Abstract:
A communications device is disclosed that includes a united integrated circuit card (UICC) that provides a single wire protocol (SWP) current signal to a near field communications (NFC) device. The SWP current signal is filtered by a filter to remove noise from the SWP current signal. The filter is precharged by a reference voltage when the SWP current signal is in a low state and then filters the SWP current signal when the SWP current signal is in a high state. A switching network provides the filter with the reference voltage to precharge when the SWP is current signal is in the low state and then provides the filter with the SWP current signal to filter when the SWP current signal is in the high state.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/260,742, filed Sep. 28, 2011, which claims the benefit of priority from prior Great Britain Patent Application No. PCT/GB2010/051491, filed on Sep. 7, 2010, all of which are incorporated herein by reference in their entirety. 
     
    
       [0002]    This invention relates to single wire, protocol (SWP) signalling and more particularly to noise rejection techniques for SWP in electronic mobile telecommunications apparatus. Still more particularly this invention relates to communication between a near field radio frequency communicator and a Universal Integrated Circuit Card (UICC) module in a telecommunications device via a single wire protocol. 
         [0003]    Aspects and preferred features of the invention are set out in the claims. 
         [0004]    In an aspect there is provided a near field RF communicator including a near field RF communicator including a coupling interface for communication using a single wire protocol, SWP, wherein an SWP employs a voltage signal (S 1 ) to transmit signals and a current signal (S 2 ) to receive signals at the interface, and the near field RF communicator further comprising a controller for controlling a switch, the controller operable, in response to a control signal, to provide a delay and then to control the switch to couple an SWP current signal to the interface. This and other examples of the invention have the advantage of providing effective noise rejection on the SWP line between a UICC and a near field RF communicator. 
         [0005]    In an aspect there is provided a coupling interface for communication using a single wire protocol SWP, wherein an SWP employs a voltage signal (S 1 ) to transmit signals from the interface and a current signal (S 2 ) to receive signals at the interface, the coupling interface including a controller for controlling a switch, the controller operable, in response to a control signal, to provide a delay and then to control the switch to couple an SWP current signal to the interface. 
         [0006]    Described herein is a near field RE communicator including a coupling interface for communication using a single wire protocol and a controller for controlling a switch configuration, the controller operable to provide a delay and then to control. the switch to couple a signal to the interface. The controller can be operable to control a switching configuration in response to a control signal. 
         [0007]    In an embodiment the interface includes at least one capacitor Wherein the controller is operable to control the switch configuration to couple the capacitor to a reference voltage. 
         [0008]    In an embodiment the controller is operable to control the switch configuration such that the capacitor is coupled to the reference voltage during the delay period and coupled to provide a filter during a second period. 
         [0009]    Preferably the delay is selected according to at least one characteristic of an SWP current signal, still more preferably the delay corresponds to the length of a transient signal current in an SWP signal. Optionally a preferred delay is provided by the use of a selected timing capacitance. 
         [0010]    In an embodiment the controller includes a timing capacitor to provide a delay which corresponds to at least one characteristic of an SWP current signal. Preferably the delay corresponds to the length of a transient current in an SWP signal, in some embodiments the length of this delay is selected according to one or more inherent capacitances of the SWP interface, for example according to a time constant of the SWP interface. 
         [0011]    In some embodiments the switch configuration comprises two switching elements. The interface may be external to the near field RF communicator while in other embodiments the interface is within the near field RE communicator. 
         [0012]    Also described herein is a coupling interface for communication using a single wire protocol including a controller for controlling a switch, the controller operable, in response to a control signal, to provide a delay and then to control the switch to couple an SWP current signal to the interface. Said coupling interface may be provided with any of the optional or preferred features set out herein for example such as the features described in any of claims  2  to  8 . 
         [0013]    An embodiment of the invention provides a near field RF communicator including an SWP coupling interface. Preferably such an SWP coupling interface corresponds to the coupling interface described in the foregoing paragraph or may be substantially as described herein with reference to the accompanying drawings. Preferably a near field RF communicator is an NFC communicator. 
         [0014]    Embodiments of the invention provide computing and mobile telecommunication devices including a near field RF communicator as set out in the preceding paragraph. 
