Abstract:
RFID tags and chips for RFID tags are capable of complying with only a limited number of remaining commands, and methods. In a number of embodiments, a counter is adjusted in association with receiving a command and complying with it. The tag complies until the counter reaches a limit, and then it can stop complying. Non-compliance can be by the tag becoming quiet, or performing other activities, or performing nothing at all, or performing only selected activities, and so on. In some embodiments non-compliance is permanent, while in others it is temporary and/or restorable.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S.A. Provisional Application Ser. No. 60/902,746, filed on Feb. 21, 2007, the disclosure of which is hereby incorporated by reference for all purposes. 
         [0002]    This application claims priority from U.S.A. Provisional Application Ser. No. 60/933,222, filed on Jun. 5, 2007, the disclosure of which is hereby incorporated by reference for all purposes. 
         [0003]    This application may be found to be related to U.S. patent application Ser. No. ______ by the same inventors, titled “CAUSING RFID TAG TO CHANGE HOW MANY REMAINING COMMANDS IT WILL COMPLY WITH”, filed by the same assignee on the same day as the instant application. 
     
    
     FIELD OF THE INVENTION 
       [0004]    The present description is about Radio Frequency IDentification (RFID) systems and more specifically for systems where RFID tags reply to readers only a limited number of times. 
       BACKGROUND 
       [0005]    Radio Frequency IDentification (RFID) systems typically include RFID tags and RFID readers. RFID readers are also known as RFID reader/writers or RFID interrogators. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package. 
         [0006]    In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be predominantly electric or magnetic in the near field. 
         [0007]    A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways. 
         [0008]    The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on. 
         [0009]    An RFID tag typically includes an antenna system, a radio section, a power management section, and frequently a logical section, a memory, or both. In earlier RFID tags, the power management section included an energy storage device, such as a battery. RFID tags with an energy storage device are known as active or semi-active tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device, and are called passive tags. 
         [0010]    It is desired to have RFID systems with additional capabilities, for improved functionality. For example, there are concerns that tags will be counterfeited without authorization. In addition, there are security concerns where tags have been attacked, so as to unravel their on-board passwords bit-by-bit. 
       BRIEF SUMMARY 
       [0011]    The invention improves over the prior art. 
         [0012]    Briefly, the present invention provides RFID tags and chips for RFID tags that are capable of complying with only a limited number of remaining commands, and methods. In a number of embodiments, a counter is adjusted in association with receiving a command and complying with it. The tag complies until the counter reaches a limit, and then it can stop complying. 
         [0013]    The present invention further provides RFID reader systems, readers, components, software and methods for causing such RFID tags to change how many remaining commands they will comply with. In a number of embodiments, this is achieved by causing a custom limiting command to be transmitted to a tag. 
         [0014]    RFID tags according to embodiments can thus become non-compliant. Non-compliance can be by the tags becoming quiet, or performing other activities, or performing nothing at all, or performing only selected activities, and so on. In some embodiments non-compliance is permanent, while in others it is temporary and/or restorable. 
         [0015]    An advantage of the invention is that this non-compliance can help resist having a tag read successively, by those who would unravel its password bit-by-bit. For example, tags can be made with a very low remaining number of subsequent commands that will be complied with, which will hamper the efforts of tag counterfeiters. 
         [0016]    These and other features and advantages of the invention will be better understood from the specification of the invention, which includes the following Detailed Description and accompanying Drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The following Detailed Description proceeds with reference to the accompanying Drawings, in which: 
           [0018]      FIG. 1  is a block diagram of components of an RFID system. 
           [0019]      FIG. 2  is a diagram showing components of a passive RFID tag, such as a tag that can be used in the system of  FIG. 1 . 
           [0020]      FIG. 3  is a conceptual diagram for explaining a half-duplex mode of communication between the components of the RFID system of  FIG. 1 . 
           [0021]      FIG. 4  is a block diagram of an implementation of an electrical circuit formed in an IC of the tag of  FIG. 2 . 
           [0022]      FIG. 5  is a flowchart illustrating methods for a tag according to embodiments. 
           [0023]      FIG. 6  is a flowchart illustrating methods for a tag according to alternate embodiments. 
           [0024]      FIG. 7A  is a diagram showing that an RFID tag can stop complying after receiving a limited number of commands according to embodiments. 
           [0025]      FIG. 7B  is a diagram showing that an RFID tag can stop complying after receiving a limited number of commands according to additional embodiments. 
           [0026]      FIG. 7C  is a diagram showing that an RFID tag can stop complying after receiving a limited number of commands according to yet other embodiments. 
           [0027]      FIG. 7D  is a diagram showing that an RFID tag can stop complying after receiving a limited number of commands, and then resume the pattern after non-compliance for another limited number of commands, according to embodiments. 
           [0028]      FIG. 7E  is a diagram showing that an RFID tag can stop complying after receiving a limited number of commands, and then resume the pattern after it receives a Restore command, according to embodiments. 
           [0029]      FIG. 7F  is a diagram showing that an RFID tag can stop complying after receiving a limited number of commands, but where the limit is changed by a Reset command according to embodiments. 
           [0030]      FIG. 7G  is a diagram showing that an RFID tag can start reaching its limit of compliance, but then the feature becomes disabled according to embodiments. 
           [0031]      FIG. 7H  is a diagram showing that an RFID tag can start by complying with an indefinite number of commands, but then transition to where it would stop complying beyond a limited number of subsequently received commands, according to embodiments. 
           [0032]      FIG. 8  is a block diagram showing a detail of an RFID reader system according to embodiments. 
           [0033]      FIG. 9  is a block diagram of a whole RFID reader system according to embodiments. 
           [0034]      FIG. 10  is a block diagram illustrating an overall architecture of an RFID reader system according to embodiments. 
           [0035]      FIG. 11  is a flowchart illustrating methods for an RFID reader system component according to embodiments. 
           [0036]      FIG. 12A  is a table illustrating the fields of the Select command of the Gen2 Spec version 1.1.0, versions of which may be used as any one or more of the custom limiting commands of  FIG. 11  according to embodiments. 
