Application Note Transceiver TH71221 Wireless Sensing Network Scope This application note describes the use of the Melexis TH71221 RF transceiver in a wireless sensing network (WSN) application. A WSN is a network, consisting of spatially distributed autonomous modules (called nodes) using sensors to monitor physical or environmental conditions such as temperature, pressure or humidity. All these devices communicate wirelessly to a main sensor node which collects back all the measurements and sent it to a main station for further processing (e.g. a computer). Figure 1: Melexis WSN Node This application note describes how the TH71221 RF transceiver has been used with other external components like microcontroller and sensors to build a WSN node. The detailed application schematics of the TH71221 and the microcontroller will be provided as example as well as the complete bill of materials. Applications Wireless sensing networks for temperature, humidity, light and RF field strength measurements. Home and building automation Related Melexis Products TH71221 - Multi-band, Multi-channel RF Transceiver Theory of operation The Melexis WSN Node (shown in figure 1) is composed of the TH71221 RF transceiver, a microcontroller type MSP430 to handle the network communication protocol, several on-board sensors for temperature, humidity, light and RF field strength measurements and a state-of-the-art USB interface for node control and programming. The Melexis TH71221 RF transceiver supports all frequency bands ranging from 300 to 930 MHz. Its high input sensitivity enables large node distances. The MSP430 microcontroller and other peripheral devices offer ultra low-power operation to allow for long battery life time. A state-of-theart USB interface supports secure and reliable programming of the node. Custom specific WSN solutions can be addressed through the on-board expansion connector. 39011 71221 06 Rev. 002 Page 1 of 6 AN71221_WSN Oct/08 Application Note Transceiver TH71221 Wireless Sensing Network Application schematic The following two pictures show the complete application schematics of the TH71221 RF transceiver and the connection to the microcontroller. The TH71221 transceiver is built with several components around it. The transmitting power amplifier is matched to the 50-ohm antenna with the components CTX0, CTX1, CTX2, CTX4, LTX0 and LTX1. For the reception, the input of the low noise amplifier (LNA) is matched with the capacitor CRX0 and the inductor LRX2. A tank circuit composed of L1 and C1 is built at the output of the LNA to provide sufficient gain at the desired frequency and to allow enough image frequency rejection. This tank circuit must be centred on the received frequency band. An external ceramic filter CERFIL provides the intermediate frequency (IF) filter on 10.7MHz. The TH71221 is used in FSK detection mode with an external ceramic discriminator CERDIS. Decoded data information is available for the microcontroller processing at the pin 8 of the device. Figure 2: Transceiver TH71221 application schematic 39011 71221 06 Rev. 002 Page 2 of 6 AN71221_WSN Oct/08 Application Note Transceiver TH71221 Wireless Sensing Network To support the Network communication protocol and to interface the TH71221 device, an external microcontroller type MSP430 is used. After power ON reset, the microcontroller configures the TH71221 transceiver through its SPI connection (pins 23, 29 and 31). According to the preprogrammed software on the node, the Melexis WSN Node will act as a master or a slave in the communication. The RF information is sent to the UART of the microcontroller through the pins 32 and 33. To ensure a reliable RF communication, the PLL lock signal is always sensed by the microcontroller at the pin 24. The RSSI signal generated from the TH71221 transceiver can be measured by one channel of the ADC of the microcontroller. This can be used to easily implement a “listen before transmit” function. The microcontroller communicates to a RS232/USB converter of type FTDI which allows easy USB connection. Figure 3: Microcontroller MSP430 application schematic 39011 71221 06 Rev. 002 Page 3 of 6 AN71221_WSN Oct/08 Application Note Transceiver TH71221 Wireless Sensing Network Component List Component Number/ Type / Distributor Board Specification Unit Price Manufacturer (100ku) Remark TH7221 (QFN32 5x5) MSP430 F1611 IRTDT (QFN64) Transceiver 1 TH71221 various 1.41€ Melexis Microcontroller 1 MSP430 F1611 Mouser 6.87€ Texas Instruments XTAL_TH Telcona 0.26€ FCA HKC 7.1505 XTAL_MSP Farnell 1.40€ EPSON TOYOCOM MC30632.768KHz 1 Crystals 1 Table 1: Component’s list (active components) Component Number/ Board RB1 RB3 RP RPS 1 1 1 1 100 Ω 100 kΩ 3.3 kΩ 33 kΩ Farnell Farnell Farnell Farnell RL0 1 390 Ω Farnell RF CB0 CB1 CB2 CB3 CB5 CB6 CB7 C0 C1 C2 C3 C4 C5 CX1_TH CX2_TH CX1_MSP CX2_MSP CF1 CF2 CTX0 CTX1 CTX2 CTX4 CRX0 L0 L1 LTX0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 33 kΩ 10 µF 10 nF 330 pF 100 nF 10 nF 100 pF 100 nF NIP 1 pF 10 pF 10 nF 330 pF 1.5 nF 12 pF 15 pF 12 pF 12 pF 1 nF NIP 10 pF 3.9 pF 4.7 pF 3.3 pF 100 pF 5.6 nH 4.7 nH 4.7 nH Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell Farnell 39011 71221 06 Rev. 002 Type/ Distributor Specification Page 4 of 6 Unit Price Manufacturer (100ku) 0.01€ various 0.01€ various 0.01€ various 0.01€ various 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.01€ 0.09€ 0.09€ 0.09€ various various various various various various various various various various various various various various various various various various various various various various various various various various various various various Remark RL for Cerdis. VCOtank C AN71221_WSN Oct/08 Application Note Transceiver TH71221 Wireless Sensing Network Type/ Distributor Specification Unit Price Manufacturer (100ku) 0.09€ various 0.09€ various Component Number/ Board LTX1 LRX2 1 1 10 nH 15 nH Farnell Farnell CERDIS 1 10.7 MHz Farnell 0.33€ Murata CERFIL 1 10.7 MHz Farnell 0.31€ Murata Remark CDSCB 10M7 GA136 SFECF 10M7 HA00 Table 2: Component’s list (passive components) Sum of active components: Sum of passive components: Total Sum: 9.64€ 1.39€ 11.03€ Conclusion With several components around , the TH71221 RF transceiver can be used in a low cost wireless sensing network application. The external microcontroller handles the network protocol of communication while the TH71221 is used to receive and transmit information over the wide frequency range of 300 to 930 MHz. 39011 71221 06 Rev. 002 Page 5 of 6 AN71221_WSN Oct/08 Application Note Transceiver TH71221 Wireless Sensing Network Reference and Related Documents TH71221 data sheet Evaluation Board for TH7122 For the latest version of this document, go to our website at: www.melexis.com Or for additional information contact Melexis Direct: Europe, Africa: Americas: Asia: Phone: +32 1367 0495 E-mail: [email protected] Phone: +1 603 223 2362 E-mail: [email protected] Phone: +32 1367 0495 E-mail: [email protected] ISO/TS 16949 and ISO14001 Certified 39011 71221 06 Rev. 002 Page 6 of 6 AN71221_WSN Oct/08