         [0015]    In an aspect there is provided a single wire protocol interface, operable according to a single wire protocol, SWP, wherein an SWP employs a voltage signal (S 1 ) to transmit signals from the interface and a current signal (S 2 ) to receive signals at the interface, wherein the current detection circuitry is disconnected from the single line during switching transients following the line being driven high. 
         [0016]    An example of the invention provides a near field RF communicator and/or an NFC communicator including an SWP coupling interface substantially as described herein with reference to  FIGS. 4 and 5 . 
     
    
     
         [0017]    Preferred embodiments of the invention will now be described in greater detail, by way of example only, with reference to the accompanying drawings, in which: 
           [0018]      FIG. 1  shows a representational diagram illustrating communication between two devices comprising NFC communicators; 
           [0019]      FIG. 2  shows a very schematic view of components of an NFC communicator; 
           [0020]      FIG. 3  shows a very schematic representation of a mobile telecommunications device; 
           [0021]      FIG. 4  shows a diagram of a circuit according to an example of the invention; 
           [0022]      FIG. 5  shows a control circuit to provide switch control signals for use in the circuit of  FIG. 4 ; and, 
           [0023]      FIG. 6  illustrates the timing of certain signals which exist in an example of the invention in use. 
       
    
    
     DESCRIPTION 
       [0024]    With reference to the drawings in general, it should be understood that any functional block diagrams are intended simply to show the functionality that exists within the device and should not be taken to imply that each block shown in the functional block diagram is necessarily a discrete or separate entity. The functionality provided by a block may be discrete or may be dispersed throughout the device or throughout a part of the device. In addition, the functionality may incorporate, where appropriate, hard-wired elements, software elements or firmware elements or any combination of these. The near field RF communicator may be provided wholly or partially as an integrated circuit or collection(s) of integrated circuits. 
         [0025]    Referring now specifically to  FIG. 1 , there is shown a representational diagram illustrating communication between two NFC communications enabled devices. In  FIG. 1  the representations of the NFC communications enabled devices have been shown partly cut-away and the functionality provided by the NFC communications enabled devices illustrated by way of a functional block diagram within the NFC communications enabled device. 
         [0026]    As shown in  FIG. 1 , one NFC communications enabled device comprises a mobile telephone (cellphone)  1  and the other NFC communications enabled device comprises a portable computer  2  such as a notebook or laptop computer. 
         [0027]    The mobile telephone  1  has the usual features of a mobile telephone including mobile telephone functionality  410  (in the form of, usually, a programmed controller, generally a processor or microprocessor with associated memory or data storage, for controlling operation of the mobile telephone in combination with a SIM card), an antenna  408  for enabling connection to a mobile telecommunications network, and a user interface  3  with a display  4 , a keypad  5 , a microphone  6  for receiving user voice input and a loudspeaker  7  for outputting received audio to the user. The mobile telephone also has a chargeable battery  11  coupled to a charging socket  412  via which a mains adapter (not shown) may be connected to enable charging of the battery  11 . The mobile telephone  1  may have an alternative or additional power supply (not shown), for example a reserve battery or emergency battery. The chargeable battery  11  forms the primary power supply for the mobile telephone and NFC communicator  15 . Given it is chargeable, it is designed to be removed at certain times. 
         [0028]    Similarly the portable computer  2  has the usual features of a portable computer including portable computer functionality  20  in the form of, usually, a processor with associated memory in the form of ROM, RAM and/or hard disk drive, one or more removable media drives such as a floppy disk drive and/or a CDROM or DVD drive, and possibly a communications device for enabling the portable computer to connect to a network such as the Internet. The portable computer  2  also includes a user interface  21  including a display  22 , a keyboard  23  and a pointing device, as shown a touchpad  24 . The portable computer  2  also has a chargeable battery  25  coupled to a charging socket  26  via which a mains adapter (not shown) may be connected to enable charging of the battery  25 . Again the chargeable battery  25  is the primary power supply for the portable computer and NFC communicator  30 . 
