WO2017141012A1

WO2017141012A1 - Power saving in near field communications

Info

Publication number: WO2017141012A1
Application number: PCT/GB2017/050327
Authority: WO
Inventors: Tore AUSTAD; Malihe Zarre DOOGHABADI; Christoffer AMLO
Original assignee: Nordic Semiconductor Asa; Samuels, Adrian James
Priority date: 2016-02-18
Filing date: 2017-02-09
Publication date: 2017-08-24
Also published as: TW201735559A; GB2547447A; GB201602844D0

Abstract

An electronic device comprises: a field strength detector (14) connected to an antenna (8), the field strength detector (14) being arranged to determine a strength of a magnetic field induced in the antenna (8) and generate a first wake up signal (30) if the determined strength exceeds a threshold; a field frequency detector (6) arranged to: determine upon receiving the first wake up signal whether a frequency of the induced magnetic field is within a predetermined range; and generate a second wake up signal if the frequency is within said predetermined range; and a near-field communication module arranged to transmit and/or receive a near-field communication message upon receiving the second wake up signal.

Description

Power Saving in Near Field Communications

The present invention relates to ways of saving power in electronic communications where continuous communication is not required, particularly in a near-field communication (NFC) system.

Near-field communication (NFC) is a technology for transmitting data between two electronic devices, where at least one of the devices is usually a portable device such as a smartphone, tablet, wearable etc. NFC uses electromagnetic induction between two loop or "coil" antennae - each device having its own antenna - to transfer data from a transmitting device to a receiving device. These devices may, for example, be an NFC tag and an NFC reader or may be a pair of portable electronic devices such as smartphones. Typically, the communications

themselves are carried out using the unlicensed 13.56 MHz industrial, scientific and medical (ISM) radio frequency band using the ISO/I EC 18000-3 standard for air interface communications.

NFC provides the possibility of using a "passive" tag that has no internal power supply of its own, instead harnessing all of the power it requires by converting some of the energy obtained from the electromagnetic field used for NFC communication that is produced by the NFC reader into a current that can then be used to power the tag itself. However, in some circumstances both devices may be powered (e.g. by an internal battery) electronic devices, with one of the devices acting as the tag, while the other acts as an NFC reader. Of course, the NFC communication may be bidirectional, e.g. using half-duplex or full-duplex communication methods.

A powered NFC device typically has two modes of operation: a sensing mode and an active mode. When there is no electromagnetic NFC field present, the device is in sensing mode and a field detection circuit is powered up. Once an NFC field is detected, the field detection circuit generates a wake up signal and turns on the rest of the NFC device, and then the device enters the active mode in which NFC communications are carried out. Relatively speaking, the sensing mode requires little power, while the active mode has larger power consumption requirements. A simple field detection method might use an envelope detector or peak amplitude detector to try and determine the presence of an NFC field. The device antenna and an associated tuning capacitor act as a crude band pass filter with notional centre frequency of 13.56 MHz, which might allow signals with frequencies of approximately 1 MHz to 100MHz to pass - though it will of course be appreciated that this will depend on the field strength and the quality factor (or "Q factor") of the antenna circuitry. This unfortunately leads the field detector to generate false wake up signals, causing the active mode circuits to be enabled in response to non-NFC fields, causing an unnecessary increase in power consumption.

When viewed from a first aspect, the present invention provides an electronic device comprising: a field strength detector connected to an antenna, the field strength detector being arranged to determine a strength of a magnetic field induced in the antenna and generate a first wake up signal if the determined strength exceeds a threshold; a field frequency detector arranged to: determine upon receiving the first wake up signal whether a frequency of the induced magnetic field is within a predetermined range; and generate a second wake up signal if the frequency is within said

predetermined range; and a near-field communication module arranged to transmit and/or receive a near-field communication message upon receiving the second wake up signal.

