Passive keyless entry system

Abstract: A keyless entry system for gaining entry to a vehicle (10) comprises a portable beacon including a transmitter for transmitting a coded beacon signal and a receiver associated with the vehicle (10), said receiver including an antenna for receiving said coded beacon signal when said beacon is within a predetermined range. Controller means are provided for decoding said beacon signal and activating a first predetermined function associated with the vehicle (10) upon receipt of a valid coded beacon signal from the beacon. With the keyless entry system the coded beacon signal is continually changing at a predetermined interval and said controller means include means for updating a receiver clock code at substantially predetermined interval to coincide with the changing coded beacon signal (Fig. 1). A respective method for performing predetermined functions associated with a vehicle is also disclosed.

Description: Background and Summary of the Invention The present invention relates to electronic keyless entry systems and in particular to a passive keyless entry system that is particularly adapted for use with automotive vehicles. Automotive keyless entry systems, when first introduced, typically included a numerical keypad located on the exterior door panel of the vehicle. The operator entered a unique multiple digit code on the keypad to automatically unlock or lock the vehicle. Recently, more sophisticated keyless entry systems for vehicles have been proposed which use a portable remote transmitter that is carried by the operator and a receiver located in the vehicle that is adapted to unlock the vehicle in response to the detection of a coded radio frequency signal or a coded optical signal received from the transmitter. Such systems require that the operator actuate a button or switch on the transmitter to initiate the transmission, similar to the operation of a conventional automatic garage door opener, in order to conserve battery life and prevent inadvertent actuation. While more convenient to operate than the keypad-type keyless entry systems, the latter transmitter/receiver-type systems nonetheless require that the operator physically locate the transmitter and actuate a button to unlock the vehicle. Hence, the convenience provided by such a system versus a conventional key and lock arrangement is not substantially improved. In addition, while other types of "keyless" entry systems are known and presently used in other applications, these systems for various reasons are generally not suitable for automotive use. This would include systems employing magnetic card readers, interrogation/transponder-type systems, and conventional automatic garage door openers. A magnetic card-type system is probably adaptable to automotive use, but provides little added benefit to justify the expense. Interrogator/transponder-type systems, though adaptable to operate in a totally passive manner, are more complex and therefore more expensive, and present a power consumption problem as the interrogator therefore must be on and interrogating at all times. In addition, the transponder in such systems must be capable of receiving as well as transmitting data, thus adding to cost. Lastly, automatic garage door systems, while appearing to employ similar technology have substantially different operating requirements which result in significantly different circuit designs. In particular, an automotive keyless entry system must possess a level of noise immunity and signal discrimination that is substantially greater than that required for an automatic garage door opener. This is due to several factors including the many different environments in which vehicles may be located, the greater number of vehicles that may be equipped with comparable systems, and the fact that large numbers of vehicles are frequently located within close proximity to one another, such as in parking lots. In addition, automatic garage door systems require that the operator actuate a transmitter, and therefore are not totally passive. Also, garage door opener-type systems have a substantial range and therefore can potentially activate a function, such as unlocking the trunk of the vehicle, when the operator is some distance away from the vehicle, and unaware the trunk has been opened. EP-A-0 343 619 discloses a passive keyless entry system for use with automotive vehicles which automatically unlocks the vehicle as the operator approaches the vehicle and locks the vehicle as the operator moves away from the vehicle. The system includes a portable beacon carried by the operator and a receiver/controller in the vehicle connected to an antenna. Transmission between the beacon and the receiver/controller is by a magnetically coupled radio frequency signal embodying differential phase encoded data with error correction coding of data to enhance noise immunity and signal discrimination. The signal transmission is of a constant code. EP-A-0 311 112 discloses a remote control system, which is not specifically intended for automotive use, in which a remote control transmitter and a remote control receiver each have a timer and the control signals passing between them consist of timer information generated from the timer of the remote control transmitter there being a comparator for comparing the timer information of the control signal with the timer information of the remote control receiver to control the latter when the compared pieces of timer information coincide. It is an object of the present invention to provide a passive keyless entry system having special anti-theft capabilities featuring a continuously changing coded transmission at predetermined intervals to deter a would-be thief from recording the transmission for later use to gain access to the vehicle. The system also has a programming capability to synchronise the coded beacon transmission with the receiver. In general, the passive keyless entry system according to the present invention comprises three basic components; a transmitter or beacon, a receiver/controller, and a receiving antenna. The beacon, which is small enough to be attached to a keychain, is carried by the operator and incorporates a motion sensor that is used to energise the transmitter portion of the beacon. The transmitter in the beacon is adapted to emit a coded radio frequency signal that contains a clock code, an identification code and function information, and an error correction code. Since the coded signal includes a clock code, the coded signal changes at predetermined intervals. The beacon is designed to continue to transmit repeatedly its coded signal until no motion is detected for a predetermined period of time. Thus, the motion sensor serves to promote beacon battery life and enables the present system to function in a totally passive manner. Neither of the prior publications referred to above transmits a coded signal which is continually changing and which therefore makes it very difficult for a recording of the signal to be used by a thief to gain entry to a vehicle fitted with the system. The receiving antenna comprises a simple coil of wire, wound to be sensitive at the transmitted frequency of the beacon. The antenna is located at a position on the vehicle to optimize the performance of the system. It has been found desirable to adapt the system so that the receiver/controller is responsive to the signal from the beacon when the beacon comes within 3 - 6 feet of the vehicle. The receiver/controller is mounted inside the vehicle and is adapted to operate on the vehicle's 12 volt negative ground battery. Upon receipt of a radio frequency signal from the antenna, the receiver/controller is adapted to process the coded radio frequency signal and evaluate the serial data contained therein. If the signal is determined to be valid, the receiver/controller automatically unlocks the driver's-side vehicle door. The receiver/controller also includes a clock which is originally synchronized with the clock code of the beacon. The receiver/controller will automatically resynchronize the receiver's clock to the clock code of the transmitted signal each time a valid signal is received. In addition, a tolerance algorithm will be incorporated in the receiver/controller such that the received coded signal will be valid within a predetermined tolerance depending on the time from the last reception of a valid beacon signal. Optionally, the beacon may be provided with function switches which, when depressed by the operator, change the function code contained in the radio frequency signal, thus directing the receiver/controller to open the vehicle trunk and/or perform other functions. In addition, the receiver/controller in the present system is adapted to automatically lock the vehicle doors when the operator leaves the vehicle and carries the beacon out of range of the receiver/controller, assuming the proper status of the ignition and doorjamb switches. The passive keyless entry system further includes a program mode by which the coded signal can be sent from the beacon to the receiver/controller in an uncoded format. The clock of the receiver/controller is originally synchronized with the clock signal of the beacon when the system is in the program mode. In the program mode, the receiver/controller can also resynchronize the uncoded clock and identification code signal from the transmitter with the clock in the receiver and store the clock and identification code in the receiver/controller. The program mode can also store a plurality of identification and clock codes in the receiver. Additional objects and advantages of the present invention will become apparent upon reading the following description of the preferred