Method and Apparatus for Use in an Access Control System

Abstract: Encoder and decoder microchips suitable for use in remote control devices, are disclosed. The encoder microchip comprises means (7) for performing an encoding function on an identification number (6) embedded in the said microchip and a combination of a unit number (CSR3,CSR2) and a stepping counter value, (CSR1,CSR0) so as to generate a transmission value which is only decodable by a related decoding function having access to the same identification number. The decoder microchip comprises means (13) for decoding the transmission value into a decoded unit number (CSR3,CSR2) and a decoded counter value (CSR1,CSR0) and means for comparing the decoded counter value with a decoder counter value range. The encoder and decoder microchips are also provided with means for synchronizing the decoder microchip with a particular encoder microchip which has generated a synchronization command.

Description: FIELD OF THE INVENTION This invention relates to security systems. More particularly, the invention relates to microchips suitable for use in remote control devices, to remote control devices comprising the said microchips and to a security system. BACKGROUND OF THE INVENTION Remote control via radio frequency or infra red media is well known and very popular for the control of car alarms, building alarms and automatic garage door equipment. Conventional remote control systems which are based on a uni-directional transmission with limited security features, are in common use and are available at relatively low prices. More sophisticated devices based on bi-directional transmission systems and extensive handshaking, are also available on the market and are known to the applicant. However, because of their high cost and certain practical disadvantages, they are not widely used in commercial remote control devices. The aforementioned conventional devices based on uni-directional transmission systems have two important shortcomings in the context of a security application, namely firstly - the codes they are able to transmit are usually fixed and - secondly, the number of combinations of codes that they can transmit, is relatively small. Either of these shortcomings can lead to access being given to unauthorised persons. Such unauthorised access can be obtained by way of an exhaustive search, in which all the different combinations are tested to see if they are accepted, something which could be done in a matter of minutes if an appropriate apparatus is used. As an alternative, a recording could be made of a transmission and this could be retransmitted to gain access. As a result, such conventional uni-directional systems can be accessed without the use of authorised remote control or other security devices. Improved security can be derived from the known principle of code stepping or code hopping. US Patents 4 835 407 and 4 847 614, German Patent No 3 244 049 and German Patent Publications DE-OS-33 20 721, DE-OS-32 34 538, DE-OS-34 07 436 and DE-OS-34 07 469 describe this principle in more detail. South African Patent Specification No 89/8225 also describes a code hopping remote control system which is similar to the one described in US Patent No 4 847 614. US Patent No 4 847 614 describes the generation, by a transmitter, of a different code word after each previous transmitting operation. Such new code word is produced anew by linking, according to a given function, starting from a stored original code word and the previously transmitted code word. The receiver operates in exactly the same way and compares the new code word it generates, by the same method, with the code word received from the transmitter. If the two code words agree, the centrally controlled locking system of the vehicle in which the apparatus is installed, is activated. If there is non-agreement, additional code words, say n code words produced in sequence by the receiver, are compared. Thereafter, if non-agreement persists after the n code words, the receiver switches over to an increased security mode wherein two successive code words transmitted in sequence must be successfully compared before the central locking system of the vehicle is activated. This double comparison must take place within the next m code words generated at the receiver. If the transmitting device and the receiving device are out of step by more than m+n, another signal is used indicate to the receiver that is must search through its entire set of codewords in an attempt to synchronize. An essential feature of this remote control apparatus is that the receiver merely compares the received code word with the code word generated by itself without decoding the received code word to its original elements. Thus, in the event of non-agreement, and this will occur very often if the system is widely used in RF-devices, because of accidental reception from other users, this apparatus changes to an increased security mode, which is user unfriendly. When it is in the high security mode, the receiver will force the user to operate his/her transmitter more than once. A further essential feature of this remote control device is that the "window" of disagreement which is still acceptable to the apparatus, is applied to the received code word and the code word generated by the receiver. If the code words are not the same with the first attempt, the receiver generates a second code word which is then compared with the received code word. This process may have to be repeated as many times as the size of the "window" which has been built into the receiver algorithm. Depending on the electronics in which this process is carried out, the size of the "window" and also the extent of disagreement between the first received code word and the first code word generated by the received, the reaction time for this apparatus could vary from transmission to transmission, and could be lengthy. However, a