Electronic Design

RFID Tag Reader Uses FSK To Avoid Collisions

The RFID (radio-frequency identification) system consists of an RFID tag, a reader, and a user-interface computer. The passive RFID tag contains a silicon chip and an LC antenna circuit. The passive tag is energized by an RF field that’s transmitted by the reader (interrogation). Therefore, the tag doesn’t require any batteries for its operation.

Passive RFID tags are used for animal tagging, asset tracking, access control applications, etc. When the tag is energized by the RF field, it transmits back the contents of its memory by modulating the incoming RF field. The reader detects and demodulates the signal and identifies the tag.

When a large volume of tags must be read together in the same RF field, the application needs an anti-collision feature that enables the reader to receive data from the each tag on a one-by-one basis. Microchip’s MCRF250 is an example of an RFID tag IC featuring anti-collision technology. When multiple tags are in the same RF field and transmit data together, the reader must communicate with the tags to prevent collisions of data. This is accomplished by transmitting a “gap pulse” for the MCRF250.

When the tag recognizes the gap pulse, it doesn’t transmit data until it counts a number that’s generated by a random number counter. Each tag will finish counting the number in a different time. Therefore each tag will retransmit its data again in a different time slot. The anti-collision algorithm of the MCRF250 is shown in Figure 1.

The reader consists of a transmitting and a receiving section. The transmitting section includes a carrier frequency generator, gap signal gate, and an antenna circuit. The receiving section includes a peak detector, a signal amplifier/filter, a signal-collision detector, and a microcontroller for data processing. While this reader is designed to read the MCRF250 anticollision RFID tag, the concepts described here may be applied to readers based on other products.

The reader also communicates with an external host computer. A basic block diagram of the typical RFID reader is shown in Figure 2.

The electronic circuitry for an anticollision FSK reader is depicted in Figure 3. The RFID reader requires a +9-V dc power supply.

The 125-kHz carrier signal is generated by dividing the 4-MHz timebase signal from a crystal oscillator. A 16-stage binary ripple counter (74HC4060) is used for this purpose. The 74HC4060 also provides a clock signal for the PIC16F84 8-bit flash microcontroller.

The 125-kHz signal from pin 5 of U6 is fed into U2 (NOR gate) and two-stage power amplifiers that are formed by U4, Q1, and Q2. The 125-kHz signal from Q1 and Q2 is fed into the antenna circuit formed by L1 (162 mH) and C22 (0.01 mF). L1 and C22 form a series-resonant circuit with a 125-kHz resonance frequency. Because C22 is grounded, the carrier signal (125 kHz) is filtered out to ground after passing the antenna coil. The circuit provides a minimum impedance at the resonance frequency. This results in maximizing the antenna current, and therefore, the magnetic field strength is maximized at the resonant frequency.

The gap signal from pin 7 of U7 controls the 125-kHz antenna driver circuit (Q1 and Q2). Q1 and Q2 are turned off during the gap signal “high.” There’s no RF signal at the antenna coil during this gap period. The MCRF250 utilizes the field gaps as described in the anti-collision algorithm (Fig. 1, again).

The reader circuit shown uses a single coil for both transmitting and receiving the rf signals. L1, C22, D8, and the other components in the bottom portion of the circuit form a signal receiving section.

In the FSK communication protocol, a “0” and a “1” are represented by two different frequencies. In the MCRF250 RFID tag, a “0” and a “1” are represented by fC/8 and fC/10, respectively, where fC is the carrier frequency. The MCRF250 sends this FSK signal to the reader via amplitude modulation of the carrier signal.

The demodulation is accomplished by detecting the envelope of the carrier signal. A half-wave, capacitor-filtered rectifier circuit (D8, D9, and C26) is used for the demodulation process. The detected envelope signal charges capacitor C26. R37 provides a discharge path for the voltage charged onto C26. This voltage passes active filters (U10:A,C,D) and the pulse-shaping circuitry (U10:B) before it’s fed into the PIC16F84 for data processing. U10 (A,D,C) forms a bandpass filter for 12- to 16-kHz signals.

When more than one tag is transmitting data at same time, there will be “wobbles” in the receiver’s data signals. This wobble is detected in U8. If the wobble occurs, C10 becomes fully charged. This will set the CLK input of U5:B, resulting in a logic “LOW” in Q of the U5:B. If the microcontroller (U7) detects the logic “LOW,” it turns on the gap control gate (U5:A) to transmit a gap signal to the competing tags.

The PIC16F84 microcontroller performs data decoding, generates the gap timing signals, and communicates with the host computer via an RS-232 serial interface.

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