Patent Publication Number: US-2022231729-A1

Title: Communication Between Devices During Wireless Power Transfer

Description:
This application is a continuation of U.S. non-provisional patent application Ser. No. 17/183,169, filed on Feb. 23, 2021, which claims priority to U.S. provisional patent application No. 63/043,711, filed Jun. 24, 2020, and to U.S. provisional patent application No. 63/043,818, filed Jun. 25, 2020, which are all hereby incorporated by reference herein in their entireties. 
    
    
     FIELD 
     This relates generally to power systems, and, more particularly, to wireless power systems for charging electronic devices. 
     BACKGROUND 
     In a wireless charging system, a wireless power transmitting device such as a charging mat or charging puck wirelessly transmits power to a wireless power receiving device such as a portable electronic device. The portable electronic device has a coil and rectifier circuitry. The coil of the portable electronic device receives alternating-current wireless power signals from the wireless power transmitting device. The rectifier circuitry converts the received signals into direct current power. 
     SUMMARY 
     A wireless power system has a wireless power transmitting device and a wireless power receiving device. The wireless power transmitting device may include a coil and wireless power transmitting circuitry coupled to the coil. The wireless power transmitting circuitry may be configured to transmit wireless power signals with the coil. The wireless power receiving device may include a coil that is configured to receive wireless power signals from the wireless power transmitting device and rectifier circuitry that is configured to convert the wireless power signals to direct current power. 
     The devices in the wireless power system may communicate using in-band communication. The wireless power transmitting device may transmit data to the wireless power receiving device using frequency-shift keying (FSK) modulation. The wireless power receiving device may transmit data to the wireless power transmitting device using amplitude-shift keying (ASK) modulation. 
     While transmitting data to the wireless power receiving device using FSK modulation, the wireless power transmitting device may monitor for ASK modulation from the wireless power receiving device. In response to detecting the ASK modulation from the wireless power receiving device, the wireless power transmitting device may abort the FSK data transmission and process the detected ASK modulation to receive data from the wireless power receiving device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative wireless charging system that includes a wireless power transmitting device and a wireless power receiving device in accordance with an embodiment. 
         FIG. 2  is a circuit diagram of illustrative wireless power transmitting and receiving circuitry in accordance with an embodiment. 
         FIG. 3  is a diagram showing an illustrative frequency-shift keying (FSK) modulation bit encoding scheme in accordance with an embodiment. 
         FIG. 4  is a diagram showing an illustrative amplitude-shift keying (ASK) modulation bit encoding scheme in accordance with an embodiment. 
         FIG. 5  is a flow chart of illustrative operations involved in operating a wireless power transmitting device in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A wireless power system has a wireless power transmitting device that transmits power wirelessly to a wireless power receiving device. The wireless power transmitting device may be a device such as a wireless charging mat, wireless charging puck, wireless charging stand, wireless charging table, or other wireless power transmitting equipment. The wireless power transmitting device may be a stand-alone device or built into other electronic devices such as a laptop or tablet computer, cellular telephone or other electronic device. The wireless power transmitting device has one or more coils that are used in transmitting wireless power to one or more wireless power receiving coils in the wireless power receiving device. The wireless power receiving device is a device such as a cellular telephone, watch, media player, tablet computer, pair of earbuds, remote control, laptop computer, electronic pencil or stylus, other portable electronic device, or other wireless power receiving equipment. 
     During operation, the wireless power transmitting device supplies alternating-current signals to one or more wireless power transmitting coils. This causes the coils to transmit alternating-current electromagnetic signals (sometimes referred to as wireless power signals) to one or more corresponding coils in the wireless power receiving device. Rectifier circuitry in the wireless power receiving device converts received wireless power signals into direct-current (DC) power for powering the wireless power receiving device. 
     An illustrative wireless power system (wireless charging system) is shown in  FIG. 1 . As shown in  FIG. 1 , wireless power system  8  includes a wireless power transmitting device such as wireless power transmitting device  12  and includes a wireless power receiving device such as wireless power receiving device  24 . Wireless power transmitting device  12  includes control circuitry  16 . Wireless power receiving device  24  includes control circuitry  30 . Control circuitry in system  8  such as control circuitry  16  and control circuitry  30  is used in controlling the operation of system  8 . This control circuitry may include processing circuitry associated with microprocessors, power management units, baseband processors, digital signal processors, microcontrollers, and/or application-specific integrated circuits with processing circuits. The processing circuitry implements desired control and communications features in devices  12  and  24 . For example, the processing circuitry may be used in selecting coils, determining power transmission levels, processing sensor data and other data, processing user input, handling negotiations between devices  12  and  24 , sending and receiving in-band and out-of-band data, making measurements, and otherwise controlling the operation of system  8 . 
