Method and system for storage and recovery of vital information on radio frequency transponders

[0002] The following patent application is a Continuation In Part application based on and claims the benefit of U.S. patent application Ser. No. 09/227,649 now abandoned, filed Jan. 8, 1999 and Provisional Patent Application Serial No. 60/070,758, filed Jan. 8, 1998, the contents of both are incorporated herein by reference.

[0003] 1. Field of the Invention

[0004] The invention relates to the field of Radio Frequency Transponders and more specifically to retaining and restoring of valid state information by the Radio Frequency Transponders upon the reapplication of power.

[0005] 2. Description of the Background of the Invention

[0006] Radio Frequency (RF) Transponders (tags) are used in a multiplicity of ways. They may be used in locating and identifying accompanying objects, as well as for transmitting information about the state of an object. It has been known since the early 60's that electronic components of transponders could be powered by a sequence of periodic signal bursts sent by a “base station” and received by a tag antenna on each of the transponders.

[0007] The RF electromagnetic field induces an alternating current in the transponder antenna that can be rectified by a RF diode of the transponder, and the rectified current can be used for a power supply for the electronic components of the transponder. The current induced in the transponder antenna from the incoming RF energy would thus be changed, and the change in the alternating current changes or modulates the RF power radiated from the transponder antenna back to the base station. This change in the radiated power from the transponder antenna is picked up by the base station antenna. Thus, the transponder antenna broadcasts a return signal without itself having a self contained power supply.

[0008] The “rebroadcast” of the incoming RF energy is conventionally called “back scattering”, even though the transponder broadcasts the energy in a pattern determined solely by the transponder antenna. Since this type of transponder carries no power supply of its own, it is called a “passive” transponder to distinguish it from a transponder containing a battery or other energy supply, conventionally called an active transponder.

[0009] As an example, consider an RF tag designed to respond equally well to signals at frequencies ranging from 2.425 GHz to 2.475 GHz in an ideal environment. In a less then ideal environment, where there are other RF tags, metal objects, water-filled objects, etc. disposed in close proximity to a particular RF tag that is attempting to communicate with the base station, the RF energy available at a particular frequency to be received by the particular RF tag may be attenuated. This situation is analogous to repositioning a TV or a radio antenna to get the strongest reception. Similar to the effect of the person repositioning the antenna on the reception of the antenna, the presence of other tags and/or objects interferes with the RF reception of a particular tag. Additionally to comply with the FCC regulations, the carrier frequency used by the base station hops over relatively narrow channels of up to 1 MHz wide in the allowed band, e.g., 2.400 to 2.483 GHz in the 2.450 GHz case, during communication.

[0010] When an array of tags is being interrogated by a base station, it is possible for very different field strengths to be available to tags depending on the carrier frequency of that channel being used by the base station at the time of the communication and on the different positions of tags in the array. For instance, a first tag at a first position may be well powered when the base station operates at 2.422 and 2.463 GHz but not at 2.447 GHz, while a second tag at a second different position may be well-powered when the base station operates at 2.463 GHz and 2.447 GHz, but not 2.422 GHz. These differences are related not to the RF tag design but to the instantaneous RF environment of the individual RF tag at the time the interrogation by the base station occurs.

[0011] If power being supplied to the RF tag has been removed for even short time duration, the state information being maintained or stored by the RF tag is lost. For example, when the RF burst powering the RF tag falls off, the tag power, which for passive tags is maintained by a storage capacitor, may be lost in as little as 100 microseconds. The state information of the RF tag is then also lost.

[0012] Losing the state information of the RF tag is particularly injurious when a base station sending a polarized RF is interrogating an array of RF tags having antennas polarized in different manners. When some RF tags may not be powered up by a particular frequency used, the communication protocol will attempt to talk to each tag in the array.

[0013] RF tags may have major and minor states. The major states may include the “ID,” “READY,” and “DATA-EXCHANGE” states. Each RF tag identifies itself to the base station in the “ID” state, lets the base station know that it is ready to transfer data in the “READY” state, and sends data in the “DATA-EXCHANGE” state. The minor states include information such as the counter value used during the identification protocol initiated from the base station.

[0014] When a RF signal burst of a first frequency is sent from the base station to an array of RF tags, some of RF tags in the array do receive sufficient power to operate from that signal burst and will proceed to operate through the stages or states of operation, such as entering the “READY” or “DATA-EXCHANGE” states. The RF tag is typically operated cyclically through those states; in each cycle the states are carried out in the order set out above. Thus if the base station knows in which state a particular tag is operating, it has an effective “book ark” as to where in the cycle this particular tag is operating. When the RF environment changes or when the base station hops to new carrier frequency, some of the tags that were previously powered, will not now receive sufficient power and will no longer be able to operate. At the same time, other RF tags in the array that previously had insufficient power to operate will now become powered up by the RF burst of the new frequency and start working.