embodiment of the present invention which makes reference to the drawings in which: Brief Description of the Drawings Figure 1 is a diagrammatical view illustrating the preferred application of the present invention; Figure 2 is a partial cutaway view of the receiving antenna used in the present invention; Figure 3 is a timing diagram illustrating the Miller encoding technique used in the preferred embodiment; Figure 4 is a perspective view of the beacon/transmitter of the present invention; Figure 5 is a circuit diagram of the beacon/transmitter shown in Figure 4; Figure 6 is a block diagram of the beacon/transmitter shown in Figure 5; Figure 7 is a timing diagram illustrating the various signals produced by the timing controller 54 shown in Figure 6; Figure 8 is a block diagram of a linear N p stage shift register used to generate the error correction code polynomial used in the preferred embodiment; Figure 9 is a diagrammatical view of the steps used to generate the transmitted beacon code; Figure 10 is a detailed circuit diagram of the antenna driver circuit 66 of Figure 6; Figure 11 is a combined timing and circuit diagram illustrating the manner in which the bipolar driver signal is generated; Figure 12 is a signal diagram illustrating the waveform of the resulting bipolar drive signal supplied to the beacon antenna; Figures 13a - 13c are circuit diagrams of the receiver/controller according to the present invention; Figure 14 is a block diagram of the bipolar front end and digital data detect custom integrated circuits used in the receiver/controller; Figure 15 is a detailed block diagram of the clock generator circuit shown in Figure 14; Figure 16 is a detailed block diagram of the carrier synchronizer circuit shown in Figure 14; Figure 17 is a detailed block diagram of the lock detector circuit shown in Figure 14; and Figure 18 is a detailed block diagram of the data extraction circuit shown in Figure 14. Detailed Description of the Preferred Embodiment Referring to Figure 1, a diagrammatic view illustrating the application of the present invention as an entry system for an automobile 10 is shown. As depicted in the drawing, the beacon (concealed) is adapted to be carried by the driver 12 and is responsive to the motion created as the driver walks to energize the transmitter portion of the beacon. While energized, the transmitter portion of the beacon continuously transmits a coded radio frequency signal which includes clock, identification and function data, and an error correction code. When the driver comes within range of the vehicle--approximately three feet in the preferred embodiment--the receiving antenna (also concealed) located on the vehicle magnetically couples the transmitted beacon energy into the receiver/controller. The antenna 14 (Figure 2) in the preferred embodiment is located in the B-pillar 16 of the vehicle as this location has been experimentally determined to produce the most favorable results. However, other locations on the vehicle have been found to be acceptable. Moreover, in view of the magnetic coupling between the beacon and receiver, it is possible to control the field of the antenna by appropriate placement, orientation, and configuration of the antenna, and in this manner provide coverage in selected areas, such as in the vicinity of the trunk lid or the driver-side door handle. The receiver/controller, which is located in the vehicle, is adapted to process the coded radio frequency signal received from the antenna and perform the function requested if the clock and identification code are determined to be "valid", where a "valid" code corresponds to the code prestored in the receiver/controller. Optionally, the beacon may be provided with one or more input switches that can be used to transmit up to sixteen different function codes to control various functions in addition to unlocking the driver's side door, such as unlocking the trunk, turning on the interior lights, etc. Preferably, however, when none of the function switches are depressed, the receiver/controller is adapted to interpret the default function code as a command for unlocking the driver's side door of the vehicle. Alternatively, the "default" function of the system can be selected to unlock all doors of the vehicle, or any other function desired by the automobile manufacturer. In addition, the system is adapted to respond to the condition where the driver leaves the vehicle by automatically locking all doors of the vehicle a predetermined period of time after the beacon signal becomes too weak to receive. Turning to Figure 2, a partial cutaway view of the receiving antenna 14 used in the preferred embodiment is shown. The antenna 14 merely comprises a single wire, such as 34-gauge magnet wire, that is wrapped as shown at 18 the number of turns required to provide the desired inductance. Preferably, the magnet wire is wrapped around a stable form to maintain the shape of the antenna rigid so that the inductance of the antenna does not vary. A plastic shield 20 may be placed over the coil of wire 18 for protection. Although the drawing shows the antenna as having a circular shape, other configurations can be used (e.g., rectangular) to accommodate space constraints at the desired mounting location. In addition, it may be desirable to add a second grounded wire to the coil for electrical shielding purposes to improve the performance of the antenna. However, this also adds to the cost of the antenna. Communication Channel In order to conserve the battery power of the beacon and the vehicle battery, the transmission frequency in the preferred embodiment is selected to be a relatively low 98.3 KHz. The frequency of the transmission signal is therefore substantially above most of the low frequency noise that emanates from an automobile and yet the fifth harmonic of the transmission frequency is still below the AM radio band. Accordingly, interference with AM radio frequency signals is avoided. The beacon antenna comprises a coil of wire as described that is tuned to the selected carrier frequency. The antenna creates a magnetic field which couples to the coil of wire used for the receiving antenna. The change in flux caused by the receipt of a signal at the carrier frequency generates a voltage across the receiving antenna terminals. When the beacon is placed in the center of the receiving antenna, the resulting voltage can be as high as 2 volts peak-to-peak. However, as the beacon moves a few feet away from the center of the receiving antenna, the voltage may drop to less than 15 microvolts peak-to-peak. This wide variation in signal level comprises one of the major constraints in the selection of appropriate communication channel architecture. Although many different types of carrier modulation schemes may be employed, including frequency shift keying, amplitude shift keying, etc., phase shift keying ("PSK") has been selected for the preferred embodiment. In PSK encoding, the frequency stays the same, but the phase is shifted exactly 180 degrees to differentiate a logical "1" from a logical "0". This scheme uses a minimum bandwidth, and is very simple to implement in the transmitter. In particular, PSK encoding may be accomplished using an exclusive-OR gate with the carrier frequency supplied to one input and the data signal provided to the other. An additional benefit of PSK encoding is energy efficiency. Specifically, PSK encoding uses all of the transmitted energy for information with no energy being separately applied to generate a carrier. Moreover, front end signal processing of PSK encoding can be accomplished at the receiver using a limiter instead of an automatic gain control circuit. Hence, since PSK encoding is defined in terms of phase, the received signal will not suffer when passed through a limiter. The primary disadvantage of PSK encoding, on the other hand, is the need to regenerate the carrier at the receiver and thus properly synchronize the receiver to the coded transmitted signal. Error Correction Code While the use of many different coding schemes is possible, the preferred embodiment of the present keyless entry system uses a coding scheme that employs an error correction code to improve system performance. Specifically, the use of an error correction code allows the system to properly receive a coded transmission which may have been partially altered by noise. This improves reception performance and the response time of the system. In addition, the use of error correction codes results in a reduction of false detections. A false detection occurs any time the receiver/controller indicates the detection of a valid beacon key code when the valid code was not transmitted. This can occur due to random noise, or the presence of another beacon with a similar code that is incorrectly decoded into a valid code. The coding scheme used in the preferred embodiment comprises a 48-bit code with two repetitions. The first four bits in the coded transmission comprise preselected synchronization bits which produce eight transitions that are illegal under the Miller code. The four SYNC bits are followed by a 4-bit FUNCTION code, and a 20-bit TRANSFORMATION code. The TRANSFORMATION code is the result of passing a 20-bit IDENTIFICATION code and a 20-bit CLOCK code through certain logic gates to get a combined identification and clock code as will be described in more detail below. A 24-bit error correction code ("ECC") is then added to the 24 bits of FUNCTION and TRANSFORMATION code. The 4-bit FUNCTION code, as previously noted, provides up to sixteen different FUNCTION codes to selectively control the activation of additional functions as desired, such as trunk unlock, unlock all doors, turn on interior lights, etc. In addition, it