The present invention extends to a method of operating an electronic device comprising: determining a strength of a magnetic field induced in an antenna of the device; generating a first wake up signal if the determined strength exceeds a threshold; determining upon receiving the first wake up signal whether a frequency of the induced magnetic field is within a predetermined range; generating a second wake up signal if the frequency is within said predetermined range; and transmitting and/or receiving a near-field communication message upon receiving the second wake up signal. Thus it will be appreciated by those skilled in the art that the present invention provides an electronic device and a method of operating the same that may provide a reduction in power consumption compared to conventional approaches by only operating the frequency detector if it is determined that a sufficiently strong field is indeed present. Moreover, further power savings are achieved by entering the active mode of the near-field communication module only if the magnetic field is varying with the appropriate frequency.

The Applicant has appreciated that the device may be in the more passive sensing mode for relatively long periods of time, e.g. days or weeks. By only waking up the more power-intensive circuits when necessary, a significant reduction in power consumption can be achieved.

It will be appreciated that there are a number of circuit arrangements for determining whether a field is present or not that are known in the art per se.

However, in some embodiments the field strength detector comprises an envelope detector. Additionally or alternatively, in at least some embodiments the field strength detector comprises an amplitude level detector.

The Applicant has appreciated that there are a number of different frequency detection circuits that could readily be implemented in accordance with the invention described herein. However, the Applicant has devised a particularly advantageous frequency detector arrangement, and in a set of embodiments the field frequency detector comprises: a ramp generator; a voltage comparator; and a counter clocked by a signal derived from the magnetic field induced in the antenna, the field frequency detector being arranged such that: upon receiving the first wake up signal, the ramp generator begins generating a voltage ramp and the counter begins a count;

the voltage comparator compares a voltage of the voltage ramp to a threshold voltage and stops the counter if the voltage of the voltage ramp reaches the threshold voltage; and

it generates the second wake up signal if the count is within a

predetermined range.

It may be appreciated by those skilled in the art that this arrangement can provide a simple, low power way of determining whether the frequency of the induced field is within the predetermined range. The predetermined frequency range may correspond to a lower count threshold and an upper count threshold. The second wake up signal may thus be generated if the count is between said lower and upper count thresholds. If the time it takes the ramp generator to reach the threshold voltage is known, the number which the count reaches in that known period of time is indicative of the frequency of the field.

At least one count comparator may be provided to compare the count to the lower and upper count thresholds.

In at least some embodiments, the ramp generator comprises at least one resistor - capacitor (RC) network. In some alternative embodiments, the ramp generator comprises a capacitor and a current source. In such embodiments, the values of capacitance and, where appropriate, resistance are chosen to set the time taken to reach the threshold voltage - which is inversely proportional to the product of the resistance and capacitance values - to a particular value. For example, if the time delay were set to 10 με, a count value of 135 (truncated from 135.6) would correspond to a frequency of approximately 13.56 MHz. This exemplary system may therefore accept a count value between 128 and 143 (i.e. the frequency detector will generate the second wake up signal if the frequency is between 12.19 MHz and 15.05 MHz, which will prevent the active mode circuits from being erroneously woken up in response to e.g. a 6.78 MHz Rezence® wireless charging field). This exemplary count value range could then be narrowed with sufficiently accurate calibration (e.g. calibrating to offset any temperature-dependent effects). ln a particular set of embodiments, the ramp generator begins generating a voltage that increases (that is, it "ramps up") over time e.g. linearly. Of course, it will be appreciated that the voltage ramp does not necessarily need to be linear, nor does the voltage need to increase - indeed the principle of the present invention applies equally to embodiments wherein the voltage decreases over time once triggered.

Similarly the count may increment or decrement and may start at any chosen value with the upper and lower count thresholds being defined accordingly. In order to generate a clock signal from the magnetic field, at least in some embodiments the frequency detector comprises a first AND gate having a first input connected to the antenna, a second input arranged to receive the first wake up signal from the field strength detector, and an output arranged to output the clock signal. As described above, a 13.56 MHz NFC field will have around 135-136 cycles every 10 με. By using the cycles of the induced field as the clock for the counter, the counter will increment the count value at the frequency of the time- varying field.

While the aforementioned clock signal may be connected directly to the counter such that the clock signal acts as the counter clock signal, in a set of embodiments, the frequency detector comprises a second AND gate having a first input arranged to take the clock signal from the output of the first AND gate, a second input arranged to take the logical inverse of the output of the voltage comparator and an output arranged to provide the counter clock signal. This assumes that the voltage ramp generator increases voltage with time. If it decreases voltage over time, there is no need to invert the output of the voltage comparator.

