Short range wireless Master
Check “channel free” Send the “value request”
Slave
Receive the “value request” Send “ACK”
Get the data from sensor Check “channel free” Send value
Receive value and transfer to microcontroller
Send ACK Table 1: The steps involved in transmitting a single wireless sensor reading
implements channel-hopping, and its interference-immunity has been proven in many wireless keyboards and mice, which have to operate in office airspace full of conflicting 2.4GHz signals. And CyFi implements an ‘AutoACK’ feature that enables the radio controller to go to sleep between transmissions, thus extending battery life. You might be forgiven for thinking that you have found the answer to your prayers. But wait: while Cypress claims a respectable maximum 1Mb/s data rate for CyFi, evaluation reveals that this is not always realised in practice. In a typical industrial application – linking wireless sensors to a control centre – the process of transmitting a sensor reading involves multiple steps (see Figure 1). The latency between each step in the process can account for far more
time than the transmission of the data itself, especially when the data is small. Total realised data rate can therefore be very different from a theoretical over-the-air data rate. And technologies, such as CyFi, that use a relatively small and slow processor will tend to exhibit greater latency than their beefier counterparts.
So while the low hardware overhead and low system cost of CyFi looks appealing, the actual data rate can in some applications turn out to be too low. In search of a higher data rate, then, you turn your sights back to……. Bluetooth? We have come full circle back to where we started, without ever necessarily finding the perfect technology for our application. And this is the biggest lesson to learn for designers evaluating short-range RF
components for a new project. However hard you look, and no matter how exhaustive your evaluation, you will certainly have to accept one or more painful trade- offs.
In a sense, the designer’s job is to find the least bad compromise. And the fact that the designer has a wide choice of trade-offs is a tribute to the confusing variety of short-range RF technologies that are available. The diversity in the market, which looks confusing and frustrating initially, allows you to choose a trade-off you can accept or mitigate, while gaining the benefit(s) that most matters to you, whether it is low system cost, low power consumption, simplicity, high data rate, long range or the ability to penetrate solid objects or to avoid interference.
So while the market will not offer you a perfect solution, it does reward
the hard graft of finding and evaluating all the technologies that could fit your application. Indeed, I hope I’ve been able to demonstrate a variety of short-range RF
technologies without even having to describe the valid products that are also available from NXP Semiconductor, Semtech, Micrel and a host of others.
What is clear is that Bluetooth and ZigBee have their uses, but they have their disadvantages too. Rigorous evaluation of the options will allow you to find the right blend of functions, benefits and trade-offs for your product and the market(s) it addresses.
Future Electronics |
www.futurelectronics.com Ulf Ziemann is technical solutions manager at Future Electronics, Germany
www.cieonline.co.uk
Components in Electronics July/August 2010 13
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