           [0037]      FIG. 12B  is a table illustrating how a number of custom commands can be enabled in a reader and a tag. 
           [0038]      FIG. 12C  is a table showing sample values that can be used for the table of  FIG. 12B . 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    The present invention is now described. While it is disclosed in its preferred form, the specific embodiments of the invention as disclosed herein and illustrated in the drawings are not to be considered in a limiting sense. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Indeed, it should be readily apparent in view of the present description that the invention may be modified in numerous ways. Among other things, the present invention may be embodied as devices, methods, software, and so on. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment, an entirely firmware embodiment, or an embodiment combining aspects of the above. This description is, therefore, not to be taken in a limiting sense. The invention is now described in more detail. 
         [0040]      FIG. 1  is a diagram of components of a typical RFID system  100 , incorporating aspects of the invention. An RFID reader  110  transmits an interrogating Radio Frequency (RF) wave  112 . RFID tag  120  in the vicinity of RFID reader  110  may sense interrogating RF wave  112 , and generate wave  126  in response. RFID reader  110  senses and interprets wave  126 . 
         [0041]    Reader  110  and tag  120  exchange data via wave  112  and wave  126 . In a session of such an exchange, each encodes, modulates, and transmits data to the other, and each receives, demodulates, and decodes data from the other. The data is modulated onto, and demodulated from, RF waveforms. 
         [0042]    Encoding the data in waveforms can be performed in a number of different ways. For example, protocols are devised to communicate in terms of symbols, also called RFID symbols. A symbol for communicating can be a delimiter, a calibration symbol, and so on. Further symbols can be implemented for ultimately exchanging binary data, such as “0” and “1”, if that is desired. In turn, when the waveforms are processed internally by reader  110  and tag  120 , they can be equivalently considered and treated as numbers having corresponding values, and so on. 
         [0043]    Tag  120  can be a passive tag or an active or semi-active tag, i.e. having its own power source. Where tag  120  is a passive tag, it is powered from wave  112 . 
         [0044]      FIG. 2  is a diagram of an RFID tag  220 , which can be the same as tag  120  of  FIG. 1 . Tag  220  is implemented as a passive tag, meaning it does not have its own power source. Much of what is described in this document, however, applies also to active tags. 
         [0045]    Tag  220  is formed on a substantially planar inlay  222 , which can be made in many ways known in the art. Tag  220  includes an electrical circuit, which is preferably implemented in an integrated circuit (IC)  224 . IC  224  is arranged on inlay  222 . 
         [0046]    Tag  220  also includes an antenna for exchanging wireless signals with its environment. The antenna is usually flat and attached to inlay  222 . IC  224  is electrically coupled to the antenna via suitable antenna ports (not shown in  FIG. 2 ). 
         [0047]    The antenna may be made in a number of ways, as is well known in the art. In the example of  FIG. 2 , the antenna is made from two distinct antenna segments  227 , which are shown here forming a dipole. Many other embodiments are possible, using any number of antenna segments. 
         [0048]    In some embodiments, an antenna can be made with even a single segment. Different points of the segment can be coupled to one or more of the antenna ports of IC  224 . For example, the antenna can form a single loop, with its ends coupled to the ports. It should be remembered that, when the single segment has more complex shapes, even a single segment could behave like multiple segments, at the frequencies of RFID wireless communication. 
         [0049]    In operation, a signal is received by the antenna, and communicated to IC  224 . IC  224  both harvests power, and responds if appropriate, based on the incoming signal and its internal state. In order to respond by replying, IC  224  modulates the reflectance of the antenna, which generates the backscatter from a wave transmitted by the reader. Coupling together and uncoupling the antenna ports of IC  224  can modulate the reflectance, as can a variety of other means. 
         [0050]    In the embodiment of  FIG. 2 , antenna segments  227  are separate from IC  224 . In other embodiments, antenna segments may alternately be formed on IC  224 , and so on. 
         [0051]    The components of the RFID system of  FIG. 1  may communicate with each other in any number of modes. One such mode is called full duplex. Another such mode is called half-duplex, and is described below. 
         [0052]      FIG. 3  is a conceptual diagram  300  for explaining the half-duplex mode of communication between the components of the RFID system of  FIG. 1 , especially when tag  120  is implemented as passive tag  220  of  FIG. 2 . The explanation is made with reference to a TIME axis, and also to a human metaphor of “talking” and “listening”. The actual technical implementations for “talking” and “listening” are now described. 
         [0053]    RFID reader  110  and RFID tag  120  talk and listen to each other by taking turns. As seen on axis TIME, when reader  110  talks to tag  120  the communication session is designated as “R→T”, and when tag  120  talks to reader  110  the communication session is designated as “T→R”. Along the TIME axis, a sample R→T communication session occurs during a time interval  312 , and a following sample T→R communication session occurs during a time interval  326 . Of course interval  312  is typically of a different duration than interval  326 —here the durations are shown approximately equal only for purposes of illustration. 
         [0054]    According to blocks  332  and  336 , RFID reader  110  talks during interval  312 , and listens during interval  326 . According to blocks  342  and  346 , RFID tag  120  listens while reader  110  talks (during interval  312 ), and talks while reader  110  listens (during interval  326 ). 
         [0055]    In terms of actual technical behavior, during interval  312 , reader  110  talks to tag  120  as follows. According to block  352 , reader  110  transmits wave  112 , which was first described in  FIG. 1 . At the same time, according to block  362 , tag  120  receives wave  112  and processes it, to extract data and so on. Meanwhile, according to block  372 , tag  120  does not backscatter with its antenna, and according to block  382 , reader  110  has no wave to receive from tag  120 . 
         [0056]    During interval  326 , tag  120  talks to reader  110  as follows. According to block  356 , reader  110  transmits a Continuous Wave (CW), which can be thought of as a carrier signal that ideally encodes no information. As discussed before, this carrier signal serves both to be harvested by tag  120  for its own internal power needs, and also as a wave that tag  120  can backscatter. Indeed, during interval  326 , according to block  366 , tag  120  does not receive a signal for processing. Instead, according to block  376 , tag  120  modulates the CW emitted according to block  356 , so as to generate backscatter wave  126 . Concurrently, according to block  386 , reader  110  receives backscatter wave  126  and processes it. 