     Control circuitry in system  8  may be configured to perform operations in system  8  using hardware (e.g., dedicated hardware or circuitry), firmware and/or software. Software code for performing operations in system  8  is stored on non-transitory computer readable storage media (e.g., tangible computer readable storage media) in control circuitry  8 . The software code may sometimes be referred to as software, data, program instructions, instructions, or code. The non-transitory computer readable storage media may include non-volatile memory such as non-volatile random-access memory (NVRAM), one or more hard drives (e.g., magnetic drives or solid state drives), one or more removable flash drives or other removable media, or the like. Software stored on the non-transitory computer readable storage media may be executed on the processing circuitry of control circuitry  16  and/or  30 . The processing circuitry may include application-specific integrated circuits with processing circuitry, one or more microprocessors, a central processing unit (CPU) or other processing circuitry. 
     Power transmitting device  12  may be a stand-alone power adapter (e.g., a wireless power transmitting device that includes power adapter circuitry), may be a wireless charging puck or other device that is coupled to a power adapter or other equipment by a cable, may be equipment that has been incorporated into furniture, a vehicle, or other system, may be a removable battery case, or may be other wireless power transfer equipment. In some cases, power transmitting device  12  may be a portable electronic device such as a cellular telephone, watch, media player, tablet computer, pair of earbuds, remote control, laptop computer, electronic pencil or stylus, or other portable electronic device. Power transmitting device  12  may also be capable of receiving wireless power (and may have similar power receiving components as power receiving device  24 ). 
     Power receiving device  24  may be a portable electronic device such as a cellular telephone, watch, media player, tablet computer, pair of earbuds, remote control, laptop computer, electronic pencil or stylus, other portable electronic device, or other wireless power receiving equipment. 
     Power transmitting device  12  may be coupled to a wall outlet (e.g., an alternating current power source), may have a battery such as battery  18  for supplying power, and/or may have another source of power. Power transmitting device  12  may have an alternating-current (AC) to direct-current (DC) power converter such as AC-DC power converter  14  for converting AC power from a wall outlet or other power source into DC power. DC power may be used to power control circuitry  16  and other components within device  12 . In some cases, a single electronic device may be configured to serve as both a power receiving device and a power transmitting device (e.g., the device has both power transmitting circuitry and power receiving circuitry). 
     The DC power may be used to power control circuitry  16 . During operation, a controller in control circuitry  16  uses power transmitting circuitry  52  to transmit wireless power to power receiving circuitry  54  of device  24 . Power transmitting circuitry  52  may have switching circuitry (e.g., inverter circuitry  61  formed from switches such as transistors) that is turned on and off based on control signals provided by control circuitry  16  to create AC current signals through one or more wireless power transmitting coils such as wireless power transmitting coils  36 . Coils  36  may be arranged in a planar coil array (e.g., in configurations in which device  12  is a wireless charging mat) or may be arranged to form a cluster of coils (e.g., in configurations in which device  12  is a wireless charging puck). In some arrangements, device  12  may have only a single coil. In other arrangements, device  12  may have multiple coils (e.g., two coils, more than two coils, four or more coils, six or more coils, 2-6 coils, fewer than 10 coils, etc.). 
     As the AC currents pass through one or more coils  36 , alternating-current electromagnetic (e.g., magnetic) fields (wireless power signals  44 ) are produced that are received by one or more corresponding receiver coils such as coil(s)  48  in power receiving device  24 . Device  24  may have a single coil  48 , at least two coils  48 , at least three coils  48 , at least four coils  48 , or other suitable number of coils  48 . 
     When the alternating-current electromagnetic fields (sometimes referred to as magnetic flux) are received by coils  48  (e.g., when magnetic flux passes through coils  48 ), corresponding alternating-current currents are induced in coils  48 . Rectifier circuitry such as rectifier circuitry  50 , which contains rectifying components such as synchronous rectification metal-oxide-semiconductor transistors arranged in a bridge network, converts received AC signals (received alternating-current signals associated with electromagnetic signals  44 ) from one or more coils  48  into DC voltage signals for powering device  24 . 
