Pulse or digital communications – Pulse position – frequency – or spacing modulation
Reexamination Certificate
2000-07-06
2004-03-02
Vo, Don N. (Department: 2631)
Pulse or digital communications
Pulse position, frequency, or spacing modulation
C375S240020, C375S240280, C329S313000
Reexamination Certificate
active
06700931
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to radio frequency identification (RFID) tag devices, and more particularly, to a read and write radio frequency identification tag device which synchronizes its decoder timing to the pulse positions of a received pulse position modulation (PPM) signal.
BACKGROUND OF THE RELATED TECHNOLOGY
A radio frequency identification (RFID) tag is a device that stores identification information and sends back this identification information, and may also include other information, when the device is powered-up by a radio frequency (RF) signal. These RFID tag devices may be used in managing inventory, automatic identification of cars on toll roads, security systems, electronic access cards, keyless entry and the like. RFID tag devices utilize radio frequencies that have much better penetration characteristics to material then do optical signals, and will work under more hostile environmental conditions then bar code labels. Therefore, the RFID tag device may be read through paint, water, dirt, dust, human bodies, concrete, or through the tagged item itself. RFID tag devices are used in conjunction with a radio frequency tag reader (interrogator) which transmits RF signals and receives data signals from the RFID tag device. The passive RFID tag device has no internal power source, rather it uses the incoming RF signal as a power source. Once the RFID tag device is activated, it sends stored data to the interrogator by modulating the amplitude of the incoming RF signal.
There are two classes of passive RFID tag devices: (1) read only, and (2) read and write (or read/write). In the read only RFID tag device, information stored in a memory array is not rewriteable once it has been written and locked. This RFID tag device normally sends the stored information as soon as it is energized by the RF signal from the interrogator. In the read/write RFID tag device, the information stored in the memory array is rewriteable at any time by an RF command sequence from the interrogator. Thus, the information in the memory array can be constantly updated and then read by the interrogator.
The RFID tag device transmits stored information to the reader-interrogator by modulating the amplitude of the RF carrier signal from the reader by detuning a resonant circuit of the RFID tag device that is initially tuned to the RF carrier signal (de-Qing or loading, for example by resistive loading, of the resonant circuit in the RFID tag device may also be used to modulate the amplitude of the RF carrier signal of the reader-interrogator). The RFID tag device comprises, for example, a parallel connected inductor and capacitor which is used as an antenna and is resonant (tuned) to the frequency of the RF carrier signal of the interrogator, an RF to direct current (DC) converter, a modulation circuit to send the stored information to the reader-interrogator, a logic circuit which stores coded information, a memory array that stores digitized information, and controller logic that controls the overall functionality of the RFID tag device.
An excellent application for RFID tag devices is item level tagging such as retail and inventory management where a large number of RFID tags may be read and written in a short period of time. Using read-write RFID tag devices, product information stored in the RFID tag device memory array such as inventory number, product expiration date, weight, and product description can be constantly updated.
Unlike the read-only RFID tag device, the read-write RFID tag device needs a specific command sequence for writing to or reading from its memory array. Therefore, the read-write RFID tag device may have, for example, two operational modes: 1) “tag talks first” and 2) “reader talks first” modes. The “tag talks first” mode is when the RFID tag device transmits its data as soon as it is energized by the RFID tag reader. The “reader talks first” mode is when the RFID tag device does not transmit data unless being commanded to do so by the RFID tag reader.
The RFID tag reader sends command signals to the RFID tag device by modulating its RF carrier signal. These command signals may be represented by appropriately timed gap pulses using, for example, pulse position odulation (PPM) of the RF carrier signal. PPM is a transmission scheme whereby data is represented by the temporal location of a pulse or pulses within a time window known as a symbol frame. Initiating and maintaining synchronization of the RFID tag device's time reference to the PPM signals from the RF)D tag reader is of the utmost importance for correct communications between the RFID tag device and reader.
There are two ways to achieve synchronization: The first uses the radio frequency carrier as a reference. This is often called the “synchronous” method. In this method, the internal power consumption of the RFID tag device increases as the carrier frequency goes higher. Therefore, this may reduce the total available power for reradiation. As a result, the effective communications range between the RFID tag device and reader becomes shorter.
The second way is to generate an internal reference signal by an internal oscillator in the RFID tag device. This is called the “asynchronous” method. This method, however, requires “calibration” of the internal timing reference of the RFID tag device to the reader's timing signal. This “time calibration” can be achieved by transmitting a calibration pulse from the reader every so often. This calibration pulse must be transmitted ahead of the PPM command signal so that the internal timing of the RFID tag device is synchronized to the reader before the PPM command is detected. Since the internal oscillator of the RFID tag device can be designed to keep the power consumption low, the total power consumption of the device can be less than a device using the synchronous method. As a result, its communication range becomes greater. This is an advantage of an RFID tag device using the asynchronous method.
Therefore, what is needed is a simple and effective way of synchronizing a PPM decoder circuit and externally transmitted PPM signal to a required timing precision for correct PPM decoding.
SUMMARY OF THE INVENTION
The invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing in an RFID tag device having a PPM decoder that is synchronized to an external PPM source such as a RFID tag reader by measuring the number of internal oscillator cycles during a calibration cycle having a plurality of pulses in a single symbol frame.
The RFID tag reader sends the command and acknowledgment signals to the RFID tag device by modulating the continuous wave (CW) carrier signal. The RFID tag reader uses, for example, a 1-of-16 PPM for data transmission. The gap pulse sequences are controlled by the time spacing between pulses to encode the command and operating parameters. The RFID tag reader also sends time reference pulses to calibrate the time base of the decoder in the RFID tag device. The time spacing between pulses for both the 1 of 16 PPM data and the calibration reference can be measured by counting the number of oscillator frequency cycles elapsing from an internal on-chip oscillator of the RFID tag device.
The RFID tag reader uses 1 of 16 PPM for control commands such as tag acknowledgment, read a tag block, write a tag block, etc. The 1 of 16 PPM uses the pulse positions in one of sixteen possible time slots as the communication mechanism for sending 4 bit symbols (2
4
=16). All communications begin with a code violating calibration sequence composed of, for example but not limited to, three pulses in pulse positions zero, six and fourteen.
The symbol frame start and end are not explicitly transmitted and may be recovered by knowledge of the last symbol received, the count to the next received pulse and counts per pulse width. An equation describing this relationship is:
(
sym
)
n
=CBP/CPP
−16+(
sym
)
n−1
where CBP=number of inte
Alexander Sam
Furey Lee
Gallagher William F.
Inui Shinichiro
Lee Youbok
Baker & Botts L.L.P.
Ghulamali Qutbuddin
Microchip Technology Incorporated
Vo Don N.
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