         [0057]    In the above, an RFID reader/interrogator may communicate with one or more RFID tags in any number of ways. Some such ways are called protocols. A protocol is a specification that calls for specific manners of signaling between the reader and the tags. 
         [0058]    One such protocol is called the Specification for RFID Air Interface—EPC™ Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz-960 MHz, which is also colloquially known as “the Gen2 Spec”. The Gen2 Spec has been ratified by EPCglobal, which is an organization that maintains a website at: &lt;http://www.epcglobalinc.org/&gt; at the time this document is initially filed with the USPTO. Version 1.1.0 of the Gen2 Spec is hereby incorporated by reference in its entirety. 
         [0059]    In addition, a protocol can be a variant of a stated specification such as the Gen2 Spec, for example including fewer or additional commands than the stated specification calls for, and so on. In such instances, additional commands are sometimes called custom commands. 
         [0060]      FIG. 4  is a block diagram of an electrical circuit  424  according to embodiments. Circuit  424  may be formed in an IC of an RFID tag, such as IC  224  of  FIG. 2 . Circuit  424  has a number of main components that are described in this document. Circuit  424  may have a number of additional components from what is shown and described, or different components, depending on the exact implementation. 
         [0061]    Circuit  424  includes at least two antenna connections  432 ,  433 , which are suitable for coupling to one or more antenna segments (not shown in  FIG. 4 ). Antenna connections  432 ,  433  may be made in any suitable way, such as using pads and so on. In a number of embodiments more than two antenna connections are used, especially in embodiments where more antenna segments are used. 
         [0062]    Circuit  424  includes a section  435 . Section  435  may be implemented as shown, for example as a group of nodes for proper routing of signals. In some embodiments, section  435  may be implemented otherwise, for example to include a receive/transmit switch that can route a signal, and so on. 
         [0063]    Circuit  424  also includes a Power Management Unit (PMU)  441 . PMU  441  may be implemented in any way known in the art, for harvesting raw RF power received via antenna connections  432 ,  433 . In some embodiments, PMU  441  includes at least one rectifier, and so on. 
         [0064]    In operation, an RF wave received via antenna connections  432 ,  433  is received by PMU  441 , which in turn generates power for components of circuit  424 . This is true for either or both R→T and T→R sessions, whether or not the received RF wave is modulated. 
         [0065]    Circuit  424  additionally includes a demodulator  442 . Demodulator  442  demodulates an RF signal received via antenna connections  432 ,  433 . Demodulator  442  may be implemented in any way known in the art, for example including an attenuator stage, an amplifier stage, and so on. 
         [0066]    Circuit  424  further includes a processing block  444 . Processing block  444  receives the demodulated signal from demodulator  442 , and may perform operations. In addition, it may generate an output signal for transmission. 
         [0067]    Processing block  444  may be implemented in any way known in the art. For example, processing block  444  may include a number of components, such as a processor, memory, a decoder, an encoder, and so on. It may also include a counter  447 , as is described later in this document. 
         [0068]    In a number of embodiments, processing block  444  includes a state machine  448 . State machine  448  retains the state of the tag, at least while circuit  424  is powered. The state of the tag dictates which of the subsequently received commands the tag would respond to and how, and so on. State machine  448  can be as is called for in the specified communications protocol, and adapted to further accommodate a custom limiting command according to embodiments, with or without contradicting the operation of the protocol. 
         [0069]    Circuit  424  additionally includes a modulator  446 . Modulator  446  modulates an output signal generated by processing block  444 . The modulated signal is transmitted by driving antenna connections  432 ,  433 , and therefore driving the load presented by the coupled antenna segment or segments. Modulator  446  may be implemented in any way known in the art, for example including a driver stage, amplifier stage, and so on. 
         [0070]    In one embodiment, demodulator  442  and modulator  446  may be combined in a single transceiver circuit. In another embodiment, modulator  446  may include a backscatter transmitter or an active transmitter. In yet other embodiments, demodulator  442  and modulator  446  are part of processing block  444 . 
         [0071]    Circuit  424  additionally includes a memory  450 , which stores data  452 . Memory  450  is preferably implemented as a Nonvolatile Memory (NVM), which means that data  452  is retained even when circuit  424  does not have power, as is frequently the case for a passive RFID tag. 
         [0072]      FIG. 5  is flowchart  500  illustrating methods according to embodiments. The methods of flowchart  500  may be practiced by different embodiments, including but not limited to RFID tag  220 , chip  224 , circuit  424 , and so on. 
         [0073]    At operation  510 , a first command is received wirelessly from an RFID reader via an antenna. According to the communication protocol in use, this first command calls for the tag to comply by performing a first operation, or a set of operations that include the first operation. Examples of such operations are provided later in this document. 
         [0074]    At optional next operation  530 , it is determined whether a counter has reached a preset limit. If not, then at next operation  540 , the first command is not complied with. This non-compliance can be by the first operation not being performed, and possibly others of the operations not being performed. For example, the first operation could be that a reply is backscattered, but that does not happen. That reply could be a specific reply, or a randomly generated reply. 
         [0075]    At an optional further operation  545 , an Out_Of_Cycles reply can be backscattered to the first command. The Out_Of_Cycles reply indicates that the tag no longer responds. The Out_Of_Cycles reply can be backscattered every time, only some of the times, only the first time, and so on. 
         [0076]    If the counter had not reached its limit, then at next operation  560 , the first command is complied with. In other words, the tag performs the first operation, and any other operations mandated by the first command. 
         [0077]    At next operation  570 , the counter is adjusted, to advance towards the limit. At this point it will be appreciated that the counter and the limit are merely a mechanism for limiting how many more commands the tag will comply with. Beyond that, the tag is out of cycles, as will be described in more detail. 
