Patent Publication Number: US-2012044058-A1

Title: Rfid tag and communication method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-183404, filed on Aug. 18, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein relate to a radio frequency identification (RFID) tag and an RFID tag communication method. 
     BACKGROUND 
     Conventionally, there exists a system in which a user is made to carry an RFID tag and information is displayed on the RFID tag by transmission between the RFID tag and an RFID reader/writer installed at a specific location such as a gate. Further, there is a system that derives a moving direction of a moving object by attaching two RFID tags to the moving object so that an RFID reader/writer can detect the order of the two RFID tags on the moving object. 
     SUMMARY 
     According to an aspect of the invention, an RFID tag that communicates information with an RFID reader/writer includes a receiving unit that receives ID information; a memory unit that stores the ID information; a table that stores a correspondence relation between a change in the ID information and a moving direction; a determining unit that refers to the table to determine a moving direction from ID information received by the receiving unit and previously received ID information stored in the memory unit; and a signal generating unit that sends an interruption signal to a control unit driven by electric power supplied from a battery when the moving direction is determined by the determining unit, and stops generating the interruption signal when the moving direction is not determined by the determining unit. 
     The object and advantages of the invention will be realized and attained by at least the features, elements, and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an RFID tag according to an embodiment. 
         FIG. 2  is a block diagram illustrating a system using an RFID tag according to an embodiment. 
         FIG. 3  is an example of setting table for the system using the RFID tag according to the embodiment. 
         FIG. 4  is a block diagram illustrating an RFID tag according to the embodiment. 
         FIG. 5  is an example of a comparison determination table for the RFID tag according to the embodiment. 
         FIG. 6  is a flow chart illustrating passage detection procedures of an RFID tag according to the embodiment. 
         FIG. 7  is a flow chart illustrating controller side procedures of an RFID tag according to the embodiment. 
         FIG. 8  is a timing diagram illustrating operations of a system using the RFID tag according to the embodiment. 
         FIG. 9  is a block diagram illustrating an RFID tag according to the embodiment. 
         FIG. 10  is a flow chart illustrating a passage detection procedure of an RFID tag according to the embodiment. 
         FIG. 11  is a flow chart illustrating controller side procedures of an RFID tag according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     For example, an RFID reader/writer includes multiple antennas and each antenna is provided with unique ID (identifier) information. When an RFID tag approaches the antennas, the unique ID information from the antennas is written in the memory inside the RFID tag. The RFID tag senses its own moving direction according to the order in which the ID information was written. With this configuration, it can be assumed that an RFID tag can determine its own moving direction and independently conduct an operation based on the moving direction. 
     However, power consumption inside the RFID tag increases due to memory access since, for example, a controller that controls applications and operations of a built-in sensor frequently accesses the memory containing the ID information to read multiple pieces of ID information. When the controller in the RFID tag accesses the memory, the memory is activated by power supplied from a battery built into the RFID tag. Therefore, a problem occurs in that the battery does not last long in a configuration where the frequency of memory access by the abovementioned controller increases. In particular, batteries may last even less time when a memory type that consumes much power such as, for example, a Ferroelectric Random Access Memory (FRAM), is used in the RFID tag. 
     The embodiments herein seek to reduce power consumption in an RFID tag. 
     Example embodiments of the RFID tag will be described herein with reference to the accompanying drawings. The RFID tag stores location information sent by the RFID reader/writer, compares the stored location information with newly received location information, and then sends an interruption signal when the moving direction is determined. Accordingly, the interruption signal is not sent when the moving direction is not determined. Power consumption by the circuit that operates the battery can be reduced by not sending the interruption signal. In the following embodiments, the same configuration elements are assigned the same reference numerals, and the description thereof is omitted here. 