[0015] An illustrative cycle of operation of the array of ten RF tags may be described as follows:

[0016] 1. The base station or the reader is on channel one and RF tags 1-8 respond by beginning their participation in the identification protocol. All eight tags are successfully identified.

[0017] 2. The reader now hops to channel two, the frequency of channel 2 allows tags 7-9 to be powered. Tag 9 will now respond by beginning participation in the identification protocol, while tags 1-6 lose their power and therefore stop participating. Since tags 7 and 8 were already identified and continue to be powered sufficiently when operating on channel, they do not participate in the protocol.

[0018] 3. The reader hops to channel 3. The frequency of channel 3 allows tags 2-10 to be powered. Tags 7-9 stay powered and do not participate in the protocol. However, tags 2-6 must be reidentified in order to identify the one truly new tag 10.

[0019] The RF tags that are not well powered lose track of state information. This state information is essentially a bookmark in the communication sequence between the tag and the base station. In running an ID protocol, for example, tags that newly enter the field as well as tags that have lost power and then regained it while remaining in the field are treated equally; they both have to be identified from scratch, wasting time. If state information could be maintained, the tags that remain in the field and are not powered sufficiently even only for brief periods of time would not have to reenter the protocol and thus system level performance with regard to tag identification would be improved.

[0020] What is needed is for the state information of each RF tag to be maintained in order to prevent that RF tag from participating in the protocol with the base station, in identifying previously unidentified tags of a tag array therefore improving system performance.

[0021] The present invention assures the integrity of state information retained by the RF tags during a loss of power. At the time of the regular operation of each RF tag, the power is provided to a voltage-storing device, such as a capacitor, powering an information retention mechanism of the RF tag. The power source for that voltage storing device may be directed through a device such as a diode or a PFET, which may be dedicated or shared with other components of the RF tag.

[0022] After the loss and reestablishing of power to the transponder but before the transponder restarts, the transponder checks the voltage-storing device powering the information retention mechanism to determine whether sufficient power is present to retain information without corruption. Additionally, after the power is restored, the delaying circuitry of the transponder may delay the restart to assure proper determination of whether sufficient power is present. If sufficient power is present, the transponder communicates a signal to indicate that fact, the stored information is restored and the transponder restarts.

[0023] The foregoing objects and advantages of the present invention may be more readily understood by one skilled in the art with reference being had to the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings wherein like elements are designated by identical reference numerals throughout the several views, and in which:

[0024] FIG. 1 is a block diagram showing an exemplary Radio Frequency Transponder circuitry for retaining state information during a period when power is not supplied to the transponder.

[0025] FIG. 2 is a block diagram showing an alternative Radio Frequency Transponder circuitry where separate power sources are provided to power the circuitry and to charge a capacitor for information retention.

[0026] FIG. 3 is a block diagram showing an alternative Radio Frequency Transponder circuitry where a PFET transistor is used.

[0027] FIG. 4 is a block diagram showing a Radio Frequency Transponder circuitry where voltage across a capacitor is measured to determine the state of retained information.

[0028] FIG. 5 is a block diagram showing an exemplary voltage check circuitry, which may be used by the present invention.

[0029] FIGS. 6a-care time/voltage diagrams showing good and bad power levels.

[0030] FIG. 1 of the present invention shows a RF tag 10 comprised of a diode 1, such as a Schottky or another type of diode, for accepting power from the main power line 6. The main power line 6 powers both, a main digital section 3 of the RF tag 10 and a capacitor C_AUX5. The power accepted by the diode 1 is generated by the RF tag antenna 4 in conjunction with a well known power receiving circuit (not shown) when RF energy is received from a base station.

[0031] The capacitor C_AUXis charged to a sufficient voltage to power a mirror latches mechanism 2 which may hold copies of state information of the RF tag 10. At any time during the operation of the RF tag 10, the processing of the main digital section 3 may place or save relevant information in to the mirror latches mechanism 6. This capacitor C_AUXis provided as a power source for the mirror latch mechanism 2 to enable it to retain the essential state indicators during the interval when the power supply to the RF tag is interrupted, for example the base station hops to a new frequency. The power from the capacitor C_AUXis provided to only a few transistors of the RF tag 10 and is not used to power up the clock or other RF tag 10 electronics. Therefore, the amount of required energy is small and may be provided by a relatively small capacitor. Furthermore, when the power is restored or reapplied and the RF tag 10 restarts, the information retained in the mirror latches mechanism 2 may be read back into the main digital section 3 to be used in the subsequent operation of RF tag 10 operation.

[0032] The information to be retained or saved may include both, the major and the minor state indicators. Some minor state information, such as the address from which the base station attempted to access data when the power is removed, need not be saved and may be allowed to be lost with little impact on the communication protocol initiated by the base station. That is because the whole command including the address must be re-sent by the base station.