         [0078]    The counter can be implemented in any number of ways. For example, the counter can be adjusted by being incremented up to a limit, or being decremented down to a limit. In some embodiments, the counter can start with a positive value, and be adjusted by being decremented down to a limit of zero, which is also the example that will be used later in this document. The counter can be a single counter. Or there can be a combination of two or more counters, with equal or different coefficients. One or more of these counters can be adjusted according to a deterministic process. Or according to a non-deterministic process, instead. For example, the counter can permit only one decrement, but that has to happen according to a condition that can be met at random, and so on. In addition, the counter can be implemented in any number of ways that are equivalent to the above, as will be evident to a person skilled in the art in view of the present description. 
         [0079]    Any one of the adjustments of operation  570  can cause the counter to reach the limit, in which case the decision at operation  530  could be different. Once the limit is reached, a flag can be set, or a state machine can transition to a different state. Accordingly, operation  530  can be performed by checking the flag or a current state of the state machine. 
         [0080]    In some embodiments, the first command includes an attempted password, as specified by the protocol. Examples include the Kill command, which can kill a tag, or the Access command, which can access more sensitive functions of the tag. Such commands include an attempted password, which the tag must deem valid before it complies with the command. 
         [0081]    An optional operation  580  can be performed within flowchart  500 . If it is determined that the attempted password is valid, flowchart  500  can be performed as before. But if it is determined that the attempted password is not valid, then the counter can be further adjusted, as a penalty. Operation  580  is useful if someone without authorization is trying to gain access to the sensitive functions of the tag, and lacks the password that a legitimate owner would have for their protection. 
         [0082]    The validity or not of the attempted password can be determined by checking the received bits of the attempted password as they correspond to respective bits of a password stored in tag memory. In addition, the counter can be adjusted by an amount in relation to how many of the attempted password bits differ from their corresponding bits of the first password. This way someone who misses only one bit, e.g. due to interference, would be penalized by less than someone who attempts a password at random. 
         [0083]    Operations  510 ,  540 ,  560 , and  570  can be implemented in different orders according to embodiments. One such example is shown in  FIG. 6 , as flowchart  600 . 
         [0084]    The operations of flowchart  500 ,  600 , and others, result in the tag having, in some embodiments only a limited number of remaining commands it will comply with. Many examples are now described, which tags according to embodiment may implement individually or in combination. 
         [0085]      FIG. 7A  is a diagram  710 , contrasting behaviors a RFID tags. In the horizontal axis there is shown a number of commands received sequentially from one or more RFID readers. In the vertical axis there is shown the remaining number of command compliances. 
         [0086]    A horizontal behavior line  712  shows what a prior art tag does. Namely, according to a comment  714 , a tag replies to all commands, as long as they are not themselves disabling commands like the Kill command. The number of remaining compliances is independent of how many commands are received, and the tag can comply indefinitely. Here the sign of infinity is used as the intercept of behavior line  712 , to denote that the number of commands could be a very large one, without compliance ever stopping. 
         [0087]    A behavior line  716  shows a behavior of a sample tag made according to embodiments. The tag starts with an initial number of remaining compliances NC, before having received any commands. With each command that is then received and complied with, behavior line  716  is decremented by one. Decrementing may take place using the above described counter. After a limited number of commands NF 1 , behavior line  716  drops down to zero. According to a comment  718 , the tag then stops complying with commands received after that. 
         [0088]    In the example of  FIG. 7A , behavior line  716  is decremented by one for every command received. This is a specific example of a case where the counter is adjusted by the same amount for all commands. As such, NF 1  is equal to NC, or a number very close to it. This need not be the case. For example, different commands can result in different adjustments. Two examples are now given. 
         [0089]      FIG. 7B  is a diagram  720 , showing a behavior line  726  of a sample tag made according to embodiments. Behavior line  726  starts from the same initial number of remaining compliances NC as behavior line  716 . According to a comment  728 , however, not all commands are counted against the limit, and thus the counter is adjusted only for some commands. For example, the only commands that are counted can be those where the tag has been singulated, a stored code is read, or passwords are attempted. As such, the final number is NF 2 , which could be larger than NC. 
         [0090]      FIG. 7C  is a diagram  730 , showing a behavior line  736  of a sample tag made according to embodiments. Behavior line  736  starts from the same initial number of remaining compliances NC as behavior line  716 . According to a comment  738 , however, some commands are counted for more against the limit than other commands, and thus the counter is adjusted differently between the commands it receives. An example for penalties to be heavier is where the tag has been singulated from a population of tags. Another example has already been given above, where penalties can be heavier if an attempted password has incorrect bits. As such, the final number is NF 3 , and the tag could comply with fewer commands than even NC. 
         [0091]    In some instances of the above examples, when a tag receives a first command shown as an intercept in the horizontal axis, either it complies with it and adjusts the counter, or it does not comply with it, depending on whether the counter had reached the limit by prior such adjusting. 
         [0092]    As will be seen in the examples of behavior lines  716 ,  726 ,  736 , lack of compliance can continue for at least some more of the commands received afterwards. This lack of compliance can be specific to some commands, or indiscriminately to all subsequently received commands. 
         [0093]    In a number of embodiments, not complying can include that the tag no longer performs the first operation, even when called for by subsequent commands. That first operation could be backscattering a reply, transitioning to a certain state, and so on. But it could perform other operations, or comply with other commands. 
         [0094]    In a number of embodiments, not complying can include that the tag no longer complies with subsequent commands, even if they are different than the first command, and/or call for an operation to be performed that is different than the first operation. In some of these embodiments, the tag has been killed. In others, this lack of compliance is temporary, and a later received command is indeed complied with, even if it is the same as the first command. 
         [0095]    The temporary lack of compliance can be implemented by resetting the counter to an updated value, or temporarily reversing how the counter counts, or equivalently using a different counter, and so on. This updated value could be determined in any number of ways. For example, it could be determined at random, or from a preset initial value stored in tag memory. 
         [0096]    In some embodiments, the counter is reset because it has reached the limit, and then a suitable intervening number of commands have since then been received. An example is now described. 
