CN117917912A - Method of contactless communication between an object and a reader using active load modulation - Google Patents

Abstract

The present disclosure relates to a contactless communication method between an object and a reader using active load modulation. In one embodiment, the object may communicate contactlessly with the reader using active load modulation. The method includes detecting a sensing magnetic field emitted by a reader, initializing a phase locked loop of the subject, detecting a stop of the sensing magnetic field emitted by the reader before the phase locked loop of the initialization subject is completed, calibrating an oscillator of the phase locked loop based on an internal clock of the subject, detecting a new sensing magnetic field of the reader after calibrating the oscillator, and implementing the phase locked loop to adjust a phase of the oscillator after detecting the new sensing magnetic field.

Description

Method of contactless communication between an object and a reader using active load modulation

Cross Reference to Related Applications

The application claims the benefit of French application No.2210964 filed on month 21 of 2022, incorporated herein by reference.

Technical Field

Embodiments and examples of the present invention relate to near field communication, that is to say contactless.

Background

Near field communication (also known by the acronym NFC) is a short-range high-frequency wireless communication technology that enables data to be exchanged between two contactless devices over a short distance (e.g. on the order of 10 cm).

NFC technology is an open technology platform standardized in standards ISO/IEC 18092 and ISO/IEC 21481, but combines many existing standards, such as type a and type B protocols defined in standard ISO-14443, which may be communication protocols that may be used in NFC technology.

Near field communication may be performed between the reader and an object that may be in a card emulation mode. The reader is then configured to generate a magnetic field through its antenna, which in the standard of conventional use is typically a sine wave of 13.56 MHz. The strength of the magnetic field is, for example, between 0.5 and 7.5 amperes/meter Root Mean Square (RMS).

Near field communication may be performed according to an active mode of operation. In this mode of operation, both the reader and the object generate an electromagnetic field. Typically, this mode of operation is used when the object is equipped with its own power source (e.g. a battery), as is the case in a mobile cellular telephone, which is then in a card emulation mode.

Disclosure of Invention

Embodiments provide a contactless communication method using active load modulation between an object and a reader.

According to one aspect, a method of contactless communication between an object and a reader uses active load modulation. The method includes transmitting a sensing magnetic field by a reader, and then detecting the sensing magnetic field by an object. The phase locked loop of the object is initialized. A stop of the field of the reader is detected before the phase locked loop of the initialization object ends. An oscillator of a phase locked loop of the object is calibrated based on an internal clock of the object. Then, a new sensed magnetic field of the reader is detected and a phase locked loop is implemented to adjust the phase of the oscillator in order to enable the subject to respond to the reader.

This method may initialize the frequency of the oscillator of the phase locked loop of the object when a fast sensing reader is detected. This initialization makes it possible to define the frequency of the oscillator of the phase locked loop based on the internal clock of the object. Thus, when the reader transmits a new field to sense the presence of an object, the phase-locked loop is prepared to transmit a response to the reader more quickly. In fact, during detection of a new field, the object may only adjust the phase of the signal generated on the phase of the field generated by the reader, since the frequency of the signal generated by the oscillator of the phase locked loop is already desired. In particular, the calibration of the frequency of the oscillator may be between 200 μs (microseconds) and 300 μs, while the adjustment of the phase of the oscillator may be simply between 10 μs and 20 μs. By responding to the reader during its sensing period, the object can initiate communication with the reader.

Thus, this approach may enable an object to communicate with a reader even though the time for initializing its phase locked loop is longer than the sensing period of the reader.

In an advantageous embodiment, the calibration of the oscillator is performed by measuring the output frequency of the oscillator, then by comparing the measured frequency with a desired frequency, and then calibrating the frequency of the oscillator according to the result of the comparison.

As a variant, the calibration of the oscillator is performed by locking the frequency of the oscillator to the frequency of the internal clock using a frequency-locked loop.

For example, calibration may include implementation of a frequency-locked loop to calibrate and lock the frequency of the oscillator based on a value stored in a register when a sensed magnetic field of the reader is detected to be stopped before initialization is completed. Preferably, the phase locked loop is implemented when a new sensing magnetic field of the reader is detected to adjust the phase of the oscillator. Advantageously, the phase locked loop is implemented when a new sensing magnetic field of the reader is detected to lock the frequency locked loop of the control oscillator.

