Circuit for transmitting a square wave signal by inductive coupling.

Abstract

Electronic transmission circuit (100) by inductive coupling of a square wave signal between a transmitter and a receiver, comprising: • an operational amplifier (150), having two inputs and one output; • a first coil (110) one terminal of which is connected to a first connection pad of the transmitter, and the other to ground; • a second coil (120), inductively coupled with the first coil (110), and whose terminals are electrically connected to one end of a first pair of electrical traces; • a third coil (130), whose terminals are electrically connected to the other end of the first pair of electrical traces; • a fourth coil (140), inductively coupled with the third coil (130), and whose terminals are connected to the inputs of the operational amplifier (150); the output of which is connected to a first connection pad of the receiver. Figure for the abbreviation: Fig. 2

Description

Title of the invention: Circuit for transmitting a square wave signal by inductive coupling.

[0001] In the field of signal transmission, it is known to connect components or circuits together by a galvanic link.

[0002] It is sometimes desirable to eliminate galvanic coupling. In this case, it is known to implement a coupling by inductive coupling, which is the domain of the present invention.

[0003] Inductive coupling implies that the shape of the signal, for example sinusoidal or square, has an influence on the transmission of said signal.

[0004] The present invention relates to the transmission by inductive coupling of a square wave signal, in particular a baseband logic square wave signal, without modulation.

[0005] Signal transmission by inductive coupling is known.

[0006] In this field, the inductive transmission of a modulated sinusoidal signal allows for very good energy transfer at the cost of a phase shift between the incoming and outgoing signals, for example for modulated radio frequency signals, used in particular in RFID systems.

[0007] Transposed into the spectral domain, the resultant of a sinusoidal signal [Ex of a carrier frequency] by inductive coupling is represented by an energy line centered on the frequency of said signal. The energy transfer is optimal.

[0008] On the other hand, in the case of a square wave transmission, an infinite number of odd harmonics are obtained, with coefficients varying as 1 / n; and in practice, the efficiency is poor. The coupling acts as a bandpass filter with a limited bandwidth.

[0009] However, the transmission of a square wave signal is essential, for example, for a data signal or a clock signal.

[0010] In any case, the complete retransmission of a square signal would require an infinite bandwidth to recover the square signal at the output of the inductive coupling.

[0011] In practice, when a square wave signal is in the input of an inductive coupling, a distorted and attenuated signal is obtained at the output of said inductive coupling.

[0012] There are solutions that address this problem, but they are based on electronic systems that aim to change the shape of said signal to transform it into a sinusoidal signal upstream of the inductive coupling and then transform the sinusoidal signal back into a square signal at the output of the inductive coupling, for example the digital isolator for SPI (Serial peripheral interface) sold under the reference ADuM3151.

[0013] The present invention aims to overcome this problem by replacing the complete retransmission of a square signal with the detection and transmission of the edges of a square signal, without changing the shape of said signal.

[0014] More specifically, the invention relates, according to a first of its objects, to an electronic transmission circuit (100) by inductive coupling of a square wave signal between a first electronic device comprising at least one connection pad, and a second electronic device comprising at least one connection pad, • One of the first electronic device and the second electronic device being the emitter of said square signal, and the other being the receiver of said square signal.

[0015] It is essentially characterized in that the electronic transmission circuit (100) comprises: • an operational amplifier (150), comprising a first input, a second input and an output; • a first coil (110) one terminal of which is electrically connected to a first connection point of the transmitter, and the other terminal is connected to ground; • a second coil (120), arranged so as to be able to be in inductive coupling with the first coil (110), and whose terminals are electrically connected to one end of a first pair of electrical tracks; • a third coil (130), whose terminals are electrically connected to the other end of the first pair of electrical tracks; • a fourth coil (140), arranged so as to be able to be in inductive coupling with the third coil (130), and whose terminals are electrically connected respectively to the first input and the second input of the operational amplifier (150); • the output of the operational amplifier (150) being electrically connected to a first connection point of the receiver.

[0016]

[0017] According to another of its objects, the invention comprises a chip-enabled document, comprising: • a first module (10), comprising at least one connection pad, • a second module (20), comprising at least one connection pad, and • a first electronic circuit (100) according to the invention, which electrically connects the first module (10) and the second module (20), in which: • The first module (10) acts as the first electronic device and the second module (20) acts as the second electronic device, • the first electronic circuit (100) being configured for the transmission of a first square wave signal from the first module (10) to the second module (20).