serious problem in the operation of the system results when the situation occurs that the transmitter and receiver are out of step by more than n+m steps. It is taught by the aforementioned patent that another signal is to be supplied to the receiver to indicate to it that a total search must be done to achieve synchronization. Because of the enormous number of possible code words (>10⁹), it could take several minutes to succeed. This patent even suggests that the user opens the transmitter and removes its batteries to facilitate a short search. Both of the above situations are user unfriendly. If this process is repeated often, it also presents a security risk. The battery removal suggestion further precludes the use of non-volatile memory elements (EEPROM) for the counter of the transmitter. The use of EEPROM in the transmitter would have offered several advantages such as the elimination of standby power requirements, a longer battery life, fewer synchronization actions required and a guaranteed forward stepping (higher security). If this system must be expanded to decode two or more transmitters it will have to step through 2 (or more) x n code words if an unauthorised code word is received. Reference is also made to US-A-4723121 (which corresponds to DE-A-3532156), which describes an electronic locking apparatus for motor vehicles featuring an electronic key and an electronic lock, which each contain a synchronized, constantly operating, precision oscillator. The output of each oscillator is applied at a predetermined counting rate to a respective number sequence generator. Both generators contain the same predetermined number sequence, which they step through at the same clock rate, applying the instantaneous value of the count to a first input of a respective computer. A second input of each computer is connected to a fixed memory which supplies a permanent, characteristic code number to the computer. Both the count state and the characteristic code number are combined using corresponding algorithms in the key and in the lock to produce a combination code. The key-produced combination code is sent from a transmitter in the key to a receiver in the lock and compared there with the lock-produced combination code. In the event of a successful comparison, a control pulse is generated, which actuates various positioning means in the lock. Reference is also made to FR-A-2606232 which describes an evolutive coding process using a pseudo-random discontinuous function for coding remote controls, such as for automobiles. A message containing at least two pseudo-random values calculated by the pseudo-random discontinuous function is transmitted. Preferably the pseudo-random values are successive values of the pseudo-random discontinuous function, and the message optionally includes a code derived as a function of a specific base code associated with the system. The general preferred form of complete code message may take the form T(C) plus FA(C,n-1), plus FA(C,n) where T is a continuous function and FA is a pseudo-random discontinuous function. SUMMARY OF THE INVENTION In a first aspect, the invention provides an encoder for an access control system, comprising: means for defining an identification number for an encoding operation; means for storing a counter value; means for performing an encoding function using the identification number, on data comprising the counter value, to generate an encoded value therefrom which can be decoded by a related decoding function using a related identification number to yield the counter value; and means for changing the counter value in association with each operation of the encoder, to vary the encoded value independently of time. The data may further comprise a unit number. For example, the unit number may represent access control information to be encoded. The encoder may be formed on a microchip. In a second aspect, the invention provides a decoder for an access control system, comprising: means for defining an identification number for a decoding operation; means for storing a value representing a reference counter value which is independent of time; means for performing a decoding function on an inputted signal according to the identification number to derive from the inputted signal decoded data comprising a decoded counter value; means for comparing the decoded counter value with the reference counter value to ascertain, as at least one condition of an acceptable signal, whether the decoded counter value falls within a predetermined counter value range based on the reference counter value; and means for updating the value representing the reference counter value, based on the decoded counter value, if the inputted signal is ascertained to be acceptable. The decoded data may further comprise a unit number. For example, the unit number may represent decoded access control information. The decoder may be formed on a microchip. The invention also contemplates a combined encoder and decoder microchip comprising both an encoder as aforesaid, and a decoder as aforesaid. In a closely related aspect, the invention provides a method for use in an access control system, for producing an encoded value during encoding, and for decoding the encoded value during decoding, comprising during encoding: (a) performing an encoding function based on a stored encoding identification number and on data comprising a stored encoding counter value, to generate said encoded value therefrom; (b) changing the stored counter value in association with each encoding operation to vary the encoded value independently of time; and the method comprising during decoding: (c) performing a decoding function on the encoded value according to a stored decoding identification number to derive decoded data comprising a decoded counter value corresponding to the encoding counter value; (d) comparing the decoded counter value with a stored value representing a reference counter