     The DC voltage produced by rectifier circuitry  50  (sometimes referred to as rectifier output voltage Vrect) can be used in charging a battery such as battery  58  and can be used in powering other components in device  24 . For example, device  24  may include input-output devices  56 . Input-output devices  56  may include input devices for gathering user input and/or making environmental measurements and may include output devices for providing a user with output. As an example, input-output devices  56  may include a display for creating visual output, a speaker for presenting output as audio signals, light-emitting diode status indicator lights and other light-emitting components for emitting light that provides a user with status information and/or other information, haptic devices for generating vibrations and other haptic output, and/or other output devices. Input-output devices  56  may also include sensors for gathering input from a user and/or for making measurements of the surroundings of system  8 . Illustrative sensors that may be included in input-output devices  56  include three-dimensional sensors (e.g., three-dimensional image sensors such as structured light sensors that emit beams of light and that use two-dimensional digital image sensors to gather image data for three-dimensional images from light spots that are produced when a target is illuminated by the beams of light, binocular three-dimensional image sensors that gather three-dimensional images using two or more cameras in a binocular imaging arrangement, three-dimensional lidar (light detection and ranging) sensors, three-dimensional radio-frequency sensors, or other sensors that gather three-dimensional image data), cameras (e.g., infrared and/or visible cameras with respective infrared and/or visible digital image sensors and/or ultraviolet light cameras), gaze tracking sensors (e.g., a gaze tracking system based on an image sensor and, if desired, a light source that emits one or more beams of light that are tracked using the image sensor after reflecting from a user&#39;s eyes), touch sensors, buttons, capacitive proximity sensors, light-based (optical) proximity sensors such as infrared proximity sensors, other proximity sensors, force sensors, sensors such as contact sensors based on switches, gas sensors, pressure sensors, moisture sensors, magnetic sensors, audio sensors (microphones), ambient light sensors, optical sensors for making spectral measurements and other measurements on target objects (e.g., by emitting light and measuring reflected light), microphones for gathering voice commands and other audio input, distance sensors, motion, position, and/or orientation sensors that are configured to gather information on motion, position, and/or orientation (e.g., accelerometers, gyroscopes, compasses, and/or inertial measurement units that include all of these sensors or a subset of one or two of these sensors), sensors such as buttons that detect button press input, joysticks with sensors that detect joystick movement, keyboards, and/or other sensors. Any of these input-output components (which form a load for device  24 ) may be powered by the DC voltages produced by rectifier circuitry  50  (and/or DC voltages produced by battery  58 ). 
     Device  12  may optionally have one or more input-output devices  60  (e.g., input devices and/or output devices of the type described in connection with input-output devices  56 ). For example, device  12  may include a display  32  and one or more sensors. 
     Device  12  and/or device  24  may communicate wirelessly using in-band or out-of-band communications. Device  12  may, for example, have wireless transceiver circuitry  40  that wirelessly transmits out-of-band signals to device  24  using an antenna. Wireless transceiver circuitry  40  may be used to wirelessly receive out-of-band signals from device  24  using the antenna. Device  24  may have wireless transceiver circuitry  46  that transmits out-of-band signals to device  12 . Receiver circuitry in wireless transceiver  46  may use an antenna to receive out-of-band signals from device  12 . In-band transmissions between devices  12  and  24  may be performed using coils  36  and  48 . 
     It is desirable for power transmitting device  12  and power receiving device  24  to be able to communicate information such as received power, states of charge, and so forth, to control wireless power transfer. However, this process need not involve the transmission of device identification information. Out of an abundance of caution, it is noted that to the extent that any implementation of this charging technology involves the use of device identification information (or more generally, personally identifiable information), implementers should follow privacy policies and practices that are generally recognized as meeting or exceeding industry or governmental requirements for maintaining the privacy of users. In particular, identification information data should be managed and handled so as to minimize risks of unintentional or unauthorized access or use, and the nature of authorized use should be clearly indicated to users. Where possible, such identification information may be abstracted, such as by using some but not all bits in a byte of information, so that the resulting identification is not globally unique but still sufficient to facilitate communication under reasonable device usage scenarios. 
     Control circuitry  16  has external object measurement circuitry  41  that may be used to detect external objects adjacent to device  12  (e.g., on the top of a charging surface). Circuitry  41  may detect foreign objects such as coils, paper clips, and other metallic objects and may detect the presence of wireless power receiving devices  24  (e.g., circuitry  41  can detect the presence of one or more coils  48 ). In arrangements in which device  12  forms a charging puck, the charging puck may have a surface shape that mates with the shape of device  24 . A puck or other device  12  may, if desired, have magnets (sometimes referred to as magnetic alignment structures) that removably attach device  12  to device  24 , in the process aligning coil  48  with coil  36  for efficient wireless charging. 