According to another aspect, the object is configured to be capable of performing contactless communication with the reader by using active load modulation. The object includes an internal clock, a detector configured to detect a sensing magnetic field of the reader, and a phase-locked loop including an oscillator and configured to lock a phase of the oscillator to the sensing magnetic field of the reader. The phase-locked loop is configured to be initialized when the detector detects the sensed magnetic field. The system for calibrating the frequency of an oscillator of a phase locked loop is configured to: when the detector detects a stop of the sensing magnetic field of the reader before the end of the phase locked loop of the initialization object, the frequency of the oscillator is calibrated based on the internal clock, and then when a new sensing magnetic field of the reader is detected, the phase of the oscillator is adjusted so as to be able to respond to the reader.

In an advantageous embodiment, the calibration system is configured to measure the output frequency of the oscillator and to compare the measured frequency with a desired frequency and then to calibrate the frequency of the oscillator based on the comparison.

As a variant, the calibration system comprises a frequency-locked loop configured to lock the frequency of the oscillator to the frequency of the internal clock.

Advantageously, the calibration system is configured to implement the frequency-locking loop to calibrate and lock the frequency of the oscillator based on a value stored in a register when the detector detects a stop of the sensing magnetic field of the reader before initializing the phase-locked loop of the object ends. A phase locked loop is then implemented when a new sensed magnetic field of the reader is detected to adjust the phase of the oscillator so that it can respond to the reader.

The calibration system may be configured to implement a phase-locked loop to lock the frequency-locked loop of the control oscillator when a new sensed magnetic field of the reader is detected.

Drawings

Other advantages and features of the invention will become apparent from a study of the detailed description of embodiments and the accompanying drawings, which are in no way limiting, wherein:

fig. 1 shows an embodiment of a near field communication system;

FIG. 2 illustrates a method implemented by an object to communicate with a reader in contactless communication;

fig. 3 shows a first embodiment of a system for controlling an oscillator of a phase locked loop;

Fig. 4 shows a method of controlling an oscillator of a phase locked loop;

fig. 5 shows a second embodiment of a system for locking an oscillator of a phase locked loop;

fig. 6 shows a frequency locked loop of a control system integrated with a phase locked loop; and

Fig. 7 shows a method of controlling an oscillator of a phase locked loop.

Detailed Description

As described above, near field communication can be performed between the reader and the object in the card emulation mode. The reader is configured to generate a magnetic field through its antenna. In the active mode, both the reader and the object generate an electromagnetic field. Typically, this mode of operation is used when the object is provided with its own power source (e.g. a battery), as is the case in a mobile cellular telephone, which is then in a card emulation mode. In particular, near field communication may be performed by using Active Load Modulation (ALM). Active load modulation requires signal synchronization between the reader and the object.

The reader is configured to emit an electromagnetic field and the object is configured to modulate the amplitude of the field without interference. In response to the reader, the object generates a signal synchronized with the reader field so as to be in phase with the reader field. The reader will generate a field that is stable enough to be able to detect low variations in its field depending on the distance between the reader and the card emulator.

In the reader mode or the card emulation mode, the object generates the most specific possible clock, and this clock enables communication to be ensured with the least amount of energy. For this reason, the presence of spurious tones (spurious tone) in the generated clock should be reduced or even avoided.

In this embodiment, the object comprises a phase locked loop. The phase locked loop includes an oscillator that is locked in phase and frequency based on a signal having a reference frequency that may be different from 13.56 MHz. The signal having the reference frequency may be a signal from a field generated by the reader. The oscillator is locked so as to obtain a signal having a desired frequency of, for example, 13.56MHz as an output of the phase locked loop. The oscillator is then locked to produce a signal having a plurality of frequencies of the desired frequency, for example, a frequency of 64 x 13.56MHz for a desired frequency of 13.56 MHz. The object may also comprise a circuit which allows the frequency of the signal generated by the oscillator to be divided in order to obtain a signal of the desired frequency, for example 13.56 MHz.

The reader is configured to periodically transmit a sensing magnetic field to initiate communication with the object. The object is configured to lock an oscillator of the phase locked loop to a frequency of a field of the reader. Once the frequency and phase of the oscillator have been calibrated, the subject is configured to respond by generating a magnetic field having the same frequency and the same phase as the sensing magnetic field of the reader based on a clock signal generated as an output of the phase locked loop to communicate with the reader.

