A Resistance Deviation-to-Time Interval Converter and its Application to a Passive RFID Tag for Security Jimann Park, Youngsoo Park, Youngsae Kim, Sungik Jun Wireless security application research team ETRI 161 Gajeong-dong, Yuseong-gu, Daejeon, 305-700 KOREA http://www.etri.re.kr Abstract:A passive RFID tag with security and measurement concepts is presented. The sensor interface module has a resistance deviation-to-time interval converter. The passive RFID tag embedded with a converter is described. A prototype RFID tag using a converter has been built with CMOS 0.35um technology. The experimental results have shown that the tag has an 8 bits resolution and a linearity characteristic when the output pulse is counted by a 3.39 MHz clock. The proposed converter is insensitive to the pulse width change regardless of environment changes. The design principle and the circuit configuration are simple. The proposed converter is expected to find wide applications in the signal processing of various sensors. Key words – RFID tag, RFID application, security, sensor signal processor, converter, analog circuit. 1 Introduction The most of Radio frequency identification (RFID) system using a sensor is applied for the instrument and measurement, and the commercial sensor tags are used at the simple work level of measurement parts. An RFID tag including a sensor function has been developed and it is presented a passive RFID tag chip embedded with a sensor interface module without battery.[1]-[2] The RFID tag for their applications can be performed the sensor with the resistance characteristics. Many resistive sensors produce small resistance variations with relatively large fixed offset resistance and sometimes need temperature compensation. The properties of resistive sensors have made resistance deviation-to-digital conversion techniques a very useful approach for implementing interface circuitry. These circuits are implemented a various analog-to-digital (A/D) converter. [3]-[5] On the other hand, the RFID systems have the security functions in a various parts. Most security RFID systems implement a digital signal processor based on the crypto algorithm. In order to improve the above mentioned function, the RFID system in this paper is realized a sensor interface circuit using a resistance deviation-to- time interval converter and its applications to a security RFID tag. The passive RFID tag in this paper plays a role in security and measurement systems, and resistive sensors can have small or large resistance variations according to applications. This paper describes a simple resistance deviation-to-time interval converter based on continuous-time ramp integration and digital time differentiation. The converter is expected to find wide applications in the signal processing of various sensors for security and measurement RFID tags. 2. Circuit Description and Operation 2.1 Security Method Fig. 1 shows the block diagram of a passive RFID tag embedded with a crypto and sensor interface module. It consists of an antenna, a radio frequency analog module (RFA), a radio frequency digital (RFD) protocol module, a control module, a crypto module, a sensor interface module, and memory. Fig. 1. Block diagram of the proposed RFID tag. Proceedings of the 11th WSEAS International Conference on CIRCUITS, Agios Nikolaos, Crete Island, Greece, July 23-25, 2007 126