[0018]

[0019] It can also be foreseen: • a second electronic circuit (100) according to the invention, which electrically connects the first module (10) and the second module (20), in which: • the second electronic circuit (100) is configured for the transmission of a first square wave signal from the second module (20) to the first module (10).

[0020]

[0021] It can also be foreseen: • a third electronic circuit (100) according to the invention, which electrically connects the first module (10) and the second module (20), in which: • the third electronic circuit (100) is configured for the transmission of a second square wave signal from the first module (10) to the second module (20).

[0022]

[0023] It can also be foreseen: • a fourth electronic circuit (100) according to the invention, which electrically connects the first module (10) and the second module (20), in which: • the fourth electronic circuit (100) is configured for the transmission of a second square wave signal from the second module (20) to the first module (10).

[0024]

[0025] It can be predicted that at least one of: • the first square wave, • the second square wave, • the third square wave and • the fourth square wave, is a periodic square wave signal at least intermittently.

[0026]

[0027] It can be foreseen that the operational amplifier (150) includes a Schmitt flip-flop function and includes a high switching threshold and a low switching threshold, the two switching thresholds being parameterizable.

[0028]

[0029] It can be predicted that the operational amplifier (150) has a gain * frequency ratio between (150) MHz and (320) MHz.

[0030]

[0031] It can be foreseen that the second module (20) is a biometric sensor, the smart document being a smart card (1000), a passport or an identity document.

[0032]

[0033] It can be foreseen that at least one connection pin (11) of the first module (10) or of the second module (20) is a synchronous serial interface.

[0034]

[0035] Based on the detection and amplification of the edges of a square wave signal, the present invention enables the transmission by inductive coupling of a square wave signal, moreover thanks to a particularly simple electronic circuit.

[0036] The present invention makes it possible to transmit a data signal or a clock signal, without changing the shape of the signal.

[0037] Other features and advantages of the present invention will become more apparent from the following description given by way of illustrative and non-limiting example and made with reference to the accompanying figures.

[0038] [Fig. 1] illustrates a smart card comprising a circuit according to the invention

[0039] [Fig.2] illustrates an electrical diagram of the circuit according to the invention,

[0040] [Fig.3] illustrates signals resulting from tests implementing a circuit according to the invention,

[0041] [Fig.4] illustrates signals resulting from implementation tests of a circuit according to the invention. Detailed description

[0042] The electrical diagrams in the figures are simplified; in particular, they do not represent the resistances or capacitances that the circuit 100 according to the invention may include.

[0043] The present invention can be implemented in a smart document, in particular such as a 1000 smart card, a passport or an identity document.

[0044] For the sake of brevity, only the case of a 1000 smart card will be described here. This example is not limiting.

[0045] A 1000 smart card according to the invention is in particular a 1000 smart bank card. However, a 1000 smart card can also have identity applications, or be a subscription card, etc.

[0046] The electronic circuit 100 for transmitting a square wave signal by inductive coupling according to the invention is referred to hereafter as the "circuit," for the sake of brevity. It is advantageously implemented for communication between a first electronic device and a second electronic device.

[0047] For example, for a 1000 bank smart card, the first electronic device is a module and the second electronic device is also a module.

[0048] The first module 10 includes at least one microcontroller, called a secure element, and the second module 20 includes a microcontroller or in this case a biometric sensor.

[0049] In order for the smart card 1000 to function effectively, it is necessary that communication can be established between the first and second module 20.

[0050] In this respect, data must be able to be exchanged between the first and second module 20, that is to say from the first module 10 to the second module 20, or from the second module 20 to the first module 10.

[0051] Similarly, it is useful, even necessary, that the first module 10 and the second module 20 share the same clock.

[0052] In the case of data or clock transfer, the associated signal is a square wave signal, which can be periodic, at least intermittently, i.e. over a set of at least one time range, for example for a clock signal; or non-periodic, for example for a data signal.

[0053] The invention therefore relates to an electronic circuit for transmitting a square wave signal, in this case by inductive coupling.