Typically, the reader's sensing magnetic field is emitted long enough, e.g., for a period of 5 milliseconds, to cause the subject to initialize its low dropout regulator and to cause the phase locked loop to end locking its oscillator to the reader's field.

Some readers have an operation that does not fully respect NFC technology and may also take advantage of aspects discussed herein.

The reader may be configured to sense the presence of the card very quickly relative to NFC technology standards. Such readers are configured to periodically sense whether the card is close to the reader. To this end, in the same manner as described above, the reader transmits the magnetic field it modulates in order to transmit a response request, and then waits for a response if the card is close. However, such readers are configured to transmit their fields for a short duration before sending their response requests. However, during this short period, the card emulation mode object has no time to initialize to lock its phase and frequency of the field to the reader's field. In this case, even if the object is close to the reader, the reader does not receive a response from the object. Thus, the object cannot start communication with the reader.

Embodiments provide a method that enables an object to communicate with a reader, quickly sensing the presence of a card.

Fig. 1 shows an embodiment of a near field communication system SYS. The system SYS comprises a reader RD and an object CE which may be in a card emulation mode for near field communication. Whether or not standard compliant, the object CE may be a multi-function phone, a connected watch, or any other device using NFC. The object CE is configured to communicate by using Active Load Modulation (ALM). Active load modulation requires signal synchronization between the reader RD and the object CE.

In particular, the reader RD comprises an antenna ant configured with a magnetic field of a given frequency, for example of the order of 13.56 MHz.

The object CE includes an antenna ANTC configured to receive a magnetic field emitted by the reader RD and to emit the magnetic field. The object also comprises a battery BAT which makes it possible to power its various elements.

The object CE comprises a phase locked loop PLL. The phase locked loop PLL comprises an oscillator DCO. The oscillator DCO may be a digitally controlled or an analog controlled oscillator. For example, the digitally controlled oscillator may be a ring oscillator. The phase-locked loop PLL is configured to lock the phase and frequency of the oscillator DCO to the frequency and phase of the field of the reader. Thus, the phase-locked loop PLL can generate a signal having the same frequency and the same phase as the reader.

The object CE further comprises a transmission chain TCH configured to form a signal to be transmitted via the antenna for communication with the reader. The transmission chain uses a clock signal to generate a signal to be transmitted, which has the same frequency and the same phase as the field of the reader.

The object CE includes at least one low dropout regulator (LDO regulator) configured to supply power to the phase locked loop.

The object CE includes a low power mode in which some of its circuitry is not powered, particularly the LDO regulator powering the phase locked loop.

The object CE is configured to exit the low power mode when it receives the magnetic field emitted by the reader RD.

In particular, the object CE is configured to initialize its phase-locked loop PLL when it receives the field of the reader RD. If the sensing magnetic field of the reader is transmitted long enough for the phase-locked loop PLL to be initialized, the object CE is configured to receive the request of the reader RD and respond thereto in order to initiate a communication by using the clock signal generated by the phase-locked loop PLL locked onto the field of the reader RD.

As a variant, the reader RD may be a fast sensing reader. The sensing magnetic field of the reader is then not emitted long enough to enable initializing the phase-locked loop PLL of the object CE.

However, in the illustrated embodiment, the object CE is even configured to communicate with a fast sensing reader RD.

In particular, the object CE is configured to detect whether the reception field stops before the initialization phase locked loop PLL ends. In this case, the object CE knows that the reader RD is a fast sensing reader.

If the object CE has detected that the reader RD is a fast sensing reader, the object CE is configured to calibrate the frequency of the oscillator DCO of the phase locked loop PLL by using the frequency of the signal generated by the internal clock CLKXO. There are several ways to perform the calibration of the oscillator DCO of the phase locked loop PLL. Fig. 3 to 6, described below, illustrate two different ways for performing this calibration.

Once the oscillator is calibrated, the object CE is configured to wait for the next emission of the sensing magnetic field of the reader RD.

Once the object CE detects a new sensed magnetic field of the reader RD, the object CE is configured to implement a phase-locked loop PLL to calibrate the phase of the oscillator of the phase-locked loop PLL on the phase of the sensed magnetic field. When the phase of the oscillator DCO is calibrated, the object CE is configured to initiate communication with the reader RD.

Fig. 2 shows a method of communicating with a reader RD in contactless communication implemented by an object CE such as described above.

The method comprises a step 20, wherein the object CE is in its low power mode.