[0033] FIG. 2 shows another embodiment of the present invention. In this embodiment, the power line 7 for the mirror latches mechanism 2 is independent. The capacitor C_AUXis connected directly to the antenna-lead-in 4 and may illustratively use a Schottky diode 8 to rectify the RF energy received by the antenna which is not shown in the figure.

[0034] In yet another embodiment shown in FIG. 3, a PFET transistor 9 may be used instead of the diode 1 (FIG. 1). The supply of voltage V_DD6 may vary very slowly, so that the transistor 9 may effectively rectify voltage for charging the capacitor C_AUX.

[0035] Turning back to FIG. 1, please note that the size of the capacitor C_AUXand the leakage current therefrom determines the time interval during which the mirror latches mechanism 2 will store a copy of the state information. When the power sent to the RF tag 10 from the antenna 4 is too low, the voltage V_DD6 may fall below a threshold value sufficient to energize the RF tag electronics reliably. In such a situation, a power-no-good signal (POK_), as shown in FIGS. 6a-cis issued by a separate RF tag circuitry to prevent the RF tag 10 from any further processing. Nevertheless, the mirror latches mechanism 2 will maintain the copy of the state information, for a period time initiated when the RF tag is powered up and continuing as long as it takes for the leakage current to drain the capacitor C_AUX5 through the mirror latches mechanism 2. Please note that when a RF signal burst of a different or second frequency is sent from the base station, other RF tags 10 in the array will be powered up. Each RF tag may be energized by the RF signal bursts of different frequencies see the example described in the Background of the invention section. Hence, some RF tags from a first set may be powered up, by the RF signal bursts of frequencies other then the first frequency and will attempt to change their state.

[0036] As explained, the inventive RF tag 10 is constructed to retain the state information that was stored in the mirror latches mechanism 2 during a first application of the RF signal burst at a frequency suitable to power up that RF tag 10. The RF tag 10 will be powered until the base station performs a frequency hop to a frequency, which may or may not be suitable to power this particular RF tag 10 depending upon the RF environment of the tag array. The state information stored in the mirror latches mechanism 2 is retained until the base station, at a later time, reapplies the RF signal burst suitable to re-power up the RF tag 10 to the set threshold level. When the RF tag 10 is re-powered, it is permitted to down load and use the state information retained in the mirror latches mechanism 2. So, when the RF tag 10 is re-powered, the state information maintained in the mirror latches mechanism 2 is read and used to reset the states of the RF tag 10. However before the state information is used, the voltage across C_AUXmust be checked to ascertain that it has not fallen to a threshold level where the information maintained in the mirror latches 2 is no longer trustworthy.

[0037] FIG. 4 shows a power-no-good signal 45 applied as an input to a flip flop circuit 41, another input to that flip flop circuit 41 being the output of a check circuit 42. When the power-no-good signal 45 falls while the V_DDon line 6 is sufficient to power the RF tag 40, the check circuit 42 determines if the voltage V_AUXacross the capacitor C_AUX5 is high enough with respect to the value of the voltage V_DD. Thereby if the determined voltage is high enough, a signal 43 is outputted to permit the use of the information stored in the mirror latches 2. If on the other hand, the voltage level of the capacitor C_AUXis determined to be less then the threshold, the integrity of the information contained in the mirror latches mechanism 2 is considered suspect and is not used. An example of a check circuit 42 for checking the voltage V_AUXacross the capacitor C_AUXis shown in FIG. 5.

[0038] FIG. 6ashows the voltage V_DDas a function of time during the energizing of the RF tag by a single burst of RF energy. When the RF burst is broadcast to the RF tag 10 (FIG. 4), the power supply V_DD(FIG. 4) starts to charge the capacitor C_AUX(FIG. 4). At time T₁the voltage may charge the capacitor C_power(FIG. 2) to a sufficiently high voltage to render the RF tag 10 operative, and the power-no-good signal POK_will be removed. This signal POK_is issued in the region 64 shown in FIG. 6b.to tell the electronic components of the RF tag 40 (FIG. 4) that there is insufficient power for reliable operation. When V_DDreaches the threshold level for stable operation at T₁, the power-no-good signal POK_falls and a power-good signal POK comes on as shown in FIG. 6c.

[0039] Returning now to FIG. 4, the power-no-good POK_signal 45 is used to prevent C_AUXfrom charging when the voltage V_DDon line 6 is falling after the power burst has been applied and when the voltage V_DDon line 6 is increasing as represented by region 64 (FIG. 6b). When the power-no-good signal POK_45 falls at T₁(FIG. 6a), the transistor 1 is rendered conductive to apply current to recharge C_AUX. Furthermore, to assure that the voltage V_AUX46 across the capacitor C_AUXis not changing while the decision is being made as to whether to use the information in the mirror latches mechanism 2 powered by the capacitor C_AUX, an optional delay circuit 44 may be used to assure that the period allocated to decision making is long enough.

[0040] While the invention has been particularly shown and described with respect to illustrative and preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention that should be limited only by the scope of the appended claims.