     EMBODIMENT 
     Description of RFID Tag 
       FIG. 1  is a block diagram illustrating an RFID tag according to an embodiment. As illustrated in  FIG. 1 , an RFID tag  2  communicates information with an RFID reader/writer  1 . The RFID tag  2  includes a receiving unit  3 , a memory unit  4 , a table  5 , a determining unit  6 , a control unit  7 , and a signal generating unit  9 . The receiving unit  3  receives location information sent by the RFID reader/writer  1 . The memory unit  4  stores the location information. The table  5  stores a correspondence relation between changes in the location information and moving directions. The determining unit  6  refers to the table  5  to determine the moving direction from location information received from the receiving unit  3  and the previously received information location stored in the memory unit  4 . The control unit  7  is driven by power supplied from a battery  8  built into the RFID tag  2 . The signal generating unit  9  sends an interruption signal to the control unit  7  when the moving direction can be determined by the determining unit  6 , and stops the generation of an interruption signal when the moving direction cannot be determined by the determining unit  6 . 
     According to the embodiment, the control unit  7  is driven by the battery  8  and the signal generating unit  9  sends an interruption signal to the control unit  7  when the moving direction of the RFID tag  2  is determined. When the moving direction of the RFID tag  2  cannot be determined, the signal generating unit  9  stops the generation of the interruption signal and hence the operations of the control unit  7  driven by the battery are reduced and consumption of power by the battery is reduced. Power in a battery of a related art RFID tag would be consumed when the moving direction is determined. A related art RFID tag would use power from the battery to conduct multiple memory accesses to read multiple pieces of location information from the memory to determine the moving direction of the RFID tag. Compared to the related art RFID tag, the RFID tag  2  that is applicable to the embodiments offers substantially reduced consumption of battery power. Therefore, the RFID tag  2  provides substantially lower power consumption. 
     Other Embodiments 
     Description of System Using RFID Tags 
       FIG. 2  is a block diagram illustrating a system using an RFID tag according to an embodiment. As illustrated in  FIG. 2 , a host  11  sets location ID information that uniquely identifies antennas, for example, as location information for each antenna of the RFID reader/writer  12  according to a setting table to be described below. The host  11  sets parameters and commands and the like to send to the RFID tag  17 . 
     The RFID reader/writer  12  is equipped with a transmission data generating circuit  13 , a transmission radio frequency (RF) circuit  14 , and multiple antennas such as an antenna  1 _ 15  and an antenna  2 _ 16 . The transmission data generating circuit  13  generates data to send to the RFID tag  17  based on the location ID information for each antenna, commands, and parameters set by the host  11 . The transmission RF circuit  14  conducts encoding and modulation processing on the data generated by the transmission data generating circuit  13 , and sends the processed data as transmission signals from the antenna  1 _ 15  and the antenna  2 _ 16 . The antenna  1 _ 15  and the antenna  2 _ 16  are arranged away from each other to allow identification by an antenna  18  of the RFID tag  17 . 
     Description of Setting Table 
       FIG. 3  is an example of setting table for the system using the RFID tag according to the embodiment. As illustrated in  FIG. 3 , location ID information given to each antenna of the RFID reader/writer  12  and a time period for sending the location ID information from the applicable antennas are set in a setting table  19 . In the example illustrated in  FIG. 3 , “01” is set as location ID information to be sent for a period of 100 ms from, for example, the antenna  1 _ 15 . Similarly, “02” is set as location ID information to be sent for a period of 100 ms from, for example, the antenna  2 _ 16 . 
     As illustrated in  FIG. 2 , when, for example, the location ID information “02” is received after receiving the location ID information “01”, the RFID tag  17  senses that it has passed near the RFID reader/writer  12  in a direction (direction A) from antenna  1 _ 15  toward antenna  2 _ 16 . When, for example, the location ID information “01” is received after receiving the location ID information “02”, the RFID tag  17  senses that it has passed near the RFID reader/writer  12  in a direction (direction B) from antenna  2 _ 16  toward antenna  1 _ 15 . 