[0054] The first module 10 includes a set of at least one connection pad 11, allowing data or energy exchanges, by means of a digital signal, from or to said first module 10, and in particular the microcontroller.

[0055] Similarly, the second module 20 includes a set of at least one connection pad 21, allowing data or energy exchanges, by means of a digital signal, from or to said second module 20, and in particular the sensor.

[0056] The electronic transmission circuit 100 according to the invention is arranged between the first module 10 and the second module 20.

[0057] Preferably, the first module 10 and the second module 20 are connected by an SPI link, for Serial Peripheral Interface in English.

[0058] To ensure inductive coupling, the circuit 100 according to the invention comprises a set of pairs of induction coils, hereinafter referred to as "coils" for brevity.

[0059] Preferably, a circuit 100 according to the invention is provided per signal.

[0060] Preferably, a "half-duplex" configuration or a a "simplex" configuration, in which a circuit 100 according to the invention is provided for the transmission of a signal from a first electronic device to a second electronic device; and another electronic circuit 100 for the transmission of another signal from the second electronic device to the first electronic device.

[0061] Thus, in this case, we preferably have: • a circuit 100 according to the invention, for the transmission of a first square wave signal, from the first module 10 to the second module 20, where in this case the first module 10 acts as a transmitter, and the second module 20 acts as a receiver. And • another circuit 100 according to the invention, for the transmission of a second square signal, from the second module 20 to the first module 10, where in this case the second module 20 plays a role of transmitter, and the first module 10 plays a role of receiver. • The first square wave is usually different from the second square wave. For example, the first square wave is a MOSI signal and the second square wave is a MISO signal.

[0062] For the transmission of another square signal, for example a clock signal, another circuit 100 according to the invention is preferably provided, so that there is one circuit 100 according to the invention per square signal.

[0063] A circuit 100 according to the invention comprises at least two pairs of coils.

[0064] The first pair of coils comprises a first coil 110 and a second coil 120, and the second pair of coils comprises a third coil 130 and a fourth coil 140.

[0065] The first coil 110 and the second coil 120 are arranged so that they can be in inductive coupling with each other; and the third coil 130 and the fourth coil 140 are arranged so that they can be in inductive coupling with each other.

[0066] Each coil includes two connection terminals, hereinafter referred to as "terminals" for brevity.

[0067] For the sake of brevity, we consider here only the transmission of a square wave signal between a first module 10 (for example a secure element) and a second module 20 (for example a biometric sensor) on a smart card 1000.

[0068] The terms "transmitter", "first electronic device" and "first module 10" are therefore understood to be used interchangeably. The same applies to the terms "receiver", "second electronic device" and "second module 20".

[0069] At the transmitter:

[0070] The first terminal of the first coil 110 is electrically connected to a first connection point 11 of the transmitter, and the second terminal of the first coil 110 is connected to ground.

[0071] The first connection pin 11 of the transmitter can be a standard connection pin, for example for the clock signal output. It is accessible either directly on the microcontroller (secure element) or on a face of the module.

[0072] A ground connection pin is easily accessible on the transmitter, in a standard manner.

[0073] The first coil 110 can be integrated into the first module 10 or remote from it.

[0074] The second coil 120 is arranged near the first coil 110, so that it can be in inductive coupling with the first coil 110.

[0075] In a smart card 1000, the first coil 110 and the second coil 120 are integrated into the card body, i.e. not accessible from outside the smart card 1000.

[0076] The terminals of the second coil 120 are electrically connected to one end of a first pair of electrical tracks.

[0077] The terminals of a third coil 130 are electrically connected to the opposite end of the first pair of electrical tracks.

[0078] The second coil 120 and the third coil 130 are connected directly by the first pair of electrical tracks, without any intermediate electronic component, so that the signal from the inductive coupling between the first coil 110 and the second coil 120 can be transferred to the third coil 130 by the first pair of electrical tracks.

[0079] Opposite the third coil 130, a fourth coil 140 is provided, arranged so as to be able to be in inductive coupling with the third coil 130.

[0080] Thus the signal from the inductive coupling between the first coil 110 and the second coil 120 can be transferred downstream of the fourth coil 140, thanks to the inductive coupling between the third coil 130 and the fourth coil 140.

