Abstract:
This invention provides a method and system for inventorying wireless transponders, specifically referred to as RFID transceiver devices. The RFID transceiver devices are certifiable by a recognized standards body, such as EPCglobal, or are compatible with recognized standards but have higher functionality than typical passive RFIDs, and yet are implemented with techniques that lower cost and battery requirements. Backscatter techniques of standard passive RFIDs are used to keep cost and battery requirements low. To provide higher functionality, a microcontroller is used in the RFID, along with a battery, but the clock frequency of the microcontroller is adjusted, based on external input, to minimize battery requirements. In one embodiment, the microcontroller initially has a zero or near zero clock frequency. A comparator compares the received RF energy to a threshold, and when the threshold is exceeded, indicating the presence of a probe signal from an RFID reader, the microcontroller clock frequency is immediately increased, and further adjusted based on the received data. In an alternate embodiment, the clock frequency of the microcontroller is adjusted based on data from sensors, to keep the clock speed at the proper speed to adequately process the data while minimizing the power requirement of the microcontroller, and then create an input to modulate the backscattered signal to be transmitted by the RFID. The received data can also cause the microcontroller to request the sensors to generate sensing data at a faster rate, which in turn requires the microcontroller clock to increase to handle the increased sensor data. In an additional embodiment, multiple sensors are multiplexed to provide a single input stream to the microcontroller, reducing the microcontroller clock speed required, along with the overall cost of the microcontroller and sensors.

Description:
GOVERNMENT INTERESTS 
       [0001]    This patent is based on work supported by the US Army under Contract No. W81XWH-08-C-0014. 
     