     Description of RFID Tag 
       FIG. 4  is a block diagram illustrating an RFID tag according to an embodiment. As illustrated in  FIG. 4 , the RFID tag  17  includes a passive tag unit  21 . The passive tag unit  21  acts as a receiving unit and includes, for example, an RF unit  22 , a logic circuit unit  23 , and a non-volatile memory  30  built into the passive tag unit  21  as, for example, a memory unit. The RF unit  22  receives signals sent from the RFID reader/writer  12  via the antenna  18 . The RF unit  22  conducts demodulation and decoding processing on the received signals and outputs the processed signals as location ID information, commands, and parameters. 
     The logic circuit unit  23  includes an interruption generating unit  24  that acts as, for example, a signal generating unit. The logic circuit unit  23  includes a comparison determining unit  25  that acts as, for example, a determining unit, and a location ID write command detecting unit  26 . The logic circuit unit  23  further includes a passage detection result writing unit  27 , a location ID information reading unit  28 , and a location ID information writing unit  29 . The logic circuit unit  23  includes some notification unit functions. The notification unit records a moving direction in a passage detection result storage unit  31  for notifying the moving direction to other functions. The moving direction is notified when a moving direction obtaining command is received from the RFID reader/writer  12  by recording the moving direction in the passage detection result storage unit  31 . 
     The location ID write command detecting unit  26  detects a location ID write command from the data outputted by the RF unit  22  and transfers the command to the comparison determining unit  25  as current location ID information. The comparison determining unit  25  reads previous location ID information stored in a location ID storage unit  32  in the non-volatile memory  30  via the location ID information reading unit  28 . The comparison determining unit  25  compares the current location ID information with the previous location ID information and refers to a table such as, for example, a comparison determination table described below to determine the moving direction of the RFID tag  17 . The comparison determination table stores changes in the location ID information and the moving direction of the RFID tag  17  in association with each other. The comparison determining unit  25  transfers the moving direction comparison result to the passage detection result writing unit  27 . 
     The passage detection result writing unit  27  transfers the moving direction comparison result to the passage detection result storage unit  31  in the non-volatile memory  30 . The passage detection result writing unit  27  outputs an interruption request signal to the interruption generating unit  24  based on the moving direction comparison result. The interruption generating unit  24  receives the interruption request signal and sends an interruption signal to an interruption detecting unit  35  of a power controller  34  to be described below. The location ID information reading unit  28  reads the previous location ID information from the location ID storage unit  32  and transfers the previous location ID information to the comparison determining unit  25 . The comparison determining unit  25  transfers the current location ID information to the location ID information writing unit  29 . The location ID information writing unit  29  writes the current location ID information into the location ID storage unit  32  as previous location ID information. 
     Moreover, the logic circuit unit  23  includes a power regenerating circuit (not illustrated). The power regenerating circuit converts radio waves received from the antenna  18  to electric power. The units in the passive tag unit  21  operate using the electric power supplied by the power regenerating circuit. 
     The non-volatile memory  30  includes the passage detection result storage unit  31  that acts as, for example, a storage unit, and the location ID storage unit  32  that acts as, for example, a storage unit in a memory unit. The passage detection result storage unit  31  stores a comparison result of the moving direction written by the passage detection result writing unit  27 . The location ID storage unit  32  stores the current location ID information and the previous location ID information written by the location ID information writing unit  29 . The non-volatile memory  30  is equipped with, for example, a dual port memory that can be accessed by both the logic circuit unit  23  and the power controller  34  to be described below. The non-volatile memory  30  may be, for example, a ferroelectric memory. 
     Moreover, the RFID tag  17  includes an active portion that is enabled by a built-in power source. The active portion includes a processor  33 , a power controller  34 , a power source switch  37 , a power source circuit  38 , a battery  39 , an application processor  40 , a sensor such as, for example, an acceleration sensor  41 , a Wi-Fi or other type of wireless module  42 , an input unit  43 , and a memory  44 . The power controller  34  controls applications and devices such as power sources, sensors, and modules according to the moving direction determination result of the RFID tag  17 . The power controller  34  is driven by power supplied from the battery  39  via the power source circuit  38 . The power controller  34  is equipped with the interruption detecting unit  35  that acts as, for example, a portion of the notification unit, and a result reading unit  36  that acts as, for example, a portion of the notification unit. 