will be appreciated that the 20-bit TRANSFORMATION code virtually eliminates the probability that two beacons with the same transformation code will be present in the same vicinity at the same time. In that the SYNC bit pattern is the only mechanism that the receiver/controller has to synchronize onto the data for the purposes of Miller decoding and extraction of the data bits, it is possible for noise to generate a pattern which will cause the receiver/controller to begin to read a code word at the wrong place. Moreover, it is the nature of cyclic codes that any shifted version of a legal code word is itself a legal code word. The improperly synchronized code word could, therefore, become a valid code word for another vehicle and thus cause an improper unlocking of a door or activation of another function. To overcome this problem, the coding scheme in the preferred embodiment employs an anti-slip pattern that is superimposed onto the 48-bit code word making an improperly synchronized code word invalid. Specifically, the 48-bit FUNCTION, TRANSFORMATION, and ECC code word is exclusive-ORed with a predetermined 48-bit anti-slip pattern. The resulting output from the exclusive-OR gate is then provided to a Miller encoder which produces the final encoded transmitted word. The reason for employing the Miller encoding technique is as follows. One of the characteristics of PSK encoding is the ambiguity at the receiver. This occurs because of the exact 180 degree phase shift. Since the signal received for a digital "1" is essentially the negative of the signal transmitted for a digital "0", it is impossible to distinguish one from the other without some prior knowledge. This problem is effectively eliminated by using an edge sensitive data encoding scheme such as Miller encoding which has been selected for the present keyless entry system. In Miller encoding, when a logical "1" is transmitted, a transition always occurs in the middle of the bit cell. When a logical "0" is transmitted, a transition occurs at the beginning of the bit cell only if the previously transmitted bit was also a logical "0". Otherwise, no transition occurs. Miller encoding is utilized in the preferred embodiment because it is relatively simple to implement at both the transmitter and receiver/controller. Miller encoding is defined by the following table: D n-1 D n TD 2n TD 2n+1 0 0 1 0 0 1 0 1 1 0 0 0 1 1 0 1 where n is an integer 0..last n. D n-1 is the previously transmitted data bit. When n = 0, D n-1 is defined to be 1. D n is the current data bit. TD 2n is the first of two transition bits for each data bit. where 1 indicates a change in the state of the encoded data signal, and 0 indicates no change. TD 2n+1 is the second of two transition bits for each data bit. where 1 indicates a change in the state of the encoded data signal, and 0 indicates no change. The Miller encoding table is illustrated for convenient reference in Figure 3. As previously noted, it is necessary for the receiver to identify the beginning of the encoded transmission word and for this purpose a 4-bit SYNC pattern is added at the beginning of the code word which presents an illegal pattern for Miller encoding. Because of this illegal nature of the SYNC pattern, it can always be differentiated by the receiver from the code word. The SYNC pattern used in the preferred embodiment is defined by the transition string: 01000001, where "1" indicates a change in the state of the encoded data signal and "0" indicates no change. In addition, it will be recognized that the receiver/controller must separate the signal coming through the data channel into individual bits. Since the bits are separated in time, this requires a clock. The preferred solution to this problem is a self-clocking code. A self-clocking code is characterized by a reasonable number of transitions that are well-defined with respect to the data bits. It will be appreciated, therefore, that since Miller encoding ensures the presence in the transmitted encoded word of a reasonable number of transitions, the use of Miller encoding also satisfies the requirement for a self-clocking code. Beacon/Transmitter Turning now to Figure 4, a perspective view of the beacon/transmitter 24 according to the present invention is shown. In the preferred embodiment, the beacon is packaged in a relatively small, generally rectangular housing 26 approximately 2 inches by 1.5 inches by 0.5 inches in size and having an opening 28 for conveniently attaching the beacon to a keychain. The beacon 24 illustrated in Figure 4 includes two manually operable switches 30 and 32 that are operative to unlock the trunk of the vehicle and unlock the passenger doors. As previously indicated, the beacon may alternatively be provided with additional function switches to control the selective activation of additional functions, such as turning on the interior lights of the vehicle. As will be described in more detail below, the function switches can also act to put the beacon in a program mode such that the IDENTIFICATION and CLOCK codes will be transmitted in an uncoded format. As also previously noted, in the default mode where none of the function switches are depressed, the beacon is adapted to transmit the "unlock driver's-side door" function code. Referring now to Figure 5, a circuit diagram of the beacon/transmitter 24 according to the present invention is shown. In a preferred embodiment, the beacon circuit is comprised of a single custom integrated circuit 40, a battery 42, a coil of wire acting as a transmitting antenna L1, a motion detector 44, a quartz crystal 46, a counter circuit 43 and the two function switches 30 and 32, as well as other electrical components, all mounted to a printed circuit board. The battery 42 utilized in the preferred embodiment is a conventional 3 volt lithium manganese dioxide watch battery. Similarly, the quartz crystal 46 used in the preferred embodiment comprises a conventional 32.768 KHz watch crystal that is used to control the operating frequency of the beacon. In particular, the energy contained in the third harmonic of the crystal frequency signal is used in the antenna driver for both antennas. The beacon antenna L1 consists of a coil of wire and a tuning capacitor C1, the values of which are tuned to the third harmonic, 98.304 KHz. The counter circuit 43 is designed to emit a signal on the "real time interrupt" line every predetermined number of clock intervals from the crystal 46 to update the continually changing CLOCK code as will become apparent from the discussion below. In one preferred embodiment, the counter circuit 43 will emit an interrupt signal approximately every five seconds. The motion detector 44 is a device that is responsive to very slight movements and is adapted to continuously cycle on and off when movement is sensed. In addition, the preferred motion detector is equally sensitive to motion in any orientation of the beacon. A motion detector suitable for use in the present application is disclosed in U. S. Patent No. 4,833,281 and assigned to the assignee of the present application. A block diagram of the beacon/transmitter 24 according to the present invention is illustrated in Figure 6. The signal from the motion sensing switch 44 is provided to a motion detector and oscillator enable circuit 50. This interface circuit 50 is always active and is adapted to monitor the state of the motion sensing switch 44 and produce a timed oscillator enable signal at its output 51 whenever a change in the state of the motion sensing switch 44 is detected. In a preferred embodiment, the timed oscillator enable signal comprises a 26-second pulse. If motion continues to be detected, successive enable signals will be produced so that the oscillator enable signal on line 51 will terminate 26 seconds after all motion of the beacon has ceased. To conserve battery life, the motion detector and oscillator enable circuit 50 is the only circuit that remains active at all times. In response to the oscillator enable signal on line 51, the oscillator circuit 52 is activated to produce the 32.768 KHz crystal clock signal ("XTAL") at its output. The 32.768 KHz crystal clock signal is provided to a timing controller circuit 54 that is adapted to produce the various timing signals used for generating the code word, initializing the various circuits, and sequence control. The various timing signals produced at the output of the timing controller 54 are illustrated in the timing diagram shown in Figure 7. In particular, the XTAL divide-by 16 and XTAL divide-by 32 clock signals are produced at the Clock [0-2] output lines; the 0-51 State Count corresponding to the 48-bit code word plus the four SYNC bits, is produced at the State Count [0-5] output lines; and the Parity, Miller, FUNCTION, and TRANSFORMATION signals are produced at the Control [0-3] output lines. The State [0-1] output line is provided to the encoded data generator 64 and supplies a sync pulse or end-of-message pulse at the end of each complete cycle. A debounce circuit 56 is shown connected to four function switches although, as previously noted, the preferred embodiment herein utilizes only two function switches 30 and 32. The four function outputs from the debounce circuit 56 are connected to the message generator circuit 58 which controls the content of the TRANSFORMATION and FUNCTION information produced at the MESSAGE output. The status of the four function inputs determines the content of the 4-bit FUNCTION code. An override output puts beacon 24 in a program mode upon the proper operation of the function switches as will be discussed in more detail below. The 20-bit TRANSFORMATION code (0 - 19) is also applied to the message generator 58. As will be described