         [0097]      FIG. 7D  is a diagram  740 , showing a behavior line  746  of a sample tag made according to embodiments. Behavior line  746  starts from the same initial number of remaining compliances NC as behavior line  716 . After a number NF 4  of commands, it has reached the non-compliance stage, and no longer complies with further received commands as per the above. After a few more received intervening commands, however, at a restarting number NS 4 , behavior line  746  jumps back up to NC, from where it continues declining. In other words, according to comment  748 , compliance has only paused complying for a certain number of commands, here at least between NF 4  and NS 4 . Behavior line  746  continues back down to zero, at which time compliance pauses again, and then it is restored, and so on. 
         [0098]    In behavior line  746 , the pause can be implemented by the tag. For example, the tag can include a pause counter, counter, after which the first counter is reset to the limit NC. 
         [0099]    In other embodiments, compliance stops, and can be restored by the reader, instead. An example is now described. 
         [0100]      FIG. 7E  is a diagram  750 , showing a behavior line  756  of a sample tag made according to embodiments. Behavior line  756  starts from the same initial number of remaining compliances NC as behavior line  716 . After a number NF 5  of commands, it has reached the non-compliance stage, and no longer complies with further received commands as per the above. In fact, it might give the Out_Of_Cycles reply, in some optional embodiments. The, according to comment  758 , a Restore command is received, at a command number CLV. This resets the counter, as seen by behavior line  756 . Some of the subsequently received commands are then complied with, as per the above. 
         [0101]    There are a number of ways of implementing the Restore command. For example, it may be implemented with a restore password, and be obeyed only if it also encodes the valid restore password. The restore password can be a separate password, or derived from other passwords stored on the tag such as an access password or a kill password. In addition, the Restore command can include a value from which the updated value is determined for resetting the counter. Other such ways are described later in this document. 
         [0102]    In some embodiments, the counter is reset responsive to receiving a reset command. Importantly, a Reset command, can be implemented before the tag has reached the end of its compliance. An example is now described. 
         [0000]      FIG. 7F  is a diagram  760 , showing a behavior line  766  of a sample tag made according to embodiments. Behavior line  766  starts from initial number of remaining compliances NC 1 . As commands are received, behavior line  766  declines, showing the declining remaining number of available compliances. At command number CLR, a Reset command is received. According to comment  768 , the counter is reset to a different value NC 2 , and therefore some of the subsequently received commands are then complied with, as per the above. Value NC 2  can be determined from the Reset command. 
         [0103]    In some embodiments, not only the counter is reset, but also the manner of how the counter is adjusted. As will be seen in the example of  FIG. 7F , behavior line  766  starts with the profile of behavior line  716 , but it continues with the profile of behavior line  726 , responsive to the Reset command. 
         [0104]    In some optional embodiments, an Inquire command is received by the tag, as part of a custom limiting command subset. If the counter had not been reached the limit, a reply can be backscattered that indicates a state of the counter with respect to the limit. This way a reader will know when to send the Reset command for maximum effect. 
         [0105]    In some embodiments, the whole feature of limiting the number of compliances is disabled. An example is now described. 
         [0106]      FIG. 7G  is a diagram  770 , showing a behavior line  776  of a sample tag made according to embodiments. Behavior line  776  starts from initial number of remaining compliances NC. As commands are received, behavior line  776  declines, showing the declining remaining number of available compliances. According to comment  778 , at command number CLD, a Disable command is received. Then behavior line  772  coincides with behavior line  712  of  FIG. 7A , performing all commands without limitation as per the above. 
         [0107]    Disabling the feature can be implemented in any number of ways. For example, the counter can be disabled, or be no longer adjusted, or no longer be paid any attention, as to whether it reached or exceeded the limit, and so on. 
         [0108]    In some embodiments, a tag starts without the feature of limiting the number of compliances, but then that feature is enabled. An example is now described. 
         [0109]      FIG. 7H  is a diagram  780 , showing a behavior line  786  of a sample tag made according to embodiments. Behavior line  786  starts from coinciding with behavior line  712  of  FIG. 7A , performing all commands without limitation as per the above. According to comment  788 , at command number CLE, an Enable command is received. Then behavior line  782  jumps down to number NC, and from then on it declines, showing the declining remaining number of available compliances. Enabling the feature can be implemented in any number of ways, for example the inverse of the ways described with reference to  FIG. 7G . 
         [0110]      FIG. 8  is a block diagram showing a detail of an RFID reader system  810 , which can be the same as reader  110  shown in  FIG. 1 . A unit  820  is also known as a box  820 , and has at least one antenna driver  830 . In some embodiments it has four drivers  830 . For each driver  830  there is an output device for a connector. The output device is typically a coaxial cable plug. Accordingly connectors  835  can be attached to the output devices of the provided respective drivers  830 , and then connectors  835  can be attached to respective antennas  840 . 
         [0111]    A driver  830  can send to its respective antenna  840  a driving signal that is in the RF range, which is why connector  835  is typically but not necessarily a coaxial cable. The driving signal causes the antenna  840  to transmit an RF wave  812 , which is analogous to RF wave  112  of  FIG. 1 . In addition, RF wave  826  can be backscattered from the RFID tags, analogous to RF wave  126  of  FIG. 1 . Backscattered RF wave  826  then ultimately becomes a signal sensed by unit  820 . 
         [0112]    Unit  820  also has other components  850 , such as hardware and/or software and/or firmware, which may be described in more detail later in this document. Components  850  control drivers  830 , and as such cause RF wave  812  to be transmitted, and the sensed backscattered RF wave  826  to be interpreted. Optionally and preferably there is a communication link  825  to other equipment, such as computers and the like, for remote operation of system  810 . 
         [0113]      FIG. 9  is a block diagram of a whole RFID reader system  900  according to embodiments. System  900  includes a local block  910 , and optionally remote components  970 . Local block  910  and remote components  970  can be implemented in any number of ways. It will be recognized that reader  110  of  FIG. 1  is the same as local block  910 , if remote components  970  are not provided. Alternately, reader  110  can be implemented instead by system  900 , of which only the local block  910  is shown in  FIG. 1 . Plus, local block  910  can be unit  420  of  FIG. 4 . 