     When an interruption signal from the interruption generating unit  24  is detected, the interruption detecting unit  35  notifies the result reading unit  36  that an interruption has been detected. When the notification from the interruption detecting unit  35  is received, the result reading unit  36  reads the moving direction determination result of the RFID tag  17  from the passage detection result storage unit  31  and transfers information that indicates whether the RFID tag  17  passed and in which direction it passed, to the processor  33 . 
     Further, the interruption detecting unit  35  detects the interruption signal from the interruption generating unit  24  and then outputs a switch signal to the power source switch  37 . The power source switch  37  supplies power from the battery  39  to the non-volatile memory  30  via the power source circuit  38  based on the input of the switch signal. That is, power is supplied to the non-volatile memory  30  from the power regenerating circuit as described above when the logic circuit unit  23  accesses the non-volatile memory  30 , and power is supplied to the non-volatile memory  30  from the battery  39  when the power controller  34  accesses the non-volatile memory  30 . 
     The processor  33  controls, for example, the application processor  40 , the acceleration sensor  41 , or the wireless module  42  based on the information that indicates whether the RFID tag  17  passed through and the moving direction when the RFID tag  17  passed through. For example, the processor  33  instructs the application processor  40  to conduct operations such as activating or stopping applications. For example, the processor  33  turns the power of the acceleration sensor  41  and the wireless module  42  on or off. Contents of the control operations by the processor  33  are prescribed in, for example, a control table. 
     For example, controlling the acceleration sensor  41 , the application processor  40 , and the wireless module  42  when the moving direction of the RFID tag  17  is the direction A (see  FIG. 2 ) may be prescribed in the control table. That is, when the previous location ID information is “01” and the current location ID information is “02”, controls such as turning the power of the acceleration sensor  41  on, activating an application by the application processor  40 , and turning the power of the wireless module  42  off may be prescribed in the control table. For example, controlling the acceleration sensor  41 , the application processor  40 , and the wireless module  42  when the moving direction of the RFID tag  17  is the direction B (see  FIG. 2 ) may also be prescribed in the control table. That is, when the previous location ID information is “02” and the current location ID information is “01”, controls such as turning the power of the acceleration sensor  41  off, turning the power of the wireless module  42  on, and terminating an application by the application processor  40  may be prescribed in the control table. 
     The input unit  43  may be, for example, a key, a button, a switch, or a touch panel. The memory  44  may store an operating system (OS) and an application program executed by the processor  33 , and may be used as a work region of the OS and the application program. A mobile phone equipped with an RFID tag is an example of the RFID tag  17 . 
     Description of Comparison Determination Table 
       FIG. 5  is an example of a comparison determination table for the RFID tag according to the embodiment. As illustrated in  FIG. 5 , a determination result of the moving direction of the RFID tag based on a combination of the previous location ID information and the current location ID information, and operations inside the RFID tag  17  to be conducted after the determination are prescribed in a comparison determination table  45 . The previous location ID information is read from the location ID storage unit  32 . The current location ID information is obtained from the location ID write command detecting unit  26 . 
     For example, the combination of the previous location ID information and the current location ID information is represented as “previous location ID information, current location ID information.” Moreover, the location ID information of a third antenna other than antenna  1 _ 15  or antenna  2 _ 16  is “03.” The third antenna is not used to detect the moving direction of the RFID tag  17 . For example, when combinations of the location ID information are represented as [00,01], [00,02], [01,01], [02,02], [00,03], [01,03], and [02,03], the determination result of the moving direction of the RFID tag  17  is “0” which indicates no passage detection. When, for example, combinations of the location ID information are represented as [00,01], [00,02], [01,01], and [02,02], no operations are conducted in the RFID tag  17  after the determination. When, for example, combinations of the location ID information are represented as [00,03], [01,03], and [02,03], writing “00” as the previous location ID information in the location ID storage unit  32  becomes the operation in the RFID tag  17  after the determination. 