below, the encoded signal is continuously being updated. The MESSAGE output from the message generator 58 comprises a 24-bit serial output signal that is provided to the error correction code (ECC) encoder 60. The ECC encoder 60 is adapted to generate a 24-bit error correction code based upon the content of the MESSAGE code received from the message generator 58. The parity bits, or error correction code, are generated using a linear N p stage shift register connected as illustrated in Figure 8, where: g i is the ith coefficient of the generator polynomial g(X) = 1 + g 1 X + g 2 X 2 + ... + g n-k-1 Xn-k-1 + g n-k Xn-k , n is the total number of bits in the code word, in this case 48, k is the number of message bits in the code word, in this case 24, b i is the value stored in the ith register, + indicates an exclusive OR-gate, AND indicates an AND-gate, switch is a multiplexer, and "Message" is the bit pattern consisting of the FUNCTION code followed by the TRANSFORMATION code. The specific generator polynomial used in the preferred embodiment is as follows: g(X) = 1 + X = X 2 + X 4 + X 5 + X 6 + X 8 + X 9 + X 10 + X 13 + X 16 + X 17 + X 19 + X 20 + X 22 + X 23 + X 24 . The anti-slip pattern generator 62 generates a fixed 48-bit pattern at the same clock rate as the serial output from the ECC encoder 60. As previously noted, the anti-slip pattern is superimposed onto the code word to prohibit the receiver from detecting a valid code from another beacon with a similar code that is shifted in position relative to the valid beacon code. This serves to reduce the probability of false detection from other beacons. The anti-slip pattern used in the preferred embodiment which begins in bit position 0 is as follows: 1100 1100 1011 0101 1111 0000 0000 1111 1010 1101 0011 0011. The two serial outputs from the anti-slip pattern generator 62 and the ECC encoder 60 are provided to the encoded data generator 64. The encoded data generator 64 is adapted to exclusively-OR the 48-bit anti-slip pattern with the 24-bit MESSAGE and 24-bit ECC code word and then Miller encode the data. In addition, the encoded data generator 64 also adds the 4-bit SYNC code to the beginning of the Miller encoded data word. The resulting encoded data word is serially provided to antenna drive circuit 66 which is connected to the tuned antenna circuit 68. In order to deter theft and virtually eliminate the possibility of receiving a valid coded frequency signal from an invalid source, a CLOCK code, which has been synchronized in both the beacon/transmitter and the receiver/controller is utilized. At predetermined intervals the valid coded signal changes in response to the changing CLOCK code. Thus, a would-be thief would have to record the current coded frequency signal, and play it back before the elapsed interval. Even though the beacon clock and the receiver clock are originally calibrated to each other, the receiver clock will be running slightly faster or slower than the beacon clock due to crystal accuracy and stability. The receiver/controller is therefore designed to resynchronize the receiver clock with the CLOCK code received from the beacon each time a valid coded frequency signal is received. However, the time between periodic uses of the system is not always accurately determinable for a wide range of applications. Therefore, the receiver can be set to allow for certain CLOCK code discrepancies or tolerances between the beacon clock and the receiver clock over a certain amount of time. Notably, the probability of failed system integrity increases with the acceptance of a wide range of inaccuracy between the beacon clock and the receiver clock. The receiver is generally set such that a valid TRANSFORMATION code can be received within a certain period of time, generally about a year, after which the receiver has to be reprogrammed to the beacon. According to one embodiment, in order to generate the changing or rolling TRANSFORMATION code in the beacon 24, an initial CLOCK code is stored in a register 57. The register 57 outputs the initial CLOCK code in a parallel format to a CLOCK code increment circuit 59. The CLOCK code increment circuit 59 receives the real time interrupt signal at regular intervals from the counter 43, as discussed above, and a 20-bit CONSTANT input from a register 61. The register 61 also receives the real time interrupt signal such that the register 61 outputs the CONSTANT to the CLOCK code increment circuit 59 at each interval. In this manner, when the counter 43 receives a predetermined number of clock pulses from the crystal 46, it generates an output on the real time interrupt line such that the initial CLOCK code from the register 57 is added to the CONSTANT in the register 61 in order to alter or update the CLOCK code at a set interval rate. The updated CLOCK code is stored in a register in circuit 59 such that at the next interval the CONSTANT is added to the previous updated CLOCK code, and so forth. In one preferred embodiment, the counter circuit 43 initiates an interrupt signal at approximately every five seconds. An output from the CLOCK code increment circuit 59 is applied to an exclusive-OR gate 65 along with the output from an IDENTIFICATION code stored in register 63. This enables each bit from the updated CLOCK code to be exclusively-ORed with the representative bit from the 20-bit IDENTIFICATION code in order to generate a new code at the output of the exclusive-OR gate 65. This new code (TEMP) is applied to a scrambler circuit 67 which, in turn, scrambles the code in a random fashion in order to reduce the probability that a would-be thief could anticipate the changing code. The resulting output of the scrambler circuit 67 is the TRANSFORMATION code which is applied to the message generator circuit 58, as discussed above. In the event that the beacon 24 needs to be resynchronized with the clock code of the receiver or a new beacon needs to be synchronized, the beacon 24 can be put into a program format. To do this, it has been proposed to set the program mode of the beacon 24 by depressing one or a series of the beacon function switches. If the proper sequence of the function switches is depressed, an override signal from the function switches of the debounce circuit 56 will be applied to the antenna drive circuit 66 and a code change override circuit 69. Also applied to the code change override circuit 69 is the IDENTIFICATION code from the register 63 and the CLOCK code from the CLOCK code increment circuit 59. In addition, the states of the function switches are applied to the code change override circuit 69. If the override line to override circuit 69 is set high, then the override circuit 69 outputs serially the function information, the IDENTIFICATION code and the CLOCK code to the antenna drive circuit 66, as shown. The override line further disables the antenna drive circuit 66 from sending the encoded data from the encoded data generator 64. Therefore, the receiver can receive the uncoded beacon data to be re-synchronized and store the new CLOCK code to maintain the systems anti-theft integrity or synchronize additional beacons or a new beacon/transmitter in the event the original one is lost. To summarize, the various steps for generating the transmitted beacon code are diagrammatically illustrated in Figure 9. Initially, the 4-bit FUNCTION code is determined in accordance with the status of the various FUNCTION switches. The 20-bit IDENTIFICATION code and the 20-bit CLOCK code are combined to form a temporary (TEMP) code. The TEMP code is scrambled which results in the TRANSFORMATION code, and is combined with the FUNCTION code, as shown. Based upon the content of the MESSAGE, 24 parity bits are generated using the ECC polynomial. The parity bits are then appended to the MESSAGE resulting in a 48-bit code word. Each of the 48 bits in the code word is then exclusive-ORed with each bit of a predetermined 48-bit anti-slip pattern. The resulting 48-bit code word is then Miller encoded and a 4-bit SYNC pattern, illegal under the Miller code, is added to the beginning of the Miller encoded word. Turning now to Figure 10, a detailed circuit diagram of the antenna drive circuit utilized in the preferred embodiment is shown. The antenna drive circuit is designed to meet the following objectives: (1) high output efficiency; (2) average battery drain of less than 40 microamps over normal operating conditions; (3) frequency tripling to allow the use of an inexpensive 32.768 KHz crystal oscillator; (4) a reasonably clean signal spectrum; (5) freedom from latch-up during normal operation; and (6) a small number of external components. The preferred embodiment of the present beacon/transmitter utilizes a bipolar antenna drive circuit 66 which serves to inject as much energy as possible into the antenna to improve the efficiency of the beacon. The bipolar drive circuit 66 produces narrow drive pulses that are provided to the antenna circuit every 1.5 cycles, alternately driving the antenna circuit toward the battery voltage and toward ground. With this approach, an output capacitor C2 is required, and the signal decays for only 1.5 cycles between pulse injection. An additional benefit of the bipolar drive circuit is that the antenna oscillates at a peak-to-peak voltage equal to the battery voltage. To generate the bipolar drive signal, the XTAL clock signal is provided through a time delay circuit 70 to produce a DELAYED XTAL clock signal. The XTAL and DELAYED XTAL clock signals are then provided to the inputs of an AND-gate equivalent 72 and a NOR-gate 74 to produce a first narrow pulse train on line 76 and a second narrow pulse train on line 78 shifted by 180 degrees relative to the signal on line 76. The waveform illustrated