Patent Application
20130078914
The present invention relates generally to Near Field Communication (NFC) devices and the operation and application thereof. More particularly, the present invention relates to methods and apparatus for measuring the strength of an NFC reader field and adjusting the load modulation of a tag reader.
Near Field Communication (NFC) involves contactless, or wireless, communication between devices where those devices are spaced apart only a small distance and coupling between a field generated by a first device and an antenna of a second device takes place. In one mode of operation, information is communicated from the second device to the first device when the second device modulates the impedance across its antenna terminals and the impact on the coupled field is detected by the first device. Since the detectable impact on the field is caused by modulating the impedance across the antenna terminals, this communications scheme is referred to as load modulation.
The ability to communicate wirelessly, but only over very short distances, provides a framework for a range of secure personal interactions with electronic devices, products, and systems.
Indeed, personal applications have gone beyond the deskbound model of interacting with a computer to a model wherein computing and communication hardware are truly personal items, are highly mobile, and are integrated into the fabric of modern living. Consistent with this usage model for powerful personal computational and communication devices, many applications of “on-the-go” computing and communication have been, and are being, developed. One class of such on-the-go applications involves NFC between devices. Applications such as conducting financial transactions with stores, banks, trains, busses, and so on may be facilitated by the near-field coupling of two devices to exchange financial and/or personal information.
It will be appreciated that in an interaction between a first and a second NFC device, wherein at least one of the devices is maneuvered into near field coupling range by a person, there is likely to be variability in the relative position of such devices from one use to the next. As a consequence of the lack of strict uniformity in positioning, variations in coupled field strength will occur.
Conventional NFC devices that communicate by way of load modulation do not account for the non-uniformity of field strength, regardless of whether that non-uniformity results from alignment variations or other reasons.
What is needed are methods and apparatus for varying load modulation in an NFC-enabled device over a range of tag reader field strength.
Embodiments of the invention are described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left most digit(s) of a reference number identifies the drawing in which the reference number first appears.
Generally, embodiments of the present invention use a field strength measurement signal to control the level of load modulation so that when the reader signal is strong, a low impedance load modulator can be used to improve the signal seen by the reader. In a weaker field a higher impedance load modulator is used to ensure that the reader clock can be recovered by the tag or tag emulator. In this way, the requirements imposed on an NFC communications device by various NFC communications specifications, and by the tag or tag emulator, can be maintained over a wider operational range than conventional NFC communication devices.
The following Detailed Description refers to accompanying drawings of illustrative embodiments of the invention. References in the Detailed Description to “one exemplary embodiment,” “an illustrative embodiment”, “an exemplary embodiment,” and so on, indicate that the embodiment referred to may include a particular feature, structure, or characteristic, but every exemplary embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same exemplary embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an exemplary embodiment, it is within the knowledge of those skilled in the relevant art(s) to affect such feature, structure, or characteristic in connection with other exemplary embodiments whether or not explicitly described.
The exemplary embodiments described herein are provided for illustrative purposes, and are not limiting. Other exemplary embodiments are possible, and modifications may be made to the exemplary embodiments within the spirit and scope of the invention. Therefore, the Detailed Description is not meant to limit the invention. Rather, the scope of the invention is defined only in accordance with the subjoined claims and their equivalents.
This Detailed Description of the exemplary embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge of those skilled in relevant art(s), readily modify and/or adapt for various applications such exemplary embodiments, without undue experimentation, without departing from the spirit and scope of the invention. Therefore, such adaptations and modifications are intended to be within the meaning and plurality of equivalents of the exemplary embodiments based upon the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.
Although the description of the present invention is to be described in terms of NFC, those skilled in the relevant art(s) will recognize that the present invention may be applicable to other communications that use the near field and/or the far field without departing from the spirit and scope of the present invention. For example, although the present invention is to be described using NFC-capable devices, those skilled in the relevant art(s) will recognize that functions of these NFC-capable devices may be applicable to other communications devices that use the near field and/or the far field without departing from the spirit and scope of the present invention.