         [0029]    In addition, as shown in  FIG. 1 , both NFC communications enabled devices  1  and  2  have an NFC communicator  415  and  30 . As shown, the NFC communicators  415  and  30  are incorporated within the larger devices and, as with the other functional blocks, may be discrete entities within the host devices or may be provided by features dispersed throughout or integrated within the host device or a part of the host device. Each NFC communicator  415  and  30  comprises NFC operational components  16  and  31  for, as will be described below, enabling control of the NFC functionality and generation, modulation and demodulation of an RF signal. Each NFC communicator  415  and  30  also comprises an antenna circuit  17  and  32  comprising an inductor or coil in the form of an antenna  18  and  33 . The antenna circuits  17  and  32  enable an alternating magnetic field (H field) generated by the antenna of one near field RF communicator  415  (or  30 ) by transmission of an RF signal (for example a 13.56 Mega Hertz signal) to be inductively coupled to the antenna of the other near field RF communicator  30  (or  15 ) when that antenna is within the near field of the RF signal generated by the one near field RF communicator  415  (or  30 ). 
         [0030]    The NFC communicators  415  and  30  are coupled to the mobile telephone and portable computer functionality  410  and  20 , respectively, to enable data and/or control commands to be sent between the NFC communicator and the host device and to enable user input to the NFC communicator. Communication between the user interface  3  or  21  and the NFC communicator  415  or  30  is via the host device functionality  11  or  20 , respectively. 
         [0031]    Each NFC communicator  415  and  30  also comprises a power provider  19  and  34 . The power providers  19  and  34  may be power supplies within the host device or specific to the NFC communicators  415  and  30 , for example a button cell battery, or other small battery. In this case as shown by dashed lines in  FIG. 1 , one or both of the power providers  19  and  34  comprise a coupling to derive power from the corresponding device battery  11  or  25  i.e. the primary power supply. 
         [0032]    It will be appreciated that  FIG. 1  shows only examples of types of host devices. A host device may be another type of electrical device such as a personal digital assistant (PDA), other portable electrical device such as a portable audio and/or video player such as an MP3 player, an IPOD®, CD player, DVD player or other electrical device. As another possibility the NFC communicator ( 15  or  3 ) may be comprised within or coupled to a peripheral device, for example in the form of a smart card or other secure element which may be stand alone or comprised within or intended to be inserted into another electrical device. For example a SIM card for use in a mobile telephone. As a further possibility such peripheral devices may comprise interfacing systems or protocols such as the single wire protocol. 
         [0033]    Also, rather than being incorporated within the host device, the NFC communicator  415  or  30  may be associated with the host device, for example by a wired or wireless coupling. In such a case, a housing of the NFC communicator may be physically separate from or may be attached to the housing of the host device; in the later case, the attachment may be permanent once made or the NFC communicator may be removable. For example, the NFC communicator may be housed within: a housing attachable to another device; a housing portion, such as a fascia of the NFC communications enabled device or another device; an access card; or may have a housing shaped or configured to look like a smart card. For example an NFC communicator may be coupled to a larger device by way of a communications link such as, for example, a USB link, or may be provided as a card (for example a PCMCIA card or a card that looks like a smart card) which can be received in an appropriate slot of the larger or host device. 
         [0034]    In addition, one or both of the NFC communications enabled devices may be a standalone NFC communicator, that is it may have no functionality beyond its NFC communications functionality. 
         [0035]      FIG. 2  shows a functional block diagram of an NFC communications enabled device  600  in accordance with the invention to illustrate in greater detail one way in which the NFC operational components of an NFC communications enabled device embodying the invention may be implemented. 
         [0036]    In this example, the NFC communications enabled device  600  comprises an NFC communicator  600   a  having NFC operational components including an antenna circuit  102 , power provider  104 , controller  107 , data store  108 , signal generator  109  modulator  117  and demodulator  114 . 
         [0037]    The power provider  604  may be any one or more of the types of power providers discussed above. In the interests of simplicity, power supply couplings from the power provider  604  to other components are not shown in  FIG. 2 . 
         [0038]    The NFC communications enabled device  600  may or may not also have or be capable of being connected or coupled with at least one of other functionality  105  (for example functionality of a host device or peripheral device such as described above) and a user interface  106 . 