    
     1. FIELD OF THE INVENTION 
       [0002]    The present invention provides a system and method for the development and implementation of low cost, power efficient, wireless transponder devices with enhanced functionality, such as, but not limited to, sensors, extended memory, range, and security, that communicate with a reader/interrogator such that the devices can be identified and additional information can be known. 
       2. DESCRIPTION OF RELATED ART 
       [0003]    The ability of actively powered, battery assisted passive (BAP), and passive wireless transponders with enhanced functionality to acquire, store and transmit additional data along with identifying data can have applicability to a wide variety of items. As such, a new class of Radio Frequency Identification (RFID) devices with enhanced functionality is emerging to effectively address the needs of these products that include, but are not limited to, medical supply items, pharmaceuticals, certain sensitive equipment and machines, chemicals, ordnance and food. Much of the enhanced functionality and data capture involves the current or past environment of the items. 
         [0004]    Purely passive wireless transponder devices, when actively energized by a reader/interrogator, use the received source of electromagnetic (EM) energy to power themselves, to enable the data reception, to power other functional devices such as sensors and to backscatter or reflect their specific identification code and other data back to the interrogator. This type of wireless transponder device is capable of performing actions such as sensing only when the received source of energy is available. 
         [0005]    U.S. Pat. No. 7,479,886 by Burr describes an RF harvesting that can be used to store energy into a capacitor such that the device can temporarily operate when the RF signal is no longer present. This patent has the limitation that its usage is quite limited since a large capacitor would be required to store enough energy to power other functions which would increase the overall size of the system. Additionally, the amount of energy which can be stored in capacitors is significantly lower than the amount of energy that can be stored in batteries. BAP wireless transponder devices use batteries to enhance their radio frequency (RF) communication abilities and provide other processing capabilities and functions. Actively powered wireless transponder devices use batteries to acquire and store data on demand as well as to enhance the reception and transmission of their RF communications. 
         [0006]    Purely passive wireless transponder devices work on the principle of “RF harvesting” in which the reader sends a high power continuous wave (CW) tone preamble that persists over a period of time and is followed by a modulated message carrying the signaling information. “RF harvesting” techniques have been described. U.S. Pat. No. 7,385,511 by Muchkaev describes a reader which generates and transmits coded sequences of short high power pulses repeatedly to a transponder which receives and stores the pulse energy. This patent has the shortcoming that such coded sequences are not commonly employed by the standard-based systems. U.S. Pat. No. 7,400,253 by Cohen describes all forms of RF signals and EM energy, not just those transmitted from or provided by an RFID reader/interrogator, are used to provide power to an RFID transponder or to charge the battery. This patent has the shortcoming that in a practical situation the energy available in all forms of RF signals is not sufficiently high to power an RF harvesting circuit. U.S. Pat. No. 6,970,089 by Carrender describes two antennas which are configured to receive ambient radiation and use this energy to reflect a modulated signal for writing and reading data to and from the devices in the system. This patent has the shortcoming that while two antennas can improve the signal reception and backscattering, they also prohibitively increase the size of a transponder. 
         [0007]    To receive a message from the reader, a passive wireless transponder device requires two essential elements, namely, an antenna and an “RF harvesting” circuit. The antenna serves both as a transducer, which converts the electromagnetic (EM) wave into an electrical waveform, and as an impedance transformer, which matches the free space impedance to the impedance of the frontend circuit in the wireless transponder device. The RF harvesting circuit, on the other hand, extracts energy from the signal and stores it in a storage element on the wireless transponder device. A typical realization of the RF harvesting circuit employs a Schottky diode to rectify the CW signal and store the energy as an electrical charge within one or more capacitors. The voltage across the storage capacitor is proportional to the amount of the electric charge. Once a sufficient charge is built up on the capacitor, the capacitor is then able to power the receiver and the processing circuit of the wireless transponder device to demodulate and decode the message from reader. RF harvesting only provides a limited amount of energy. Circuits on the wireless transponder device can only operate in the presence of a reader signal and for a small additional “persistence” time after the end of reader signal. 
         [0008]    Purely passive wireless transponder devices are able to send information to a reader by a process called “backscattering”. Backscattering is achieved by changing (modulating) the antenna impedance with the signaling waveform. When the antenna impedance is matched to an RF frontend circuit, the RF signal impinged on the antenna is absorbed by the RF frontend circuit, whereas, when the coupling between the antenna and the RF frontend circuit is open (or shorted), a higher amount of RF signal impinged on the antenna is reflected back or “backscattered” to the reader. A control signal is used to create the “matched” or “open (or shorted)” circuit conditions based on the polarity of the signal. The reader detects the RF power level changes in the reflected or “backscattered” signal from the passive wireless transponder device and decodes the message. 
         [0009]    To store certain information permanently, a passive wireless transponder device incorporates a small amount of non-volatile memory to retain information in the absence of the reader signal. A typical realization of such non-volatile memory is a “flash” memory. Writing to the flash memory requires a higher voltage. A charge pump voltage multiplying circuit coupled to the RF harvesting circuit boosts the voltage high enough to allow the flash programming. Due to the limited available power, only a limited memory capacity is available on the passive wireless transponder device. Simple information such as an ID and serial number is stored in the device. 
         [0010]    With a purely passive wireless transponder device implementation no power supply or battery is required. This significantly simplifies the wireless transponder device complexity and reduces its cost as compared to the conventionally powered wireless transponder device. Typical commercial passive wireless transponder devices are implemented by attaching a silicon die containing the required circuitry to a printed antenna strip on a flexible substrate using a thin film material. The passive wireless transponder device can then be affixed onto merchandise in a similar way as conventional “bar-coded” labels. In order to achieve a cost target approximating a “bar-coded” label, the available commercial passive wireless transponder devices are made with the minimum functionality that satisfies the current EPC global and/or ISO specifications. The limited on-chip processing capability is implemented with hard-wired circuits, which realistically cannot be altered without incurring a high non-recoverable engineering (NRE) cost and long re-design cycle. The construction is not flexible for expansion to include other functions as it does not provide any external interfaces for adaptation, adding memory capacity, or expansion in functionality. These restrictions make it difficult for the purely passive devices to be adapted to the specific needs of different customers and applications. 