The method further comprises a test 21, wherein the object CE detects a field of the reader RD.

Subsequently, if the object CE detects a field of the reader RD during test 21, the method comprises a step 22 of initializing the object CE. During this step of initializing the object CE, the latter activates the various elements necessary to initiate the communication with the reader RD. For example, object CE activates an LDO regulator. Activation of the LDO regulator makes it possible to power the phase locked loop PLL. During an initialization step 22, the object CE performs a calibration and locking of the phase-locked loop PLL to the frequency and phase of the magnetic field.

The method then comprises a test 23 for determining the presence of a field of the reader RD. During this step, the object CE evaluates whether the fields of the reader RD are still detected.

If the object CE detects that the field of the reader RD is still present, the object CE deduces therefrom that the reader RD is configured to sense for a time long enough to enable initializing the object CE.

Thus, in this case the method comprises a test 24 which makes it possible to know whether the calibration and locking of the phase locked loop PLL is complete.

Once the frequency and phase of the oscillator DCO of the phase locked loop is locked to the frequency and phase of the magnetic field and if the field of the reader RD is still present, the method comprises a communication step 25, wherein the object CE communicates with the reader RD.

Once the communication is over, the object CE detects the end of the field of the reader RD during test 26 and then returns to step 20 in its low power mode until the next detection of the magnetic field.

If the field vanishes during step 22 (initializing the object CE, including calibration of the phase locked loop), the object CE no longer detects the field of the reader RD during test 23. The object CE deduces therefrom that the reader RD is a sensing reader that is not configured to sense long enough to enable initializing the object CE.

In this case the method comprises a step 27 in which a phase locked loop PLL is implemented to lock the oscillator DCO at a desired frequency, for example on the order of 64 x 13.56mhz, based on the internal clock of the object CE. In a particular example, the internal clock may be configured to generate a clock signal having a frequency between 13MHz and 76 MHz. The method includes a test 28 that allows verification that the oscillator is properly calibrated at the desired frequency.

Subsequently, once the oscillator DCO has been calibrated at the desired frequency, the method comprises a step 29, in which the object CE starts a timer while waiting for the next field detection.

The method then includes a test 30 in which the value of the time counter is evaluated with respect to a predetermined duration. If the time counter reaches a predetermined duration and the object CE does not detect a magnetic field during test 31, the object CE considers the reader RD to be no longer in proximity. The object CE then returns to its low power mode of step 20.

If the object CE detects a magnetic field during the test 31 before the end of the predetermined duration, the method comprises a step 32, in which the phase-locked loop PLL locks the frequency and phase of the oscillator DCO to the frequency and phase of the magnetic field.

Once the frequency and phase of the oscillator DCO are locked, the method comprises a communication step, in which the object CE communicates with the reader RD.

Once the communication is over, the subject CE detects the end of the field of the reader RD during test 26 and then returns to step 20 in its low power mode until the next detection of the magnetic field.

This method makes it possible to initialize the frequency of the oscillator DCO of the phase locked loop of the object CE when the fast sensing reader RD is detected. This initialization makes it possible to define the frequency of the oscillator DCO of the phase locked loop PLL based on the internal clock CLKXO of the object. Thus, the phase-locked loop PLL is prepared so that the object CE can more quickly issue a response to the reader when the object CE issues a new response request for sensing the presence of the card. In fact, during the detection of a new field, the object CE can only adjust the phase of the signal generated on the phase of the field generated by the reader, since the frequency of the signal generated by the oscillator DCO of the phase-locked loop PLL is already desired. In particular, the calibration of the frequency of the oscillator may be between 200 μs (microseconds) and 300 μs, while the adjustment of the phase of the oscillator may be simply between 10 μs and 20 μs. By responding to the reader RD during the sensing period of the reader, the object can communicate with the reader.

Thus, this method enables the object CE to communicate with the reader RD even if the time for initializing its phase-locked loop PLL is longer than the sensing period of the reader RD.

Fig. 3 shows a first embodiment of a system for controlling an oscillator DCO of a phase locked loop PLL, such that the oscillator DCO can be locked based on an internal clock when the detected field comes from a fast sensing reader RD. Specifically, the object CE is configured to control the frequency of the oscillator DCO of the phase-locked loop PLL with the control value dco_cal. The oscillator DC0 has an output connected to a first input of the counter divider CDIV 1. The counter divider CDIV1 further comprises a second input configured to receive a clock signal having a frequency f_ref. The clock signal is generated by the internal clock of the object CE. The counter divider CDIV1 is configured to generate a signal f_dco/f_ref.