         [0114]    Local block  910  is responsible for communicating with the tags. Local block  910  includes a block  951  of an antenna and a driver of the antenna for communicating with the tags. Some readers, like that shown in local block  910 , contain a single antenna and driver. Some readers contain multiple antennas and drivers and a method to switch signals among them, including sometimes using different antennas for transmitting and for receiving. And some readers contain multiple antennas and drivers that can operate simultaneously. A demodulator/decoder block  953  demodulates and decodes backscattered waves received from the tags via antenna block  951 . Modulator/encoder block  954  encodes and modulates an RF wave that is to be transmitted to the tags via antenna block  951 . 
         [0115]    Local block  910  additionally includes an optional local processor  956 . Processor  956  may be implemented in any number of ways known in the art. Such ways include, by way of examples and not of limitation, digital and/or analog processors such as microprocessors and digital-signal processors (DSPs); controllers such as microcontrollers; software running in a machine such as a general purpose computer; programmable circuits such as Field Programmable Gate Arrays (FPGAs), Field-Programmable Analog Arrays (FPAAs), Programmable Logic Devices (PLDs), Application Specific Integrated Circuits (ASIC), any combination of one or more of these; and so on. In some cases some or all of the decoding function in block  953 , the encoding function in block  954 , or both, may be performed instead by processor  956 . 
         [0116]    Local block  910  additionally includes an optional local memory  957 . Memory  957  may be implemented in any number of ways known in the art. Such ways include, by way of examples and not of limitation, nonvolatile memories (NVM), read-only memories (ROM), random access memories (RAM), any combination of one or more of these, and so on. Memory  957 , if provided, can include programs for processor  956  to run, if provided. 
         [0117]    In some embodiments, memory  957  stores data read from tags, or data to be written to tags, such as Electronic Product Codes (EPCs), Tag Identifiers (TIDs) and other data. Memory  957  can also include reference data that is to be compared to the EPC codes, instructions and/or rules for how to encode commands for the tags, modes for controlling antenna  951 , and so on. In some of these embodiments, local memory  957  is provided as a database. 
         [0118]    Some components of local block  910  typically treat the data as analog, such as the antenna/driver block  951 . Other components such as memory  957  typically treat the data as digital. At some point there is a conversion between analog and digital. Based on where this conversion occurs, a whole reader may be characterized as “analog” or “digital”, but most readers contain a mix of analog and digital functionality. 
         [0119]    If remote components  970  are indeed provided, they are coupled to local block  910  via an electronic communications network  980 . Network  980  can be a Local Area Network (LAN), a Metropolitan Area Network (MAN), a Wide Area Network (WAN), a network of networks such as the internet, or a mere local communication link, such as a USB, PCI, and so on. In turn, local block  910  then includes a local network connection  959  for communicating with network  980 . 
         [0120]    There can be one or more remote component(s)  970 . If more than one, they can be located at the same location, or in different locations. They can access each other and local block  910  via network  980 , or via other similar networks, and so on. Accordingly, remote component(s)  970  can use respective remote network connections. Only one such remote network connection  979  is shown, which is similar to local network connection  959 , etc. 
         [0121]    Remote component(s)  970  can also include a remote processor  976 . Processor  976  can be made in any way known in the art, such as was described with reference to local processor  956 . 
         [0122]    Remote component(s)  970  can also include a remote memory  977 . Memory  977  can be made in any way known in the art, such as was described with reference to local memory  957 . Memory  977  may include a local database, and a different database of a Standards Organization, such as one that can reference EPCs. 
         [0123]    Of the above-described elements, it is advantageous to consider a combination of these components, designated as operational processing block  990 . Block  990  includes those that are provided of the following: local processor  956 , remote processor  976 , local network connection  959 , remote network connection  979 , and by extension an applicable portion of network  980  that links connection  959  with connection  979 . The portion can be dynamically changeable, etc. In addition, block  990  can receive and decode RF waves received via antenna  951 , and cause antenna  951  to transmit RF waves according to what it has processed. 
         [0124]    Block  990  includes either local processor  956 , or remote processor  976 , or both. If both are provided, remote processor  976  can be made such that it operates in a way complementary with that of local processor  956 . In fact, the two can cooperate. It will be appreciated that block  990 , as defined this way, is in communication with both local memory  957  and remote memory  977 , if both are present. 
         [0125]    Accordingly, block  990  is location agnostic, in that its functions can be implemented either by local processor  956 , or by remote processor  976 , or by a combination of both. Some of these functions are preferably implemented by local processor  956 , and some by remote processor  976 . Block  990  accesses local memory  957 , or remote memory  977 , or both for storing and/or retrieving data. 
         [0126]    Reader system  900  operates by block  990  generating communications for RFID tags. These communications are ultimately transmitted by antenna block  951 , with modulator/encoder block  954  encoding and modulating the information on an RF wave. Then data is received from the tags via antenna block  951 , demodulated and decoded by demodulator/decoder block  953 , and processed by processing block  990 . 
         [0127]    The invention also includes methods. Some are methods of operation of an RFID reader or RFID reader system. Others are methods for controlling an RFID reader or RFID reader system. 
         [0128]    These methods can be implemented in any number of ways, including the structures described in this document. One such way is by machine operations, of devices of the type described in this document. 
         [0129]    Another optional way is for one or more of the individual operations of the methods to be performed in conjunction with one or more human operators performing some of them. These human operators need not be collocated with each other, but each can be only with a machine that performs a portion of the program. 
         [0130]    The invention additionally includes programs, and methods of operation of the programs. A program is generally defined as a group of steps or operations leading to a desired result, due to the nature of the elements in the steps and their sequence. A program is usually advantageously implemented as a sequence of steps or operations for a processor, such as the structures described above. 