         [0011]    Actively powered wireless transponders or active transponders do provide the capability for adaptation, adding memory capacity or expansion in functionality such as sensing. The onboard power also provides the means to significantly increase the communication range as compared to that of the purely passive transponder devices. 
         [0012]    The typical implementation of active transponders consists of an analog circuit and a digital circuit. These devices need to be active at all times to monitor the environment and therefore need a constant power supply. From a circuit implementation perspective, the power consumption of the analog circuit depends on the signal frequency. The higher the signal frequency, the higher the power consumption that is required. Thus, the RF transceiver analog circuit in an active transponder device consumes the most power. The power consumption of the digital logic circuit is proportional to the operational frequency of the logic gates (i.e., how often the logic state is flipped or the polarity is changed). For active transponders, the amount the power consumed by the digital logic circuit can often be controlled by adjusting its operating clock frequency. When active transponders are in either the receive or transmit mode, the peak power consumption is several orders of magnitude higher than that of purely passive transponders powered by RF harvesting. 
         [0013]    Battery supply voltage for an active transponder depends not only on its state of charge, but also on the discharge current. This IR voltage drop is caused by discharge current (I) flowing through the internal resistance (R) of the battery. The voltage drop is higher at high currents and lower temperatures which exhibit increased resistance. When a battery is discharged, the voltage gradually decreases until it reaches the minimal voltage acceptable for the device which is called the end of discharge voltage (EDV), or the voltage where continuing discharge causes damage to the battery and an accelerated drop in discharge voltage. To support high peak power active transponders, large batteries with low internal resistance are required to prevent an excessive IR drop. The large batteries significantly increase the overall packaging size of the devices which in turn places severe constraints on the merchandise, boxes, or cargo to which the devices can be affixed. 
         [0014]    To minimize the power consumption and thus the size of the power source, the system design for active transponders often minimizes the duty cycle in which the transponders are active. As an example, ISO 18000-7 active transponders used by the US Department of Defense (DoD) wake up for a duration of a few milliseconds every 2 seconds to achieve a low duty cycle in the standby mode. In order to wake up the active transponders, the reader first sends a long wake-up signal lasting more than 2 seconds. After all active transponders are in an “awake” state, the reader can read the tag contents. Once in the “awake” state, the duty cycle of the active transponders significantly increases to speed up the system response. After the reader/interrogator has interrogated all active transponders within a session, the reader/interrogator issues a command to lower the duty cycle of all active transponders. 
         [0015]    The low duty cycle (long sleep period) of an active transponder system severely limits the response time, and the system level power management operation places constraints on the system design. Since the active transponders consume large amounts of power during their active state, the number of probes or reads by readers/interrogators affects how long a battery can last. Only very limited read/write cycles are typically supported by the battery of the active transponders. The battery requirements severely limit the size and weight reduction possibilities of the active transponder devices, which in turn places severe constraints on the merchandise, boxes, or cargo to which the devices can be affixed. 
         [0016]    In additional to the problem of high power consumption, active transponder systems suffer from significantly higher costs than passive wireless transponder systems due to their higher complexity. It is also often difficult for a conventional microcontroller to interface to a variety of sensors or other external data collection devices. 
         [0017]    The ability to be able to use an off-the-shelf commercial reader/interrogator with its back-end logistics system could significantly shorten the time to implement a system when higher functionality is required by an application, enabling a faster deployment as well as improving the reader/interrogator infrastructure return on investment (ROI). 
         [0018]    It is desirable to provide a passive wireless transponder system which provides increased functionality, higher memory capacity, enhanced flexibility, and configurability similar to that of actively powered wireless transponders or active transponders. Such a passive wireless transponder system can eliminate the need for a large battery and therefore provide a small product form factor which reduces the packaging complexity. Without the required active battery power saving operations, such a wireless transponder system can respond to readers/interrogators quickly without the need for a complex system level power management operation. To further reduce the cost and complexity of a wireless transponder, the monitoring, recording and storage of data must be executed in an efficient manner without occupying a large amount of memory, but, at the same time, providing adequate data visibility to the end user. 
         [0019]    It is desirable to allow the use of the same reader/interrogator for both extremely low cost, off-the-shelf, purely passive wireless transponder devices and wireless transponder devices with enhanced functionality, such as, but not limited to, added sensors, extended memory, extended range, and enhanced security. 
       SUMMARY OF THE INVENTION 
       [0020]    The present invention provides a method and system for inventorying wireless transponders, referred to as RFID transceiver devices that have a higher functionality than typical passive RFIDs, and yet are implemented with techniques that provide lower cost and battery requirements. The RFID transceiver devices can be certifiable by a recognized standards body, such as EPCglobal, or are compatible with recognized standards Backscatter techniques of conventional passive RFIDs are used in the present invention to keep cost and battery requirements low. To provide higher functionality, a microcontroller is used in the RFID, along with a battery, but the clock frequency of the microcontroller is adjusted, based on external input, to minimize battery requirements. In one embodiment, the microcontroller initially has a zero or near zero clock frequency. A comparator compares the received RF energy to a threshold, and when the threshold is exceeded, indicating the presence of a probe signal from an RFID reader, the microcontroller clock frequency is immediately increased. The comparator threshold is also adjusted, allowing for reliable detection of data on the probe signal, which is input to the microcontroller. The clock frequency of the microcontroller can be further adjusted based on the received data. 
