Battery-Less NFC/RFID Temperature Sensing Patch

Battery-Less NFC/RFID Temperature Sensing Patch

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.

Project collateral discussed in this application report can be downloaded from www.ti.com/lit/zip/sloc322.

Trademarks

Bluetooth is a registered trademark of Bluetooth SIG, Inc.

All other trademarks are the property of their respective owners.

1 Reference Design Hardware

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.

Figure 1. RF430FRL152H NFC Temperature Sensor Schematic

Figure 2. RF430FRL152H NFC Temperature Sensor Board Layout

2 Firmware Implementation

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.

• Sensor Control Register
  The Sensor Control Register is used to select the thermistor to be sampled. The reference resistor (R3) is also sampled and used for Calibration.
  • Reference/ADC1 Sensor enabled
  • Thermistor/ADC2 Sensor enabled
  • Thermistor enabled, number of passes = 1
• Reference/ADC1 Sensor Configuration Register
  This register is used to configure the ADC1 for the reference resistor. The configuration below sets the ADC for 10-bit resolution and requires 128 ms for each sample. In this application example, 10-bit resolution was used for this for faster conversions.
  • Gain = 2
  • Filter Type = CIC Filter
  • Oversampling = 128
• Thermistor/ADC2 Sensor Configuration Register
  This register is used to configure the ADC1 for the Thermistor. The configuration below sets the ADC for 10-bit accuracy and requires 128 ms for each sample. In this application example, 10-bit resolution was used for this for faster conversions.
  • Gain = 2
  • Filter Type = CIC Filter
  • Oversampling = 128