     When the combination is [01,02] for example, the determination result of the moving direction of the RFID tag  17  is “A” which indicates that the RFID tag  17  moved in the direction A. After the determination, writing “A” in the passage detection result storage unit  31  and outputting an interruption request signal from the passage detection result writing unit  27  to the interruption generating unit  24  are conducted as operations in the RFID tag  17 . When the combination is [02,01] for example, the determination result of the moving direction of the RFID tag  17  is “B” which indicates that the RFID tag  17  moved in the direction B. After the determination, writing “B” in the passage detection result storage unit  31  and outputting an interruption request signal from the passage detection result writing unit  27  to the interruption generating unit  24  are conducted as operations in the RFID tag  17 . 
     Description of Passage Detection Processing 
       FIG. 6  is a flow chart illustrating passage detection procedures of an RFID tag according to the present embodiment. As illustrated in  FIG. 6 , when the passage detection processing is started in the RFID tag  17 , the location ID information reading unit  28  first conducts processing (step S 1 ), and then the location ID write command detecting unit  26  conducts processing (step S 2 ). Until the location ID write command detecting unit  26  detects a location ID write command (step S 2 : No), the location ID information reading unit  28  reads data (location ID information) from the location ID storage unit  32  and sends the data to the comparison determining unit  25  (step S 1 ). 
     When the location ID write command detecting unit  26  detects a location ID write command (step S 2 : Yes), the comparison determining unit  25  and the location ID information writing unit  29  conduct processing (step S 3 ). The comparison determining unit  25  receives new location ID information (current location ID information) included in the location ID write command from the location ID write command detecting unit  26 , and sends the new location ID information to the location ID information writing unit  29 . The location ID information writing unit  29  overwrites the new location ID information received from the comparison determining unit  25  in the location ID storage unit  32  (step S 3 ). 
     Next, the comparison determining unit  25  compares the previous location ID information and the current location ID information and refers to the comparison determination table  45  to determine the moving direction of the RFID tag  17  (step S 4 ). When the result of the determination indicates that the RFID tag  17  moved in the direction A or the direction B (step S 4 : passage detected), the comparison determining unit  25  sends the determination result to the passage detection result writing unit  27 . The passage detection result writing unit  27  writes the determination result into the passage detection result storage unit  31  (step S 5 ). 
     Furthermore, the passage detection result writing unit  27  outputs an interruption request signal to the interruption generating unit  24  (step S 6 ). As a result, the interruption generating unit  24  outputs an interruption signal to the interruption detecting unit  35 . Next, processing by the power controller  34  is conducted (step S 7 ). Then the RFID tag  17  processing returns to step S 1  to repeat the steps from step S 1  to step S 7 . On the other hand, when the result of the determination of the moving direction of the RFID tag  17  does not indicate that the RFID tag  17  passed through (step S 4 : passage not detected), the processing returns to step S 1 . 
     Description of Controller Side Processing 
       FIG. 7  is a flow chart illustrating controller side procedures of an RFID tag according to an embodiment. As illustrated in  FIG. 7 , on the power controller  34  side of the RFID tag  17 , the power controller  34  waits in a sleep state until the interruption detecting unit  35  detects an interruption signal (step S 11 : No). When the interruption detecting unit  35  detects an interruption signal (step S 11 : Yes), a switch signal is outputted to the power source switch  37 . As a result, the power source switch  37  is switched so that the power source circuit  38  supplies power to the non-volatile memory  30  from the battery  39  (step S 12 ). 
     Next, the result reading unit  36  reads the determination result of the moving direction of the RFID tag  17  from the passage detection result storage unit  31  (step S 13 ). When the reading of the determination result from the passage detection result storage unit  31  is completed, the power supply from the battery  39  to the non-volatile memory  30  is terminated. That is, the power supply to the non-volatile memory  30  is disabled (step S 14 ). Next, the processor  33  receives information indicating whether the RFID tag  17  passed through and indicating the moving direction from the result reading unit  36 , and conducts processing according to the contents of the control table (step S 15 ). 
