Title: Method of communication using an encoder microchip and a decoder microchip
Abstract: Encoder and decoder microchips suitable for use in remote control devices, are disclosed. The encoder microchip comprises means for performing an encoding function on an identification number embedded in the said microchip and a combination of a unit number and a stepping counter value, 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 for decoding the transmission value into a decoded unit number and a decoded counter value and means for comparing the decoded counter value with a decoder counter value range. The encoder and decoder microchips are provided with means for changing, e.g. in a preferred mode incrementing, the counter values by a number greater than one after a period of time, subsequent to the encoder microchip being activated or the decoder microchip receiving a transmission value. 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.
Patent Number: 6,985,472 Issued on 01/10/2006 to Bruwer
| Inventors:
|
Bruwer; Frederick (Chandler, AZ)
|
| Assignee:
|
Microchip Technology Incorporated (Chandler, AZ)
|
| Appl. No.:
|
701216 |
| Filed:
|
November 4, 2003 |
| Current U.S. Class: |
370/342; 340/825.1; 340/825.56; 342/42 |
| Current Intern'l Class: |
H04Q 1/00 (20060101) |
| Field of Search: |
370/342
340/825.1,825.56,825.69,825.72
342/42
|
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| |
Primary Examiner: Zimmerman; Brian
Attorney, Agent or Firm: Baker Botts L.L.P.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation of, and claims priority to, commonly owned
U.S. patent application Ser. No. 09/563,787, filed May 2, 2000, entitled, "Improved
Microchip and Remote Control Devices Comprising Same", by Frederick J. Bruwer,
now abandoned which itself is a continuation patent application of commonly owned
U.S. patent application Ser. No. 08/853,328, filed May 8, 1997, entitled, "Microchips
and Remote Control Devices Comprising Same," by Frederick J. Bruwer, now issued
as U.S. Pat. No. 6,108,326, all of which are hereby incorporated by reference herein
for all purposes.
Claims
I claim:
1. A method of communication using an encoder microchip and a decoder microchip,
said method comprising the steps of:
storing a first identification number in an encoder microchip;
storing a counter value in the encoder microchip;
changing the counter value when the encoder microchip is operated;
non-linearly encoding the counter value with the first identification number;
generating a transmission value from the nonlinearly encoded counter value;
storing a second identification number in a decoder microchip;
receiving the transmission value;
decoding the transmission value with the decoder microchip and the second identification number;
generating a present decoded counter value from the decoded transmission value;
storing a previously decoded counter value from a previous transmission; and
scanning the received transmission value to determine if it conformances to a
specific format.
2. A method for encoding a transmission value using an encoder microchip, said
method comprising the steps of:
storing an identification number in an encoder microchip;
storing a counter value in the encoder microchip;
changing the counter value when the encoder microchip is operated; and
non-linearly encoding the counter value with the first identification number
to generate a transmission value that is decodable to provide the counter value.
3. The method of claim 2, wherein the identification number is used in the step
of decoding the transmission value.
4. The method of claim 2, further comprising the step of transmitting a radio
frequency signal having the transmission value.
5. The method of claim 2, further comprising the steps of:
forming a unit number selected from the group consisting of information representing
a command, information representing an input value, information representing a
transmitter number and a constant value; and
non-linearly encoding the counter value and the unit number with the identification
number to generate the transmission value.
6. The method of claim 5, wherein the step of non-linearly encoding generates
the transmission value decodable to provide the counter value and the unit number.
7. A method for decoding a transmission value using an decoder microchip, said
method comprising the steps of:
storing an identification number;
storing a first counter value;
receiving a transmission value;
decoding the received transmission value with the identification number to generate
a second counter value;
comparing the second counter value with the first counter value;
activating an output if the comparison of the second counter value with the first
counter value is within a defined range; and
storing information relating to the second counter value when the output is activated.
8. The method of claim 7, further comprising the step of scanning the received
transmission value for identifying and acting when the transmission value is in
a valid format.
9. The method of claim 7, wherein the output, when activated, provides an indication
of information contained in the decoded transmission value.
10. The method of claim 7, wherein the output is activated only when the second
counter value is within a forward range of the first counter value.