[0081] The connection of circuit 100 according to the invention is not symmetrical between the transmitter and the receiver.

[0082] Indeed, on the receiver side, the terminals of the fourth coil 140 are not electrically connected directly to a connection point 21 of the receiver but are connected to an operational amplifier 150.

[0083] The operational amplifier 150 comprises two inputs IN+ and IN- and one output. The first input is electrically connected to one terminal of the fourth coil 140 and the second input of the operational amplifier 150 is electrically connected to the other terminal of the fourth coil 140.

[0084] The output of the operational amplifier 150 is electrically connected to a connection pin 21 of the receiver.

[0085] Thus the signal from the inductive coupling between the first coil 110 and the second coil 120 can be transferred upstream of the operational amplifier 150 by means of the inductive coupling between the third coil 130 and the fourth coil 140, then amplified and filtered by means of the operational amplifier 150.

[0086] The operational amplifier 150 acts in fact not only as an amplifier but also as a filter.

[0087] As such, it combines the functions of an operational amplifier 150 and a Schmitt flip-flop, which includes two switching thresholds, a high threshold and a low threshold, in this case configurable.

[0088] When the input signal of the operational amplifier 150 exceeds the value of the upper threshold, then the output signal of the operational amplifier 150 is a constant signal whose value is parameterized; and when the input signal of the operational amplifier 150 falls below the value of the lower threshold, then the output signal of the operational amplifier 150 is at 0; which acts as a noise filter.

[0089] For example, an operational amplifier 150 is a high-speed comparator.

[0090] In particular, an electronic component of the LTC6752 type can be used, which has a very good common-mode rejection ratio, and is known, for example, at https: / / www.analog.com / media / en / technical-documentation / data-sheetsZ6752fc.pdf, where "very good" means greater than a predetermined threshold value. It advantageously features a CMOS output. It also allows for signal amplification.

[0091] In this case, the square wave signal is transmitted on the first input IN+. On the second input IN-, a reference signal is transmitted, in this case a hysteresis reference signal.

[0092] Preferably, the second IN- input is also connected to a voltage divider bridge (not shown), so as to create a virtual ground, which in this case is uncorrelated with the ground of the power supply.

[0093] Thanks to this virtual ground, it is possible to supply the operational amplifier 150 in an asymmetrical, and positive manner, for example between 0 V and 3.3 V, while retaining the possibility of triggering output edges on a high threshold of positive voltage and a low threshold of negative voltage.

[0094] Advantageously, the operational amplifier 150 allows the hysteresis to be adjusted.

[0095] Thanks to this operational amplifier 150, each time the square wave signal at the input crosses the predetermined high threshold, the operational amplifier 150 triggers an edge at the output, for example a clock edge.

[0096] Preferably, it is also provided that each time the crossing falls back below another threshold value (low threshold), for example the high threshold value -5mV or -10mV, then the output signal of the operational amplifier 150 goes to 0.

[0097] Thus, from a noisy input signal with peaks, thanks to a high threshold and a low threshold, we have an output signal which is a switching signal with a square shape.

[0098] Thanks to the present invention, it is possible to obtain CMOS logic levels of 1.8V or 3.3V at the output of the electronic circuit 100, at the same frequency as the input signal. The operating frequency of the circuit 100 according to the invention, for example 2.5 MHz, is distinct from that of the chip containing it, for example 13 MHz, so as not to create interference.

[0099] Indeed, in order to optimize the inductive coupling, it is necessary to adjust the resonant frequency of the coils of circuit 100. To this end, a set of at least one capacitor is generally added in parallel with the first coil 110 and the fourth coil 140, so that the inductance / capacitor pair causes circuit 100 to resonate at a resonant frequency close to the pseudo-periodic frequency of the square wave signal; this is particularly true for the clock line. This allows the response curve and the transfer of the first harmonics of the signal to be optimized.

[0100] Tests were carried out by the applicant on a 1000 smart card in which the first electronic device is a module comprising a secure element and the second electronic device is a biometric sensor (sometimes also called a module), which are mounted on the same side of the 1000 smart card.

[0101] The secure element and the biometric sensor communicate via an I2C interface (points A and D on [Fig.2]).

[0102] To transmit the signal from one component to another, it is advantageous to use the fact that the first component and the second component each include a respective coil (inductor).