         [0039]    The NFC operational components include a demodulator  114  coupled between the antenna circuit  602  and the controller  107  for demodulating a modulated RF signal inductively coupled to the antenna circuit  602  from another near field RF communicator in near field range and for supplying the thus extracted data to the controller  107  for processing. Rectifier  700  is coupled to provide a rectified output to regulator  310 . Rectifier  700  and regulator  310  are coupled to the outputs AC 1  and AC 2  of the antenna circuit. The regulator  310  sets or regulates a voltage supply level (pin voltage) and the rectifier  700  provides rectified voltage to remainder of NFC circuit. The regulator  310  sets or regulates the voltage between the outputs AC 1  and AC 2  of the antenna circuit based on the voltage supply level (pin voltage) provided by the rectifier  200 . As shown the demodulator  114  is coupled to the antenna circuit outputs AC 1  and AC 2 . As another possibility, as shown in dashed line in  FIG. 2 , the demodulator may receive its input from the regulator  310 . As a further possibility, the demodulator  114  may receive its input from the rectifier  200 . In one possibility the regulator  310  regulates the voltage between the outputs AC 1  and AC 2  of the antenna circuit based on that voltage rather than the rectified voltage. 
         [0040]    The NFC operational components include a modulator  117  coupled to the controller  107  and to the regulator  310  so that a modulation signal may be applied to the regulator  31  to cause the regulator to vary the load on the antenna circuit  102 . 
         [0041]    A clock deriver  115  is coupled to receive the voltage AC 1 -AC 2  of the antenna circuit and to derive a clock signal from the voltage AC 1 -AC 2  and is coupled to provide the derived clock signal to the demodulator  114  and can be coupled to provide the derived clock signal to any of the controller  107 , the signal generator  109 , the modulator  117  and/or other functionality  105  of the near field RF communicator. Any appropriate clock derivation may be used such as, for example, a clock recovery. 
         [0042]    Together the rectifier  700  and regulator  310  protect the NFC operational components from high voltages received at antenna circuit  102 . For example the regulator may limit the voltage to 3.3 or 1.8 volts dependent on the voltage tolerance of the NFC operational components. Any suitable regulator and rectification circuit can be used for this. The NFC operational components may also include an amplifier for amplifying an RF signal inductively coupled to the antenna circuit  102 . 
         [0043]    In addition the NFC operational components include components for enabling modulation of an RF signal to enable data to be communicated to another near field RF communicator in near field range of the NFC communicator  100   a.  The data to be communicated is provided from controller  107  to modulator  117  and as shown in  FIG. 2 , modulator  117  is arranged to control regulator  310  to modulate the effective impedance of the antenna circuit  602  in order to load modulate an RF H-field inductively coupled to the antenna circuit  162 . Drive elements are also provided for providing a modulated RF signal to the antenna these components comprise a signal generator  109  coupled via a driver  111  to the antenna circuit  102 . In this example, the signal generator  110  causes modulation by gating or switching on and off the RF signal in accordance with the data to be communicated. The NFC communicator may use any appropriate modulation scheme that is in accordance with the standards and/or protocols under which the NFC communicator operates. As another possibility a separate or further signal controller may be incorporated within the NFC operational components to control modulation of the signal generated by the signal generator  109  in accordance with data or instructions received from the controller  107 . 
         [0044]    The NFC operational components also include a controller  107  for controlling overall operation of the NFC communicator. The controller  107  is coupled to a data store  108  for storing data (information and/or control data) to be transmitted from and/or data received by the NFC communications enabled device. The controller  107  may be a controller of a host device and/or a microprocessor, for example a RISC processor or other microprocessor or a state machine. Program instructions for programming the controller and/or control data for communication to another near field RF communicator may be stored in an internal memory of the controller and/or the data store. 
         [0045]    The NFC communicator  600   a  may operate in an initiator mode (that is as an initiating near field RF communicator) or a target mode (that is as a responding near field RF communicator), dependent on the mode to which the NFC communicator is set. The mode may be determined by the controller  107  or may be determined in dependence on the nature of a received near field RF signal. When in initiator mode, an NFC communicator initiates communications with any compatible responding near field RF communicator capable of responding to the initiating NFC communicator (for example an NFC communicator in target mode or an RFID tag or transponder) that is in its near field range, while when in target mode an NFC communicator waits for a communication from a compatible initiating near field RF communicator (for example an NFC communicator in initiator mode or an RFID initiator or transceiver). As thus used, compatible means operable at the same frequency and in accordance with the same protocols, for example in accordance with the protocols set out in various standards such as ISO/IEC 18092, ISO/IEC 21481, ISO/IEC 14443 and ISO/IEC 15693. NFC communicators commonly operate at or around 13.56 MHz. 