     During object detection and characterization operations, external object measurement circuitry  41  can be used to make measurements on coils  36  to determine whether any devices  24  are present on device  12 . Additional coils (that are not used for power transmission) and/or other additional sensors may be used for object detection and characterization operations if desired. 
     In an illustrative arrangement, measurement circuitry  41  of control circuitry  16  contains signal generator circuitry (e.g., oscillator circuitry for generating AC probe signals at one or more probe frequencies, a pulse generator that can create impulses so that impulse responses can be measured to gather inductance information, Q-factor information, etc.) and signal detection circuitry (e.g., filters, analog-to-digital converters, impulse response measurement circuits, etc.). During measurement operations, switching circuitry in device  12  (e.g., in the puck of device  12 ) may be adjusted by control circuitry  16  to switch each of coils  36  into use. As each coil  36  is selectively switched into use, control circuitry  16  uses the signal generator circuitry of signal measurement circuitry  41  to apply a probe signal to that coil while using the signal detection circuitry of signal measurement circuitry  41  to measure a corresponding response. Measurement circuitry  43  in control circuitry  30  and/or in control circuitry  16  may also be used in making current and voltage measurements (e.g., so that this information can be used by device  24  and/or device  12 ). 
       FIG. 2  is a circuit diagram of illustrative wireless charging circuitry for system  8 . As shown in  FIG. 2 , circuitry  52  may include inverter circuitry such as one or more inverters  61  or other drive circuitry that produces wireless power signals that are transmitted through an output circuit that includes one or more coils  36  and capacitors such as capacitor  70 . In some embodiments, device  12  may include multiple individually controlled inverters  61 , each of which supplies drive signals to a respective coil  36 . In other embodiments, an inverter  61  is shared between multiple coils  36  using switching circuitry. In yet another embodiment, device  12  includes a single inverter and a single corresponding coil  36 . 
     During operation, control signals for inverter(s)  61  are provided by control circuitry  16  at control input  74 . A single inverter  61  and single coil  36  is shown in the example of  FIG. 2 , but multiple inverters  61  and multiple coils  36  may be used, if desired. In a multiple coil configuration, switching circuitry (e.g., multiplexer circuitry) can be used to couple a single inverter  61  to multiple coils  36  and/or each coil  36  may be coupled to a respective inverter  61 . During wireless power transmission operations, transistors in one or more selected inverters  61  are driven by AC control signals from control circuitry  16 . The relative phase between the inverters can be adjusted dynamically (e.g., a pair of inverters  61  may produce output signals in phase or out of phase (e.g., 180 degrees out of phase). 
     The application of drive signals using inverter(s)  61  (e.g., transistors or other switches in circuitry  52 ) causes the output circuits formed from selected coils  36  and capacitors  70  to produce alternating-current electromagnetic fields (signals  44 ) that are received by wireless power receiving circuitry  54  using a wireless power receiving circuit formed from one or more coils  48  and one or more capacitors  72  in device  24 . 
     If desired, the relative phase between driven coils  36  (e.g., the phase of one of coils  36  that is being driven relative to another adjacent one of coils  36  that is being driven) may be adjusted by control circuitry  16  to help enhance wireless power transfer between device  12  and device  24 . Rectifier circuitry  50  is coupled to one or more coils  48  (e.g., a pair of coils) and converts received power from AC to DC and supplies a corresponding direct current output voltage Vrect across rectifier output terminals  76  for powering load circuitry in device  24  (e.g., for charging battery  58 , for powering a display and/or other input-output devices  56 , and/or for powering other components). A single coil  48  or multiple coils  48  may be included in device  24 . 
     As previously mentioned, in-band transmissions using coils  36  and  48  may be used to convey (e.g., transmit and receive) information between devices  12  and  24 . With one illustrative configuration, frequency-shift keying (FSK) is used to transmit in-band data from device  12  to device  24  and amplitude-shift keying (ASK) is used to transmit in-band data from device  24  to device  12 . Power may be conveyed wirelessly from device  12  to device  24  during these FSK and ASK transmissions (e.g., at least some wireless power is conveyed during the in-band communications, whether or not devices  12  and  24  have completed a handshake process and agreed upon a sustained power transfer level). While power transmitting circuitry  52  is driving AC signals into one or more of coils  36  to produce signals  44  at the power transmission frequency, wireless transceiver circuitry  40  may use FSK modulation to modulate the power transmission frequency of the driving AC signals and thereby modulate the frequency of signals  44 . In device  24 , coil  48  is used to receive signals  44 . Power receiving circuitry  54  uses the received signals on coil  48  and rectifier  50  to produce DC power. At the same time, wireless transceiver circuitry  46  monitors the frequency of the AC signal passing through coil(s)  48  and uses FSK demodulation to extract the transmitted in-band data from signals  44 . This approach allows FSK data (e.g., FSK data packets) to be transmitted in-band from device  12  to device  24  with coils  36  and  48  while power is simultaneously being wirelessly conveyed from device  12  to device  24  using coils  36  and  48 . 