value which is independent of time, to ascertain, as at least one condition of acceptance, whether the decoded counter value falls within a predetermined counter value range based on the reference counter value; and (e) updating the value representing the reference counter value, based on the decoded counter value, if the decoded counter value is ascertained to be acceptable; whereby the decoding operation permits successful decoding even if a certain number of encoding operations have been performed without decoding, provided that the encoding counter value responsive to the number of encoding operations performed is within said predetermined range of the reference counter value. In the above, the encoding function may be described by the following equation: fencode (Identification number, (unit number, counter value) = transmission value. The coding and related decoding functions may be non-linear functions. This type of function is often used in the field of cryptography and is usually chosen for its characteristics which prevent or at least inhibit the prediction of its next output even though the non-linear function as well as previous outs thereof may be known as long as the identification number (PIN) remains unknown. The unit number may be at least a one bit value. Although it may extend into thousands of bits and even more, it will be appreciated that the longer the unit number, the greater the security it offers but more expensive the microchip becomes. The counter value is also preferably of more then a one bit length and may also extend into thousands of bits and even more, which will as would be appreciated, increase the security. The longer the counter value, however, the higher the cost. It has been found that a 16 bit unit number and a 16 bit counter value, when combined, give adequate security because they could each individually be combined in more than 65,000 different combinations and together they could be combined in more than 4000 million combinations. Similarly, the identification number is preferably of more than one bit length and is preferably as long as 64 bits in which case more that 10¹⁹ different combinations are possible. The transmission value is preferably at least 16 bits long. It will be appreciated that if it is of a length less then 16 bits, it will be less secure and consequently it will be easier to be decoded. The decoder may comprise means for performing a format scan on signals so as t identify and respond to valid transmission values. The decoding function performed by the decoder is preferably such as to ensure that the decoded unit number and the decoded counter value are the same as, respectively, the unit number and the counter value encoded by an encoder having the same identification number as the decoder. The decoder preferably also comprises distinguishing means for distinguishing between a decoded unit number for normal operation and a synchronisation command. The decoder counter value may conveniently not be accepted by the decoder as a valid counter value unless it is greater than the previously received valid counter value but less than the previously received valid counter value plus a value n, the value would still accept. alternatively, in the event that the decoded unit number comprises a valid synchronisation command, the decoder may be adapted to store the decoded counter value plus one as the decoder counter value for subsequent use. The decoder may, in addition, comprise means for comparing the counter value with a value obtained from a uni-directional synchronisation process to which the decoder microchip may be subjected. The encoder means may be adapted to generate a stepping counter value through a uni-directional synchronisation process for the synchronisation of the encoder means of the receiver remote control device. The receiver remote control device is preferably provided with means for providing an output indicative of or in response to a valid transmission value it has received. The receiver remote control device may further comprise means for comparing the decoded counter value with a decoded counter value obtained from a uni-directional synchronisation process pre-performed on the receiver remote control device by a transmitter remote control device. The counter values of both the encoder means and the decoder means may be retained by batteries or alternatively, by memory means. In a preferred embodiment of the invention, electronic remote control apparatus is provided comprising encoder means for generating, when activated, a multibit code word by performing a function on a personal identification number (PIN) and a combination of a unit number and a counter value. Preferably, the counter value is incremented every time the apparatus is activated. The electronic remote control means preferably comprises transmitter means for generating a transmission comprising the multibit code word. Conveniently, the encoder means is further adapted to generate, upon activation of a synchronization process, a synchronization multibit code word, wherein the synchronization multibit code word is a function of a personal identification number embedded in the encoder means, and a combination of a synchronization command word and a new counter value. The encoder means may further comprise panic means adapted to generate a panic command. Additionally, the encoder means may comprise electrically erasable programable memory means or read and write memory means with standby mode means in the said encoder means to store the last counter value. In order to facilitate the programming of a multibit personal identification number (PIN) into the memory means, the apparatus may comprise program means. As an additional safety feature, the encoder means may comprise verification means for verifying the personal identification number without being able to read it, and means for locking an interface with the personal identification