The object CE is configured to implement a machine having a limited number of states. Such a machine with a limited number of states makes it possible to define the control value dco cal. In particular, fig. 4 shows a method that can be implemented by a machine with a limited number of states to control the oscillator DCO of the phase locked loop PLL. In this embodiment, the method comprises a step 40, wherein the control system is in a low power mode and the calibration control of the oscillator dco cal is initialized to its minimum value, the method then comprising a test 41, which test 41 enables the calibration of the phase-locked loop required in step 22 described above.

The method then comprises a step 42 of measuring the output frequency of the oscillator DCO of the phase locked loop PLL.

The method then comprises a test 43, in which the measured output frequency of the oscillator DCO is compared with a desired frequency, for example in the order of 64 x 13.56 mhz. If the measured frequency is lower than the desired frequency, the method comprises a step 44, in which the control value is incremented to increase the frequency of the oscillator DCO. After the incrementing, the measurement of the frequency is performed again in order to verify whether the frequency of the oscillator has reached its value.

If the frequency of the oscillator reaches the desired frequency, the calibration ends in step 45.

Fig. 5 shows a second embodiment of a system for locking the oscillator DCO of a phase locked loop PLL, so that the oscillator DCO can be locked based on an internal clock when the detected field comes from the fast sensing reader RD.

The control system is configured to receive the value FWW _TYP as an input to the frequency-locked loop FLL. This value FWW _TYP may be stored in a register, among other things. The value FWW _TYP is a predetermined value equal to F_ (DCO_ trgt)/F_ref, where F_ (DCO_ trgt) corresponds to the desired frequency of the oscillator, e.g., 64 x 13.56MHz, and F_ref corresponds to the frequency of the internal clock.

The control system is generated by a frequency-locked loop FLL controlling the same oscillator DCO as the phase-locked loop. The frequency-locked loop FLL includes a counter divider CDIV2. The counter divider CDIV2 includes a first input connected to the output of the oscillator DCO, and a second input configured to receive a clock signal of a reference frequency f_ref generated by the internal clock CLKXO. The counter divider CDIV2 is configured to generate a signal f_dco/f_ref. The frequency-locked loop FLL also includes a comparator CMP2 having a first non-inverting input configured to receive a value FWW _type and a second input configured to receive a signal f_dco/f_ref. The frequency-locked loop FLL also includes a loop filter fll_f having an input connected to the output of the comparator and an output connected to the oscillator DCO.

Fig. 6 shows a frequency locked loop of a control system integrated with a phase locked loop to control the same oscillator DCO. In this example, the oscillator DCO is configured to output an output oscillator signal having a frequency equal to N x 13.56 MHz. N is preferably an integer equal to or greater than 1, for example equal to 64.N may also advantageously be fractional in combination with the delta-sigma converter to recover the average.

The phase-locked loop PLL comprises a loop filter PLL f, a controlled oscillator DC0 and a first counter divider CNT1, the first counter divider CNT1 receiving on the one hand the output signal of the oscillator and on the other hand the clock signal CLKEX extracted from the field of the reader.

The output of the first counter divider CDIV1 is looped back via the first comparator CMP 1. Further, the adder ADD1 makes it possible to ADD a phase offset Φoffset, which represents a phase offset caused by a circuit provided between the antenna ANTC and the phase-locked loop PLL, alternatively.

The frequency locked loop FLL here includes the loop filter fll_f, the oscillator DC0, and the counter divider CDIV2 as described above, and the counter divider CDIV2 receives the output signal of the oscillator DC0 at one end and the reference clock signal CLKX0 of the reference frequency f_ref transmitted by the internal clock CLKX0 of the object CE at the other end.

The two loop filters pll_f and fll_f here are filters of conventional construction (usually integrated with a stabilizing filter) and are preferably designed in such a way that the cut-off frequency of the loop FLL is greater than the cut-off frequency of the loop PLL in order to avoid any stability problems as much as possible. Thus, the time response of the two loop PLLs and FLL is managed by the loop PLL.

The output of the counter CDIV2 loops back to the input of the loop filter fll_f via the comparator CMP 2.