Terminology
As used herein, the expression “Near-field communicator” refers to a product that includes at least the resources to provide NFC tag and NFC tag reader functionality. Such products may be sometimes referred to as NFC-enabled devices.
The terms, chip, die, integrated circuit, semiconductor device, and microelectronic device, are often used interchangeably in the field of electronics. The present invention is applicable to all of the above as these terms are generally understood in the field.
With respect to chips, it is common that power, ground, and various signals may be coupled between them and other circuit elements via physical, electrically conductive connections. Such a point of connection may be referred to as an input, output, input/output (I/O), terminal, line, pin, pad, port, interface, or similar variants and combinations. Although connections between and amongst chips are commonly made by way of electrical conductors, those skilled in the art will appreciate that chips and other circuit elements may alternatively be coupled by way of optical, mechanical, magnetic, electrostatic, and electromagnetic interfaces.
The acronym FET refers to a Field Effect Transistor.
The term “gate” is context sensitive and can be used in two ways when describing integrated circuits. As used herein, gate refers to a circuit for realizing an arbitrary logical function when used in the context of a logic gate. Gate refers to the insulated gate terminal of a three terminal FET when used in the context of transistor circuit configuration. The expression “gate terminal” is generally interchangeable with the expression “gate electrode” and either of these can be used to refer to the insulated gate terminal of a three terminal FET. Although a FET can be viewed as a four terminal device when the semiconductor body is considered, for the purpose of describing illustrative embodiments of the present invention, the FET will be described using the traditional gate-drain-source, three terminal model.
Source/drain terminals refer to the terminals of a FET, between which conduction occurs under the influence of an electric field, subsequent to the inversion of the semiconductor surface under the influence of a perpendicular electric field resulting from a voltage applied to the gate terminal. Generally, the source and drain terminals are fabricated such that they are geometrically symmetrical. With geometrically symmetrical source and drain terminals it is common to simply refer to these terminals as source/drain terminals, and this nomenclature is used herein. Designers often designate a particular source/drain terminal to be a “source” or a “drain” on the basis of the voltage to be applied to that terminal when the FET is operated in a circuit.
The term “varactor” refers to a voltage variable capacitor. By way of non-limiting examples, voltage variable capacitors may be formed by devices such as diodes, FETs, and MEMS devices. A voltage variable capacitor may be formed by a diode since the junction capacitance between the anode and cathode of the diode is related to the magnitude of the reverse bias applied across the diode junction. Similarly, a FET can be used as voltage variable capacitor. It is well-known that the capacitance between the gate terminal and the semiconductor body of the FET is related to the voltage applied to the gate terminal and the effect of that applied voltage on the depth of the depletion region. That is, FET gate capacitance is highest after inversion of the channel and lowest when the depletion region under the gate has reached its maximum depth. MEMS devices can also be used as voltage variable capacitors. A MEMS varactor typically functions by varying the distance between two capacitor plates under control of an applied voltage.
The expression “load modulation” refers generally to the modulation of a field generated by a first device by modifying the impedance across an antenna, or a coil, coupled to that field. As used in connection with NFC devices herein, load modulation refers to a process by which an NFC tag modifies an NFC reader field such that the reader may extract data from the modified field. Typically the tag modifies the reader field by modulating the impedance, i.e., the load, across the tag antenna during a time when it is coupled to the reader field. The modulation of the load is performed in accordance with the data that the tag is communicating to the reader. The reader device detects this data-dependent load modulation and extracts the data from the modulated reader field.
The term “smartcard” refers to a physical substrate, such as a credit card-sized piece of plastic, having an integrated circuit embedded therein. Typically, smartcards are used for financial transactions or secure access to locked facilities. An active smartcard is one that includes an embedded power supply such as a battery. A passive smartcard is one that requires power to be supplied from an external source. In some instances, the external source is an energization field from which the passive smartcard harvests the energy needed to carry out its desired function. Smartcards that are NFC-enabled can communicate with other devices in a contactless, or wireless manner.