     In-band communications between device  24  and device  12  may use ASK modulation and demodulation techniques. Wireless transceiver circuitry  46  transmits in-band data to device  12  by using a switch (e.g., one or more transistors in transceiver  46  that are coupled coil  48 ) to modulate the impedance of power receiving circuitry  54  (e.g., coil  48 ). This, in turn, modulates the amplitude of signal  44  and the amplitude of the AC signal passing through coil(s)  36 . Wireless transceiver circuitry  40  monitors the amplitude of the AC signal passing through coil(s)  36  and, using ASK demodulation, extracts the transmitted in-band data from these signals that was transmitted by wireless transceiver circuitry  46 . The use of ASK communications allows ASK data bits (e.g., ASK data packets) to be transmitted in-band from device  24  to device  12  with coils  48  and  36  while power is simultaneously being wirelessly conveyed from device  12  to device  24  using coils  36  and  48 . 
     The example of FSK modulation being used to convey in-band data from power transmitting device  12  to power receiving device  24  and ASK modulation being used to convey in-band data from power receiving device  24  to power transmitting device  12  is merely illustrative. In general, any desired communication techniques may be used to convey information from power transmitting device  12  to power receiving device  24  and from power receiving device  24  to power transmitting device  12 . In general, wireless power may simultaneously be conveyed between devices during in-band communications (using ASK or FSK). 
     The power transmission frequency used for transmission of wireless power may be, for example, a predetermined frequency of about 110 kHz, about 125 kHz, about 175 kHz, at least 80 kHz, at least 100 kHz, between 100 kHz and 205 kHz, less than 500 kHz, less than 300 kHz, or other desired wireless power frequency. In some configurations, the power transmission frequency may be negotiated in communications between devices  12  and  24 . In other configurations, the power transmission frequency may be fixed. 
     It has been described that power may be simultaneously conveyed between devices while using in-band communication for data transmission between the devices. In other words, in some examples in-band communications may rely on modulation of the power transmission signal (e.g., modulating the power transmission frequency or modulating amplitude of a signal at the power transmission frequency). 
     However, it should be noted that in-band communication may occur between devices before the devices agree upon a power transfer rate, power transmission frequency, etc. After initial detection and inductive coupling, devices may go through a handshake process to determine compatibility, negotiate power transfer frequency, negotiate power transfer rate, etc. During this process, in-band communication may involve FSK and/or ASK modulation of signals at the power transmission frequency. Therefore, wireless power is transmitted during this process. This is advantageous as it allows the devices to complete the handshake process even if the power receiving device has little or no remaining battery power. This transmission of wireless power during in-band communications may occur during the handshake process even if, ultimately, the negotiations between the devices result in no sustained transmission of wireless power (e.g., even if the devices do not enter a dedicated power transfer phase). 
     The aforementioned FSK and ASK modulation and demodulation techniques may be used to transmit data packets between any two devices within system  8 . Each data packet may include numerous data bits (sometimes referred to as bits). The data bits may be grouped into bytes, with each byte including any desired number of bits (e.g., 8 bits). 
     Communication of a bit using FSK modulation may take a longer period of time than communication of a bit using ASK modulation. This is illustrated in  FIGS. 3 and 4 . 
     During FSK modulation, power transmitting device  12  may switch its operating frequency between a first operating frequency (e.g., unmodulated operating frequency f op ) and a second operating frequency (e.g., modulated operating frequency f mod ). The difference between the two frequencies has both a polarity (indicating whether the difference between f mod  and f op  is positive or negative) and a depth (indicating the magnitude of the difference between f mod  and f op ). 
     Using the unmodulated operating frequency and the selected modulated operating frequency, the power transmitter may transmit bits using FSK modulation. The power transmitter may use a bit encoding scheme to transmit the bits using FSK modulation. In one illustrative example, the power transmitter may use a differential bi-phase encoding scheme to modulate data bits using the power signal. This type of bi-phase encoding scheme is shown in  FIG. 3 . 