     For example, if the moving direction of the RFID tag is direction A (step S 15 : direction A), the processor  33  controls the application processor  40 , the acceleration sensor  41 , and the wireless module (Wi-Fi)  42 . The processor  33  activates applications to be conducted by the application processor  40  (step S 16 ). The processor  33  turns the power of the acceleration sensor  41  on (step S 17 ) and turns the power of the wireless module (Wi-Fi)  42  off (step S 18 ). On the other hand, if, for example, the moving direction of the RFID tag  17  is direction B (step S 15 : direction B), the processor  33  controls the application processor  40 , the acceleration sensor  41 , and the wireless module (Wi-Fi)  42 . The processor  33  terminates applications to be conducted by the application processor  40  (step S 19 ). The processor  33  turns the power of the acceleration sensor  41  off (step S 20 ) and turns the power of the wireless module (Wi-Fi)  42  on (step S 21 ). Then the power controller  34  side processing of the RFID tag  17  is completed. 
     Description of System Using RFID Tags 
       FIG. 8  is a timing diagram illustrating operations of a system using the RFID tag according to the embodiment. As indicated by reference numeral  51 , the RFID reader/writer  12  continuously sends location ID information from each of the antennas while switching between the antenna  1 _ 15 , the antenna  2 _ 16 , and the third antenna (not shown) using time divisions based on, for example, a setting table  19 . The RFID reader/writer  12  represents the side transmitting the location ID information. 
     As indicated by reference numeral  52 , the RFID tag  17 , in other words the location ID information receiving side, is in an interruption waiting state, in other words a sleep state, until the RFID tag  17  approaches, for example, the antenna  1 _ 15 . The electric power consumed by the battery is low during the sleep state. When the RFID tag  17  approaches, for example, the antenna  1 _ 15 , the RFID tag  17  receives a location ID interruption command from the antenna  1 _ 15  as indicated by the reference numeral  53 . Next, the RFID tag  17  determines whether passage was detected based on the previous location ID information, the current location ID information, and the comparison determination table  45  as indicated by reference numeral  54 . The RFID tag  17  determines that passage was not detected based on the previous location ID information (in this case “00”) stored in the location ID storage unit  32 , the current location ID information (in this case “01”) from the location ID write command, and the comparison determination table  45 . Therefore, the portion that consumes power of the battery  39  in the RFID tag  17  does not conduct any operations. Further, the RFID tag  17  writes “01” in the location ID storage unit  32 . The RFID tag  17  then returns to the sleep state as indicated by the reference numeral  55 . 
     When the RFID tag  17  in the sleep state approaches, for example, the antenna  2 _ 16 , the RFID tag  17  receives a location ID write command sent from the antenna  2 _ 16  as indicated by the reference numeral  56 . Next, the RFID tag  17  determines whether passage was detected based on the previous location ID information, the current location ID information, and the comparison determination table  45  as indicated by reference numeral  57 . The RFID tag  17  determines that passage was detected based on the previous location ID information (in this case “01”) stored in the location ID storage unit  32 , the current location ID information (in this case “02”) from the location ID write command, and the comparison determination table  45 . Power is supplied from the battery  39  to the non-volatile memory  30  as indicated by the reference numeral  58 . In this state, the result reading unit  36  reads the determination result of the moving direction of the RFID tag  17  from the passage detection result storage unit  31  and notifies the processor  33 . Next, the power for the non-volatile memory  30  is disabled as indicated by the reference numeral  59  and the processor  33  conducts processing according to the control table. Further, the RFID tag  17  writes “02” in the location ID storage unit  32 . The RFID tag  17  then returns to the sleep state as indicated by the reference numeral  60 . 