number (PIN), in order to bar all further attempts to change or verify the personal identification number. In another preferred embodiment of the invention there is provided electronic remote control apparatus comprising decoder means for decoding the multibit code word received from the transmitter means. The decoder means may be adapted to apply a function on the multibit code word received from the receiver in such a manner as to yield the unit number and the counter value to which the encoding function had been applied. Preferably the personal identification number (PIN) of the encoder means is the same as that of the decoder means, otherwise the unit number and the counter value window of the decoder means would most probably not compare with the unit number and counter value to which the encoder means had applied the function and the received code word would then be ignored. The decoder means is preferably adapted to compare the decoded unit number of the transmitted code word with its pre-embedded unit number, and upon agreement, to check that the counter value falls inside a valid range of counter numbers, and if both conditions are satisfied, to give an indication thereof to the outside, in the form of a flag, and to store the received counter value if it was found to be valid. The decoder means may further be adapted, if one of the conditions is not satisfied, to ignore the received multibit code word and to scan its input for another multibit code word. Each of the encoder and decoder means may comprise means for programming, verifying and locking a personal identification number (PIN). In addition the decoder means may comprise means for storing the latest valid received counter value. Further according to the invention, the encoder means may comprise means for recognising, within a sequence of counter values, a false counter value, and means responsive thereto for preventing desynchronization. The means for preventing desynchronization may be adapted to also give a battery low indication. Furthermore, the encoder means may comprise means for stepping the synchronization command word to prevent the same synchronization command word from being used illigitimately. Also according to the invention, the decoder means may comprise means for recognising a panic command generated in the encoder means, and means for responding thereto. The decoder means may comprise means for recognising other commands and/or more than one unit number with independent counters, without having to perform the decoding process more than once. Referring especially to the preferred embodiment, the present invention can provide encoder and decoder microchips for use in a remote control system of increased security, of which the user friendliness has not been unduly sacrificed, comprising a transmitter remote control device and a receiver remote control device, wherein the transmitter remote control device comprises the encoder microchip, the encoder microchip forming part of an electronic circuit adapted to transmit a coded transmission value decodable by the decoder microchip, and wherein the receiver remote control device comprises the decoder microchip, the decoder microchip forming part of an electronic circuit adapted to receive and to decode the coded transmission. The invention can also provide a security system in which synchronisation of the transmitter and receiver remote control devices can be achieved by a simple yet reliable and secure manner. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described, by way of a non-limiting example, with reference to the accompanying drawings in which: Figure 1 is a block diagram of an encoder microchip in accordance with the invention; Figure 2 is a block diagram of a decoder microchip in accordance with the invention; Figure 3 is a flow diagram for the functions which the encoder microchip can perform; Figure 4 and 4a is a flow diagram for the functions which the decoder microchip can perform; Figure 5 is a preferred format for the unit number and the counter value; Figure 6 is a preferred format for the transmission value. DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Figure 1, the encoder microchip receives an input from a pair of switches (1), and comprises a control unit (2), a mode unit (3), a transmit counter (4), an input register (5) for holding an input word, an ID register (6) for holding an identity number, logic means (7) for performing a non-linear function, a shift register (8) for holding an encoded value and repeatedly feeding the encoded value to a transmitter (10), and a status register (9) for holding the configuration of the encoder microchip. The status register (9), the identity number (6) and the transmitter counter (4) are all registers or memory elements that can be programmed into the microchip and may be non-volatile (EEPROM) or volatile (RAM) memory with battery backup. As will be appreciated by those skilled in the art, the functions of the encoder microchip can be implemented in dedicated logic although a microprocessor based implementation is also possible. Referring to Figure 2, there is shown a receiver (11) for receiving a transmission value from the transmitter (10). The output of the receiver (11) is fed into a shift register (12). The value in the shift register (12) is decoded by decoding logic (13) using an identity number obtained from an ID register (14). The result obtained from the decoding logic (13) comprises a decoded unit number and a decoded counter value and is stored in a decoded result register (15). All the aforementioned steps are carried out under the control of a control unit (16). The decoder microchip also comprises four counter registers respectively numbered (17), (18), (19) and (20) in which decoder counter values may be stored and from where they may be compared with docoded counter values obtained by the control unit (16) from the decoded result register (15). Four outputs