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MCRF450 Datasheet(PDF) 3 Page - Microchip Technology |
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MCRF450 Datasheet(HTML) 3 Page - Microchip Technology |
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3 / 50 page  2003 Microchip Technology Inc. DS40232H-page 3 MCRF450/451/452/455 1.0 DESCRIPTION OF DEVICE FEATURES The MCRF450/451/452/455 is a contactless read/write passive RFID device that is optimized for 13.56 MHz RF carrier signal. The device needs an external LC resonant circuit to communicate wirelessly with the Interrogator. The device is powered remotely by rectifying an RF signal that is transmitted from the Interrogator and transmits or updates its contents from memory-based on commands from the Interrogator. The device is engineered to be used effectively for item level tagging applications, such as retail and inventory management, where a large volume of tags are read and written in the same Interrogator field. The device contains 32 blocks (B0-B31) of EEPROM memory. Each block consists of 32 bits. The first three blocks (B0-B2) are allocated for device operation, while the remaining 29 blocks (B3-B31: 928 bits) are for user data. Block 1 contains unique 32 bits of Tag ID. The Tag ID is preprogrammed at the factory and write protected. All blocks, except for the Tag ID (Block 1), are contact- lessly writable block-wise by Interrogator commands. All data blocks, with the exception of bits 30 and 31 in Block 0, are write-protectable. The device can be configured as either Tag-Talks-First (TTF) or Interrogator-Talks-First (ITF). In TTF mode, the device transmits its fast response data (160 bits max., see Example 9-1) as soon as it is energized, then waits for the next command. In ITF mode, the device requires an Interrogator command before it sends any data. The control bits for TTF and ITF modes are bits 30 and 31 in Block 0. All downlink commands from the Interrogator are encoded using 1-of-16 Pulse Position Modulation (PPM) and specially timed gap pulses. This encoded information amplitude modulates the Interrogator’s RF carrier signal. At the other end, the MCRF450/451/452/455 device demodulates the received RF signal and then sends data (from memory) at 70 Kbit/s back to the Interrogator in Manchester format. The communication between Interrogator and device takes place asynchronously. Therefore, to enhance the detection accuracy of the device, the Interrogator sends a time reference signal (time calibration pulse) to the device, followed by the command and program- ming data. The time reference signal is used to calibrate timing of the internal decoder of the device. There are device options for the internal resonant capacitor between antenna A and V SS: (a) no internal resonant capacitor for the MCRF450, (b) 100 pF for the MCRF451, (c) two 50 pF in series (25 pF in total) for the MCRF452 and (d) 50 pF for the MCRF455. The internal resonant capacitors for each device are shown in Figures 2-2 through 2-5. The MCRF450 needs an external LC resonant circuit connected between antenna A, antenna B and V SS pads. See Figure 2-2 for the external circuit configura- tion. The MCRF452 needs a single external antenna coil only between antenna A and V SS pads, as shown in Figure 2-4. This external circuit, along with the internal resonant capacitor, must be tuned to the carrier frequency of the Interrogator for maximum performance. When a tag (device with the external LC resonant circuit) is brought to the Interrogator’s RF field, it develops an RF voltage across the external circuit. The device rectifies the RF voltage and develops a DC voltage (V DD). The device becomes functional as soon as V DD reaches the operating voltage level. The device then sends data stored in memory to the Interrogator by turning on/off the internal modulation transistor. This internal modulation transistor is located between antenna B and V SS. The modulation transistor has a very small turn-on resistance between Drain (antenna B) and Source (V SS) terminals during its turn-on time. When the modulation transistor turns on, the resonant circuit component between antenna B and V SS, which is in parallel with the modulation transistor, is shorted due to the low turn-on resistance. This results in a change in the LC value of the circuit. As a result, the circuit no longer resonates at the carrier frequency of the Interrogator. Therefore, the voltage across the circuit is minimized. This condition is called “cloaking”. When the modulation transistor turns off, the circuit resonates at the carrier frequency of the Interrogator and develops maximum voltage. This condition is called “uncloaking”. Therefore, the data is sent to the Interrogator by turning on (cloaking) and off (uncloaking) the modulation transistor. The voltage amplitude of the carrier signal across the LC resonant circuit changes depending on the amplitude of modulation data. This is called an ampli- tude modulation signal. The receiver channel in the Interrogator detects this amplitude modulation signal and reconstructs the modulation data for decoding. The device includes a unique anti-collision algorithm to be read or written effectively in multiple tag environ- ments. To minimize data collision, the algorithm utilizes time division multiplexing of the device response. Each device can communicate with the Interrogator in a different time slot. The devices in the Interrogator’s RF field remain in a nonmodulating condition if they are not in the given time slot. This enables the Interrogator to communicate with the multiple devices one at a time without data collision. The details of the algorithm are described in Section 6.0 “Read/Write Anti-Collision Logic”. |
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