above the narrow pulse train waveforms represents the phase-shifted difference between the XTAL and DELAYED XTAL clock signals. The Encoded Data signal on line 80 is clocked through a D flip-flop 82 by the XTAL clock signal to provide non-inverted and inverted Encoded Data signals on output lines 84 and 86, respectively. The non-inverted and inverted Encoded Data signals on lines 84 and 86 and the first and second narrow pulse train signals on lines 76 and 78 are provided to a logic gate network 90 having two output lines 92 and 94. The logic gate network 90 functions in the following manner. When a clock pulse is present on line 76, the presence of a logic "1" in the Encoded Data word will result in a HI signal pulse being produced on output line 94 and the presence of a logic "0" in the Encoded Data word will result in a LO signal pulse being produced on output line 92. Conversely, when a clock pulse is present on line 78, the presence of a logic "1" in the Encoded Data word will result in a HI signal pulse being produced on output line 92 and the presence of a logic "0" in the Encoded Data word will result in a LO signal pulse being produced on output line 94. The output signal on line 92 is inverted by invertor 96 and the two resulting parallel data signals are then provided to the gates of two N-type and P-type FET transistors Q1 and Q2, as illustrated in Figure 11. Note that the period of the two resulting signals provided to the FETS Q1 and Q2 is equal to 1/32.768 KHz or 30.52 microseconds. The resulting bipolar drive signal supplied to the antenna circuit 68 is illustrated in Figure 12. As can be seen from the waveform illustrated in Figure 12, the bipolar drive circuit 66 injects energy into the tuned antenna circuit 68 every one-and-a-half cycles, thereby optimizing the amount of energy injected into the antenna L1 and improving the efficiency of the beacon. Receiver/Controller Referring now to Figures 13a - 13c, a circuit diagram of the receiver/controller 100 according to the present invention is shown. The receiver/controller 100 is adapted to receive the radio frequency signal received by the antenna, detect the presence of a beacon signal, demodulate the signal, and determine the content of the serial information being transmitted. If the transmission is determined to be a valid beacon code, the receiver/controller 100 further performs the instructed function corresponding to the function code in the received transmission. In general, the receiver/controller 100 comprises a microcomputer 102 for performing logical and mathematical calculations, receiver circuitry for receiving and detecting the presence of a beacon transmission, a non-volatile memory device 120 for storing valid IDENTIFICATION codes and CLOCK codes, and output circuitry for controlling various vehicle features such as door and trunk locks. In addition, the microcomputer 102 is programmed to resynchronize the microcomputer's CLOCK code to the beacon's CLOCK code when the receiver/controller receives a valid beacon transmission or the system is in the program mode. The two inputs 116 and 118 from the receiving antenna are provided to an analog receiver circuit that is implemented in the preferred embodiment with a custom integrated circuit 104. The analog bipolar receiver circuit 104 comprises a single conversion superheterodyne receiver having an intermediate frequency of 4.274 KHz. The signal received from the tuned antenna coil comprises a voltage proportional to the magnetic field received from the beacon. This signal is amplified by a preamplifier, filtered, and mixed with a voltage controlled oscillator ("VCO") frequency signal to create an intermediate frequency ("IF"). The intermediate frequency signal is further amplified, filtered, and limited to CMOS signal levels for processing by the digital data detector circuit 106. The digital data detector circuit 106, which is also implemented in the preferred embodiment with a custom integrated circuit, is adapted to detect the presence of a beacon signal and produce an output signal to "wake up" the microcomputer 102. The digital data detection circuit 106 also demodulates the PSK encoded signal from the beacon. Data synchronization circuitry extracts the clock from the resulting Miller encoded signal and clocks the data into the microcomputer. To minimize average power consumption of the receiver/controller 100, the digital data detection circuit 106 also signals the microcomputer 102 to go into a stand-by mode when the beacon is out of range. A counter circuit 127 receives the receiver/controller clock pulses from a crystal 110 and generates a signal on the real time interrupt line after a predetermined number of clock pulses, as described above for the counter circuit 43 in Figure 5. In this manner, the receiver/controller 100 can maintain the same rate of changing CLOCK codes as with the beacon 24. More particularly, the microcomputer 102 performs the same CLOCK code update procedure as does the beacon 24 in order to update the CLOCK code in the same manner as the beacon 24. In other words, the microcomputer 102 stores the initial or updated CLOCK code, and the CONSTANT added to the CLOCK code in order to maintain a rolling CLOCK code which is synchronized with the rolling beacon CLOCK code. The microcomputer 102 includes a built-in tolerance by which an extracted CLOCK code can be deemed valid with a predetermined number of errors. Typically, these allowable errors will increase the longer the receiver/controller 100 has not received a valid signal from the beacon 24. Once a valid signal is received, the tolerance is reset to zero, hence synchronizing the CLOCKS. In this regard, the bipolar front end receiver circuit 104, the digital data detection circuit 106, and the voltage regulator 112 are the only circuits that remain active at all times. In the preferred embodiment the total quiescent current draw for the receiver/controller 100 is less than one milliampere. Thus, vehicle battery power is conserved. The microcomputer 102 used in the preferred embodiment comprises an 840 Series 8-bit microcomputer manufactured by National Semiconductor. The microcomputer is programmed to decode the Miller encoded signal received from the digital data detection circuit 106 and compare the resulting bit patterns with the data previously stored in the non-volatile memory 120. In addition, the microcomputer 102 takes the extracted CLOCK code and compares it to the CLOCK code being monitored by the microcomputer 102. If the extracted CLOCK code is within a predetermined tolerance of the previously stored microcomputer CLOCK code, the microcomputer 102 will automatically resynchronize and store the extracted CLOCK code, and reset the tolerance to zero. Based upon the results of these comparisons, the microcomputer 102 is further programmed to control the activation of various functions such as door and trunk locks. A separate algorithm stored in the microcomputer is provided for programming new identification codes into the non-volatile memory 120. Significantly, in the preferred embodiment, the EEPROM 120 has the capacity for storing and the microcomputer 102 is programmed to accept and check for more than one valid beacon code. In this manner, several different beacons can be validated and used in conjunction with a single receiver/controller 100. In addition, the receiver/controller 100 includes interface circuitry 112 including a power regulator and a transient suppressor to isolate the receiver/controller circuitry from noise on the 12-volt battery lines. The interface circuitry 112 also provides various regulated power supply voltages. The various additional inputs to the receiver/controller circuit 100 serve the following functions. The Key Switch input is grounded when the key is in the ignition and serves to inhibit the keyless entry system by grounding the various output lines from the microcomputer 102 to the relay driver circuits that activate the doors and trunk lock mechanisms. The Door-jamb input is used by the microcomputer 102 in combination with the Key Switch input to automatically lock all of the vehicle doors when the beacon is out of receiving range. In particular, the microcomputer 102 is programmed to automatically lock all of the vehicle doors a predetermined period of time after the beacon is out of range and all of the vehicle doors have been closed, but only if the key is not in the ignition. The Hatch or trunk lid input signal is provided as a feedback signal to the microcomputer 102 to prevent repetitive actuation of the trunk or hatch unlock solenoid. The Manual Lock and Manual Unlock inputs are provided to the receiver/controller 100 to manually override the system regardless of the state of the microcomputer 102. In particular, actuation of the manual lock and/or unlock buttons on the vehicle will override the microcomputer 102 and activate the appropriate lock and/or unlock solenoids regardless of the state of the control outputs from the microcomputer 102. The Program input is used to activate the receiver/controller programming mode. This is generally accomplished by depressing a switch or combination of switches on the vehicle's dashboard. The program mode is also only activated if the key is in the run position. Once the program mode is activated and the beacon 24 is brought into range of the antenna 14 while the beacon 24 is transmitting data in an uncoded format, a new IDENTIFICATION code can then be programmed into the non-volatile memory 120 of the receiver/controller 100 and the receiver/controller CLOCK code can be synchronized with the CLOCK code of the beacon. Specifically, to program a new beacon IDENTIFICATION code into the controller the Program input line is grounded