         [0131]    Performing the steps, instructions, or operations of a program requires manipulation of physical quantities. Usually, though not necessarily, these quantities may be transferred, combined, compared, and otherwise manipulated or processed according to the steps or instructions, and they may also be stored in a computer-readable medium. These quantities include, for example, electrical, magnetic, and electromagnetic charges or particles, states of matter, and in the more general case can include the states of any physical devices or elements. It is convenient at times, principally for reasons of common usage, to refer to information represented by the states of these quantities as bits, data bits, samples, values, symbols, characters, terms, numbers, or the like. It should be borne in mind, however, that all of these and similar terms are associated with the appropriate physical quantities, and that these terms are merely convenient labels applied to these physical quantities, individually or in groups. 
         [0132]    The invention furthermore includes storage media. Such media, individually or in combination with others, have stored thereon instructions of a program made according to the invention. A storage medium according to the invention is a computer-readable medium, such as a memory, and is read by a processor of the type mentioned above. If a memory, it can be implemented in a number of ways, such as Read Only Memory (ROM), Random Access Memory (RAM), etc., some of which are volatile and some non-volatile. 
         [0133]    Even though it is said that the program may be stored in a computer-readable medium, it should be clear to a person skilled in the art that it need not be a single memory, or even a single machine. Various portions, modules or features of it may reside in separate memories, or even separate machines. The separate machines may be connected directly, or through a network such as a local access network (LAN) or a global network such as the Internet. 
         [0134]    Often, for the sake of convenience only, it is desirable to implement and describe a program as software. The software can be unitary, or thought in terms of various interconnected distinct software modules. 
         [0135]    This detailed description is presented largely in terms of flowcharts, algorithms, and symbolic representations of operations on data bits on and/or within at least one medium that allows computational operations, such as a computer with memory. Indeed, such descriptions and representations are the type of convenient labels used by those skilled in programming and/or the data processing arts to effectively convey the substance of their work to others skilled in the art. A person skilled in the art of programming may use these descriptions to readily generate specific instructions for implementing a program according to the present invention. 
         [0136]    Embodiments of an RFID reader system can be implemented as hardware, software, firmware, or any combination. It is advantageous to consider such a system as subdivided into components or modules. A person skilled in the art will recognize that some of these components or modules can be implemented as hardware, some as software, some as firmware, and some as a combination. An example of such a subdivision is now described. 
         [0137]      FIG. 10  is a block diagram illustrating an overall architecture of an RFID reader system  1000  according to embodiments. It will be appreciated that system  1000  is considered subdivided into modules or components. Each of these modules may be implemented by itself, or in combination with others. It will be recognized that some aspects are parallel with those of  FIG. 9 . In addition, some of them may be present more than once. 
         [0138]    RFID reader system  1000  includes one or more antennas  1010 , and an RF Front End  1020 , for interfacing with antenna(s)  1010 . These can be made as described above. In addition, Front End  1020  typically includes analog components. 
         [0139]    System  1000  also includes a Signal Processing module  1030 . In this embodiment, module  1030  exchanges waveforms with Front End  1020 , such as I and Q waveform pairs. In some embodiments, signal processing module  1030  is implemented by itself in an FPGA. 
         [0140]    System  1000  also includes a Physical Driver module  1040 , which is also known as Data Link. In this embodiment, module  1040  exchanges bits with module  1030 . Data Link  1040  can be the stage associated with framing of data. In one embodiment, module  1040  is implemented by a Digital Signal Processor. 
         [0141]    System  1000  additionally includes a Media Access Control module  1050 , which is also known as MAC layer. In this embodiment, module  1050  exchanges packets of bits with module  1040 . MAC layer  1050  can be the stage for making decisions for sharing the medium of wireless communication, which in this case is the air interface. Sharing can be between reader system  1000  and tags, or between system  1000  with another reader, or between tags, or a combination. In one embodiment, module  1050  is implemented by a Digital Signal Processor. 
         [0142]    System  1000  moreover includes an Application Programming Interface module  1060 , which is also known as API, Modem API, and MAPI. In some embodiments, module  1060  is itself an interface for a user. 
         [0143]    All of these functionalities can be supported by one or more processors. One of these processors can be considered a host processor. Such a processor would, for example, exchange signals with MAC layer  1050  via module  1060 . In some embodiments, the processor can include applications for system  1000 . In some embodiments, the processor is not considered as a separate module, but one that includes some of the above-mentioned modules of system  1000 . 
         [0144]    A user interface  1080  may be coupled to API  1060 . User interface  1080  can be manual, automatic, or both. It can be supported by a separate processor than the above mentioned processor, or implemented on it. 
         [0145]    It will be observed that the modules of system  1000  form something of a chain. Adjacent modules in the chain can be coupled by the appropriate instrumentalities for exchanging signals. These instrumentalities include conductors, buses, interfaces, and so on. These instrumentalities can be local, e.g. to connect modules that are physically close to each other, or over a network, for remote communication. 
         [0146]    The chain is used in opposite directions for receiving and transmitting. In a receiving mode, wireless waves are received by antenna(s)  1010  as signals, which are in turn processed successively by the various modules in the chain. Processing can terminate in any one of the modules. In a transmitting mode, initiation can be in any one of these modules. Ultimately, signals are transmitted internally, for antenna(s)  1010  to transmit as wireless waves. 
         [0147]    The architecture of system  1000  is presented for purposes of explanation, and not of limitation. Its particular subdivision into modules need not be followed for creating embodiments according to the invention. Furthermore, the features of the invention can be performed either within a single one of the modules, or by a combination of them. 
         [0148]    An economy is achieved in the present document in that a single set of flowcharts is used to describe methods in and of themselves, along with operations of hardware and/or software and/or firmware. This is regardless of how each element is implemented. 
         [0149]    Methods are now described more particularly according to embodiments. Such methods may be practiced by different embodiments, including but not limited to RFID reader system components as described above. In addition, individual operations of such methods may be practiced by different readers, at different phases of the lifetime of an RFID tag, with or without interruptions between them, and so on. 
         [0150]      FIG. 11  is flowchart  1100  illustrating a method according to an embodiment. The method of flowchart  1100  may be practiced by different embodiments, including but not limited to the embodiments listed above, and with tags that may or may not support the feature of limiting the number of replies. 