An Illustrative Near Field Communications Environment
Improvements in manufacturing technologies and digital architecture have resulted in a number of products and product categories that were not previously possible or practical to implement. The emerging developments in the area of Near Field Communication (NFC) circuits, systems and applications is making new products and product categories possible. Products incorporating Near-Field Communication capabilities are sometimes referred to as NFC-enabled. For example, mobile phones, smart cards, key fobs, secure access cards, tablet computers, or other electronic products that include NFC capabilities are referred to as NFC-enabled. Near-field communication allows data to be communicated from a first NFC-enabled device to a second NFC-enabled device over short distances. Although a strict definition for the range of short distances is not agreed upon in the field, short range for NFC usually is thought of as being less than 4 cm, or within one wavelength of the selected communication frequency, typically 13.56 MHz.
Typical NFC arrangements involve a pair of devices in which a first device acts as a target or “tag” to respond to a communication and a second device within a near-field coupling distance of the first device acts as a “reader” to initiate the communication. In various embodiments of the present invention the first device may be equipped with the circuitry for acting as both a tag and a reader, commonly referred to as a communicator. Electronic products that include NFC tag circuitry along with circuitry for other functionality may be referred to as tag emulators, or to have the capability of operating in “tag emulation mode”, Similarly, electronic products that include NFC reader circuitry along with circuitry for other functionality may be referred to as reader emulators, or to have the capability of operating in “reader emulation mode”,
As will be described in greater detail below, NFC-enabled devices and applications have utility in at least consumer electronics and industrial products.
In connection with the following illustrative embodiments, it is noted that any reference to a computational platform is intended to include similar computational devices and computers regardless of their form factor or input/output configuration. By way of example, and not limitation, a smartphone is a computational platform.
An Illustrative Field Strength-Dependent Load Modulator
Referring more particularly to
Variable load-modulation load sub-circuit 204 is responsible for communication between the tag and a tag reader (not shown) by changing, that is modulating, the load across antenna terminals 210, 212 responsive to the state of signal Tx. Some or all of the components shown in
In the illustrative embodiment of
In typical embodiments of the present invention, a default value for the load modulation load is chosen for each specific antenna design. Again, in typical embodiments, subsequent to a communication in which the load-modulation load has been varied, the default load-modulation load will be re-selected for the next communication. Various embodiments of the present invention use predetermined values of load impedance in the load-modulation load sub-circuit, where the selected load is based, at least in part, on the specific antenna design to be used with the corresponding load-modulation load sub-circuit.
Referring to
In various alternative embodiments, where this tag, or tag emulator, is to be used with two or more different antennas, a programmable reference 304 may be used to provide an updated reference value for use with each of the different antennas. In another alternative, reference 304 contains a plurality of fixed reference values to accommodate a selected one or more of the two or more different antennas. In this embodiment reference 304 would be provided with a selection signal indicating which of its plurality of reference values should be output for comparison with output signal 303 of shunt regulator 302.
Still referring to reference 304, in a further embodiment, the reference value that is output may be changed based, at least in part, on error rates in an on-going communication. In a still further embodiment, a communications protocol stack (typically implemented in software) provides control information such that the reference value that is output by reference 304 may be changed based on the determinations of the communications protocol stack. In some embodiments reference 304 is addressable and the address that selects a particular output value is placed in a reference address register (not shown) coupled to reference 304. In some embodiments the reference address register is visible to the protocol stack software and may be directly controlled by the software of the protocol stack.
A comparator 306 is coupled to receive as inputs both the output signal 303 of shunt regulator 302 and output signal 305 of reference 304. Comparator 306 generates an output signal 307 that is a logical one when the low impedance load-modulation load should be used and a logical zero otherwise. That is, when the strength of the coupled reader field exceeds a predetermined value, then the state of output signal 307 is such that a low impedance modulation load is used for communicating with the reader.