     When the RFID tag  17  in the sleep state approaches, for example, the third antenna, the RFID tag  17  receives a location ID write command sent from the third antenna as indicated by the reference numeral  61 . Next, the RFID tag  17  determines whether passage was detected based on the previous location ID information, the current location ID information, and the comparison determination table  45  as indicated by reference numeral  62 . The RFID tag  17  determines that passage was not detected based on the previous location ID information (in this case “02”) stored in the location ID storage unit  32 , the current location ID information (in this case “03”) from the location ID write command, and the comparison determination table  45 . Further, the RFID tag  17  writes “00” in the location ID storage unit  32 . The RFID tag  17  then returns to the sleep state as indicated by the reference numeral  63 . 
     According to the embodiment, the determination of the moving direction of the RFID tag  17  is conducted in the passive tag unit  21  using electric power converted from radio waves received via the antenna  18 . As a result, if the determination result is not notified to the power controller  34  side, electric power of the battery  39  is not consumed. If the determination result is read by the power controller  34  side, electric power of the battery  39  is consumed. Therefore, a related art RFID tag would use battery power to conduct multiple memory accesses, for example two memory accesses, on the passive memory to read, for example, two pieces of location ID information to determine the moving direction of the RFID tag  17 . Compared to the related art RFID tag, the RFID tag  17  that is applicable to the embodiments offers reduced consumption of battery power. Essentially, the RFID tag  17  achieves a substantial reduction in power consumption. 
     Other Embodiments 
     Description of RFID Tag 
       FIG. 9  is a block diagram illustrating an RFID tag according to an embodiment. As illustrated in  FIG. 9 , the difference between the RFID tag of the present embodiment and the RFID tag according to the embodiment illustrated in  FIG. 4  is that the RFID tag of the present embodiment stores the new location ID information obtained when receiving the location ID write command in a memory such as a buffer immediately after receiving the location ID write command. The RFID tag  17  includes, for example, a location ID storage buffer  46  as a buffer in the non-volatile memory  30 . 
     The location ID write command detecting unit  26  transfers the new location ID information to the location ID information writing unit  29 . The location ID information writing unit  29  writes the new location ID information in the location ID storage buffer  46 . The location ID information reading unit  28  reads the new location ID information from the location ID storage buffer  46  and transfers the new location ID information as current location ID information to the comparison determining unit  25 . Other configurations of the passive tag unit  21  are substantially the same as the previous embodiment. 
     Moreover, the present embodiment is provided with, for example, the acceleration sensor  41  and a temperature sensor  47  as sensors controlled by the processor  33 . Moreover, for example, the application processor  40  and the wireless module  42  are not provided in the present embodiment. According to this configuration, when, for example, the moving direction of the RFID tag  17  is direction A (see  FIG. 2 ), turning power to the temperature sensor  47  on and turning the power to the acceleration sensor  41  off may be prescribed in the control table. Moreover, when, for example, the moving direction of the RFID tag  17  is direction B (see  FIG. 2 ), turning power to the temperature sensor  47  off and turning the power to the acceleration sensor  41  on may be prescribed in the control table. 
     Description of Passage Detection Processing 
       FIG. 10  is a flow chart illustrating a passage detection procedure of an RFID tag according to the present embodiment. As illustrated in  FIG. 10 , when the passage detection processing is started in the RFID tag  17 , first the location ID information writing unit  29  is idle until the location ID write command detecting unit  26  detects the location ID write command (step S 31 : No). When the location ID write command detecting unit  26  detects a location ID write command (step S 31 : Yes), the location ID write command detecting unit  26  transfers new location ID information included in the location ID write command to the location ID information writing unit  29 . The location ID information writing unit  29  writes the new location ID information received from the location ID write command detecting unit  26  in the location ID storage buffer  46  (step S 32 ). 
     Next, the location ID information reading unit  28  reads the data from the location ID storage buffer  46  and transfers the data as current location ID information to the comparison determining unit  25 . The location ID information reading unit  28  reads the data from the location ID storage unit  32  and transfers the data as the previous location ID information to the comparison determining unit  25  (step S 33 ). Next, the comparison determining unit  25  transfers the current location ID information to the location ID information writing unit  29 . The location ID information writing unit  29  writes the current location ID information received from the comparison determining unit  25  in the location ID storage unit  32  (step S 34 ). 