It will be appreciated that in accordance with the foregoing, when the field strength detector has determined that the strength of a field proximate to the antenna is sufficiently high, the field frequency detector can be enabled and the time-varying field itself can be used as a clock to trigger the counter - but the counter is otherwise disabled if the field strength detector determines that no field with sufficient strength is present. ln some embodiments, the frequency detector further comprises an AND gate (i.e. the third AND gate if the first and second AND gates referred to above are provided) having a first input connected to an output of the count comparator, a second input connected to the output of the voltage comparator, and an output arranged to provide the second wake up signal. This ensures that both the field strength and the field frequency are appropriate before waking up the near-field communication module.

Typically, the antenna is a coil antenna or a loop antenna. In at least some embodiments a tuning element is connected to the antenna. In some such embodiments, the tuning element comprises a tuning capacitor. The tuning capacitor could be variable or multiple of them could be provided to allow the device to be tuned to different frequency values if necessary. In some embodiments, the field frequency detector is arranged to determine upon receiving the first wake up signal whether a frequency of the induced magnetic field is within one of a plurality of predetermined ranges e.g. by using a plurality of count comparators. In some such embodiments, upon determining that the frequency of the induced magnetic field is within a selected range, the tuning element is tuned to a selected frequency associated with said selected range. This provides the capability for such devices in accordance with the foregoing to support multiple frequencies (and, by extension, different communication standards) by using a broadband antenna that can receive fields having a variety of frequencies or ranges of frequencies and subsequently tuning to a detected one thereof. Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 is a circuit diagram of an electronic device in accordance with an embodiment of the present invention; and

Fig. 2 is a timing diagram illustrating the typical operation of the device of Fig. 1.

Fig. 1 is a circuit diagram of an electronic device 2 in accordance with an embodiment of the present invention. The device comprises a coil antenna 8 in parallel with a tuning capacitor 10 which is connected across two NFC input pins 12a, 12b of an NFC circuit 4. The NFC circuit 4 itself comprises a field strength detection circuit 14 which has its inputs connected to the NFC input pins 12a, 12b and its output connected to a frequency detection circuit 6.

The frequency detection circuit 6 comprises: a ramp generator 16; a voltage comparator 18; three two-input Boolean AND gates 20, 22, 28; a counter 24; and a count comparator 26. The output of the field strength detection circuit 14 is connected to an enable input of the ramp generator 16, which in turn has its output connected to the positive input of the voltage comparator 18. The voltage comparator 18 also takes a reference voltage 34 on its negative input, and has an enable input 33 connected to the output of the field strength detection circuit 14.

The output of the field strength detection circuit 14 is further connected to one of the inputs of the first AND gate 20, which takes its other input from one of the NFC input pins 12a. The output of the first AND gate 20 is connected to the second AND gate 22. The output of the voltage comparator 18 is negated and connected to the other input of the second AND gate 22.

The output of the second AND gate 22 is connected to the clock input of the counter 24, which has its enable input connected to the output of the field strength detection circuit 14. The output of the counter 24 is connected to the input of the count comparator 26, the output of which is connected to the third AND gate 28, which takes its second input from the output of the voltage comparator 18. The third AND gate 28 produces an output 50 which can be provided to additional NFC circuitry that is used for transmission and reception functionality. The count comparator 26 also has two further inputs: a lower threshold input 44 and an upper threshold input 46.

Fig. 2 is a timing diagram illustrating the typical operation of the device 2 of Fig. 1. At an initial time t₀, there is no NFC field present and thus the field strength detector 14 does not detect a field induced in the antenna 8. Accordingly the output of the field strength detector 14, i.e. the first wake up signal 30, is set to logic low.