         [0046]    When in initiator or target mode, the NFC communicator may communicate in accordance with an active or passive protocol. When using an active protocol the initiating NFC communicator will transmit an RF field and following completion of its data communication turn off its RF field. The responding near field RF communicator (target) will then transmit its own RF field and data before again turning off the RF field and so on. When using a passive protocol the NFC communicator (initiator) will transmit and maintain its RF field throughout the entire communication sequence. The protocol used will depend on instructions received from the controller  107  and the response received from a responding near field RF communicator. 
         [0047]    In  FIG. 2  control of operation of the NFC communicator is through controller  107 . As another possibility where the NFC communicator is comprised as part of a host device, control of the operation of the NFC communicator may be directed by the host device, for example through other functionality  105 . In such circumstances all or part of the control may be provided by other functionality  105 . For example the NFC communicator controller  107  may control modulation and modulation protocols whereas the data to be transmitted may be directed by the host device through other functionality  105  or through controller  107 . In these circumstances the voltage levels of the modulation signal are set by the host device. 
         [0048]    The NFC communicator also comprises an antenna circuit  102 . The design of the antenna circuit will depend on the NFC communicator  600  and the environment in which it operates. For example the antenna circuit may be in the form described for co-pending international patent application number PCT/GB2008/000992 (which claims priority from GB 0705635.1). 
         [0049]      FIG. 3  shows a mobile telecommunications device  50  having a long range RF antenna  51  and a telecoms modem  52  to support mobile telecommunications, for example GSM, GPRS, UMTS, or HSDPA communication using a cellular telephone network. The mobile telecommunications device  50  includes a UICC  53  which typically includes one or more secure elements for supporting transaction or billing functions associated with the device and/or a user of the device. Mobile telecommunications devices according to some examples of the invention include a near field RF communicator  54  and a short range RF antenna  55  arranged to couple inductively with the H-field of another short range RF antenna in near field range. This near field RF communicator is coupled to communicate with the UICC by means of an SWP communication interface. As will be appreciated, SWP provides bidirectional communication along a single wire using current and voltage signalling. SWP signals in one direction are provided by variations in the mark space ratio of a voltage whilst in the other direction signalling is by means of a current variation. 
         [0050]    The SWP interface employs a voltage signal, denoted S 1  and a current signal, denoted S 2 . S 1  is a voltage signal provided by the master to the slave. S 2  is a current signal provided by the slave to the master. The signal S 2  is only measured when the signal S 1  is high. In the present example the master is a near field RF communicator, such as an NFC communicator, which may be referred to as a contactless front end (CALF); and the slave is the UICC. 
         [0051]    A logical ‘one’ of the S 1  signal is provided by a 0.75 duty cycle waveform i.e. S 1  is high for 0.75 of the waveform period. A logical ‘zero’ of the S 1  signal is provided by a 0.25 duty cycle waveform, i.e. S 1  is high for 0.25 of the waveform period. 
         [0052]    The S 2  signal is only valid when S 1  is high. A logical ‘one’ of the S 2  signal is indicated by the slave (in this case the UICC) drawing a current of between 600 μA and 1000 μA logical ‘zero’ of the S 2  signal is indicated by the slave (in this case the UICC) drawing a current of between 0 and 20 μA. 
         [0053]    SWP signalling can be characterised by the bit duration, the duration of a cycle in the S 1  signal. Rapid SWP signalling can employ bit durations of 0.59 μs. As set out above signalling on the S 1  line is provided by 0.25 and 0.75 duty cycle square waves, to support signalling at this rate it is therefore necessary to resolve signalling changes which occur over 0.25 of a 0.59 μs cycle. To meet the Nyquist criterion in this case requires a sampling bandwidth of 13.56 MHz. 
         [0054]    SWP is designed for signalling along a single wire. The single wire coupling between a CLF (near field RF communicator) and the UICC has some inherent inductance, therefore electromagnetic interference (EMI) can couple inductively with this wire. In an environment with high levels of EMI such as within a mobile telecommunications device, due to its length and the absence of any shielding, the single communication line between an NFC component and a UICC communicating via a single wire protocol will couple with relatively high amplitude, broad band, noise signals. These noise signals can substantially degrade communication on the line, for example providing a signal to noise ratio (SNR) of 10% or less. 