11. A method of communication using an encoder microchip and a decoder microchip,
said method comprising the steps of:
storing an identification number in an encoder microchip;
storing a counter value in the encoder microchip;
changing the counter value each time the encoder microchip is operated;
nonlinearly encoding the counter value with the identification number;
generating a transmission value from the nonlinearly encoded counter value;
changing the changed counter value after a given period of time subsequent to
the encoder microchip being operated, wherein if the encoder microchip is operated
more than once during the given period of time, the counter value is only changed once;
storing a second identification number in a decoder microchip;
receiving the transmission value;
decoding the transmission value with the decoder microchip and the second identification number;
generating a decoded counter value from the decoded transmission value;
storing a second decoded counter value obtained from the decoding of a previous
transmission value;
changing the stored second decoded counter value after a period of time subsequent
to each time the decoder microchip receives a transmission value; and
scanning the received transmission value to determine if it conformances to a
specific format.
12. A method for encoding a transmission value using an encoder microchip, said
method comprising the steps of:
storing an identification number in an encoder microchip;
storing a counter value in the encoder microchip;
changing the counter value when the encoder microchip is operated;
encoding the counter value with the identification number to generate a transmission
value; and
changing the changed counter value after a period of time subsequent to the encoder
microchip being operated.
13. The method of claim 12, further comprising the step of transmitting a radio
frequency signal having the transmission value.
14. A method for decoding a transmission value using an decoder microchip, said
method comprising the steps of:
storing an identification number in a decoder microchip;
storing a first counter value;
receiving a transmission value;
decoding the received transmission value with the identification number to generate
a second counter value;
comparing the second counter value with the first counter value;
activating an output if the comparison of the second counter value with the first
counter value is within a defined range;
storing information relating to the second counter value when the output is activated; and
changing the second counter value after a period of time subsequent to the step
of receiving the transmission value.
15. The method of claim 14 wherein the step of activating the output is performed
only when the second counter value is within a forward range of the first counter value.
16. The method of claim 14, further comprising the step of storing a plurality
of counter values, each being related to a different encoder microchip.
17. A method of remote control using a transmitter and a receiver, the method
comprising the steps of:
storing a first identification number in a transmitter;
storing a counter value which is related to a number of times the transmitter
is activated;
encoding the first identification number and the counter value to generate a
transmission value;
transmitting the transmission value and indicating an activation of the transmitter
to a user;
changing the stored counter value after a period of time subsequent to transmitter activation;
storing a second identification number in a receiver, the second identification
number being the same as the first identification number;
receiving the transmission value with the receiver; and
performing a decoding function on the received transmission value with the second
identification number so as to generate a decoded counter value.
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 unauthorized persons. Such
unauthorized 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 authorized remote control or other security devices.
Improved security can be derived from the known principle of code stepping
or code hopping. U.S. Pat. Nos. 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 U.S. Pat. No. 4,847,614.
U.S. Pat. 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 to indicate to the receiver that it must search through
its entire set of code words 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 receiver, 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 (>10
9), 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)×n code words if an unauthorized code word is received.
In addition, the above-described systems are also vulnerable to a newly developed
sophisticated "code grabber." The new code grabber intercepts a piece or portion
of the code word being transmitted when an authentic transmitter, e.g. the transmitter
of a standard one button garage door opener remote control, is activated and jams
the remaining portion of the code word being transmitted. During the same transmission,
the code grabber then jams the portion of the code word it has already "grabbed"
or recorded and then intercepts and records the remaining portion of the code word
previously jammed. The code grabber then completely jams the signal until the user
releases the button on the authentic transmitter. As a result, the code grabber
now has one full complete authentic code word and the receiver in the garage door
opener has not received a signal transmission. The above process is repeated by
the code grabber until the user releases the button a second time, at which time
the code grabber has two valid code words and the garage door opener receiver has
received nothing. After the user releases the button the second time, the code
grabber transmits the first code word it has captured and the door closes. The
user thinks that the first transmission was simply noise, i.e., not received, and
drives away to work for instance. The code grabber now has a second valid code
word that can be transmitted in the future to open the garage door.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to 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.
It is a further object of the invention to provide a security system in which
synchronization of the transmitter and receiver remote control devices can be achieved
by a simple yet reliable and secure manner.
According to one aspect of the present invention, there is provided an
encoder microchip comprising: (1) means for performing a non-linear encoding function
on an identification number embedded in said microchip and a combination of a unit
number and a stepping counter value, so as to generate a transmission value which
is only decodable by a related decoding function having access to the same identification
number; and (2) means for generating, upon a synchronization command being given
thereto, a counter value which is encodable together with the synchronization command,
to generate a synchronization transmission value which will facilitate the synchronization
of a related decoder microchip having the same identification number. The encoder
microchip may further comprise means for changing, e.g. incrementing or decrementing
the counter value by a number greater than one, after a given period of time subsequent
to the encoder microchip being operated.
