could be operating in the same physical space at the same time without clashing.
Competing with Bluetooth and ZigBee
A designer looking for a low-cost, point-to-point and multi-point-to-point wireless solution could consider the IEEE-ratified standard wireless technologies, Zigbee and Bluetooth. Both these technologies offer similar benefits to Wireless USB in terms of interference avoidance.
Bluetooth, however, doesn’t really suit relatively simple solutions such as keyboards, mice and gamepads that only require low data rate communications. It was designed for mobiles, and other relatively demanding short-range networking applications. Bluetooth is a subset of the 802.15 “wireless personal area network” (WPAN) standard, and has a range up to 10 m or so, and a data exchange rate of around 1 Mbit/s.
While ZigBee is being touted as a “low cost” alternative to Bluetooth, it is still expensive compared with Wireless USB for exactly the same reasons as bigger brother; it has to comply with a strictly-defined interoperability standard. Just to complicate matters further, the full standard is far from complete, and it will be another year before it is fully ratified. Nonetheless, a designer working with ZigBee faces a more difficult task even meeting the draft standard than he would when not having to meet a standard at all.
In addition, ZigBee is primarily designed to be a multi-node networking solution with mesh network capabilities, rather than a point-to-point system. ZigBee is said to achieve raw data throughput rates of 250 kbit/s using 2.4 GHz (10 channels), 40 kbit/s at 915 MHz (6 channels) and 20 kbit/s at 868 MHz (single channel).
For both Bluetooth and ZigBee the sophistication demands complex protocol stacks, both of which are unnecessary for simple wireless applications. And these stacks demand expensive memory, which further increases the cost. Bluetooth requires 128 kByte of memory for its protocol stack and ZigBee needs 32 kBytes.
In contrast, Wireless USB’s protocol stack typically uses just 1 kByte for a one-way application, and 2 to 4 kBytes for bi-directional link.
It is the requirement for interoperability defined in the Bluetooth and ZigBee profiles that increases the design overhead by demanding comprehensive compatibility testing. While this is important for applications where the device would be expected to communicate seamlessly with those from other manufacturers, it makes no sense when communication will be restricted to just one other device in a point-to-point network, for example, a mouse talking to a keyboard. That’s when Wireless USB becomes the more compelling solution.
Extended to longer range
Cypress Semiconductor’s WirelessUSB LS devices communicate at distances of up to 10 m, have an average latency of less than 4 ms and provide bi-directional or uni-directional RF transmission at 62.5 kbit/s. These SoCs feature a radio transceiver plus digital baseband. The LR version extends the Wireless USB family into long range and multipoint-to-point applications.
The LS family includes a simple SPI interface, radio and baseband modem. While the CYWUSB6932 is a transmit-only IC, the CYWUSB6934 is a transceiver IC.
Both chips have 34 8 bit-registers that are addressable through the SPI interface.
Other registers control channel selection, enable wakeup for power safe mode, transmit data and receive data. By using these registers, features like 1- or 2-way communication, basic, automatic and semi-automatic bind procedures can be enabled.
The SPI interface gives the user the option to use almost any inexpensive 8-bit micro. The protocol code for a sensor application, for example, requires 4 kbit of memory on the microcontroller. All the reference designs use Cypress MicroSystems PSoC or Cypress enCoRe chips. The application runs on the micro and communicates with the Wireless USB chip via the SPI interface. And with –90 dBm receive sensitivity and up to 0 dBm output power the device has sufficient range for many applications.
Cypress’ LR family is a good solution if longer range is required in applications such as building automation and industrial control but sophisticated networking control isn’t necessary. The CYWUSB6935 is a transceiver with enhanced receive sensitivity of -95dBm, which extends the range up to 50 metres or more. Using an external power amplifier can further extend the range.
The development kit includes two Wireless USB platform boards, two single antenna radio modules and a listener tool. One platform board supports wireless peripherals and the other acts as a USB HID Bridge. A “chip kit” is also included which features several pre-programmed functions,