In the present case, the output of the first counter CDIV1 delivers a phase offset between the output signal of the oscillator and the clock signal CLEX extracted from the field of the reader, whereas one of the stages (stages) of the counter CDIV1 makes it possible to deliver a clock signal DVCLK of frequency divided by N, for example here 64. In this way a frequency of 13.56MHz is obtained with the closest tolerance with respect to the frequency of the signal DCOS.

The switch is further arranged between the loop filter pll_f and the comparator CMP 2. The switch includes a first input '0' configured to be connected to an output of the loop filter pll_f, and a second input '1' configured to receive the value FWW _typ. The switch also includes an input configured to receive a select signal start_on_ dfll. The switch has an output connected to an input of the frequency-locked loop. Thus, when the value of the select signal start_on_ dfll is equal to 0, the output signal of the switch corresponds to the output value of the loop filter pll_f, and when the value of the select signal start_on_ dfll is equal to 1, the output signal of the switch corresponds to the value of FWW _typ.

The switch is configured such that it is possible to operate only the frequency-locked loop without the field of the reader (the value of the selection signal then equals 1).

The switch is configured to enable operation of the phase locked loop when a new sensed magnetic field of the reader is detected.

Fig. 7 shows a method implemented by the control system for controlling the oscillator DCO of the phase locked loop PLL by using the frequency locked loop FLL. Specifically, the method includes step 70, wherein the control system is in a low power mode and the control of the oscillator dco_cal is initialized to a minimum value. The method then comprises a test 71 which allows to start the calibration of the phase locked loop required in step 22 described above. To calibrate the phase locked loop, the control system performs a frequency locked loop for a predetermined duration based on the value FWW _TYP in step 72. Subsequently, the method comprises a test 73, wherein the control value dco_ctrl of the oscillator is compared with the value of the center frequency dco_mid, in particular 64 x 13.56mhz. Test 73 makes it possible to allow the time of the frequency-locked loop to stabilize. If the control value dco_ctrl of the oscillator is lower than the value dco_mid, this means that the frequency of the oscillator is lower than the desired frequency. If the frequency of the oscillator is lower than the desired frequency, the method comprises a step 74 in which dco cal is incremented to concentrate the frequency of the oscillator at the center frequency, in particular 64 x 13.56mhz, before again allowing the frequency-locked loop to try to stabilize the frequency of the oscillator. If the control value dco_ctrl is no longer lower than the value dco_mid during the comparison test 73, this means that the oscillator has reached the desired frequency. The calibration is then ended at step 75.

In this embodiment, the oscillator is controlled by a frequency-locked loop FLL. However, as a variant, the oscillator may be controlled by a phase-locked loop PLL only.

Claims (20)

1. A method of operating an object using active load modulation for contactless communication between the object and a reader, the method comprising:

detecting a sensing magnetic field emitted by the reader;

Initializing a phase-locked loop of the object;

Detecting a stop of the sensing magnetic field emitted by the reader before the phase-locked loop initializing the object is completed;

Calibrating an oscillator of the phase-locked loop based on an internal clock of the object;

after calibrating the oscillator, detecting a new sensed magnetic field of the reader; and

After detecting a new sensed magnetic field, the phase-locked loop is implemented to adjust the phase of the oscillator.

2. The method of claim 1, further comprising responding to the reader after implementing the phase-locked loop to adjust the phase of the oscillator.

3. The method of claim 1, wherein calibrating the oscillator comprises:

Measuring an output frequency of the oscillator;

comparing the measured output frequency with a desired frequency; and

The frequency of the oscillator is calibrated according to the result of the comparison.

4. The method of claim 1, wherein calibrating the oscillator comprises using a frequency-locked loop to lock the frequency of the oscillator to the frequency of the internal clock.

5. The method of claim 4, wherein calibrating the oscillator comprises: after detecting a stop of the sensing magnetic field emitted by the reader, the frequency-locking loop is implemented to calibrate and lock the frequency of the oscillator based on a value stored in a register.

6. The method of claim 5, wherein the phase-locked loop is implemented in response to detecting the new sensed magnetic field of the reader.

7. The method of claim 5, wherein the phase-locked loop is implemented by locking the frequency-locked loop that controls the oscillator in response to detecting the new sensed magnetic field of the reader.