     Subsequent processing is substantially similar to the processing from step S 4  to step S 7  in the flow chart illustrated in  FIG. 6  (step S 35  to step S 38 ). 
     Description of Controller Side Processing 
       FIG. 11  is a flow chart illustrating example controller side procedures of an RFID tag according to the embodiment. As illustrated in  FIG. 11 , when the processing of the power controller  34  side of the RFID tag  17  starts, first processing substantially similar to step S 11  to step S 15  of the flowchart illustrated in  FIG. 7  of the previous embodiment are conducted (step S 41  to step S 45 ). If the moving direction of the RFID tag  17  is, for example, direction A (step S 45 : direction A), the processor  33  turns the power to the temperature sensor  47  on (step S 46 ), and turns the power to the acceleration sensor  41  off (step S 47 ). On the other hand, if the moving direction of the RFID tag  17  is, for example, direction B (step S 45 : direction B), the processor  33  turns the power to the temperature sensor  47  off (step S 48 ), and turns the power to the acceleration sensor  41  on (step S 49 ). Then the power controller  34  side processing of the RFID tag  17  is completed. 
     Description of Operation of System Using RFID Tags 
     A timing diagram illustrating operations of a system using the RFID tag according to the present embodiment is substantially similar to the diagram illustrated in  FIG. 8 . However, differences between the present embodiment and the previous embodiment are described below. 
     At the timing indicated by reference numeral  53  in  FIG. 8 , the RFID tag stores location ID information “01” of the antenna  1 _ 15  in the location ID storage buffer  46  when the location ID write command is received from the antenna  1 _ 15 . Next, the RFID tag  17  determines whether passage was detected based on the previous location ID information, the current location ID information, and the comparison determination table  45  as indicated by reference numeral  54  in  FIG. 8 . The RFID tag  17  determines that passage was not detected based on the previous location ID information (in this case “00”) stored in the location ID storage unit  32 , the current location ID information (in this case “01”) stored in the location ID storage buffer  46 , and the comparison determination table  45 . Moreover, the RFID tag  17  writes the location ID information (in this case “01”) stored in the location ID storage buffer  46  into the location ID storage unit  32 . 
     At the timing indicated by reference numeral  56  in  FIG. 8 , the RFID tag stores location ID information “02” of the antenna  2 _ 16  in the location ID storage buffer  46  when the location ID write command is received from the antenna  2 _ 16 . Next, the RFID tag  17  determines whether passage was detected based on the previous location ID information, the current location ID information, and the comparison determination table  45  as indicated by reference numeral  57  in  FIG. 8 . The RFID tag  17  determines that passage was detected based on the previous location ID information (in this case “01”) stored in the location ID storage unit  32 , the current location ID information (in this case “02”) stored in the location ID storage buffer  46 , and the comparison determination table  45 . Moreover, the RFID tag  17  writes the location ID information (in this case “02”) stored in the location ID storage buffer  46  into the location ID storage unit  32 . 
     At the timing indicated by reference numeral  61  in  FIG. 8 , the RFID tag  17  stores location ID information “03” of the third antenna in the location ID storage buffer  46  when the location ID write command is received from the third antenna. Next, the RFID tag  17  determines whether passage was detected based on the previous location ID information, the current location ID information, and the comparison determination table  45  as indicated by reference numeral  62  in  FIG. 8 . The RFID tag  17  determines that passage was not detected based on the previous location ID information (in this case “02”) stored in the location ID storage unit  32 , the current location ID information (in this case “03”) stored in the location ID storage buffer  46 , and the comparison determination table  45 . Further, the RFID tag  17  writes “00” into the location ID storage unit  32 . 
     Results similar to the previous embodiment are achieved with the present embodiment. Moreover, immediately after receiving the location ID write command, the RFID tag  17  stores the received location ID information in the location ID storage buffer  46  thus substantially reducing or preventing the loss of the new location ID information during the determination of the moving direction of the RFID tag  17  even when the state of communication is poor. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present invention(s) has (have) been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.