respectively numbered (21), (22), (23) and (24) are also provided and may be used by the control unit (16) to indicate what kind of information has been received. The decoder microchip further comprises format scan means (26) for scanning and verifying the format of any transmissions received by the receiver (11). In order to prevent a synchronization value which has been used for one synchronization command, from being used for subsequent synchronization commands, synchronization memory means (28) are provided. The identity number (6) of the encoder is programmed by the user with a secret value to provide the security of the system. Although there may be millions of other users of exactly the same encoder (Figure 1) and decoder (Figure 2) microchips, every user will have a very high degree of security. A decoder microchip will only be able to correctly decode a transmission value that has originated in an encoder microchip when the same identity number is programmed into it. Furthermore, a specific encoder will also be defined by the value of its status register (9). The format of the status register is shown in Figure 5. There can also be more than one input (1) into the control device, which will also influence the status register value (9). The transmit counter (4) can be programmed with an initial value and will then increment every time the encoder is used to transmit a value. The input word (5) to the encoding function comprises the unit number (CSR3 and CSR2 in the input register (5)) and the transmit counter value (CSR1 and CSR0 in the input register (5)). The non-linear encoding function (7) will use the identity number (6) to map the input word (5) to a transmission value that is stored in the transmission shift register (8). This value can be further encoded to form the transmission format as shown in figure 6. The non-linear function may be any non-linear function of sufficient complexity and which has a related decoding function i.e., if the non-linear decoder function is applied to the transmission value using the same identity number, it will produce as a result the value that was in the input word (5). The encoder operation is explained in the flow diagram of figure 3. When power is applied to the encoder microchip it would perform its functions in the sequence indicated. It would first reset itself to a defined state in order to start with normal operation (31). It is important to recognise that the operation of the encoder can at any time be terminated. If it is not terminated, it will sequentially execute the functions indicated in Figure 3, until the wait loop (45) is reached. The encoder microchip will suspend its activities at this point and will perform no further functions until a reset function (31) is performed. The test shown in step (32) could be based on the inputs received from the switches (1) to the control unit (2). The transmission word (8) must be at least as long as the input word (5), but need not be the same length as the identity number (6). Security requirements dictate that the transmit counter (4) should be at least 16 bits long and so too the unit number. This indicates that a good length for the transmission word is 32 bits. This provides ample security and is also practical in terms of transmission time and implementation costs. The functions and operation of the decoder microchip are substantially more complex and would be described with the help of simple examples. The block diagram in Figure 2 shows the functional elements of the decoder microchip and the flow diagram in Figures 4 and 4a shows its operation. It should be clear from the encoder description that all information bits to be transmitted are encoded with the non-linear encoding function. This has the effect that the transmission value (8) bears no obvious resemblance to the input word (5). However, at the decoder the information embedded in the input word must be recovered. The receiver (11) turns the transmitted signals, whether they are in the form of radio frequency, infra red waves or any other suitable medium, into a digital signal. This digital signal in the receiver (11) is continuously scanned (26, 47) from a word that conforms to the format such as shown in Figure 6. Another format may be chosen if it has advantages. When a valid transmission word is recognised, it is moved into the decoder input shift register (12). The control (16) of the decoder microchip would then apply the decoding function (13) with inputs from the preprogrammed decoder identity number (14), to the value in the input shift register (12). The result of this decoding operation (48) is stored in the decoded unit number and decoded counter value result register (15). The next operation (49) is to compare the value in the CSR2 (see Figure 5) which is part of the unit number which is in turn part of the decoded result with the code for a synchronization command. If they compare, the decoder will proceed with operation (50) along the path on the flow diagram that shows the uni-directional synchronization operation. If they do not compare, it will proceed to get (56) the transmitter identity from the decoded result register (15). The control (16) will then calculate the difference (58) between the decoded counter value and the corresponding Rx counter value (17, 18, 19, 20). If the difference is greater than or equal to zero but less than a value n, the decoded value is accepted as the result of an authorised or valid transmission. The value n is the number of lost codes which the system may be set up to handle. In practice, this means that a remote control system comprising a transmitter and a receiver, i.e. an encoder and decoder set with identical identity numbers (6, 14), does not have to remain in perfect synchronization. For example, if n is say 100, then the transmitter, once it has been synchronized can be activated, for instance, 98 times out of range of the receiver (dummy