         [0055]    One way to improve the SNR might be to use a low pass filter. However in order to provide adequate SNR improvement the time constant of the filter would need to be relatively long. 
         [0056]    Referring to the diagram of  FIG. 4 , comparator  100  has two inputs  100   a  and  100   b  and an output which provides an output signal SWP_RX to near field RF communicator  12 . Comparator input  100   a  is coupled to one plate of filter capacitor  101 , the other plate of filter capacitor  101  is coupled to a supply or reference voltage V DD . A switch  103  is coupled between comparator inputs  100   a  and  100   b  to provide a switchable conducting path between the comparator inputs controlled by control signal SW 2 . 
         [0057]    UICC card  9  is coupled to provide signal SWP_IO to the drain of NMOS transistor  15  and to the drain of PMOS transistor  13 . The source of NMOS transistor  15  is coupled to a ground or reference voltage. The source of I′M OS transistor  13  is coupled to a supply or reference voltage V DD  by sense resistance  12  and is coupled by switch  102 , in series with resistance  104 , to comparator input  100   a.  A control signal SW 1  is arranged to control switch  102 . 
         [0058]    Resistance  10  is coupled between the supply or reference voltage V DD  and the comparator input  100   b.  Current sink/source  14  is connected between comparator input  11   a  and a ground or reference voltage. 
         [0059]    Current sink/source  14  determines a current through resistance  10  to provide a reference voltage at comparator input  100   b.    
         [0060]    When SWP_IO is high, a current variation signal SWP_IO provided by UICC  9  develops a voltage across the sense resistance  12 . Comparator  100  provides an output signal SWP_RX which depends upon the difference between the voltages at comparator inputs  100   a  and  100   b.    
         [0061]    The gate connections of transistors  13  and  15  provide voltage control of the SWP_IO line. Dependent on the gate voltages applied to transistors  13  and  15 , the SWP_IO line may be coupled to a ground or reference voltage (low) or to the supply voltage V DD  by resistor  12  (high). When the SWP_IO voltage is high (transistor  13  is conducting) the UICC may draw a current to provide signalling from the UI CC to the CLF. However, the physical input/output wire  8  of the UICC  9  has an inherent capacitance. This results in a transient current surge when the SWP_IO voltage changes. This current surge can cause a transient filter response which, in the presence of a low pass filter with a long time constant, is likely to mask any current signalling provided on the line. In other words the filter capacitor  101  will tend to smooth-out the initial current surge and cause it to persist too long for the filter to be useful with low current signals which change on sub-microsecond timescales. 
         [0062]    Digital signal processing techniques provide one solution to this problem. However such techniques require a suitably high frequency clock signal. in NFC applications, typically it is required that the NFC must be able to operate and communicate with a UICC while in a “battery-oft” mode. In “battery-off” mode the NFC host (for example the mobile phone  50  of  FIG. 3 ) has no battery power available and therefore an external clock signal may not be available. 
         [0063]    In the example of  FIG. 4  control signal SW 2  is arranged to control switch  103  to switch on and off a conducting path between comparator inputs  100   a  and  100   b.  When SW 2  controls switch  103  to be closed (and SW 1  controls switch  102  to be open), the voltage difference between  100   b  and the supply or reference voltage level V DD  is applied to capacitor  101  which accumulates charge dependent on this voltage difference, its capacitance and the length of time for which the switches are held in this configuration. 
         [0064]    When switch  102  is closed and switch  103  is open the voltage at  100   b  depends on the voltage across resistor  12  and on the charge on filter capacitor  101 . 
         [0065]    When capacitor  101  is appropriately charged the voltage at comparator input  100   a  can be controlled by a current drawn by SWP_IO. 
         [0066]    In this example switches  102  and  103  are provided by appropriately biased field effect transistors but may be provided by any suitable voltage controlled impedance. Optionally current sink/source  14  is provided by mirroring a reference current from a band gap reference. Optionally the gate voltages of transistors  13  and  15  are controlled using a tri-state driver. 