The encoding function may be described by the following equation:
fencode (Identification number, (unit number, counter value))=transmission value.
The encoding and related decoding functions are, as stated above, 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
outputs 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 than 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 a one bit length and is preferably as long as 64 bits in which case
more than 10
19 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 than 16 bits, it will be less secure and consequently
it will be easier to decode.
According to another aspect of the invention, there is provided a decoder
microchip comprising: (1) means for performing a decoding function on a received
transmission value and an identification number embedded in the decoder microchip,
so as to generate from the transmission value, a decoded unit number and a decoded
counter value; (2) means for comparing the decoded counter value with a decoder
counter value range; and (3) means, upon a valid synchronization command having
been decoded by the decoder microchip, for synchronizing the decoder counter value
with the counter value of an encoder microchip which has generated the synchronization command.
According to a further aspect of the invention, there is provided a decoder
microchip comprising: (1) means for performing a decoding function on a received
transmission value and an identification number embedded in the decoder microchip,
so as to generate from the transmission value, a decoded unit number and a decoded
counter value; (2) means for comparing the decoded counter value with a decoder
counter value range; (3) means for recognizing, in the decoded unit number, a synchronization
command; and (4) means for storing the decoded counter value in the event of a
valid transmission value having been received. The decoder microchip may comprise
means for changing, e.g. according to a preferred embodiment incrementing or decrementing
the stored decoded counter value by a number greater than one, after a period of
time subsequent to the receipt of a valid transmission value. The decoder microchip
may comprise means for performing a format scan on signals so as to identify and
respond to valid transmission values.
The decoding function performed by the decoder microchip 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 microchip
having the same identification number as the decoder microchip.
The decoder microchip preferably also comprises distinguishing means for distinguishing
between a decoded unit number for normal operation and a synchronization command.
The decoder counter value may conveniently not be accepted by the decoder microchip
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 n constituting the number of lost codes the encoder microchip
would still accept. Alternatively, in the event that the decoded unit number comprises
a valid synchronization command, the decoder microchip may be adapted to store
the decoded counter value plus one as the decoder counter value for subsequent use.
The decoder microchip may, in addition, comprise means for comparing the counter
value with a value obtained from a uni-directional synchronization process to which
the decoder microchip may be subjected.
Also according to the invention, there is provided a combined encoder and decoder
microchip comprising: (1) means for performing a non-linear encoding function on
an identification number embedded in said microchip and a combination of a unit
number and a stepping counter value, so as to generate a transmission value which
is only decodable by a related decoding function having access to the same identification
number; (2) means for generating, upon a synchronization command being given thereto,
a counter value which is encodable together with the synchronization command, to
generate a synchronization transmission value which will facilitate the synchronization
of a related decoder microchip having the same identification number; (3) means
for performing a decoding function on a received transmission value and an identification
number embedded in the microchip, so as to generate from the transmission value,
a decoded unit number and a decoded counter value; (4) means for comparing the
decoded counter value with the decoded counter value range; and (5) means, upon
a valid synchronization command having been decoded by the microchip, for synchronizing
the decoder counter value with the counter value of an encoder microchip which
has generated the synchronization command.
According to a further aspect of the invention, there is provided a transmitter
remote control device comprising encoder means and transmission means adapted to
transmit a transmission value receivable by a receiver remote control device capable
of responding thereto, the encoder means comprising means for performing an encoding
function on an identification number embedded in the encoder means and a combination
of a unit number and a variable counter value so as to generate a transmission
value incorporated in the transmission, the transmission value being decodable
through a related decoding function performed by the receiver remote control device.
The encoder means may be adapted to generate a stepping counter value through
a uni-directional synchronization process for the synchronization of the encoder
means of the receiver remote control device.
Also according to the invention, there is provided a receiver remote control
device comprising decoder means comprising means for performing a decoding function
on a combination of a transmission value and an identification number, so as to
generate a decoded unit number and a decoded counter value; and means for comparing
the decoded counter value number with a counter value range.
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
synchronization 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 programmable
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 has 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 has applied the function and
the received code word would then be ignored.
The decoder means is preferably adapted to: (1) compare the decoded unit number
of the transmitted code word with its pre-embedded unit number, and upon agreement,
(2) check that the counter value falls inside a valid range of counter numbers,
and if both conditions are satisfied, (3) give an indication thereof to the outside,
in the form of a flag, and (4) 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 recognizing, 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 illegitimately.
