6.115 Final Project Proposal: An RFID Access Control System

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1 6.115 Final Project Proposal: An RFID Access Control System Christopher Merrill April 24, 2012 Abstract The goal of this nal project is to implement a device to read standard 125 khz RFID cards using the 8051 microcontroller and supporting hardware, and use the data from the RFID reader to implement an access control system. The 8051 will instruct a 8254 timer to produce a 125 khz carrier wave, and will continuously sample the resulting envelope of the signal to obtain the data stored on the RFID card. This data will be stored in RAM. Once the data is captured, the system will perform a lookup of access parameters in a data EEPROM and use this to allow or deny access to a storage chest. A log will be kept of all successful and failed access attempts in a separate log EEPROM. The system will also implement, over serial, a terminal that allows for reprogramming access controls and viewing the log le. 1 Background RFID readers work by driving an LC circuit at its resonant frequency, producing a large oscillating magnetic eld in the inductive component. A card is read when the magnetic eld from the reader is coupled with a similarly tuned circuit in the inductor, resulting in a transfer of power. By connecting and disconnecting additional load to the LC resonant antenna, the card is able to aect the eciency of the power transfer and produce an AM modulated signal at a measurement node on the reader module. RFID is a common access control mechanism, as cards can be created and programmed to produce dierent modulated data patterns. The reader can determine a unique ID for every card and associate this with specic permissions for access and control of the security system. These systems also often contain an internal log, which keeps an audit trail of all actions in order to allow security sta to check up on the system and verify everything is acting the way it should. 2 System Description The system can be divided into three main functional blocks: an RFID reader, a re-programmable access control system, and a solenoid-locking physical enclosure. The RFID reader is composed of a resonant antenna circuit, an envelope lter and comparator to extract data, an amplier to produce the carrier wave, and software to parse the tag data and store it in memory. The access control system consists of a log EEPROM, a conguration EEPROM, and software to allow verication of and modication of access parameters. The access control system determines if a user can access an electronics parts box held closed by a solenoid. 2.1 RFID Reader Antenna The RFID reader antenna is a series LC circuit with a resonant peak at 125 khz. The inductor will be made from a square spiral of wire on a printed circuit board, with a desired inductance of 700 uh. The capacitor will be made from an array of ceramic capacitors totaling 100 uf. Ceramic is used to ensure 1 Figure 2: RFID Reader Antenna

2 Figure 1: Basic RFID Reader Circuit high surge current ratings, low ESR, and low ESL all of which will increase the predictability of the resulting antenna circuit and minimize the required tuning. The antenna impedance at 125 khz is estimated to be around 550 Ω, with almost all of the voltage dropped across the inductor. The produces a max output power level of slightly less than V2 /Z antenna = 260 mw. Data Acquisition The data acquisition block consists of an envelope RC lter with a cuto frequency of 30 khz and a comparator to extract the high and low logic levels from the amplitude modulated envelope. The diode before the envelope lter ensures that the the comparator is protected from negative voltage spikes. The lter cuto of 30 khz is chosen to eliminate the 125 khz carrier wave while maintaining unity gain at the AM modulation frequencies of interest, which are typically 25 khz or lower. The comparator is set to output a high value for envelope voltages larger than V ref, and a low value when the envelope voltage is smaller. V ref will be set using a potentiometer and calibration data from test RFID cards. Output Amplier The antenna output stage will consist of a MOSFET totem driven by a buer circuit. The MOSFET totem will provide the high power levels needed for the antenna to drive the coupled RFID card. The buer circuitry will either be a simple logic-level buer or a more complicated MOSFET driver chip (such as those used in the uorescent lamp ballast or in 6.131). Decoding in Software The user will be able to specify one of many possible RFID encoding schemes using a serial terminal. Once one is selected, the 8051 will sample the signal at 125 khz, triggered by the falling edge of the antenna driver waveform. Each of the bits of envelope data will be composed together into 8-bit chunks and stored into external memory. The 8051 will then post-process the raw data, using the transitions between low and high values to recover clock data and matching the transmitted bit pattern to the appropriate numerical values. The resulting value will then be sent back to the user over serial as a string of hexadecimal numbers. Common RFID encoding schemes include simple constant-period square wave modulation, and variable frequency AM modulation. In constant-period modulation, each bit has a constant period and results in either no attenuation of a signal (logic high) or additional attenuation of the signal (logic low). Typical bit lengths are somewhere around 5 clock cycles at 125 khz, or approximately 25 khz. Variable frequency AM 2

3 Figure 3: Diagram of Access Control System modulation uses dierent frequencies of square wave AM modulation of the carrier wave to represent logic high and logic low. Typical values include 12.5 khz to represent logic low and 15 khz to represent logic high. 2.2 Access Control System Conguration EEPROM & Access Control The conguration EEPROM holds mappings between decoded card data and access permissions. After a card is read, the conguration EEPROM is searched for an entry matching the card data. If one is found, either access is given right away or the ne grained access control settings are checked for the user. If the card matches the stored control card, the system will begin listening on the serial port. If the user sends a character, the device will launch a programming interface that will allow for customizing the current system conguration. The user can add or remove cards from the access list, and optionally modify any custom access settings that have been implemented. Log EEPROM In order to allow for a security audit of the RFID security system, any action taken on the device (trying and/or failing to open the door, changing control settings, viewing the log) will add an entry to a log EEPROM. This EEPROM will act as a circular buer, storing as much of the door's history as possible given the space allowed. When the user is in control mode, he or she will be able to download and view the log data over serial to verify that everything is working properly. Real-time Clock Optionally, a real time clock IC may be used to allow time-restricted access to the box, and provide timestamps for the log entries. 2.3 Physical Enclosure The physical enclosure will be a set of drawers that can be used to hold useful electronic parts. Solenoid Driver The solenoid driver will be a simple, high current switch. The 8051 will ensure that the solenoid is only opened when an allowed card is presented to the reader, and it will then open the solenoid only for a specialized period of time. 3

4 3 Project Scope Modest Risk RFID reader with wire wrapped antenna. Obtain data from sample RFID cards. Decode constant-period modulated RFID cards. Conrm access permissions and open solenoid. Log successful and failed access attempts. More Risk All of the above, plus the ability to: Use a control card to put the device in programming mode, allowing the user to add cards for access. Dump log data over serial if control card is used. Reprogram the device to remember dierent control cards. High Risk All of the above, plus the ability to: Set access permissions for limited-use cards. Set access permissions based on time of day. Allow users to give guests access without requiring the control card. 4 Specialized Components These are components required beyond those available in the lab kit. SPI EEPROMs for conguration and log data storage. PCB with traces for inductor and pads for capacitor arrays. Solenoid for access control. Physical enclosure. RFID cards. MOSFET driver IC. Real time clock (for high-risk plan). 5 Timetable Week 1 (4/23) Mill PCB for antenna circuitry and assemble antenna. Assemble MOSFET driver circuitry. Generate 125 KHz waveform with signal generator and determine response to RFID cards. Build comparator circuit to detect high and low logic levels. Interface MOSFET driver with 8254, and send comparator output to

5 Week 2 (4/30) Write code to begin polling an RFID card and to store the returned data. Write code to decode RFID format and send data to the user over serial. Implement SPI programming interface for EEPROM. Build physical enclosure. Debug and test. Determine if reader obtains same data as commercial readers. Week 3 (5/7) Test basic access control mechanisms. Write code to modify EEPROM data over serial using control card. Test control card log viewing. Test adding new users using control card. Build solenoid driver. Test solenoid driver. Week 4 (5/14) Implement decoding of alternative RFID encoding formats. Implement customized access permissions. 5