       FIG. 3  shows the power signal frequency over time during FSK modulation. The power signal frequency transitions between frequencies f 1  and f 2  to encode bits. Frequencies f 1  and f 2  may be equal to f op  and f mod  as discussed previously, with either f op  or f mod  being the higher of the two frequencies. As shown, in the encoding scheme of  FIG. 3 , a transition between the two frequencies occurs at the start of each bit. To encode a ‘one’ bit, there are two transitions in the power signal frequency. To encode a ‘zero’ bit, there is one transition in the power signal frequency. 
     For example, at t 1  the operating frequency (power signal frequency) transitions from f 2  to f 1 . This indicates the start of encoding the one bit. The operating frequency may remain at f 1  for a given number of cycles of the power signal (e.g., 256 cycles) then transition back to f 2  at t 2 . The operating frequency remains at f 2  for the given number of cycles. At t 3 , the encoding of the one bit is complete. 
     At t 3 , the operating frequency (power signal frequency) transitions from f 2  to f 1 . This indicates the start of encoding the zero bit. The operating frequency may remain at f 1  for a given number of cycles (e.g., 512 cycles) then transition back to f 2  at t 4 . At t 4 , the encoding of the zero bit is complete. 
     To summarize, each bit (either a ‘one’ or ‘zero’) is transmitted over the same period of time (e.g., duration T 2  in  FIG. 3 ). For a zero bit, the operating frequency transitions once at the beginning of the period of time and then remains at the same operating frequency for the entire period of time (T 2 ). For a one bit, the operating frequency transitions once at the beginning of the period of time and again halfway through transmission of the bit. During encoding of a one bit, the operating frequency is therefore at both frequencies f 1  and f 2  for an equal duration of time T 1  that is half of T 2 . 
     During encoding of bits using the differential bi-phase encoding scheme of  FIG. 3 , the frequency remains constant for either a duration of time T 2  or T 1  before transitioning to the other frequency. T 1  is half of T 2 . These periods of time where the frequency is constant may be referred to as modulation states. The modulation states (sometimes referred to as bit periods) are used to convey bits using the bit encoding scheme. 
     Herein, an example will be described where the duration of each bit transmission (e.g., T 2 ) is 512 cycles total. T 1  is therefore 256 cycles. For encoding a zero bit, the operating frequency transitions and then is held constant for 512 cycles. For encoding a one bit, the operating frequency transitions, is held constant for 256 cycles, transitions again, and is then again held constant for 256 cycles. The modulation states (where the operating frequency is constant) are therefore either 512 cycles or 256 cycles. 
     Using an illustrative power transmission frequency of 110 kHz and the example of 512 cycles per bit, the total time to transmit each bit (e.g., the length of time for 512 cycles at 110 kHz) is 4.65 milliseconds. The modulation states (where the operating frequency is constant) are therefore either 4.65 milliseconds (for 512 cycles) or 2.33 milliseconds (for 256 cycles). 
     During ASK modulation, the power receiving device similarly uses an encoding scheme to modulate data bits onto the power signal. In one example, the power receiving device may use a differential bi-phase encoding scheme (similar to as in FSK modulation described above). However, the ASK modulation may be asynchronous with the power signal. Instead of modulating a power signal parameter in synchronization with the power signal (as with FSK modulation), the ASK modulation may be performed according to an internal clock that has a constant frequency. 
       FIG. 4  shows the power signal amplitude over time during ASK modulation. The power signal amplitude transitions between amplitudes A 1  and A 2  to encode bits. As previously discussed, during ASK modulation the power receiving device  24  may use a switch (e.g., one or more transistors in transceiver  46  that are coupled coil  48 ) to modulate the impedance of power receiving circuitry  54  (e.g., coil  48 ). This, in turn, modulates the amplitude of signal  44  and the amplitude of the AC signal passing through coil(s)  36 . The amplitude of the signal passing through coil(s)  36  may vary between two amplitude magnitudes (e.g., A 1  and A 2 ). 
     As shown, in the encoding scheme of  FIG. 4 , a transition between the two amplitudes occurs at the start of each bit. To encode a ‘one’ bit, there are two transitions in the power signal amplitude. To encode a ‘zero’ bit, there is one transition in the power signal amplitude. 
     With the ASK modulation of  FIG. 4 , the transitions in the power signal amplitude coincide with transitions in the clock signal (also depicted in  FIG. 4 ). The clock signal may cycle through high and low values in a series of clock cycles, with each clock cycle taking a duration of time T CLK  (shown in  FIG. 4 ). For a zero bit, there is a single transition in the power signal amplitude that coincides with the rising edge of the clock signal (e.g., at t 3  in  FIG. 4 ). For a one bit, there is a first transition in the power signal amplitude that coincides with the rising edge of the clock signal (e.g., at t 1  in  FIG. 4 ) then a second transition in the power signal amplitude that coincides with the falling edge of the clock signal (e.g., at t 2  in  FIG. 4 ). 