for a predetermined period of time. The ID and CLOCK code from the beacon is thereupon read by the microcomputer 102 and the ID code is put into the EEPROM 120 and the microcomputer 102 stores the new CLOCK code. In this manner the receiver can be resynchronized or, if a beacon is lost, a new beacon with a different IDENTIFICATION code can be provided and the new beacon ID and CLOCK code conveniently programmed into the receiver/controller 100. If the receiver or beacon loses power, the system will have to be reprogrammed. The output terminals from the microcomputer 102 are provided to various relay driver circuits that serve to activate the various lock and unlock door and trunk mechanisms. In addition, it is preferred that the relay driver circuits that interface with external relays or solenoids include short circuit protection circuitry to protect the microcomputer 102 and the relay driver circuits in the event of a short in the external relays or solenoids. The various relay driver circuits function in essentially the same manner. Therefore, the following description of the Unlock Driver's Door relay driver circuit can be considered applicable to the remaining relay driver circuits as well. When the microcomputer 102 determines from decoding the function code from a valid beacon transmission that the driver's door is to be unlocked, the microcomputer 102 produces a logic HI signal at output port L5 on pin 16. The HI signal on line 124 serves to bias the Darlington transistor Q4 into full conduction, thereby energizing the relay coil of relay RX1. Energization of relay coil RX1 in turn serves to energize a motor (not shown) that is operatively connected to the lock mechanism of the driver's door and is effective to unlock the door. As previously noted, if the manual lock or unlock buttons in the vehicle are actuated, the receiver/controller 100 is overridden regardless of the state of the microcomputer 102. In particular, if the manual unlock button is actuated, a positive signal pulse is produced on line 126 which results in a corresponding positive signal pulse being provided at node 128, designated "Point A" in the circuit diagram. The positive signal pulse at node 128 is effective to immediately turn on Darlington transistor Q4 as well as Darlington transistor Q5 to thereby energize both relays RX1 and RX2 and unlock the driver's side and passenger doors. Similarly, if the manual lock button is actuated, a positive signal pulse is provided on line 130 which results in a positive signal pulse being provided at node 132, designated "Point B" in the circuit diagram. The positive signal pulse at node 132 is effective to turn on Darlington transistor Q6 which in turn energizes relay coil RX3 and locks all of the vehicle doors. As also previously noted, the receiver/controller 100 in the present keyless entry system is adapted to inhibit system operation whenever the ignition key is in the ignition. Specifically, upon insertion of the key in the ignition, a LO signal pulse is produced on line 134, designated "Point C" in the circuit diagram, which results in a corresponding LO signal being provided on line 136 to the positive input of a comparator 138. This in turn causes the output of comparator 138 to go LO, thereby pulling nodes 140 and 142 to ground potential and inhibiting the output ports L5 - L7 and G0 of the microcomputer 102. Accordingly, it will be appreciated that when the key is in the ignition, the microcomputer 102 is inhibited from activating switching control transistors Q4 - Q6 and Q9 in the various relay driver circuits, thereby effectively inhibiting operation of the system. The additional circuitry 144 shown in the "unlock hatch" drive circuit is provided to detect a short circuit in the remotely located unlock hatch solenoid (not shown) and, in such event, pulse width modulate the Darlington transistor Q9. Optionally, additional circuitry 114 may also be provided at the antenna input 118 to provide a continuity test for the antenna connections. Turning now to Figure 14, blocked diagrams of the custom integrated circuits 104 and 106 utilized to process and demodulate the incoming signal from the antenna are shown. The analog receiver IC 104 comprises a very low power amplifier, filter, and converter circuit. The circuit accepts a narrow band PSK signal at 98.304 KHz from the tuned antenna circuit. The signal is amplified and mixed down to an intermediate frequency ("IF") of 4.274 KHz in the preferred embodiment. The resulting IF signal is filtered and amplified with the last stage performing a limiting function. The resulting output signal is a 0 - 5 volt square wave signal with an approximately 50 percent duty cycle. With particular reference to the drawing, the two output lines, 116 and 118, from the antenna are provided to a differential amplifier 150 which minimizes common mode noise and amplifies the antenna signal. The output from the differential amplifier 150 is provided to a high Q, very narrow bandwidth, bandpass filter having a center frequency of 98.304 KHz, the frequency of the beacon transmission. The output from the bandpass filter 152 is in turn provided to a mixer 154 which converts the incoming signal down to an intermediate frequency signal of 4.274 KHz in the preferred embodiment. More particularly, the mixer 154 is adapted to take the difference in frequency between the incoming signal from the bandpass filter 152 and the output signal from the voltage controlled oscillator (VCO) circuit 160. The voltage controlled oscillator 160 in the preferred embodiment is designed to lock onto a frequency signal of 102.578 KHz, thus providing the 4.742 KHz differential intermediate frequency between the VCO frequency signal and the beacon frequency signal. The IF signal on line 155 is amplified by an amplifier circuit 156 and thereafter provided to a limiter circuit 158 which converts the signal to a square wave signal having approximately a 50 percent duty cycle. The resulting output signal from the limiter circuit 158 on line 159 is provided to the digital data detection circuit 106. The digital data detector 106 performs the following functions. A clock generator circuit 162, shown in greater detail in Figure 15, has an oscillator circuit 184 which uses the 32.768 KHz crystal 110 (Figure 13a) to generate a 32.768 KHz crystal clock signal (XTAL CLK). The clock generator circuit 162 also receives the VCO clock signal, and contains divider circuits 163 and 165 which divide down the VCO clock signal. These signals are used in the other blocks as timing signals. The carrier synchronizer 164 combines with the voltage controlled oscillator 160 in the bipolar front end chip 104 to make up a phase lock loop circuit (PLL). The PLL circuit recreates the carrier frequency necessary to decode the PSK signal. Rapid frequency acquisition and phase synchronization with a detected beacon signal are enhanced by a frequency sweep circuit that is controlled by a counter circuit which compares the VCO frequency to the crystal oscillator frequency when a beacon is not in range. In this manner, the frequency of the VCO circuit is made to gradually sweep up and down around the 102.578 KHz frequency to thereby keep the frequency of the VCO signal within the vicinity of the expected frequency of a beacon transmission. The lock detector circuit 166 is adapted to produce a "µP wake-up signal" when a beacon signal has been detected and the receiver is properly synchronized to the beacon signal. In other words, the lock detector circuit 166 is adapted to activate the microcomputer 102 when it is determined that a beacon is present and the beacon signal has been "locked" onto. The microcomputer 102 is programmed thereafter to determine if the received beacon signal contains a valid beacon code. The lock detector circuit 166 essentially comprises a quadrature lock detector and a digital filter. When the phase lock loop circuit acquires both phase and frequency, the lock detector output from the lock detector circuit 166 goes HI. The µP wake-up signal similarly comprises a pulse which causes the microcomputer 102 to enter its active state. The data extraction circuit serves to demodulate the received signal to recover the data bit stream. A matched filter is included to remove noise from the signal and additional circuitry decodes the Miller encoded recovered transition bit stream. The decoding circuitry includes a clock synchronizer to regenerate and synchronize to the data sample clock which is required to demodulate the Miller encoded signal. Referring now to Figure 16, a more detailed block diagram of the carrier synchronizer circuit 164 is shown. As previously noted, the purpose of the carrier synchronizer 164 is to recreate the carrier frequency signal in proper phase with the received beacon transmission. The regenerated carrier signal is required to demodulate the PSK encoded beacon transmission. The carrier synchronizer 164 includes the circuit components which, together with the VCO circuit 160 in the bipolar front end IC 104, comprise a long loop phase locked loop circuit with frequency doubling. In particular, the output signal from the bipolar front end IC 104 on line 159 is provided to a shift register 170 which shifts the phase of the incoming signal by approximately 90 degrees. The incoming signal on line 159 together with the phase shifted signal on line 171 are then provided to a frequency doubler 172 which mixes the two signals to provide a resulting output signal on line 174 having a frequency double that of the input frequency signal on line 159. The doubled frequency output signal (DBL OUT) on line 174 is provided to one input of a phase error circuit 176 which has its other input connected to the output of a 