         [0021]    In an alternate embodiment, the clock frequency of the microcontroller can be further adjusted based on data from sensors, in order to keep the clock speed at the proper speed to adequately process the data while minimizing the power requirement of the microcontroller, and then create an input to modulate the backscattered signal to be transmitted by the RFID. The received data can also cause the microcontroller to request that sensors generate sensing data at a faster rate, which in turn requires the microcontroller clock to increase to handle the increased sensor data. In an additional embodiment, multiple sensors are multiplexed to provide a single input stream to the microcontroller, reducing the microcontroller clock speed required, along with the overall cost of the microcontroller and sensors. In another embodiment, the interface between the sensors and the microcontroller contains logic circuitry to reduce the required microcontroller clock speed. In another embodiment, the microcontroller clock is increased by a timer, i.e., the microcontroller is turned on at a specific time, or periodically, to process sensor data, or the clock speed is increased to handle additional sensor data at specific times. 
         [0022]    The invention will be more fully described by reference to the following drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a schematic diagram of an embodiment of the RFID transceiver device in accordance with the teachings of the present invention. 
           [0024]      FIG. 2  is a schematic diagram of an embodiment of an RF detection and backscattering frontend of the RFID transceiver device. 
           [0025]      FIG. 3  is a schematic diagram of an alternate embodiment of the RFID transceiver device with a special purpose logic circuit. 
           [0026]      FIG. 4  is a schematic diagram of data encoding, tag to interrogator, for a data- 0  and a data- 1  without a subcarrier. 
           [0027]      FIG. 5  is a schematic diagram of an embodiment of the RFID transceiver device for software modulation for data encoding, tag to interrogator, for a data- 0  and a data- 1  without a subcarrier. 
           [0028]      FIG. 6  is a schematic diagram of data encoding, tag to interrogator, for a data- 0  and a data- 1  with a subcarrier. 
           [0029]      FIG. 7  is a schematic diagram of data encoding, tag to interrogator, for a data- 0  and a data- 1  with a subcarrier using an exclusive-or gate. 
           [0030]      FIG. 8  is a schematic diagram of an embodiment of the RFID transceiver device for data encoding, tag to interrogator, for a data- 0  and a data- 1  with a subcarrier using an exclusive-or gate. 
           [0031]      FIG. 9  is a schematic diagram of data encoding, interrogator to tag, for a data- 0  and a data- 1  without a subcarrier. 
           [0032]      FIG. 10  is a schematic diagram of an embodiment of the RFID transceiver device for data decoding, interrogator to tag, for a data- 0  and a data- 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Reference will now be made in greater detail to preferred and additional embodiments of the invention. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. 
         [0034]      FIG. 1  illustrates an embodiment of RFID transceiver device  100 . Antenna  102  receives probe  201  from RFID reader  200 , and transmits signal  101  back to RFID reader  200 . RF detection and backscattering frontend  103  receives signal  201  from antenna  102  and sends RF signal RFIN  104  to comparator circuitry  105 . When RF signal RFIN  104  exceeds a threshold, comparator circuitry  105  sends interrupt signal  106  to microcontroller  107  for increasing the speed of clock  108 . If microcontroller  107  initially was turned off with a clock speed of zero, interrupt signal  106  increases the clock speed to turn on microcontroller  107 . Microcontroller  107  also outputs control signal  109 , which causes switch  110  to provide DC power  111  from battery  112  to RF detection and backscattering frontend  103 . DC power  111  turns on a comparator in RF detection and backscattering frontend  103  whose output signal  113  provides data from signal  201  to microcontroller  107 . Microcontroller  107  also provides input signal  114  to RF detection and backscattering frontend  103  to modulate signal  201  to provide the backscattered signal  101  that is transmitted to RFID reader  200 . This embodiment supports a low power and compact size implementation of a wireless transponder device by employing a passive RF frontend similar to that of a purely passive wireless transponder device, which derives its entire operational power by rectifying energy from an RF signal. The key advantages of such an RF frontend are that no standby current is required for the circuit and the wireless transponder device can provide an instant response to a reader/interrogator. Additionally, since no RF transceiver that requires power is being used, the peak power requirement can be significantly lowered, allowing the use of a smaller battery for powering advanced or additional transponder device functionality. 
         [0035]    RF detection and backscattering frontend  103  employs a diode rectifying circuit to demodulate and recover the reader baseband modulating signal.  FIG. 2  shows an embodiment of this circuitry, which includes comparator circuitry  105 . Antenna  102  is connected to diode  301 , RF choke inductor  302 , and inductor  303 . The other end of inductor  302  is connected to capacitor  304  and resistor  305 , which receives input signal  114  from microcontroller  107  shown in  FIG. 1 . The other end of inductor  303  is connected to capacitor  306  and capacitor  307 . A first pair of Schottky diodes  308  and  309  is used to rectify the RF signal from antenna  102  and store the charge in capacitor  310 . A second pair of Schottky diodes  311  and  312  is added to the first pair to increase the output voltage of RF signal RFIN  104  which is stored in capacitor  313 . These diodes provide rectifying with to charge capacitor  313  on both the positive and negative swing of the received signal voltage. The output of the Schottky diodes  311  and  312 , voltage of RF signal RFIN  104 , is connected to comparator circuitry  105  for RF signal detection, which uses low speed comparator  314  to detect the presence of the RF probe signal by comparing voltage of RF signal RFIN  104  to VCC voltage  315  which is connected via resistor  316 , grounded through resistor  317 , to comparator  314 . Low speed comparator  314  consumes very low power such that it could be powered by an RF harvesting circuit. The output of comparator circuitry  105  is interrupt signal  106 , as also shown in  FIG. 1 , which is used to trigger the microcontroller  107  shown in  FIG. 1  and turns on the receiver operation. The output of Schottky diodes  311  and  312 , voltage of RF signal RFIN  104 , is also connected to slicer  318  inside RF detection and backscattering frontend  103 . Low pass filter  319 , which contains resistors  320 ,  321 , and  322 , voltage  315 , and capacitor  323 , and couples to negative input  324  of comparator  325 , provides the detecting reference level of slicer  318 . Since low-pass filter  319  averages out the RF modulation, reference level  324  settles at the average level of the signal voltage of RF signal RFIN  104 . This allows comparator  325  to slice RF signal RFIN  104  and provides a digital baseband signal at RF detection and backscattering frontend  103 , which is coupled to microcontroller  107 . Slicer  318  typically operates at a fast speed and consumes a higher amount of power. Accordingly, slicer  318  can be powered by battery  112 . Comparator circuitry  105  provides interrupt signal  106  to connect the battery power to the slicer to activate its operation, as shown in  FIG. 1 . 