Also according to the invention, the decoder means may comprise means for recognizing
a panic command generated in the encoder means, and means for responding thereto.
In addition, the decoder means may comprise means for recognizing other commands
and/or more than one unit number with independent counters, without having to perform
the decoding process more than once.
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:
FIG. 1 is a block diagram of an encoder microchip in accordance with the invention;
FIG. 2 is a block diagram of a decoder microchip in accordance with the invention;
FIG. 3 is a flow diagram for the functions which the encoder microchip can perform;
FIGS. 4A and 4B is a flow diagram for the functions which the decoder microchip
can perform;
FIG. 5 is a preferred format for the unit number and the counter value; and
FIG. 6 is a preferred format for the transmission value.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to FIG. 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 FIG. 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 decoded 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, e.g., indicating an activation of the transmitter (
10) to
a user.
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.
Referring to FIG. 1, 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 (FIG. 1) and decoder
(FIG. 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 FIG. 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 change, e.g., increment or decrement, every time the encoder is used to transmit
a value. In addition, the encoder may include a timing means or circuit, not shown,
which times out a given period of time, e.g., from about 30 seconds to about 60
seconds, after a transmission, and after said time the transmit counter (
4)
value is changed, e.g. according to a preferred embodiment incremented by a number
greater than one, e.g., 8 or 16, to provide further security against certain code
grabbing devices. If, for example, the counter value of the encoder microchip is
designed to change 30 seconds after a transmission or activation, and the encoder
microchip is operated twice during a 30 second period, the counter value, according
to a preferred embodiment, is only changed once.
The input word (
5) to the encoding function comprises the unit number
(CSR
3 and CSR
2 in the input register (
5)) and the transmit
counter value (CSR
1 and CSR
0 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 FIG. 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 FIG. 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 recognize 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 FIG. 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).
A more detailed description of the encoder operation shown in FIG. 3 will now
be
provided. Upon activation the encoder will perform the following actions sequentially
until terminated. The first action (
31) will be to reset itself. Then the
encoder will increment the transmitter counter value, an action that is repeated
every time the encoder is activated to transmit a value.
Next the inputs will be tested (
32) to determine whether a normal or
other (utility) command is required. The inputs will also influence the status
register. Based on the inputs the appropriate command value would be loaded into
the CSR
2 register part of the unit number. If in (
32) a normal mode
is determined CSR
2 is set to AAH, otherwise CSR
2 is set to XXH. The
encoding operation (
34) will now take place to create the transmission value
from the input word (
5). The transmission value will be transmitted for
four seconds (
35). If the encoder is still activated after this time it
will proceed to increment the transmitter counter value again (
36) and to
load the CSR
2 register with a different value, A5H, for example the "panic"
command value (
37) before encoding the input word again (
38). The
resulting transmission value will again be transmitted for a period of time (1
second) (
39).
If the transmission has still not been terminated, the encoder operation will
proceed to perform a synchronization sequence. This may include incrementing the
transmitter counter value by 256 (
40) and setting the CSR
2 value
to 55H to indicate a synchronization command (
41). It will then again encode
the input word before transmitting it (
42,
43). After one second
(
43) the encoder will terminate all further transmissions and will perform
an endless wait loop until it is deactivated (
45). It should be noted that
the transmit sequence may be terminated at any time.
The synchronization sequence may perform some other tests on the counter to further
establish it as a synchronization counter value for example the lower 8 bits of
the counter value must be forced to zero.
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 FIG.
2 shows the functional elements of the decoder microchip and the flow diagram in
FIGS. 4A and 4B shows it 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
FIG. 6. Another format may be chosen if it has advantages. When a valid transmission
word is recognized, 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 CSR
2 (see
FIG. 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 authorized 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 changed and stored (
66) in the corresponding
Rx counter register (
17,
18,
19,
20). In a preferred
embodiment, the decoded counter value of a valid transmission is incremented by
a number greater than one, e.g., 8 or 16, and stored (
66) in the corresponding
Rx counter register (
17,
18,
19,
20). Of course, the
receiver should be set to change or increment its stored counter value by the same
value as the transmitter. The decoded counter value should only be incremented
after a certain given period of time subsequent to the receipt of a valid transmission.
This embodiment provides additional security against newly developed code grabbing
devices as described above. 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 decoder microchip.
The unidirectional synchronization process is essential for establishing synchronization
between a matched transmitter and receiver. If in operation (
49) the transmission
is recognized 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 recognizes 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-volatil