90-degree delay circuit 173. The 90-degree delay circuit 173 delays the VCO DIV 12 signal from the clock generator circuit 162 by 90 degrees. The phase error block 176 in the preferred embodiment comprises an exclusive-OR gate which, in a digital system, provides a very close approximation to the mixer used in analog phase lock loop circuits. The duty cycle of the output signal from the phase error circuit 176 reflects the amount of phase error between the VCO DIV 12 signal and the doubled IF signal on line 174. Exactly 50 percent duty cycle corresponds to a zero phase error. Any other duty cycle causes the charge pump circuit 178 to produce a voltage signal on output line 180 which is fed back to the VCO circuit 160 to correct the VCO frequency and reduce the phase error. When the phase lock loop circuitry has acquired both the frequency and phase of the beacon signal, the LOCK DETECT signal provided to the charge pump circuit 178 to disable the frequency sweep function. In order to minimize the amount of time it takes for the receiver/controller to lock onto the beacon transmission, a 32.768 KHz watch crystal 110 (Figure 13a) comparable to the watch crystal 46 in the beacon, is included in the receiver circuitry to provide a fixed frequency signal that closely approximates the expected frequency of the beacon transmission. Returning to Figure 15, the clock generator circuit 162 includes a 32.768 KHz oscillator circuit 184 that is connected to the watch crystal 110 and is adapted to produce a fixed frequency output signal on line 185 (XTAL CLK) substantially equal to the crystal frequency of the beacon signal. The XTAL CLK signal on line 185 is provided through a pulse generator 186 which divides the frequency of the XTAL clock signal by a factor of 3. The resulting pulse generator output signal on line 182 is provided to the carrier synchronizer circuit 164 and is used to keep the frequency of the output signal from the voltage controlled oscillator 160 within the vicinity of 102.578 KHz. In particular, the pulse generator output signal on line 182, together with the VCO DIV 3 signal, are provided to a VCO frequency counter 178 and VCO frequency counter decoder 179 which essentially compare the frequency of the VCO signal to the frequency of the XTAL clock signal. The output is then provided to a sweep control latch circuit 180 which in turn directs the charge pump circuit 178 to slowly sweep the frequency of the output signal from the voltage controlled oscillator 160 up and down within a couple hundred Hertz of 102.578 KHz. In other words, the VCO frequency counter, decoder, and sweep control latch circuits 179 - 180 serve to provide a feedback loop for the charge pump circuit 178 when no beacon is within range of the receiver antenna to thereby slowly dither the frequency of the VCO signal within the vicinity of 102.578 KHz. In this manner, the receiver/controller 100 is able to rapidly lock onto the phase of the beacon frequency signal when a valid beacon signal is detected. Once a beacon comes within range of the receiving antenna, the charge pump circuit 178 is primarily influenced by the phase error circuit 176 and thus functions as a conventional phase lock loop circuit. Accordingly, circuits 178 - 180 have no effect on the operation of the phase lock loop circuitry once a beacon is within range. This is accomplished by the lock detector circuit 166 which produces a LOCK DETECT output signal on line 190 when a beacon signal is detected that is provided to the charge pump circuit 178 and is effective to disable the frequency sweep function. Referring now to Figure 17, a detailed block diagram of the lock detector circuit 166 is shown. As previously noted, the lock detector circuit 166 determines when the receiver has detected and properly synchronized to a beacon signal and produces a µP wake-up signal to turn on the microcomputer 102. The microcomputer 102 thereupon determines if the received signal is from a valid beacon. The lock detector circuit 166 comprises an exclusive-OR quadrature circuit 192, a lock detector interval counter 194, a lock detector low pass filter and comparator 196, a hold-on circuit 198, and a µP wake-up circuit 200. In particular, the DBL OUT signal on line 174 from the carrier synchronizer circuit 164 and the VCO DIV 12 from the clock generator circuit 162 are provided to the inputs of the exclusive-OR quadrature circuit 192. It will be recalled that the DBL OUT signal on line 174, which has a frequency twice the frequency of the IF signal on line 159, has been phase shifted 90 degrees relative to the IF signal. Thus, the VCO DIV 12 signal is 90 degrees out of phase with the DBL OUT signal on line 174, thereby making the circuit a true quadrature detector. When a beacon signal is present and the PLL circuit has acquired both proper frequency and phase, the exclusive-OR quadrature circuit 192 will provide a HI output signal on line 193 most of the time. When the PLL circuit is unlocked, the output signal on line 193 will toggle up and down with an average HI time of approximately 50 percent. Lock detector circuits 194 and 196 are adapted to sample the quadrature output line 193 over a fixed period of time and count the number of samples which are in quadrature. In the preferred embodiment, lock is indicated when 384 or more samples are in quadrature out of a possible 511. Specifically, the lock detector counter 196 and interval counter 194 are reset at the same time and clocked by the same VCO DIV 6 clock signal. Interval counter 194 is incremented each clock pulse while the lock detector counter 196 is incremented at a clock pulse only if the output signal on line 193 is HI. If the count total in lock detector counter 196 is equal to or greater than 384 at the time of the next reset pulse, the hold-on circuit 198 is activated. Hold-on circuit 198 comprises a retriggerable one-shot circuit that is adapted to maintain the lock condition to the microcomputer 102 for approximately 500 milliseconds after it is no longer indicated by the counters 194 and 196. In this manner, the lock detector circuit 166 ignores dropouts in the beacon signal or short-term noise bursts. The µP wake-up circuit 200 comprises a D flip-flop that produces a 7.8 millisecond output pulse on line 202 that is provided to the microcomputer 102 in response to the production of the LOCK DETECT output signal from the hold-on circuit 198. Turning now to Figure 18, a detailed block diagram of the data extraction circuit 168 is shown. As previously noted, the data extraction circuit 168 demodulates the incoming PSK encoded signal and runs the resulting data bit stream through a matched filter to remove noise. Initial decoding of the Miller encoded data is performed and the synchronizing data clock is generated to decode the Miller code and clock the data bit stream into the microcomputer 102. Specifically, the data extraction circuitry 168 includes a data extraction circuit 204 which comprises an exclusive-OR gate having the PSK encoded signal on line 159 provided to one input and the synchronized VCO DIV 24 signal provided to its other input. Once locked onto the beacon signal, it will be appreciated that the frequency of the VCO signal (102.578 KHz) divided by 24 equals the same 4.274 KHz signal as the intermediate frequency (IF) signal on line 159. It will further be appreciated, therefore, that the synchronized VCO DIV 24 clock signal contains the recreated carrier phase locked to the intermediate frequency signal on line 159. The data extraction exclusive-OR gate 204 demodulates the PSK encoded signal and the output is filtered through a digital matched filter circuit 206. The early-late gate data synchronizer circuit 208 generates the clock signal used to decode the Miller encoded data and clock the data bit stream into the microcomputer 102. Initial decoding of the Miller encoded transition bit stream is then performed by the transition data bit stream generator 210. The serial data bit stream is provided to the microcomputer 102 on output line 214. The µP data handshake circuit 212 interfaces with the microcomputer to control the serial inputting of the data bit stream into the microcomputer 102. In the preferred embodiment, final decoding of the data bit stream is performed in the microcomputer 102 by software. While the above description constitutes the preferred embodiment of the invention, it will be appreciated that the invention is susceptible to modification, variation, and change without departing from the proper scope or fair meaning of the accompanying claims. A passive keyless entry system has been disclosed that is specifically adapted for use with automotive vehicles. The system includes a portable beacon that is carried by a vehicle operator, a receiver/controller located in the vehicle, and an antenna connected to the receiver/controller for receiving an encoded transmission from the beacon. The system is designed to automatically unlock the vehicle as the operator approaches the vehicle. The system is further designed to automatically lock the vehicle as the operator, carrying the beacon, moves away from the vehicle. The beacon includes a motion sensor to conserve battery life when the beacon is stationary. Transmission between the beacon and the receiver/controller is characterized by a magnetically coupled radio frequency signal embodying differential phase encoded data with error correction coding of the data to enhance noise immunity and signal discrimination. In addition, the transmitted signal between the beacon and the receiver is continually changing in order to deter theft. A clock in the receiver/controller is automatically reset to a clock code of the encoded transmission if the transmitted signal is valid.