         [0036]    Pin-diode  301  provides an open or short connection to antenna  102 , and serves as the backscattering circuit. The open or short circuit control of pin diode  301  is under the control of a microcontroller. RF detection and backscattering frontend  103  adapts itself to different input RF signal levels, powers and consumes no, or extremely low, power during the standby mode. The combination of the diode circuit with comparator  314  to provide interrupt signal  106  allows a “close-to zero” power standby mode with a wake up in a sufficiently short amount of time for receiver operation. RF frontend circuit implementations, fulfilling the same requirements, could be used with the teachings of the present invention. 
         [0037]    In one embodiment, a software radio architecture is employed in which the recovered baseband waveform is sampled, digitized and processed with a microcontroller. Microcontroller  107  is in a standby mode when no RF signal is detected. In the standby mode, microcontroller  107  operates using clock  108  at a very low frequency and consumes virtually no power. Microcontroller  107  shuts off most of its internal circuits except for certain peripheral circuits that allow it to detect interrupt signals or timer circuits that can be programmed such that microcontroller  107  wakes up at certain time intervals. Microcontroller  107  requires little computing processing power during this low power state. 
         [0038]    In order to detect the presence of an RF signal, in  FIG. 2  RF signal RFIN  104  from the RF detection circuitry is connected to comparator  314 . Comparator  314  is used to detect the rectified RF level against a threshold voltage. If the RF level exceeds the threshold voltage, the presence of the RF signal is detected. Interrupt signal  106  which is the output of comparator  314  is used to interrupt microcontroller  107  and allows microcontroller  107  to transition from a standby mode to an active mode. Power consumption of comparator  314  depends on its response time. For a slow response time, on the order of hundreds of microseconds, comparator  314  can consume extremely low power, less than a microamp. Comparator  314  in comparator circuitry  105  can be powered by either an onboard power source, such as battery  112  or an RF harvesting circuit as part of RF detection and backscattering frontend  103 , as shown in  FIG. 1 . 
         [0039]    Once the presence of the RF signal is detected, comparator  314  generates interrupt signal  106  for microcontroller  107 . Microcontroller  107  then transitions from a standby mode to an active mode to process the received waveform. In RF detection and backscattering frontend  103 , self-biasing slicer  318  is used to convert the analog baseband waveform into a hard limited signal consisting of high and low pulses which are sampled by microcontroller  107  directly. Slicer  318  is used to detect a binary digitally-modulated AM signal which is commonly used in readers/interrogators, including readers conforming to EPCglobal, ISO or other known standards. The preferred embodiment includes the use of a slicer, but other embodiments may not. For other types of signals, an analog-to-digital converter can be used to sample the analog baseband waveform coming out of the RF detection circuit. It is also possible for a microcontroller to directly sample the analog waveform of a binary, digitally modulated AM signal as long as the analog baseband waveform resembles a digital stream with its high level close to the supply voltage, its low level close to ground, and a transition time from the high level to the low level that is short relative to the duration of the high or low level. Slicer  318  can be turned off in the absence of an RF signal to conserve battery power. As soon as an RF signal is detected, slicer  318  can be turned on to convert the waveform. 
         [0040]    Comparator  314  performs essentially the same function as a slicer. In an alternate embodiment, comparator  325  in slicer  318  and comparator  314  in comparator circuitry  105  can be the same comparator. Accordingly, the comparator and the slicer are the same circuit with a different biasing current. The comparator, biased with a low current, is used to monitor for the presence of the RF signal. When the RF signal is detected, the comparator is biased at an adequate current to support the faster speed of the AM modulated waveform. 
         [0041]    In microcontroller  107 , the sampled baseband waveform is parsed to detect the preamble or frame sync and to detect the signaling data and message from RFID reader  200 . In the preferred embodiment that supports the decoding of an EPCglobal C1G2 signal, the preamble or frame sync starts with a delimiter. Following the delimiter, a data_ 0  symbol with a duration equal to Tari, and an RTcal symbol with a duration equal to the sum of the duration of a data_ 0  and a data_ 1  symbol is detected. RTcal allows the tag to set the pivot duration equal to RTcal/2. The pivot duration is used for detection of a data_ 0  and data_ 1  symbol. Any symbol with a duration longer than the pivot is interpreted as data_ 1  and any symbol with a duration shorter than a pivot is interpreted as data_ 0 . For the preamble only, a TRcal symbol follows the RTcal symbol. The TRcal symbol duration is used by the RFID transceiver device to set the backscattering link frequency. For the frame sync, there is no TRcal symbol following the RTcal symbol. By detecting data_ 0  and data_ 1 , a digital stream carrying a message can be decoded following the EPCglobal C1G2 specification. The RFID transceiver device could be altered to decode other standard specifications. 
         [0042]    In the preferred embodiment, clock  108  is derived from a low frequency clock oscillator using an internal phased-locked loop. Clock  108  can be configured to different frequencies. At a lower clock frequency, microcontroller  107  can operate at a lower supply voltage. It is desirable to operate at as low a clock frequency as possible such that microcontroller  107  can still operate at a supply voltage close to the EDV (end of discharge voltage) of battery  112 . The lower the clock frequency, the greater the battery life and device operational temperature range provided by battery  112 . The power consumption of microcontroller  107  is proportional to its clock frequency. Accordingly, another benefit of the lower clock speed is the reduction in the current consumption. 
         [0043]    In RFID transceiver device  100 , the receiver function is handled in software by microcontroller  107 . Typical microcontrollers are designed to handle serial processing operations. To perform parallel processing operations, the clock frequency of the microcontroller needs to increase. A majority of microcontroller operations are designed to simultaneously handle multiple bits with bus widths typically equal to 8, 16, 32, or 64 bits. This operation represents a waste of power if the number of bits to be processed is less than the bus width. It is more power efficient to employ a special purpose logic circuit to handle the receiver operations that require a high clock speed.  FIG. 3  illustrates an alternative embodiment of the RFID transceiver device  115 . RFID transceiver device  115  employs special purpose logic circuit  116  to provide the processing of the receiver functions such as, for example, the decoding of the preamble or the frame sync, the detection of the bit duration, and the decoding and generation of messages from and to a reader/interrogator. Input and output lines  113  and  114 , shown in  FIGS. 1 and 2 , are replaced by bidirectional lines  117  and  118  from special purpose logic circuit  116 . This architecture lowers the overall power consumption by using special purpose logic circuit  116  to handle the receiver function. The use of special purpose logic circuitry also allows microcontroller  107  to operate at significantly lower clock speeds for sensor operation. The addition of the special purpose logic circuit only marginally increases the overall system cost. 