Still referring to
A Q output terminal of flip-flop 308 is coupled to a first input terminal of a logic gate 310. In this illustrative embodiment, logic gate 310 is a two-input AND gate. Transmit data, Tx, is coupled to a second input terminal two-input AND gate 310. The output of AND gate 310 is a load-modulation control signal 208. Referring back to
As noted above, shunt regulator 302 provides an input signal to comparator 306.
Still referring to
It is noted that a signal from the shunt regulator is not necessarily the only source of information that may be used in connection with varying the load-modulation load. For example, in an NFC communicator device, even though an IQ demodulator is generally not powered up when the tag emulator is being used, in principle the IQ demodulator could be used as the source of information regarding the strength of the field coupling between tag and reader, and thus could be used in determining changes to the value of the load-modulation load.
In this high-level representation of variable load-modulation load 204 a first switchable impedance 502, directly controlled by transmit data signal Tx, is coupled between antenna terminals 210, 212; and a second switchable impedance 504, directly controlled by load modulation control signal 208, is coupled between antenna terminals 210, 212. In this way, when the strength of the coupled reader field is greater than a predetermined level, the parallel pathway of switchable impedance 504 is activated so that the low impedance portion of the transmit operation is an even lower impedance than can be achieved by activating switchable impedance 502 alone. Typical values for the load impedances are 16 ohms or less, but the present invention is not limited to any particular value of load impedance.
Those skilled in integrated circuit design will understand that as an alternative to turning on two parallel FETs in order to reduce the impedance of the load-modulation load, it is possible to use a single FET and apply two different voltages to the gate. In the case of n-channel FETs a higher voltage on the gate results in a lower on-resistance. In such an alternative implementation, signal 208 would enable the application of a higher gate voltage rather than turning on a second transistor.
Applying a voltage that varies over time with the logical combination of transmit data and coupled field strength to the gate of a FET coupled between the antenna terminals is one method of controlling the impedance of a load-modulation load. But in a FET, drain current (IDS) does not have a linear relationship to drain voltage (VDS). This can present additional design complexity. An alternative approach is to use a selected set of FETs in their fully on (i.e., low resistance state) to couple one or more circuit elements (e.g., resistors) into the pathway between antenna leads 210, 212.
Embodiments using resistors for impedance control in the pathway between antenna leads 210, 212, typically use resistors having fixed, voltage invariant resistance values. In some embodiments, the resistors may be trimmable so that a manufacturer may adjust these resistors for a particular application subsequent to the formation of the resistors. Trimming of resistors to modify their resistance values is well-known as are various methods of trimming.
Still referring to
In an alternative arrangement, each of FETs 904-1, 904-2, . . . , 904-n may be coupled in series with a fixed or variable impedance element. In such alternative arrangements an impedance element may be coupled between antenna terminal 210 and the drain of a FET, between the source of a FET and antenna terminal 212, or both.
In further alternative embodiments the impedance elements may be selected from two or more sets of impedance elements by means of electrical programming. For example, fuses and/or anti-fuses may be electrically programmed to select among the aforementioned two or more sets of impedance elements and permanently connect the selected set to the switchable pathway that forms the field strength dependent variable load-modulation load.
Similarly, a non-volatile memory that can be reprogrammed may be used to specify which one of the two or more sets of impedance elements are selected.
It is noted that the load-modulation load is an impedance value and is not required to be a purely resistive load. In addition to the illustrative embodiments described above, wherein transistors and resistors are coupled between antenna terminals 210, 212, other circuit elements such as, but not limited to, capacitors may also be switched in and out of the path between the antenna terminals (i.e., the load modulation path). Capacitors may be fixed value capacitors or voltage variable capacitors (varactors).
In another alternative, when load modulation impedance is to be set to a particular nominal value, the default switching path is de-selected and a new set of switchable paths is enabled for use with transmit data.