Claims: A keyless entry system for a vehicle (10), comprising: a portable beacon (24) including a transmitter (52-66) for transmitting a coded beacon signal, said coded beacon signal including a clock code; a receiver (100) associated with the vehicle (10), said receiver (100) including an antenna (14) for receiving said coded beacon signal when said portable beacon (24) is within a predetermined range, said receiver including a receiver clock; and controller means (106, 102, 120) for decoding said beacon signal and activating a first predetermined function associated with the vehicle (10) upon receipt of a valid coded beacon signal from the portable beacon (24), characterised in that the coded beacon signal is continually changing at a predetermined interval and said controller means (106, 102, 120) includes means (102, 120) for updating a receiver clock code at substantially the predetermined interval to coincide with the changing coded beacon signal, said controller means synchronising the receiver clock to the clock code upon receipt of the valid coded beacon signal. The keyless entry system of Claim 1 wherein the coded beacon signal includes a beacon identification code (JD), wherein the beacon clock code changes at the predetermined interval and wherein the controller means (106, 102, 120) separates the beacon identification code from the beacon clock code to determine a valid transmitted signal. The keyless entry system of Claim 1 or Claim 2 wherein the beacon includes function means (56, 69) for transmitting a beacon identification code (JD) and a beacon clock code in an uncoded format and wherein the controller means (106, 102, 120) includes storage means (120) for storing the beacon identification code and beacon clock code upon receipt by the receiver (100). The keyless entry system of any of Claims 1-3 wherein the controller means (106,102, 120) includes first storage means (120) for storing a beacon identification code (JD) associated with the beacon and second storage means (102) for storing the receiver clock code, said controller means (106, 102, 120) separating the beacon identification code (JD) and the beacon clock code from the encoded beacon signal and comparing them respectively to the identification code (JD) in the first storage means (120) and to the clock receiver code in the second storage means (102) to determine a valid beacon signal. The keyless entry system of Claim 4 wherein the controller means (106, 102, 120) include storage means (120) for storing the beacon identification code and the beacon clock code, and wherein the receiver clock code is updated in the storage means (120) upon receipt of a valid beacon signal. The keyless entry system of any of Claims 1-5, wherein said portable beacon (24) includes beacon clock means (43, 46, 59) for continually updating at predetermined intervals the beacon clock code to produce a transmitted beacon signal which varies with time; and wherein said controller means (106, 102, 120) includes the receiver clock (110, 127) which is initially synchronised to the beacon clock code, said receiver clock (110, 127) generating a receiver clock code to coincide with the beacon clock code of the coded beacon signal in order to determine if the transmitted signal is valid. The keyless entry system of any of Claims 1-6 wherein the controller means (106, 102, 120) includes means (102) to generate a tolerance such that the controller means (106, 102, 120) will indicate a valid beacon signal over more discrepancies between the coded beacon signal and the receiver clock as the time between receipt of valid signals increases. The keyless entry system of Claim 7, wherein the tolerance means (102) generates a tolerance between the beacon clock code and the receiver clock code. A method of performing predetermined functions associated with a vehicle (10), said method comprising the steps of: transmitting (66, 68) a coded signal from a portable transmitter (24) to a receiver (100) associated with the vehicle (10), said coded signal including a transmitter clock code and said receiver including a receiver clock; and decoding (106, 102) the coded transmitted signal in the receiver (100) to determine if the coded signal is a valid signal and performing a vehicle function if the coded signal is valid; said method being further characterised by the steps of; altering (59, 67) the coded signal at predetermined intervals in the portable transmitter (24) in order to transmit an updated coded signal; updating (102, 120) the receiver clock in the receiver (100) in order to coincide with the updated coded signal; and synchronising the receiver clock to the clock code upon receipt of the valid coded signal. The method according to Claim 9 wherein the coded signal includes a transmitter identification code (JD) and wherein the step (59, 67) of altering the coded signal includes continually updating (59) the transmitter clock code with respect to a transmitter clock (57) at the predetermined intervals. The method according to Claim 10 wherein the step of updating (59) the transmitter clock code includes updating (59) the clock code by adding a constant value (61) to an initial clock code (57) to produce an updated clock code and continuously adding (59) the constant (57) to the updated clock code to form subsequent updated clock codes at set intervals. The method according to any of Claims 9-11 wherein the step of decoding (106, 102) the coded signal includes the steps of separating (106) a transmitted identification code (JD) and a transmitted clock code from the coded signal in the receiver (100), comparing (102) the identification signal with a stored (120) identification code in the receiver (100) and comparing (102) the transmitted clock code with a stored (120) clock code in the receiver (100) and wherein the stored (120) clock code is updated in substantially the same way as the updated coded signal. The method according to Claim 12 wherein the step of decoding (106, 102) the coded signal includes the step of generating (102) a tolerance factor in the receiver (100) for enabling discrepancies between the transmitted coded signal and the receiver clock code to increase over time between accepted valid transmitted signals and still indicate that the transmitted coded signal is valid. The method according to any of Claims 9-14 further comprising the steps of transmitting (66, 68) the coded signal in an uncoded format (69) in order to calibrate the receiver clock.

Inventors: MICHAELS PAUL A, POIRIER JAMES A, REA IRVIN B, SLAUGHTER SHERRI A, WARAKSA THOMAS J, MICHAELS, PAUL A., POIRIER, JAMES A., REA, IRVIN B., SLAUGHTER, SHERRI A., WARAKSA, THOMAS J.
Applicants: LECTRON PRODUCTS, LECTRON PRODUCTS, INC.
Application year: 1993
Main classification: E05B49
IPC codes: G07C9, B60R25