         [0067]    One example of a control circuit to provide switch control signals SW 1  and SW 2  for a circuit according to  FIG. 4  is shown in  FIG. 5 . An input control signal SWP_TX is derived from a controlling logic, for example from controlling logic of the host device and is coupled to the input of an inverter  208 . The output of inverter  208  is coupled to the gate of PMOS transistor  201  and NMOS transistor  206 . The source and drain of NMOS transistor  206  are coupled across timing capacitor  209 . The source of NMOS transistor  206  is coupled to the drain connection of PMOS transistor  201 . Resistance  205  provides a conduction path between supply or reference voltage V DD  and the source connection of PMOS transistor  201 . The drain of PMOS transistor  201  is coupled to comparator input  200   a.    
         [0068]    Resistances  207  and  204  provide a potential divider which couples supply or reference voltage V DD  to a ground voltage. The coupling between resistance  204  and resistance  207 , the mid-point of this potential divider, is coupled to comparator input  200   b.    
         [0069]    Comparator output  210  is coupled to the input of inverter  211  and to provide output signal SW 1 . The output  212  of inverter  211  is coupled to provide output signal SW 2 . 
         [0070]    In this example, the output signal SW 1  of  FIG. 5  is coupled to control switch  102  (shown in  FIG. 4 ) and the output signal SW 2  of  FIG. 5  is coupled to control switch  103  (shown in  FIG. 4 ). 
         [0071]    When the SWP_TX voltage is high, the output voltage of inverter  208  goes low, PMOS transistor  201  is biased into a conducting state and NMOS transistor  206  is not biased into a conducting state. This causes timing capacitor  209  to charge with the current drawn from V DD  through resistance  205 . 
         [0072]    The comparator input  200   a  is coupled to the timing capacitor  209  and comparator input  200   b  is coupled to the midpoint of the potential divider  204 ,  207 . Therefore, whenever the voltage across the timing capacitor  209  exceeds or drops below a level determined  20  by the potential divider  204 ,  207 , the comparator output  210  will change polarity and therefore provide a change in the timing signals SW 1  and SW 2 . When signal SWP_TX is low, the output of inverter  208  is high and NMOS transistor  206  is biased into a conducting state which causes timing capacitor  209  to discharge through NMOS transistor  206 . This provides a reset of the timing capacitor  209 . Thus the circuit of  FIG. 5  controls the timing of the switch control signals SW 1  and SW 2 . 
         [0073]    In this example there is no particular relationship between the chosen capacitance of timing capacitor  209  and the capacitance of the input/output line  8  of the UICC  9 . However the resistance  205  and transistor  201  of  FIG. 6  are chosen to correspond to the sense resistance  12  and the PMOS transistor  13 . For example the transistors  201  and  13  may be matched and resistance  205  may be chosen to have a resistance in proportion to sense resistance  12  so that the current in resistance  205  is proportional to the current in sense resistance  12 . 
         [0074]    Generating the switch control signals SW 1  and SW 2  in the manner described enables the filter capacitor  101  to be pre-charged for a selected period while a current surge is diverted through resistor  12  prior to current mode communication over the SWP. As will be apparent to the skilled practitioner having read the present application, by an appropriate choice of components the duration of this period can be selected to match the duration of the current surge on SWP_IO. In some examples matching is provided by selecting this duration to be: not less than; substantially equal to; or equal to within a determined tolerance, for example 20%, 10%, 5%, 1% or less than 1% of the duration of the current surge on SWP_IO. Advantageously this allows the pre-charge time to be reduced to a minimum to ensure the initial surge current has ended whilst allowing the maximum time for the filter capacitor to settle. Advantageously allowing filter capacitor  101  to be charged prior to UICC to NFC communication reduces problematic filter transients while enabling noise rejection on the SWP line between a UICC and a near field RF communicator. Still more advantageously this noise rejection is achieved without the need for an external clock signal and by making use of a minimum number of electronic components. 
         [0075]      FIG. 6  shows the operation and the timing of signals in the circuits of  FIGS. 2 and 3  in the event that the S 2  signal (current mode signalling from the UICC to the CLF) signals a logical ‘1’ by drawing a current. In  FIG. 6 : 
         [0076]    plot  300  shows a voltage against time graph of the signal SWP_TX, a control signal (for example applied by the controlling logic) which is applied to an input of the control circuit of  FIG. 6 ; 
         [0077]    plot  301  shows a graph of current against time for the current in the sense resistance  12  of  FIG. 4 , the transient current surge which results from the capacitance of the UICC input/output line  8  can be clearly seen; 
         [0078]    plots  306  and  307  provide an indication of the control signal voltages SW 1  and SW 2 . 
