Pulse or digital communications – Transmitters – Keying circuits
Reexamination Certificate
2000-01-31
2001-11-06
Chin, Stephen (Department: 2734)
Pulse or digital communications
Transmitters
Keying circuits
C375S322000
Reexamination Certificate
active
06314143
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a circuit configuration for receiving an on-off-keying-modulated signal (OOK-modulated signal), in particular for use in a data carrier of an identification system. The configuration has a reception circuit, a demodulator circuit which is connected downstream of the latter, and a decoding circuit which is connected downstream of the latter and a sequential controller for controlling in particular the reception, demodulation and processing of received data.
Such a circuit configuration is known from Published, European Patent Application EP 0 669 591 A2.
In the case of identification systems, data are exchanged between a stationary or quasi-stationary unit and a mobile or portable unit—referred to below as a data carrier—which has at least one memory. Power is also frequently transmitted from the stationary unit to the data carrier, preferably by inductive coupling or electromagnetic radiation. Examples of this are access control devices with a portable identifier, such as a chip card for example. However, electronic immobilizers with a key-lock system are also examples of the identification systems of the generic type.
In identification systems of the generic type, data are transmitted from the stationary unit to the data carrier by OOK-modulation. The high-frequency carrier signal is used directly as a clock signal—possibly after suitable division and/or conditioning for digital signal processing—with the result that the blanking intervals occurring in the carrier signal owing to the modulation also appear in the clock signal. In addition, the power that is necessary to supply the data carrier is acquired by rectification and smoothing of the transmitted signal.
The OOK-modulated carrier signal is demodulated either directly from the transmitted and received signal or from the clock signal that is acquired from the signal. This is possible because the clock signal also has the blanking intervals, and thus the modulation. The circuits in the data carrier of the known identification system are configured here in such a way that a brief gating out of the clock is tolerated.
The reception of data takes place here according to a predefined protocol that is controlled by a sequential controller (state machine). Interruption in the clock at a time other than that permitted according to the rules of the reception protocol causes the reception to be aborted.
Published, European Patent Application EP 0 387 071 A1 also discloses how a reset signal is derived in a transponder from the clock signal which has been derived from an OOK-modulated carrier signal. The reset signal resets circuit components of the transponder if the pause between two time periods in which the carrier signal occurs exceeds a specific time period.
After the reception of the data that may constitute instructions or else values that are to be processed, the data are processed. Such processing contains reading and writing data or values in the nonvolatile memory of the data carrier and also the transmission of data to the stationary unit, and possibly the encryption of data. During the processing of the received data, the data carrier does not expect any further data, and thus any interruption in the clock, because the carrier signal is not modulated by the stationary unit.
However, this leads to a security risk because in principle the system tolerates the clock being stopped. A hacker could then stop the clock, or gate it out, and examine the respective state of the circuit and thus find out about the structure and method of operation in order to enable himself to simulate it.
In order to counter this problem, there is already the tendency in the specialist world to use amplitude modulation with a relatively low modulation index instead of a 100% OOK modulation. Therefore, less robust demodulation has to be implemented on the data carrier, which leads to increased costs due to a lower yield, more complex measurement method or poorer compatibility.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a circuit configuration for manipulation-protected reception of an OOk-modulated signal that overcomes the above-mentioned disadvantages of the prior art devices of this general.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for receiving an on-off-keying-modulated signal, including:
a demodulator circuit having an output;
a decoding circuit connected downstream of the demodulator circuit and having an input;
a sequential controller for controlling a reception, demodulation and processing of received data, the sequential controller having a reset input; and
a switching element driven by the sequential controller and disposed between the demodulator circuit and the decoding circuit, the switching element having an input connected to the output of the demodulator circuit, a first output connected to the input of the decoding circuit, and a second output connected to the reset input of the sequential controller, the switching element being driven by the sequential controller such that the output of the demodulator circuit being connected via the switching element to the input of the decoding circuit in a first operating state in which the data are received, and to the reset input of the sequential controller in a second operating state in which no data are received.
The switching element is disposed between the demodulator circuit and the decoding circuit of the data-carrier circuit configuration of the identification system. The switching element is used to connect—controlled by the sequential controller—the output of the demodulator circuit to the reset input of the sequential controller in the operating state in which no data are received.
If no data are transmitted from a reading station of the identification system to a data carrier, the carrier signal, which still has to be transmitted in order to transmit power, is not modulated with the result that the demodulator circuit outputs a constant signal, while the clock signal, which is also derived from the carrier signal, does not have any intervals.
If a hacker now interrupts the carrier signal, and thus the clock signal, in order to be able to examine the state of the circuit configuration in this interruption time period, the demodulator circuit would change the state at its output. Which then, according to the invention, brings about a reset of the circuit configuration so that it is not possible to examine instantaneous states of the circuit configuration by interrupting the clock.
The derivation of the clock signal from the high-frequency carrier signal can be effected by, for example, limitation and transformation at the switching level. Depending on the logic selected, the clock signal will then have a low or a high level in a pause in the clock signal. If the demodulator circuit has a first pause recognition circuit, which is advantageously formed with a retriggerable monoflop because this is simpler and thus more cost-effective, it is necessary to know the state of the signal which is to be demodulated while a carrier signal is gated out. The signal that is to be demodulated can be here, as already stated, either the clock signal that is derived from the signal that has already been transmitted by a reading station and received by the data carrier, or a signal which is derived from the received signal in the demodulator circuit and transformed to the switching level.
In order to ensure better protection against attacks by hackers, the quiescent state of the signal which is supplied to the first pause recognition circuit can be advantageously undefined. In this case, according to the invention, the first pause recognition circuit which is contained in the demodulator circuit has connected in parallel with it a further pause recognition circuit with a negating input. The output of the further pause recognition circuit is connected to the reset input of the sequential controller. The
Al-Beshrawi Tony
Chin Stephen
Greenberg Laurence A.
Infineon - Technologies AG
Lerner Herbert L.
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