NFC/RFID enables communication with transponders. The transponders include memory that can store various information including URLs, Bluetooth® connection handover, contact information, diagnostics, and now sensor measurement data. Battery-less sensor measurements that utilize energy harvesting from the RF field can be implemented in applications ranging from medical, health and fitness, and industrial where instantaneous measurements are required and a battery is not feasible or desired. This application report discusses the implementation of a single chip NFC/RFID field powered temperature sensor system with the RF430FRL152H.
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The schematic and layout are shown in Figure 1 and Figure 2. The design keeps a low component count by using integrated features of the RF430FRL152H such as the internal A/D converter and ROM functions for acquiring sensor measurements from a thermistor (RT1). This design also offers the flexibility to include a 1.5 V battery at TP1/TP2 if data logging is required and the RF field will not always be available to provide power. The hatched ground pour is used for improved RF performance.
One benefit of this design is that firmware does not need to be programmed into the device. This design utilizes the ROM functions inside of the RF430FRL152H. The device is initialized “over the air” by a NFC/RFID Reader/Writer device that writes the configuration registers. Subsequently, the thermistor measurements are read out by this same Reader/Writer device. When utilizing the graphical user interface (GUI) software as discussed in Section 3, the RF430FRL152H will be configured as shown below. It is important to note that with this specific configuration, each sample requires 128 ms for acquisition. The reference resistor and thermistor must be sampled for each measurement, which brings the total conversion time to 256 ms. This is just one implementation of the ADC configuration register that can be modified to meet the needs of a given application.
NOTE
The configuration of the device can also be read out using the RF430FRL152H GUI “Read All Tabs” button and viewing the tabs that contain the affected registers.