Still referring to
In one embodiment of the present invention, a method of operating an NFC-enabled device, includes selecting, at the communications device, a first load-modulation load; determining, by the communications device, a field strength of a field coupled to the communications device; selecting, at the communications device, a second load-modulation load, if the field strength is greater than a threshold value.
In another embodiment of the present invention, a method of operating a communications device, includes generating a signal indicative of a field strength of a tag reader field; comparing the signal to at least a first threshold and producing a comparison result signal; and producing a load-modulation load based at least in part on the comparison result signal.
In a still further embodiment, a method of operating an NFC-enabled device, includes providing a first load-modulation load and a second load-modulation load; generating a signal indicative of whether a strength of a field to which the NFC-enabled device is coupled is greater than a predetermined threshold value; selecting the first load-modulation load if the strength of the field is not greater than the predetermined threshold; and selecting the second load-modulation load if the strength of the field is greater than the predetermined threshold; wherein the second load-modulation load has a lower impedance than the first load-modulation load.
Those skilled in the art will appreciate that the present invention is not limited to the use of positive logic versus negative logic. Similarly, the present invention is not limited to any particular range of power supply voltages.
In typical embodiments of the present invention, there is no specific speed requirement for a transition or settling time for the change in load impedance to take effect. However, it must take effect in the time between LoadModEn being asserted and the transmission by load modulation starting. It is noted that this time period can be set by the system design engineer. And, since switching, i.e., set-up, of the load-modulation load takes place prior to a transmission beginning, there are no undesired sideband artifacts that result from load switching.
In an alternative implementation of the present invention, the field strength and threshold are in digital format. It is noted that a digital format presents a designer with various trade-offs, including but not limited to increased power consumption from converting one or more analog signals to the digital domain.
In some embodiments a continuously varying load-modulation load is used. A continuously varying load-modulation load is one that is able to change the magnitude of its impedance at the time. However, even though this presents a benefit in the theoretical sense, in practice the frame duration and speed of tag movement are such that any possible field change during transmission is very small.
In some embodiments a continuously variable load-modulation load is used. A continuously variable load-modulation load is one that is able to change to any value, rather than a limited number of discrete values. This requires more circuitry to implement, but has the advantage of getting closer to an optimum value of impedance across the antenna terminals for a given field strength.
In one illustrative embodiment, a near-field communication device, includes a load-modulation load sub-circuit operable to provide a continuously variable load-modulation load impedance, wherein the load-modulation load impedance provided by the load-modulation load sub-circuit is dependent on the data being transmitted, and a magnitude of a field strength coupled to the near-field communication device. The impedance of the load-modulation load sub-circuit is set by an analog signal derived from a sampled and held measurement of the field strength prior to data being transmitted. Typically, the field strength is a reader field strength.
It is noted that load modulation can be done by de-tuning, and therefore voltage variable components, including but not limited to varactors, may be used in the load-modulation paths between the antenna terminals.
Varying the load-modulation load impedance in accordance with the strength of the coupled reader field may be used for communication in both passive and active tags. As long as the tag is using load-modulation for communication changing the load impedance in accordance with the strength of the coupled reader field may be used.
Embodiments of the present invention have utility in at least NFC communication devices. NFC communication devices are known to be used in many application environments such as NFC tags, tag emulators, contactless cards, proximity cards, smart phones, computer tablets, key fobs, and so on. It is noted that varying a load-modulation load in a manner that depends on the strength of a coupled field in accordance with present invention is not limited to any particular application environment. That is, the present invention may be employed in any application, device, system or environment in which communication by way of load modulation is used.
It is to be appreciated that the Detailed Description section, and not the Abstract of the Disclosure, is intended to be used to interpret the claims. The Abstract of the Disclosure may set forth one or more, but not all, exemplary embodiments of the invention, and thus, is not intended to limit the invention and the subjoined claims in any way.
The invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries may be defined so long as the specified functions and relationships thereof are appropriately performed.
It will be apparent to those skilled in the relevant art(s) that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the subjoined claims and their equivalents.