         [0044]    The architecture of RFID transceiver device  115  shown in  FIG. 3  also provides an enhanced capability to the transponders. Sensor  351  and other external data collection device  352  have digital interfaces and microcontroller  107  can easily be programmed to provide signaling for serial or parallel interface  353 . Sensor  354  and other external data collection device  355  have only analog interfaces  356  and  357 , corresponding to sensor  354  and other data collection device  355 , and microcontroller  107  can control internal ADC  358  to sample the outputs from analog interfaces  356  and  357 . Many low cost commercial microcontrollers are equipped with on-chip ADCs. With on-chip ADC  358 , multiplexer  359  is used to provide multiplexed inputs from the analog interfaces  356  and  357  to ADC  358 , such that the single ADC  358  can be used to sample the multiple external data collection devices  354  and  355 . Multiplexer  359  can be further enhanced to be reconfigurable to handle a variety of sensors and different sets of sensors simultaneously. In another embodiment, an external ADC can be used. 
         [0045]    Most sensors other external data collection devices only need to be sampled at some time interval. For example, a temperature sensor does not need to sample its environment at a frequency of several thousand times per second since the temperature typically does not change that fast. Another embodiment is the use of timer  360  on microcontroller  107 , to schedule periodic sampling of sensors  351  and  354 , and other external data collection devices  352  and  355 . Microcontroller  107  can program timer  360  with a sensing interval and then immediately enter a low power standby mode. As soon as timer  360  expires, timer  360  generates interrupt signal  106  for the microcontroller  107  to enter an active state and process the sampling operation of sensors  351  and  354  and other external data collection devices  352  and  355 . 
         [0046]    In an alternate embodiment, a reader/interrogator command/signal  361  from reader  200  is used to change the reading frequency of sensors  351  and  354  and other external data collection devices  352  and  355  Sensor applications can require data collection over a period of, for example, one day, with a sensing frequency of about once every half a minute while other applications need data collection over a period of months, for example, about three months, with a sensing frequency of about once every 5 minutes. The use of timer  360  allows sensors  351  and  354  and other external data collection devices  352  and  355  to be easily configured based on the application scenario. 
         [0047]    Microcontroller  107  can include two types of memory, nonvolatile and volatile. The non-volatile memory can be used to store critical information such as ID, serial number, label information or other data pertaining to, for example, merchandise to which RFID transceiver  100  device is affixed. The information can survive an event such as when a battery becomes disabled or the wireless transponder device is unable to continue to operate under conditions such as extreme out of bound temperatures. The non-volatile and volatile memory can provide increased storage capability for the processing and storage of sensor or other data. This allows more flexibility to configure the RFID transceiver device to suit different applications. 
         [0048]    In ultralow power management, the action of saving to flash on a microcontroller too frequently can be a drain on the battery. In order to lengthen the battery life, in one embodiment, data is saved in the volatile memory of microcontroller  107  and is only saved to flash on microcontroller  107  when microcontroller  107  is awakened for other purposes. 
         [0049]    In applications involving the use of a large volume of wireless transponder devices equipped with sensors or other added functionality, the cost of wireless transponder device is critical. It is therefore important to include only the sensors or other added functionality needed for a specific application and to remove the sensors or other added functionalities which are not needed to reduce overall cost of the system. RFID transceiver device  100  can be easily configured to do this and lower the overall cost. The application software can be configured to have different footprints depending on which sensors or other added functionalities are incorporated. A footprint corresponding to a smaller number of sensors or other added functionality results in reduced code memory and lower cost. Since multiplexed inputs to the ADC are employed to interface to multiple sensors or other added functionality, the unwanted sensors or other added functionality can be removed without affecting other parts of the system operation to provide flexibility and configurability to adapt the RFID transceiver device to different applications with optimized cost. 
         [0050]    In order to optimize power consumption, microcontroller  107  in is put in standby mode when not reading a sensor or processing other added functionality. A timer is used to alert microcontroller  107  to read the sensor or process other added functionality at a given time interval. Sensors or other added functionality, while powered, can require some settling time before they reach a steady state condition. Since the sensor or other added functionality settling time could be quite long, it is desirable to configure the clock speed of microcontroller  107  to as low a clock speed as is sufficient to process the sensor reading or other added functionality operation to match the settling time of the sensor and thereby reduce the overall power consumption. 
         [0051]    Since RFID transceiver device  100  and  115  can have a very long battery life, periodic sensing of sensor or processing of other added functionality can create a huge amount of data. A preferred embodiment employs configurable thresholds to monitor for sensor reading data. In one embodiment, for sensitive temperature shipments, high and low temperature thresholds are programmed into microcontroller  107  such that microcontroller  107  monitors and records the events and the event durations for which the ambient temperature exceeds the thresholds. Microcontroller  107  is programmed to record/store only important conditions from the sensor or other added functionality devices which can cause critical merchandise breakdown. For example, the events in which the temperature or shock of the merchandise exceeds a certain temperature or shock threshold can be stored. If the merchandise is damaged after a set number of events or a given duration of event or events, microcontroller  107  only records or stores that event data in its memory. Such threshold values are programmed either over the air or during initial microcontroller programming at the factory. If RFID transceiver device  100  and  115  samples at a regular interval, there may not be enough granularity and some of the events might be missed. Thus, in one embodiment, if RFID transceiver device  100  or  115  detects that it is close to an out of bounds limit, it reduces its sampling interval and samples more frequently. This action reduces the chance of missing some events. In the case of a shock sensor, the typical shock event is normally preceded with vibration, a free fall or high acceleration condition. The shock sensor is programmed to detect these conditions and issues interrupt signal  106  to wake up microcontroller  107  to activate the sensor operation. This increases battery life. An embodiment of the storage for sensor data is shown in Table 1. 
         [0000]    
       