8. An object capable of performing contactless communication using active load modulation, the object comprising:

An internal clock;

A magnetic field detector; and

A phase locked loop comprising an oscillator and configured to lock a phase of the oscillator onto the detected magnetic field;

wherein the object is designed to:

initializing the phase-locked loop when sensing of a magnetic field is detected by the magnetic field detector;

calibrating the frequency of the oscillator of the phase-locked loop based on the internal clock when the detector detects that sensing of the magnetic field has ceased before initialization of the phase-locked loop is complete; and

The phase of the oscillator is then adjusted when a new sensed magnetic field is detected.

9. The object according to claim 8, wherein the object is designed to measure the output frequency of the oscillator, to compare the measured output frequency with a desired frequency and to calibrate the frequency of the oscillator according to the result of the comparison.

10. The object of claim 9, further comprising a frequency-locking loop configured to lock the frequency of the oscillator to the frequency of the internal clock.

11. The object of claim 10, wherein the object is designed to:

Implementing the frequency lock loop to calibrate and lock the frequency of the oscillator based on a value stored in a register in which the detector detects that sensing of a magnetic field has ceased; and then

The phase-locked loop is implemented to adjust the phase of the oscillator when a new sensed magnetic field is detected.

12. The object according to claim 11, wherein the object is designed to implement the phase-locked loop to lock the frequency-locked loop controlling the oscillator when the new sensed magnetic field is detected.

13. The object of claim 9, further comprising a counter divider having a first input coupled to an output of the oscillator and a second input coupled to a node carrying a reference signal operating at a desired frequency.

14. The object of claim 8, further comprising:

A counter divider having a first input coupled to an output of the oscillator and a second input coupled to a node carrying a reference signal;

A register;

A comparator having a first input coupled to the register and a second input coupled to an output of the counter divider; and

A frequency-locked loop is coupled between the output of the comparator and the input of the oscillator.

15. The object of claim 8, further comprising:

A first counter divider having a first input coupled to an output of the oscillator and a second input coupled to the internal clock;

A second counter divider having a first input coupled to an output of the oscillator and a second input coupled to receive a clock signal extracted from the magnetic field;

A first loop filter;

A first comparator having a first input coupled to the output of the second counter divider and an output coupled to the first loop filter;

A register;

A multiplexer having a first input coupled to the register and a second input coupled to an output of the first loop filter;

A second comparator having a first input coupled to an output of the multiplexer and a second input coupled to an output of the first counter divider; and

A second loop filter is coupled between the output of the second comparator and the input of the oscillator.

16. The object of claim 8, further comprising a system configured to calibrate a frequency of an oscillator of the phase-locked loop based on the internal clock when the detector detects that sensing of a magnetic field has ceased before initialization of the phase-locked loop is complete, and then adjust a phase of the oscillator when the new sensed magnetic field is detected.

17. An object configured to enable contactless communication with a reader using active load modulation, the object comprising:

An internal clock;

means for detecting a sensing of a magnetic field of the reader;

A phase locked loop comprising an oscillator and configured to lock a phase of the oscillator onto a detected magnetic field of the reader, the phase locked loop being configured to be initialized when sensing of the magnetic field is detected;

The means for calibrating the frequency of the oscillator of the phase locked loop is configured to calibrate the frequency of the oscillator based on the internal clock when the sensing of the magnetic field by the reader is stopped before initializing the phase locked loop is completed, and then adjust the phase of the oscillator when a new sensed magnetic field by the reader is detected so as to be able to respond to the reader.

18. An object capable of performing contactless communication using active load modulation, the object comprising:

An internal clock;

A magnetic field detector;

a phase locked loop including an oscillator;

A first counter divider having a first input coupled to an output of the oscillator and a second input coupled to the internal clock;

a second counter divider having a first input coupled to an output of the oscillator and a second input coupled to receive a clock signal extracted from a magnetic field detected by the magnetic field detector;

A first loop filter;

A first comparator having a first input coupled to the output of the second counter divider and an output coupled to the first loop filter;

A register;

A multiplexer having a first input coupled to the register and a second input coupled to an output of the first loop filter;

A second comparator having a first input coupled to an output of the multiplexer and a second input coupled to an output of the first counter divider; and

A second loop filter is coupled between the output of the second comparator and the input of the oscillator.

19. The object of claim 15, wherein the first loop filter comprises a phase-locked loop, and wherein the second loop filter comprises a frequency-locked loop.

20. The object of claim 15, further comprising an adder coupled between the first comparator and the first loop filter.