[0103] The gain * frequency ratio of the circuit 100 according to the invention is for example between 150 MHz and 320 MHz, and in particular equal to 280 MHz plus or minus 10%.

[0104] The [Fig.3] in the upper part represents a square signal at the input of the circuit 100 according to the invention, at point A.

[0105] The [Fig.3] in the lower part represents the square wave signal at the input of the operational amplifier 150, at point B.

[0106] The [Fig.4] in the lower part represents the square wave signal at the output of the operational amplifier 150, at point C.

[0107] The [Fig.4] in the upper part represents the square wave signal, at point D, "seen" by a logic analyzer.

[0108] We thus clearly have a transmission of a square wave signal by inductive coupling thanks to the circuit 100 according to the invention.

[0109] Nomenclature

[0110] 10 First module [YES]

[0112]

[0113]

[0114]

[0115]

[0116]

[0117]

[0118]

[0119] 11 Connection pad of the first module 20 Second module 21 Connection pad of the second module 100 Electronic transmission circuit 110 First coil 120 Second coil 130 Third coil 140 Fourth coil 150 Operational amplifier 1000 Smart card

Claims

Demands

1. Electronic transmission circuit (100) by inductive coupling of a square wave signal between a first electronic device comprising at least one connection pad, and a second electronic device comprising at least one connection pad, One of the first electronic device and the second electronic device being the transmitter of said square wave signal, and the other being the receiver of said square wave signal, characterized in that the electronic transmission circuit (100) comprises: • an operational amplifier (150), comprising a first input, a second input and an output; • a first coil (110) one terminal of which is electrically connected to a first connection pad of the transmitter, and the other terminal is connected to ground;• a second coil (120), arranged so as to be able to be inductively coupled with the first coil (110), and whose terminals are electrically connected to one end of a first pair of electrical tracks; • a third coil (130), whose terminals are electrically connected to the other end of the first pair of electrical tracks; • a fourth coil (140), arranged so as to be able to be inductively coupled with the third coil (130), and whose terminals are electrically connected respectively to the first and second inputs of the operational amplifier (150); • the output of the operational amplifier (150) being electrically connected to a first connection pin of the receiver.

2. Chip-enabled document, including: • a first module (10), comprising at least one connection pad, • a second module (20), comprising at least one connection pad, and • a first electronic circuit (100) according to claim 1, which electrically connects the first module (10) and the second module (20), in which: the first module (10) acts as the first electronic device and the second module (20) acts as the second electronic device, the first electronic circuit (100) being configured for the transmission of a first square wave signal from the first module (10) to the second module (20).

3. Chip document according to claim 2, further comprising: • a second electronic circuit (100) according to claim 1, which electrically connects the first module (10) and the second module (20), in which: the second electronic circuit (100) is configured for the transmission of a first square wave signal from the second module (20) to the first module (10).

4. Chip document according to any one of claims 2 or 3, further comprising: • a third electronic circuit (100) according to claim 1, which electrically connects the first module (10) and the second module (20), wherein: the third electronic circuit (100) is configured for the transmission of a second square wave signal from the first module (10) to the second module (20).

5. Chip-based document according to claim 4, further comprising: • a fourth electronic circuit (100) according to claim 1, which electrically connects the first module (10) and the second module (20), in which: the fourth electronic circuit (100) is configured for the transmission of a second square wave signal from the second module (20) to the first module (10).

6. Chip document according to any one of claims 2 to 5, wherein at least one of: • the first square wave, • the second square wave, • the third square wave and • the fourth square wave, is a periodic square wave at least intermittently.

7. Chip document according to any one of claims 2 to 6, wherein the operational amplifier (150) includes a Schmitt flip-flop function and includes a high flip-flop threshold and a low flip-flop threshold, both flip-flop thresholds being configurable.

8. Chip document according to any one of claims 2 to 7, wherein the operational amplifier (150) has a gain * frequency ratio between (150) MHz and (320) MHz.

9. Smart document according to any one of claims 2 to 8, wherein the second module (20) is a biometric sensor, the smart document being a smart card (1000), a passport or an identity document.

10. Chip document according to any one of claims 2 to 9, wherein at least one connection pad (11) of the first module (10) or of the second module (20) is a synchronous serial interface.