     For example, at t 1  the amplitude (power signal amplitude) transitions from A 2  to A 1 . This indicates the start of encoding the one bit. The amplitude may remain at A 1  for half of one full clock cycle (e.g., T CLK /2) then transition back to A 2  at t 2 . The amplitude remains at A 2  for the other half of the clock cycle. At t 3 , the encoding of the one bit is complete. 
     At t 3 , the amplitude transitions from A 2  to A 1 . This indicates the start of encoding the zero bit. The amplitude may remain at A 1  for a complete clock cycle (e.g., T CLK ) then transition back to A 2  at t 4 . At t 4 , the encoding of the zero bit is complete. 
     To summarize, each bit (either a ‘one’ or ‘zero’) is transmitted over the same duration of time (e.g., duration T CLK  in  FIG. 3 ). For a zero bit, the amplitude transitions once at the beginning of the bit and then remains at the same amplitude for the entire period of time (T 4  which is equal to T CLK ). For a one bit, the amplitude transitions once at the beginning of the bit and again halfway through transmission of the bit. During encoding of a one bit, the amplitude is therefore at both amplitudes A 1  and A 2  for an equal duration of time T 3  (that is half of T 4 /T CLK ). 
     During encoding of bits using the differential bi-phase encoding scheme of  FIG. 4 , the amplitude remains constant for either a duration of time T 3  or T 4  before transitioning to the other amplitude. T 3  is half of T 4 . These periods of time where the amplitude is constant may be referred to as modulation states. The modulation states (sometimes referred to as bit periods) are used to convey bits using the bit encoding scheme. 
     The frequency of the clock signal in  FIG. 4  may be 2 kHz, as one example. In this example, each clock cycle has a duration of time (T CLK ) equal to 0.5 milliseconds. Half of the clock cycle (T 3 ) is therefore equal to 0.25 milliseconds. The modulation states (where the amplitude is constant) are therefore either 0.25 milliseconds or 0.5 milliseconds. 
     The examples of  FIGS. 3 and 4  illustrate how the modulation states of the FSK modulation may be longer than the modulation states of the ASK modulation. In the examples of  FIGS. 3 and 4 , the shortest FSK modulation state is 2.33 milliseconds whereas the longest ASK modulation state is 0.5 milliseconds. Each of the FSK modulation states may be greater than each of the ASK modulation states by a factor of two or more, three or more, four or more, etc. For both the ASK and FSK modulation states, each modulation state may have a length that is an integer multiple of the shortest modulation state. 
     It should be noted that the specific values used in  FIGS. 3 and 4  are merely illustrative. In general, any desired values may be used for the operating frequencies, clock frequencies, modulation state lengths, etc. However, in some communication schemes the FSK modulation states may be longer than the ASK modulation states as discussed in connection with  FIGS. 3 and 4  above. 
     Consider a scenario where a power transmitting device attempts to send information (e.g., a packet) to a power receiving device using FSK modulation. Ideally, the power receiving device would detect the start of the FSK packet and receive/process the entirety of the FSK packet before taking further action (e.g., responding with an ASK packet). However, in some cases the power receiving device may fail to detect the start of the FSK packet. In these cases, the power receiving device may transmit an ASK packet to the power transmitting device, even while the power transmitting device is still attempting to transfer the FSK packet to the power receiving device. 
     In some communication schemes, the power transmitting device may have no way of knowing that the power receiving device failed to detect the start of an attempted FSK packet. Without the power transmitting device knowing this, the power transmitting device may continue to transfer the FSK packet until transmission is complete. Only after the FSK transmission is complete will the power transmitting device attempt to detect reception of an ASK packet from the power receiving device. This may cause delays in the reception of the ASK packet. The power transmitting device needlessly waits until the failed FSK transmission is ‘complete’ to receive the ASK packet. Moreover, the power transmitting device may miss some or all of the ASK packet because the ASK packet was transmitted to the power transmitting device while the power transmitting device was attempting to transmit the FSK packet (and not monitoring for a received ASK packet). By the time the power transmitting device is looking for the ASK packet, it may be too late to receive the ASK packet. 
     To avoid these types of problems (e.g. missed or delayed communications due to attempted simultaneous FSK and ASK packet transfer), it may be desirable for the power transmitting device to detect incoming ASK packets even while transmitting FSK packets. The presence of received ASK bits while transmitting FSK bits is indicative that the power receiving device failed to detect the start of the FSK packet. Therefore, in response to detecting incoming ASK bits while transmitting FSK bits, the power transmitting device may cease the FSK transmissions and demodulate/process the incoming ASK bits. 