However, some time later at t_t, the device 2 has been brought within the proximity of an NFC reader, which is producing an NFC field. Of course, at this stage, the device 2 does not know for certain that the field induced in the antenna 8 is indeed an NFC field, as it does not yet know the frequency because the frequency detection circuit 6 is still disabled by the first wake up signal 30 being low. Once the field strength detection circuit 14 determines that a sufficiently strong field is present (e.g. by observing the amplitude or the envelope of the voltage induced the antenna 8), the first wake up signal 30 is set to logic high.

Once the first wake up signal 30 goes high, the ramp generator circuit 16 is enabled, and begins increasing its output ramp voltage 32. While Fig. 2 shows the ramp voltage 32 increasing linearly, in practical implementations there may be some nonlinearity to the ramp - for example if using a resistor-capacitor (RC) ramp generator, the capacitor does not charge linearly.

Since the first wake up signal 30 is high, the output of the first AND gate 20 goes high each time the voltage induced in the antenna 8 (i.e. the voltage at the first NFC input pin 12a) goes high, producing a 'clock' signal 38 that follows the time-varying voltage induced in the antenna 8. As the ramp voltage 32 is less than a threshold voltage 34, the output of the voltage comparator 18 - i.e. stop signal 36 - is low. The negated output is therefore high, meaning that both inputs to the second AND gate 22 are high every time the 'clock' signal 38 goes high, leading to a gated clock signal 40 - illustrated as a series of pulses 52 - which drives the counter 24.

This means that while the ramp generator is enabled and its output 32 is below the threshold voltage 34, the counter 24 is enabled (due to its enable input being connected to the first wake up signal 30 which is high) and increments a count on its output 42, for example at every rising clock edge of the gated clock signal 40

(though it will be appreciated that this of course could be implemented using falling clock edges instead) which is passed to the count comparator 26.

After a period of time T, the ramp voltage 32 reaches the threshold voltage 34. This causes the stop signal 36 to go high, which sets the output of the second AND gate 22 to logic low regardless of the value of the clock signal 38 coming from the first AND gate 20, thus preventing any further counting by the counter 24.

The count comparator 26 then compares the count value from the counter output 42 to the lower and upper thresholds 44, 46. If the count value 42 is between the thresholds 44, 46, the output 48 of the count comparator 26 is set to logic high. This indicates that the count value reached corresponds to the frequency of the induced voltage in the antenna 8 an NFC frequency. Providing the stop signal 36 and the count comparator output 48 are both high, the third AND gate 28 produces a logic high on its output 50, indicating that an NFC field of the proper strength and frequency has been picked up by the antenna 8 and that NFC communications may now be carried out.

The period T is set during design by the choice of components - e.g. the resistor and capacitor values used in an RC ramp generator implementation and the value of the threshold voltage. For example, the time constant of an RC circuit is given by Equation 1 below: τ = RC

Equation 1: Time constant of an RC circuit wherein: τ is the time constant of the RC circuit; R is the resistance of the RC circuit; and C is the capacitance of the RC circuit. As T must be a fixed value for correct operation of the device, R and C must be calibrated such that τ is approximately equal to T. This calibration may be carried out either during the production of the frequency detector, in use (e.g. periodically), or both. It should be understood however, that T need not be exactly equal to τ as the ramp voltage 32 is relatively linear up to and around that point - i.e. the threshold voltage 34 need not be exactly the voltage the ramp voltage 32 reaches after a single time constant τ has elapsed.

Thus it will be appreciated that the described embodiments of the present invention provide an electronic device having NFC functionality that has a lower power consumption requirement in the sensing mode than conventional devices. Although particular embodiments have been described in detail, it will be appreciated by those skilled in the art that many variations and modifications are possible using the principles of the invention set out herein.

Claims

Claims:

1. An electronic device comprising:

a field strength detector connected to an antenna, the field strength detector being arranged to determine a strength of a magnetic field induced in the antenna and generate a first wake up signal if the determined strength exceeds a threshold; a field frequency detector arranged to:

determine upon receiving the first wake up signal whether a frequency of the induced magnetic field is within a predetermined range; and

generate a second wake up signal if the frequency is within said predetermined range; and

a near-field communication module arranged to transmit and/or receive a near-field communication message upon receiving the second wake up signal.