         [0079]    plot  309  shows a voltage against time plot of the voltage applied to the comparator input  100   a  as a result of an SWP_IO signal and dashed line  304  indicates the reference signal SREF applied to the other comparator input  100   b;  and 
         [0080]    dashed lines  303 ,  305  and  310  indicate the timing of the rising edge of the SWP_TX signal, the end of the pre-charge period, and the falling edge of the SWP_TX signal respectively. 
         [0081]    Referring now to the timings of  FIG. 6  and to the circuit diagrams of  FIGS. 2 and 3 , at time  303  SWP_ 10  is conductively coupled to resistance  12  by transistor  13  ( FIG. 4 ). At this time  303 , as shown in  FIG. 6 , switch  102  is open so any charge held on the line  8  will discharge through resistance  12  ( FIG. 4 ). At time  305 , the circuit of  FIG. 5  applies switch signals SW 1  and SW 2  to switches  102  and  103  to open switch  102  and close switch  103  ( FIG. 4 ). Therefore, prior to time  305  any current surge associated with the discharge of the SWP_IO line will not be applied to filter capacitor  101  and after time  305  current drawn by the signal S 2  will affect the voltage at comparator input  101   b  unmodified by any transient that would otherwise have arisen from the current surge. The output of comparator  100 , SWP_RX, will therefore mirror the S 1  signal. Clearly, when the comparator inputs are coupled together by switch  103  the comparator output will be indeterminate. All that is required is that SWP_RX be valid on the falling edge  310  of SWP_TX and for a short time thereafter. 
         [0082]    Voltages and currents on the plots shown in  FIG. 6  have been shown on axes indicating either positive or negative currents and/or changes in these currents or voltages of one particular polarity. As will be appreciated no particular voltage level is required (i.e. positive or negative voltage levels) and changes of one polarity could equivalently be represented as changes of an opposite polarity. As will be appreciated, as an alternative to the signal SWP_TX the voltage signal S 2  may be used to gate the control circuit of  FIG. 5 . 
         [0083]    Throughout the description reference has been made to PMOS and NMOS transistors. As will be appreciated in the context of the present application, by making appropriate modifications other transistors or voltage controlled impedances may be used instead. 
         [0084]    In particular the drawing of  FIG. 5  shows a circuit configured to control PMOS transistors as the switches SW 1  and SW 2  of  FIG. 4 . As will be appreciated by the skilled practitioner in the context of the present application, by making appropriate modifications to the circuit of  FIG. 5  alternative switching devices could be used for SW 1  and SW 2 . The diagram of  FIG. 4  shows a circuit in which a filter capacitor is coupled between connection  100   a  and supply or reference voltage V DD , in one possibility a capacitor is also coupled between connection  100   b  and supply or reference voltage V DD  to provide a balanced filter. 
         [0085]    Although a capacitor has been described as a separate component as will be appreciated any suitable capacitance may be employed, for example inherent or parasitic capacitances. 
         [0086]    The present invention has been described with particular reference to mobile telecommunications devices such as mobile telephones. As will be appreciated by the skilled practitioner the techniques methods and apparatus described herein can be applied equivalently in other devices which employ SWP communication and will provide particular advantages where there is a need to perform SWP communication over an unshielded wire and at high data transfer rates (short bit duration). 
         [0087]    As used herein the term single wire protocol relates to any communications protocol which operates bidirectional communication along a single wire and relates, in a particular example to an ETSI single wire protocol such as the protocol described in ETSI TS 102613 v7.6.0. “Smart Cards; UICC—Contactless Front-end (CLF) Interface; Part 1: 20 Physical and data link layer characteristics” (June 2008). 
         [0088]    A near field communicator is described having a coupling interface for communication using a single wire protocol, the interface having a controller for controlling a switch, the controller operable, in response to a control signal, to provide a delay and then to control the switch to couple an SWP current signal to the interface. The interface itself is also described as are NFC communicators. The interface may include a capacitor which can be controllably coupled by means of a switch to charge the capacitor from a reference voltage before the capacitor is employed as a filter. 
         [0089]    The invention extends to methods and/or apparatus substantially as herein described with reference to the accompanying drawings. 
         [0090]    Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa.