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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to an antitheft method for a vehicle having an immobilizer which prohibits the engine from starting when a key with an ID code, which does not match an ID code previously registered in a controller mounted on the vehicle is used and to a method and device for providing an antitheft operation in the event a person attempts to defeat the theft by replacement of the controller.  
         [0002]     Typical antitheft devices for vehicle as shown the U.S. Pat. Nos. 6,525,433 and 6,683,391 and co-pending U.S. application Ser. No. 10/707689, filed Jan. 5, 2004, of the assignee hereof and comprises a key cylinder and a controller on a vehicle body, that control an ignition immobilizer. The operation is such that when a key having a built-in transponder is inserted into the key cylinder, the controller performs an ID check through a circuit that communicates with the transponder via an antenna located in the vicinity of the key cylinder to read an ID code of the key. The immobilizer only permits the engine to start if the ID code matches the one previously registered in the controller.  
         [0003]     In accordance with the disclosure thereof backup power is constantly supplied to the immobilizer generally through the vehicle battery even when the vehicle is not running. Thus even when the main switch is off the immobilizer is kept in an alert mode for theft through illegal means.  
         [0004]     When a vehicle is equipped with such an antitheft device, the engine cannot be started without a key with an ID code which matches the ID code previously registered in the controller. Thus, the vehicle cannot be illegally driven by someone other than the owner of the vehicle and is prevented from being stolen.  
         [0005]     However, such an antitheft device are commercially available and the coupler for coupling the device and the on-board battery is a standardized product. Therefore a prospective thief can steal the vehicle by removing the entire antitheft device could be removed and replacing it with another device that matches a key possessed by the prospective thief.  
         [0006]     One conventional preventive measure against such theft of a vehicle is to make it difficult to steal the vehicle by complicating the structure in which an antitheft device is attached to the vehicle, for example, by increasing the number or types of attaching devices or encasing the device, so that it cannot be removed quickly. However these remedies add to the cost of both assembly and servicing and also can be defeated by skilled thieves.  
         [0007]     Therefore it is a principal object of this invention to provide an easily attached, serviced and removed system and method of antitheft protection that nevertheless makes it difficult to remove the device within a short period of time.  
         [0008]     It is a further an object of the invention is to provide antitheft method and device for a vehicle which does not complicate the structure of the attachment but nevertheless prevents theft.  
       SUMMARY OF THE INVENTION  
       [0009]     A first feature of the invention is adapted to be embodied in an antitheft method for a vehicle equipped with an immobilizer having a controller which transmits an ignition prohibition canceling signal to an ignition control unit of an engine when an ID code of a transponder built in a key matches an ID code previously registered in the controller on the vehicle. The method comprises the steps of maintaining the ignition control unit in an ignition prohibition mode for a predetermined period of time after the immobilizer has been disconnected from a power source and is subsequently connected to a backup power source.  
         [0010]     Another feature of the invention is adapted to be embodied in an antitheft device for a vehicle having an engine comprising an immobilizer having a controller that checks an ID code of a transponder built in a key against an ID code previously registered in the controller. A timer circuit counts the connection time that has elapsed after the immobilizer has been disconnected from a power source and is connected to a backup power source for prohibiting starting of said engine when the connection time is shorter than a predetermined period of time. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic view illustrating the components of an embodiment of the invention.  
         [0012]      FIG. 2  is a block diagram illustrating the embodiment  
         [0013]      FIG. 3  is a circuit diagram of the antitheft device.  
         [0014]      FIG. 4  is a flowchart illustrating the operation of the device and its method.  
     
    
     DETAILED DESCRIPTION  
       [0015]     Referring now to the drawings and initially to  FIGS. 1 and 2  these illustrate the configuration of an antitheft device of the present invention. The antitheft device of the present invention is comprised of a vehicle mounted immobilizer  11  connected to an on-board battery  12  with a voltage of 12V, for example, an ignition control unit (ECU)  13 , a key cylinder assembly, indicated generally at  14 , and a cooperating key  15 . The immobilizer  11  has an antenna  16 , generally incorporated in the body of the key cylinder  14 , and a controller, indicated generally at  17 . The antenna  16  and the controller  17  may be integrated with each other.  
         [0016]     A main switch  18  is built in a key cylinder  14 . When the appropriate key  15  is inserted into the key cylinder  14  and operated, the main switch  18  is turned on or off. In addition to its keying, if employed, the key  15  has a built-in transponder  19  in which a unique ID code is recorded.  
         [0017]     The controller  17  is formed on a printed circuit board and has an ID reading circuit  21 , a CPU  22 , a power source circuit  23 , a timer circuit  24  and a memory  25 . The controller  17  is connected to the antenna  16  and the ignition control unit, and has a function of communicating with the transponder  19  built in the key  15  and the ignition control unit  13  of an engine  12 . The ignition control unit may be incorporated in the controller  17 . The battery  12  is connected to the immobilizer  11  via a coupler  26  ( FIG. 3 ) as described later, and supplies backup power with a low voltage (5V, for example) to keep the controller  17  operational regardless of the on or off condition of the main switch  18 .  
         [0018]     When the key  15  is operated and the main switch  18  is turned on, the battery  12  is connected to load components such as lights and an ignition circuit (not shown), and an ID code signal is transmitted from the key  15  to the controller  17  via the antenna  16  of the key cylinder  14 .  
         [0019]     The timer circuit  24  counts the connection time, that is, the amount of time which has elapsed after the last initial connection of the immobilizer  11  to the battery  12  (the amount of time from the point at which the backup power starts to be supplied from the battery  12  by connecting the coupler  26 ). Since backup power is kept supplied to the immobilizer  11  even when the main switch is off, the timer circuit  24  can count the connection time after the immobilizer  11  has been attached.  
         [0020]     The immobilizer  11  is programmed not to read the transponder  19  even if the key  15  is turned to the on position until a predetermined period of time elapses. Each of the immobilizer  11  and the ignition control unit  13  is connected to the on-board battery  12  via a respective coupler  26  as shown in  FIG. 3 , and the counting of the connection time is automatically started when the immobilizer  11  is connected to the coupler  26 . Although shown separately in  FIG. 3 , the couplers  26  may be contained in a common body. Thus, the connection time cannot be changed illegally from the outside.  
         [0021]     The ignition control unit  13  may be also counted the connection time and controlled not to be activated until a predetermined period of connection time elapses as in the case with the immobilizer  11 .  
         [0022]     The ID code recorded in the transponder  19  of the key  15  is recorded in the memory  25 , and the CPU  22  determines whether the ID code, read by the ID reading circuit  21  when the key  15  is inserted into the key cylinder  14 , matches the ID code recorded in the memory  25 . When the ID codes match with each other, an ignition prohibition canceling signal is transmitted from the CPU  22  to the ignition control unit  13  to activate the ignition control unit  13 , allowing a starter motor  27  and ignition coil  28  of the of an associated engine  29  to start.  
         [0023]     When the ID codes do not match with each other, the ignition control unit  13  is maintained in an ignition prohibition mode and the engine  29  cannot be started. At this time, an alarm or indication lamp indicates the improper operation.  
         [0024]     The implementation procedure of the present invention will be described with reference to  FIG. 4 . When the key  15  is inserted into the key cylinder  14  at the step S 1 ) and turned to the on position, the main switch  18  is turned on and the controller  17  detects this at the step S 2 . The controller  17  then determines at the step S 3  whether a predetermined period of time has elapsed after the connection of the immobilizer  11  to the on-board battery  12  via the coupler  26 . If the connection time is determined to be insufficient, jumps to the step S 8  and no further operations are performed and the engine  29  cannot be started.  
         [0025]     If, however, the predetermined period of time has elapsed, the program moves to the step S 4  where an ID code check is performed. Radio waves are transmitted from the battery  12  to the key  15  via the antenna  16  to supply electric power to the transponder  19  to perform the ID code check. When a predetermined amount of electric power is charged in the transponder  19 , the unique ID code of the transponder  19  is transmitted to the controller  17  via the antenna  16 . The controller  17  reads the ID code with the ID reading circuit  21  and transmits it to the CPU  22 , where the read ID code is compared with the ID code registered in the memory  25  to determine whether they match with each other, that is, whether the key is the correct key for the controller  17  at the step S 5 .  
         [0026]     If the key  14  is determined to be the correct key, a canceling signal for canceling the start prohibition mode of the engine  29  and permitting the engine  29  to start is transmitted to the ignition control unit at the step S 6 . Then at the step S 7  the ignition control unit starts the engine  29 .  
         [0027]     On the other hand the code is different from the previously registered ID code, the key is determined to be an incorrect key and the engine  29  is maintained in the start prohibition mode by jumping to the step S 8 . Then, the operation is completed.  
         [0028]     Alternatively, the step S 3  may be programmed to be performed after the step S 5 . That is, the determination of the connection time of the backup power source may be conducted after the ID check.  
         [0029]     As described above, the vehicle cannot be illegally driven by someone other than the owner of the vehicle having the correct key  15 . Even if the immobilizer  11  mounted on the vehicle is illegally removed and replaced with a new one, the vehicle cannot be driven within a short period of time due to the necessity of the time set at step S 3  is reached. Thus, it is very difficult to steal the vehicle during temporary parking.  
         [0030]     When the key  15  is turned to the off position to turn off the main switch and then removed from the key cylinder  14  after the vehicle has been stopped, the engine  29  is stopped but backup power is kept supplied to the controller  17 .  
         [0031]     Thus from the foregoing description it should be readily apparent that the described constructions and methods a very effective and difficult to defeat vehicle antitheft protection is provided without interfering with the simplicity of instillation and servicing as well as reducing the cost thereof. Of course those skilled in the art will readily understand that the described embodiments are only exemplary of forms that the invention may take and that various changes and modifications may be made without departing from the spirit and scope of the invention, as defined by the appended claims.

Summary:
Simple but very effective vehicle antitheft methods and apparatus that avoid theft by replacement of the antitheft device by determining the time it has been before the device is powered up and prohibiting starting if it is not long enough.