2. The device as claimed in claim 1 wherein the field strength detector comprises an envelope detector.

3. The device as claimed in claim 1 or 2 wherein the field strength detector comprises an amplitude level detector.

4. The device as claimed in any preceding claim wherein the field frequency detector comprises: a ramp generator; a voltage comparator; and a counter clocked by a signal derived from the magnetic field induced in the antenna, the field frequency detector being arranged such that:

upon receiving the first wake up signal, the ramp generator begins generating a voltage ramp and the counter begins a count;

it generates the second wake up signal if the count is within a

predetermined range.

5. The device as claimed in claim 4 wherein the ramp generator comprises at least one resistor - capacitor network.

6. The device as claimed in claim 4 wherein the ramp generator comprises a capacitor and a current source.

7. The device as claimed in claim 4, 5 or 6 wherein the ramp generator is arranged to begin generating a voltage that increases or decreases over time.

8. The device as claimed in any of claims 4 to 7 wherein the field frequency detector comprises a first AND gate having a first input connected to the antenna, a second input arranged to receive the first wake up signal from the field strength detector, and an output arranged to output the clock signal.

9. The device as claimed in claim 8 wherein the field frequency detector comprises a second AND gate having a first input arranged to take the clock signal from the output of the first AND gate, a second input arranged to take the logical inverse of the output of the voltage comparator and an output arranged to provide the counter clock signal.

10. The device as claimed in any of claims 4 to 9 wherein the frequency detector further comprises an AND gate having a first input connected to an output of the count comparator, a second input connected to the output of the voltage comparator, and an output arranged to provide the second wake up signal.

11. The device as claimed in any preceding claim comprising a tuning element connected to the antenna.

12. The device as claimed in claim 1 1 wherein the tuning element comprises a tuning capacitor.

13. The device as claimed in any preceding claim wherein the field frequency detector is arranged to determine upon receiving the first wake up signal whether a frequency of the induced magnetic field is within one of a plurality of predetermined ranges.

14. The device as claimed in claim 13 arranged such that upon determining that the frequency of the induced magnetic field is within a selected range, the tuning element is tuned to a selected frequency associated with said selected range.

15. A method of operating an electronic device comprising:

determining a strength of a magnetic field induced in an antenna of the device;

generating a first wake up signal if the determined strength exceeds a threshold;

determining upon receiving the first wake up signal whether a frequency of the induced magnetic field is within a predetermined range;

generating a second wake up signal if the frequency is within said predetermined range; and

transmitting and/or receiving a near-field communication message upon receiving the second wake up signal.

16. The method as claimed in claim 15 wherein the field frequency detector comprises: a ramp generator; a voltage comparator; and a counter clocked by a signal derived from the magnetic field induced in the antenna, the method comprising:

upon receiving the first wake up signal, the ramp generator beginning to generate a voltage ramp and the counter beginning a count;

the voltage comparator comparing a voltage of the voltage ramp to a threshold voltage and stopping the counter if the voltage of the voltage ramp reaches the threshold voltage; and

generating the second wake up signal if the count is within a predetermined range.

17. The method as claimed in claim 16 wherein the ramp generator begins generating a voltage that increases or decreases over time.

18. The method as claimed in claim 16 or 17 wherein the field frequency detector comprises a first AND gate having a first input connected to the antenna, a second input receiving the first wake up signal from the field strength detector, and an output outputting the clock signal.

19. The method as claimed in claim 18 wherein the field frequency detector comprises a second AND gate having a first input taking the clock signal from the output of the first AND gate, a second input taking the logical inverse of the output of the voltage comparator and an output providing the counter clock signal.

20. The method as claimed in any of claims 16 to 19 wherein the frequency detector further comprises an AND gate having a first input connected to an output of the count comparator, a second input connected to the output of the voltage comparator, and an output providing the second wake up signal.

21. The method as claimed in any of claims 15 to 20 comprising the field frequency detector determining upon receiving the first wake up signal whether a frequency of the induced magnetic field is within one of a plurality of predetermined ranges.

22. The method as claimed in claim 21 comprising upon determining that the frequency of the induced magnetic field is within a selected range, the tuning element being tuned to a selected frequency associated with said selected range.