Medical Electronics
Figure 4: Example blood pressure monitor application
reducing packets. With the recent launch of the Bluetooth low energy (BLE) profile it is anticipated that BLE will become the dominant standard, taking the preference away from ZigBee and its low power consumption despite the fact that BLE has a slower data rate approaching that of ZigBee.
The high numbers of portable consumer devices, such as smart phones and tablets, will continue to increase BLE’s adoption during the forthcoming years. Figure 3 shows the consumption profile of an example data transfer using BLE. The consumption example plot is from a Murata LBCA2ZZVZE BLE wireless module. The decision to design-in a readily available certified wireless module or to design your own discrete solution needs careful consideration. Let’s start with reviewing the likely steps you would need to take assuming the selection of the wireless standard has already been made. Initial research would need to be conducted to identify likely chipset candidates that might suit the application requirement. A detailed specifications / datasheet would need to be sought in order to consider the board space, schematic and layout design requirements. Availability of a quick-to- implement evaluation or development board would greatly assist in this process as would any reference design software that could run on the chipset and interact with your host microcontroller platform. In parallel with the technical evaluation of the chipset a detailed BOM costing would need to be prepared taking into account all necessary additional discrete components to create the wireless function and interfacing to the host. Compliance testing to the required
standard (BLE, ZigBee etc) would also have to be conducted, this part of the process potentially introduces additional costs and time into the overall development budget and time-to-market forecast. The need to source or hire suitable test equipment for this stage is often over-looked as is the need to be realistic as to the number of iterations needed for the prototype design. For engineering teams not used to developing their own wireless applications the prospect of working up their own discrete design can be a daunting one.
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Design iterations required to achieve certification, being diligent with EMI and having to consider antenna design can deter engineers to consider an alternative. Rather than adopt a discrete approach
the alternative is to use a readily available wireless module. The consideration for this approach is not just a financial one. Typically, such modules come fully certified, occupy less board space, have passive components embedded, and are usually guaranteed to work. All these factors represent a significant saving in project time and risk, and even more when you consider that the product’s future will be decided by the consumer’s user experience. Figure 4 shows an example application of a blood pressure monitor communicating readings to a tablet computer using a wireless module. An MCU instigates reading blood pressure data and displaying it on the built-in LCD display and then passes it to the BLE module via the UART.
One possible example of a BLE wireless module is Murata’s LBCA2ZZVZE.
Figure 5: Message sequence example of data transfer
Incorporating the CC2541 chipset from Texas Instruments the module measures just 20 x 13 x 2.4 mm, it also incorporates all the BLE protocol stacks, wireless approval certification, a UART interface and chip antenna. Output power is –2 dBm typical. The average power consumption is less than 100uA for 500msec of connection interval. Figure 5 shows the communication
process involved in sending temperature read from a sensor to a mobile phone handset. Advertising, a defined state of the Bluetooth Low Energy communication specification is controlled by the generic access profile (GAP) and the link layer (LL).
Murata Europe |
www.murata.eu
Mac Tochigi is product manager, connectivity modules, Murata Europe
Components in Electronics October 2012 27
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