         
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Last Sampled Value (1 Byte) 
               
               
                   
                 Total Samples (2 Bytes) 
               
               
                   
                 Number Of Time Crossed Limit (2 Bytes) 
               
               
                   
                 Max Temp Reached (1 Byte) 
               
               
                   
                 Min Temp Reached (1 Byte ) 
               
               
                   
                 Total Time Out Side Limit (4 Bytes) 
               
               
                   
                 Sampling Time (2 Bytes) 
               
               
                   
                 Default Sampling Time (2 Bytes) 
               
               
                   
                 Threshold Sampling Time (2 Bytes) 
               
               
                   
                 Max Temp Limit (1 Byte) 
               
               
                   
                 Min Temp Limit (1 Byte ) 
               
               
                   
                 Max Threshold Temp (1 Byte) 
               
               
                   
                 Min Threshold Temp (1 Byte) 
               
               
                   
                 Hysteresis (1 Byte) 
               
               
                   
                 Time when Outside the limit (4 Bytes) 
               
               
                   
                 Time When came back from out (4 Bytes) 
               
               
                   
                 Unix Time Stamp (4 Bytes) 
               
               
                   
                   
               
             
          
         
       
     
         [0052]    Microcontroller  107  can also be programmed to change the sensing or other added functionality processing interval based on the in situ conditions. Interval changes can be programmed either over the air or during initial microcontroller programming at the factory. In some applications, it is important to monitor the sensors or other added functionality devices with a higher frequency as the data readings get closer to the thresholds. This provides the benefit that if an event could exceed a data threshold within the time of a standard wake-sleep period; the shortened interval reduces the chance that the data reading might miss such an event. In this case, microcontroller  107  can be configured to shorten its timer period to provide more frequent readings as the data readings approach the thresholds. 
         [0053]    In some situations, the battery voltage can drop below a level at which RFID transponder device  100 , 115  ceases to be able to communicate effectively with RFID reader/interrogator  200 . This can be due to an ambient temperature drop that causes the supply voltage of the battery to drop accordingly or to a battery simply being discharged due to use. In this situation, however, the battery supply voltage might still be able to support data readings or other onboard functionality that only require a low clock frequency for microcontroller  107 . In another embodiment, recorded sensor data or other onboard functionality data is read and the stored in the non-volatile memory of the microcontroller if the battery supply voltage is too low. As soon as the battery issue is resolved, the recorded sensor or other onboard functionality data can be retrieved by a reader. 
         [0054]    Since the processing of receiving and sending data to and from reader can drain battery more rapidly, when low on battery, in another embodiment the RFID transceiver device  100 , 115  is put into a hibernating mode such that it only monitors and stores the sensor data while the battery can still sustain it without responding to any signals from RFID reader  200 . This increases critical battery life. RFID transceiver device  100 , 115  can subsequently be placed in a docking fixture to restore its battery power. The data stored can then be read from the battery in the docking station or over the air if the battery power allows. 
         [0055]    In another embodiment, a real-time clock (RTC), on microcontroller  107  is used to time-tag the data in RFID transceiver device  100 , 115 . The RTC starts in an arbitrary state and provides a time stamp to the sensor data using the free-running RTC. When reader/interrogator  200  reads the data on RFID transceiver device  100 , 115 , it reads the RTC on RFID transceiver device  100 , 115  and compares this RTC with its own RTC to find the offset. When RFID reader  200  retrieves any data and the associated time stamp, RFID reader  200  adds the offset back to the timestamp to restore the real time clock reading. With this embodiment, there is no need to initialize the RTC of RFID transceiver device  100 , 115 . 
         [0056]    In another embodiment, the read and write commands in memory of RFID transceiver device  100 , 115  that are specified in the EPCglobal, ISO or another standard are modified so as to embed the protocol for sensor or other data reading. Predefined specific memory locations are used for passing read and write sensor or other data commands and storing sensor data. Specific memory locations are used as mailboxes for both commands as well as for the data. In order to provide a method to identify RFID transceiver device  100 , 115 , one or two unique words are stored in a certain memory location for identification. The unique words are unique patterns and are different from the EPC or Tag ID (TID). Once RFID reader  200  finds the unique words in specific memory locations, it associates RFID transceiver device  100 , 115  as a special RFID transceiver. Using this approach, the sensor reading or other data processing operation is standards-compliant, and an off-the-shelf standards-compliant reader can be used to read and write the sensor or other data in the higher functionality wireless transponder device. Another embodiment to identify a special type of RFID transceiver device is to store data in a high memory location which is not normally available in a passive RFID transceiver device. If the reader reads this memory location from an off-the-shelf passive RFID transceiver device, it will yield an error code, such as error code: 00000011: memory overrun or unsupported C value. If the reader reads this memory location from a special RFID transceiver device, it will yield a value and no error code. 
         [0057]    In another embodiment, scrambling of the data location is performed at RFID transceiver device  100 , 115 . The address is common for all conventional reader and graphical user interface software, which means that all readers can try to read the information at this address but only a special type of RFID transceiver device  100 , 115  recognizes and translates the special address. Accordingly, RFID reader  200  can identify a special type of RFID transceiver device. The special type of RFID transceiver device  100 , 115  replies with valid information, but other conventional passive RFIDs will reply with error codes because this address does not exist in their memory. 