transmissions) and if on the 99th time it is activated, the transmission is within range of the receiver, the decoder performs one decoding operation and will then accept the transmission as valid. If however, more than 100 (for n = 100) dummy transmissions have taken place, the receiver will ignore all further transmissions from that transmitter until it receives a transmission value that decodes into a valid synchronization command. If the decoded result was accepted as valid (59), the control can then determine what command was transmitted (60, 62, 64) and can then take the desired action (61, 63, 65), before returning to a state where it scans (47) for a valid word. The decoded counter value of a valid transmission will be incremented and stored (66) in the corresponding Rx counter register (17, 18, 19, 20). This means that once a transmission has been received as valid, the counter value of that transmission and all previous counter values will become unacceptable to that encoder microchip. The uni-directional synchronization process is essential for establishing synchronization between a matched transmitter and receiver. If in operation (49) the transmission is recognised as possibly a synchronization command, the control will proceed to perform further tests to verify (50) that the format of the counter conforms to the requirements for a synchronization command. For example, the lower 8 bits of the decoded counter value must be zero. If the format does not conform to specifications for a synchronization command, the control takes the decoder microchip back to operation (47) and the decoded value is ignored. If the decoded value passes test (50) the decoder will proceed to test the synchronization counter value against a previous valid synchronization operation (51, 52) and if it recognises a repeat, the decoder will ignore this decoded word and will return to (47). However, if the command passes to (54) the decoder counter value will immediately be modified to the decoded value plus one. This value may be any possible value within the constraints of counter length and of course the format requirements of the synchronization command. The decoder will give an indication (55) that it has accepted the new counter value. In an automotive application, this might be used to turn the flicker lights on and off as an indication to the user that synchronization has been achieved. In terms of security, it should be noted that although the decoder counter has been synchronized, the decoder will still need to receive a valid transmission based on the new counter value before it will indicate a valid reception (61, 63, 65). The synchronization command and for that matter any other command cannot be determined from an investigation of the transmission value, because of the non-linear effect of the encoding function and the fact that it forms part of the input word which gets encoded. It is very important to achieve the highest possible security in the synchronization process because it is always a weak point in a uni-directional system. Because the window n can be large and EEPROM can be used to store counter values, synchronization will only rarely be required. Other users will have no effect on the operation of a matched encoder/decoder set. This set will automatically keep in step without any actions by the user. Synchronization is a very simple and straightforward process with very limited impact on the user, since it takes only a few seconds and does not require any additional signals or actions. Because of the fact that synchronization values cannot be repeated in a non-volatile memory application, a high degree of security is offered by the system.

Claims: An encoder for an access control system, comprising: means (6) for defining an identification number for an encoding operation; means (4,5) for storing a counter value; means (7) for performing an encoding function using the identification number, on data comprising the counter value, to generate an encoded value therefrom which can be decoded by a related decoding function using a related identification number to yield the counter value; and means (2,4) for changing the counter value in association with each operation of the encoder, to vary the encoded value independently of time. An encoder according to claim 1, wherein the data comprises the counter value and a unit number representing control information to be encoded, and the encoding function is such that the encoded value can be decoded by a said related decoding function to yield the counter value, and the unit number. An encoder according to claim 2, wherein the unit number comprises information selected from the group consisting of: input information (TX1, TXO) representing a transmitter number; information (PB) from an external input; information (PB, CSR2) representing a command; and a constant value (CSR3). An encoder according to claim 2 or 3, further comprising means (2, 4; 36, 37; 40, 41) for changing at least one of the unit number and the counter value when the encoder is operated continuously for a predetermined period of time. An encoder according to any of claims 1 to 4, further comprising means (1,2, CSR0, CSR1) operable to feed to the means (7) for performing the encoding function, a synchronisation command signal and a counter value, to generate therefrom an encoded synchronisation output signal. An encoder according to any preceding claim, wherein the means (7) for performing the encoding function comprises means for performing a non-linear encoding function. An encoder according to any preceding claim, further comprising means for providing an indication in the event of a low power supply voltage. An encoder according to any preceding claim, further comprising interface means for programming a user's identification number in the encoder, and means for preventing the identification number from being read out from the encoder. An encoder according to any