         [0151]    At optional operation  1110 , a command to identify is caused to be transmitted to the tag. 
         [0152]    At optional next operation  1120 , an identifying reply is backscattered in response to the command to identify. The identifying reply can help the reader in identifying the tag, or its manufacturer, and therefore determine whether the feature of limiting the number of replies exists, and therefore on whether to send or not the command of the next operation. 
         [0153]    At next operation  1160 , a custom limiting command is caused to be transmitted to the tag. The tag therefore changes the remaining number of compliances, above and beyond an adjustment of the remaining number responsive to receiving the custom limiting command. The custom limiting command can be any one, or any combination of the Restore, Reset, Disable, and Enable commands. Such commands would be intermingled with other commands used to perform tag operations, and work with tags in view of the declining number of compliances. 
         [0154]    In some embodiments, the Inquire command is caused to be transmitted. The received reply can be used to determine whether and when to send the custom limiting reply. 
         [0155]    In the above, a number of custom limiting commands were mentioned, such as Restore, Reset, Inquire, Disable, Enable, and so on. Such commands can be considered as custom commands, by not being specified in a particular communication protocol. 
         [0156]    Such custom commands can be constructed in any number of ways. In some instances they would be standalone commands, made by a sequence of bits chosen so that they do not conflict with other commands of the protocol. In other instances, they can be commands with a custom payload. Such commands can be known to the protocol or not, and the payload can be used to distinguish among different custom commands, and optionally further transfer a parameter for the commands. 
         [0157]    When commands are used that are known to the protocol, a section of their payload can be advantageously used for the purpose of implementing the custom command. For example, it can be a custom limiting payload, such as a restore payload, a reset payload, an inquire payload, a disable payload, an enable payload, etc. Such a section in the payload can be a mask field, according to embodiments. For the Gen2 Spec, two such commands are the Select command and the BlockWrite command. Between these two candidate commands, it should be considered that the Select command can be transmitted before or after a tag is singulated out of its population, while the BlockWrite is better suited for singulated tags. In addition, the BlockWrite command is optional to the Gen2 Spec, and the tag would probably have to have a controller that can accept it. 
         [0158]    Each one of the custom commands can thus be constructed as an implementation of this Select command or the BlockWrite command. In addition to responding to the payload implementing the custom command, the tag may further, or may not, also respond to the underlying Select command or BlockWrite command. An example is now described in terms of the Select command, but would apply equally to the BlockWrite command. 
         [0159]      FIG. 12A  is a table illustrating the fields of the Select command of the Gen2 Spec. Version 1.1.0 of the Gen2 Spec is hereby incorporated by reference in its entirety. The fields of this Select command are explained in more detail in the above mentioned Gen2 Spec. In addition, the implementation of this Select command can have a custom payload so that it operates as any one of the individual custom commands. 
         [0160]      FIG. 12B  is a table illustrating how a number of custom commands can be enabled in a reader and a tag. EBV stands for Extensible Bit Vector. The Mask Field can be partitioned as shown, into two primary subfields, named FEF and FCF. 
         [0161]    The Feature Enabling Field (FEF) enables the tag to verify that it is a proper recipient for the command, by comparing the transmitted FEF value against a value in Membank. In this case, Membank can be EPC, TID or USER memory. As can be seen, the FEF can be further partitioned into subfields, for better clarity. Such subfields can include a Class Identifier, the MDID, and an Indicator Bit. 
         [0162]    The Class Identifier can be two bits. For example, EPCglobal can correspond to a value of 10. This would allow the custom command to apply, for example, only to EPCglobal tags. 
         [0163]    The MDID is the tag manufacturer&#39;s ID, which is stored in the tag&#39;s TID memory. For Impinj tags, this number is 000000000001 or 100000000001. The MDID allows a reader to select tags of only the manufacturer of interest. So, even if this Select command is transmitted and received before singulation, the Select command can select also according to the tag manufacturer&#39;s ID. This will cause the manufacturer&#39;s tags to be selected, and thus the reader can ensure prior knowledge of the tag manufacturer&#39;s identification. 
         [0164]    The Indicator Bit can be set to 0 or 1. In the Gen2 Spec, a tag model number follows the MDID. A bit of this model number can serve as the Indicator Bit, and can be interpreted as follows: If it is 0, the tags can interpret the command as an “ordinary” Select, and execute it per the Gen2 Spec. Else, if it is 1, the tags can interpret the Select command as a custom instruction, and execute according to the FCF. 
         [0165]    The Feature Command Field (FCF) can have a command code that indicates the number of the custom instruction. For example, a command code of 00000 could be the custom timing command. This permits  31  possible custom commands. In addition, a command code of 11111 could indicate an extended command code that extends into the subsequent data field. 
         [0166]    The data field can contain data needed to implement the custom instruction, if any. Not all commands will use it. The data field can be variable in size. Its meaning will derive from the command codes. 
         [0167]    In some embodiments, the tag may ignore the Target and Action field in the Select command, depending on whether these fields are relevant to the CI. In other embodiments, the tag may also set the appropriate flag. 
         [0168]    In preferred embodiments, the entire Select command must be valid for the tag to accept and execute the custom command. That means valid values for Membank, Length, Pointer, Mask, CRC-16, etc. An example is now described. 
         [0169]      FIG. 12C  is a table showing sample values that can be used for the table of  FIG. 12B . These could be used for a custom timing command. The Membank having a binary value of 10 point to the TID memory. The pointer having a hex value of 06 h points to the last two bits of the class identifier. The length having a hex value of 14 h points to a length of 20 bits that follow thereafter. 
         [0170]    Numerous details have been set forth in this description, which is to be taken as a whole, to provide a more thorough understanding of the invention. In other instances, well-known features have not been described in detail, so as to not obscure unnecessarily the invention. 
         [0171]    The invention includes combinations and subcombinations of the various elements, features, functions and/or properties disclosed herein. The following claims define certain combinations and subcombinations, which are regarded as novel and non-obvious. Additional claims for other combinations and subcombinations of features, functions, elements and/or properties may be presented in this or a related document.