       FIG. 5  is a flow chart of illustrative operations involved in operating a power transmitting device (e.g., device  12  in  FIG. 1 ). During the operations of block  102 , the power transmitting device  12  may initiate a frequency-shift keying (FSK) communication. Specifically, transceiver circuitry  40  in device  12  may modulate the operating frequency of coil  36  according to a bit encoding scheme to convey bits to the power receiving device while wireless power is simultaneously being transferred to the power receiving device. The transceiver circuitry may modulate the operating frequency of coil  36  according to the bi-phase bit encoding scheme of  FIG. 3 , as one example. Bits transmitted using FSK modulation in this way may be part of larger bytes (e.g., groups of bits) which are in turn part of a larger data packet. The FSK communication initiated in block  102  may be intended to transmit one or more data packets containing any desired information. 
     During the operations of block  104 , the power transmitting device may determine if amplitude-shift keying (ASK) modulation is present on the power signal (e.g., the signal transmitted from coil  36  in device  12  to coil  48  in device  24 ). The power transmitting device may monitor for ASK modulation while simultaneously transmitting bits to the power receiving device using FSK modulation (e.g. the ongoing communication of block  102 ). 
     It should be noted that the power transmitting device may optionally compensate for changes in frequency when monitoring for received ASK modulation signals in the power signal. During an FSK modulation state, the frequency of the power signal may be constant. Therefore, the amplitude of the power signal should also remain constant. Changes in the amplitude of the power signal within a single FSK modulation state may therefore be attributed to ASK modulations. 
     In an example where each FSK modulation state is longer than each ASK modulation state, the ASK modulation may be detected without any compensation. In other words, due to the disparity in the lengths of the modulation states, there will be multiple amplitude changes (e.g., multiple ASK modulation states) detected during a given FSK modulation state (without needing to compensate). However, in some cases, it may be desirable to detect amplitude changes from ASK communications across multiple FSK modulation states. This type of functionality may enable an entire ASK byte to be demodulated even while FSK communications are ongoing. Alternatively, detecting amplitude changes from ASK communications across multiple FSK modulation states may be useful in cases where the FSK modulation states are shorter than (or not much longer than) the ASK modulation states. 
     When the frequency of the power signal changes during FSK communications, there may be an expected change in the amplitude of the power signal. The power transmitting device may therefore compensate the detected amplitude of the power signal across different frequencies for purposes of ASK demodulation. Consider an example where a power signal is modulated between f 1  and f 2  during FSK communications. There may be an expected change in amplitude between corresponding amplitudes A 1  and A 2  each time the frequency changes. However, ASK modulation from the power receiving device may cause additional amplitude changes. The additional amplitude changes may be detected by identifying different amplitude values (e.g., such as A 3 ) and/or amplitude changes that occur asynchronously with the frequency changes. 
     The wireless power transmitting device may therefore optionally take into account expected amplitude changes caused by the FSK modulation when monitoring for amplitude changes caused by ASK modulation indicating communication from the power receiving device. An illustrative example of this comprises the operations of block  104  including an adjustment to a change in amplitude of the power signal synchronous (i.e. coinciding) with the transition to a new FSK modulation state, causing such synchronous changes in amplitude not to be erroneously detected as ASK modulation. 
     Ultimately, the wireless power transmitting device may sometimes detect the presence ASK modulation during the operations of block  104 . The presence of ASK modulation is indicative that the power receiving device  24  failed to detect the FSK communication initiated in the operations of block  102 . Therefore, during the operations of block  106 , the power transmitting device may abort the FSK communication due to the presence of the ASK modulation. Because the power receiving device is not receiving/processing the FSK communication anyway, the power transmitting device ceases the FSK communication. After ceasing the FSK communications, the power transmitting device may still transmit a power signal to the power receiving device. In other words, wireless power continues to be transferred even though the frequency modulation for communication purposes is ceased. 
     Additionally, after detecting the presence of ASK modulation, the power transmitting device may process the detected ASK modulation during the operations of block  108 . Specifically, transceiver circuitry  40  may demodulate the ASK modulated power signal to identify encoded bits in the power signal (transmitted from the power receiving device). The power transmitting device (e.g., the transceiver circuitry) may process the bits to identify and interpret a packet from the power receiving device and may then take corresponding action (e.g., attempt to restart FSK communication, change a wireless power transfer parameter, etc.) based on the content of the packet. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.