preceding claim, wherein the means for changing the counter value in association with each operation of the encoder comprises means for incrementing the counter value. An encoder according to any preceding claim, wherein the encoder is formed on a microchip. A transmitter comprising an encoder as defined in any preceding claim, the transmitter further comprising means (10) for transmitting the output from the encoder. An access control system which includes an encoder according to any of claims 1 to 10, and a related decoder which comprises: means (17-20) for storing a value representing a reference counter value; means (13,16) for performing a decoding operation on an input signal to derive therefrom decoded data comprising a decoded counter value; and means (15,16) for comparing the decoded counter value with the reference counter value to ascertain, as at least one condition of an acceptable signal, whether the decoded counter value falls within a predetermined counter value range based on the reference counter value. A decoder for an access control system, comprising: means (14) for defining an identification number for a decoding operation; means (17-20) for storing a value representing a reference counter value which is independent of time; means (13,16) for performing a decoding function on an inputted signal according to the identification number to derive from the inputted signal decoded data comprising a decoded counter value; means (15,16) for comparing the decoded counter value with the reference counter value to ascertain, as at least one condition of an acceptable signal, whether the decoded counter value falls within a predetermined counter value range based on the reference counter value; and means (15,16) for updating the value representing the reference counter value, based on the decoded counter value, if the inputted signal is ascertained to be acceptable. A decoder according to claim 13, wherein the decoded data further comprises a decoded unit number representing control information. A decoder according to claim 14, further comprising means (15; CSR2, CSR3) for storing decoder reference information, and means (15,16) for comparing at least a part of the decoded unit number with at least a part of the reference information, as at least another condition of an acceptable signal. A decoder according to claim 14 or 15, wherein the means (17-20) for storing a value representing the reference counter value comprises means for storing a plurality of values representing reference counter values each related to a different encoder, and means (16) for selecting one of said values for comparison from the plurality of values, based on information derived from the decoded unit number. A decoder according to claim 14, 15 or 16, further comprising means (16) for distinguishing between different commands contained in the unit number. A decoder according to any of claims 13 to 17 further comprising output means (21-24) for providing an indication of information contained in an accepted decoded signal. A decoder according to any of claims 13 to 18, further comprising means (16) for synchronising the reference counter value to a predetermined synchronisation value in response to a valid synchronisation command being decoded. A decoder according to claim 19, wherein the predetermined synchronisation value is derived from the inputted signal. A decoder according to any of claims 13 to 20, further comprising interface means for programming a user's identification number in the decoder, and means for preventing the identification number from being read out from the decoder. A decoder according to any of claims 13 to 21, wherein the decoder is formed on a microchip. A receiver comprising a decoder as defined in any of claims 13 to 22, the receiver further comprising receiver input means (11) for receiving a transmitted signal, and for providing the received signal as the input to the decoder. An access control system comprising a transmitter as defined in claim 11, and a receiver as defined in claim 23. A microchip comprising an encoder as defined in claim 10 and a decoder as defined in claim 22. A method for use in an access control system, for producing an encoded value during encoding, and for decoding the encoded value during decoding, comprising during encoding: (a) performing an encoding function based on a stored encoding identification number and on data comprising a stored encoding counter value, to generate said encoded value therefrom; (b) changing the stored counter value in association with each encoding operation to vary the encoded value independently of time; and the method comprising during decoding: (c) performing a decoding function on the encoded value according to a stored decoding identification number to derive decoded data comprising a decoded counter value corresponding to the encoding counter value; (d) comparing the decoded counter value with a stored value representing a reference counter value which is independent of time, to ascertain, as at least one condition of acceptance, whether the decoded counter value falls within a predetermined counter value range based on the reference counter value; and (e) updating the value representing the reference counter value, based on the decoded counter value, if the decoded counter value is ascertained to be acceptable; whereby the decoding operation permits successful decoding even if a certain number of encoding operations have been performed without decoding, provided that the encoding counter value responsive to the number of encoding operations performed is within said predetermined range of the reference counter value.

Inventors: BRUWER FREDERICK JOHANNES, KUEHN GIDEON JACOBUS, SMIT WILLEM, BRUWER, FREDERICK JOHANNES,, KUEHN, GIDEON JACOBUS, SMIT, WILLEM, Kühn, Gideon Jacobus
Applicants: MICROCHIP TECH INC, MICROCHIP TECHNOLOGY INC.
Application year: 1991
Main classification: E05B49
IPC codes: G07F7, G07C9