         [0058]    In another embodiment, a special type of RFID transceiver device  100 , 115  is identified by using a false address which does not exist in any conventional passive RFID transceiver device. The false address could have 4 or 6 bytes, the first two bytes could be the special RFID transceiver device signature. When the special RFID transceiver device  100 , 115  receives this address, it translates this false address into the address where the information is stored. 
         [0059]    In another embodiment, modulation and demodulation is implemented in software in microcontroller  107  to further lower costs. The EPCglobal air interface defines a “Tag to Interrogator” interface using backscatter modulation with or without a subcarrier, and an “Interrogator to tag” interface using double sideband amplitude shift keying (DSB-ASK) with pulse interval (PIE) encoding. The low speed microcontroller with common peripherals, such as a timer and serial peripheral interface (SPI) is used to implement the interfaces, thus providing significant flexibility with low cost. 
         [0060]    Implementing a modulator circuit using microcontroller  107  requires the precise control of software timing. A serial communication interface can be used to output a precise timing modulation signal. The serial communication interface typically provides a buffer for loading multiple bits of data. Typical serial interfaces have buffers in multiple of 8 bits, depending on the depth of the buffer. As long as the buffer is refilled before it is depleted, a continuous and precise timing modulating signal can be output from the serial interface. This embodiment allows a much higher signaling rate to be achieved as compared to a conventional bit-banging technique. It should be noted that the serial interface clock rate can be slaved to external or internal clock sources. An example of such a clock source is a conventional pulse width modulation (PWM) generator which can be used in microcontroller  107 . The use of these clock sources allows the symbol time to be changed, which provides a variable symbol rate. 
         [0061]    For data encoding from the tag to the interrogator without the subcarrier, data- 0  and data- 1  are encoded as shown in  FIG. 4 , where a data- 0  is encoded as positive pulse  401  followed by negative pulse  402 , and a data- 1  is encoded as two positive pulses  403  and  404 . In software, data- 0  is represented as 2 binary bits: 10 and data- 1  is represented by binary bits: 11.  FIG. 5  shows a block diagram of the modulator. Software in microcontroller  107  pre-calculates the binary stream and uses buffer  501  to transmit the binary stream with an 8 bit SPI interface  502  using input signal  114  to the RF modulator in the RF detection and backscattering frontend  103  to transmit without interruption, i.e., independent of interrupt  503  from buffer  501 . For EPCglobal, the highest link frequency is 640 kHz, and thus interrupt  503  of buffer empty will be generated every 4 data bits (8 binary bits) which is 6.25 μs. During this period, microcontroller  107  loads the next 8 bits for transmitting. 
         [0062]    For data encoding from the tag to the interrogator with the subcarrier, the baseband signal is Miller-modulated by a subcarrier. In this embodiment, the reversal of the baseband signals for data- 0  and data- 1 , as in  FIG. 4 , is used where data- 0  is two positive pulses and data- 1  is a positive pulse followed by a negative pulse. The frequency of the subcarrier is 2, 4 or 8 times of the data rate, which corresponds to a Miller rate of 2, 4, and 8.  FIG. 6  shows the waveforms for data- 0  and data- 1  for a Miller rate of 2. A data- 0  is encoded as positive pulse  601 , followed by negative pulse  602 , followed by positive pulse  603 , and followed by negative pulse  604 . Data- 1  is encoded as positive pulse  605 , followed by two negative pulses  606  and  607 , and followed by positive pulse  608 . Accordingly, data- 0  is encoded as 1010 and data- 1  is 1001. 
         [0063]    If there is not enough memory, an alternative embodiment is to add an exclusive- or (XOR) gate to modulate the baseband signal. This has the additional benefit of also slowing down the SPI interrupt rate on interrupt  503 .  FIG. 7  shows the process of modulating data- 1  onto a subcarrier with a Miller rate of 2. Data- 1 , which is positive pulse  701 , followed by negative pulse  702 , is XOR-ed with a subcarrier with positive  703 , negative  704 , positive  705 , and negative  706  pulse sequence, to generate a modulated data- 1  with positive  707 , two negative  708  and  709 , and positive  701  pulse sequence, as shown in  FIG. 6 . 
         [0064]    An embodiment to enable the subcarrier to have the same phase as the data is shown in  FIG. 8 . Since most conventional peripheral timers with a Pulse Width Modulator (PWM) can generate a 50% duty cycle waveform, PWM timer  801  is used as a subcarrier generator for input  802  into XOR  803 , along with SPI data output  804 . SPI clock output  805  is used to interrupt microcontroller  107  for phase information which adjusts the PWM phase to align with SPI data output  804 . For different Miller modulation rates, the PWM frequency is changed. 
         [0065]      FIG. 9  shows interrogator to tag encoding using pulse interval encoding (PIE) modulation. Data- 0  is positive pulse  901  followed by negative pulse  902  of duration PW, with a total duration of Tari. Data- 1  is positive pulse  903  that is longer than a Tari, followed by negative pulse  904  of duration PW, with a total duration of 1.5 to 2 Tari. A Tari could be 6.25 to 25 μs. The difference between data- 0  and data- 1  is the time interval between rising edges of the pulses. 
         [0066]    An embodiment of the data demodulator is shown in  FIG. 10 , which uses a capture function of the hardware timer for accurate measurements. Output signal  113  from RF detection and backscattering frontend  103  is fed into latch register  1001 , which uses timer counter  1002 , and rising edge of output signal  113  is used also as interrupt signal for microcontroller  107 . Upon an interrupt signal, microcontroller  107  reads the latch time from latch register  1001  to decode the data. 
         [0067]    A man-machine interface (MMI) or programming application interface (API) can be provided for the user which can control RFID reader  200  for inventory, sensor data reading, and the configuration of the tag. The preferred embodiment is to include middleware which deals with the lower layer operation of RFID reader  200 , the on-the-air protocol, and retrieving and writing of data to and from specific locations in the tag memory, while presenting a high (application) level interface to the MMI or API. The programming API has similar command and response messages to the command and response in the MMI. The middleware resides in a host device and can communicate with RFID reader  200  using a reader specific API. This allows users to adapt and integrate RFID reader  200  and tags into their operation quickly. This also allows users not to have to deal with different readers having different APIs. One example use is to retrieve the sensor out-of-bound indicators. In this example, a simple API controls RFID reader  200  to find the specific tag (or a number of tags) and retrieve the sensor out-of-bound indicators from a specific memory location. 
         [0068]    It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments, which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.