Patent Publication Number: US-10313068-B1

Title: Signal monitoring and measurement for a multi-wire, multi-phase interface

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to high-speed data communications interfaces, and more particularly, to monitoring and measuring characteristics of a signal on a multi-wire, multi-phase data communication link. 
     INTRODUCTION 
     Manufacturers of mobile devices, such as cellular phones, may obtain components of the mobile devices from various sources, including different manufacturers. For example, an application processor in a cellular phone may be obtained from one manufacturer, while an imaging device or camera may be obtained from another manufacturer, and a display may be obtained from yet another manufacturer. The application processor, the imaging device, the display controller, or other types of device may be interconnected using a standards-based or proprietary physical interface. In one example, an imaging device may be connected using the Camera Serial Interface (CSI) defined by the Mobile Industry Processor Interface (MIPI) Alliance. In another example, a display may include an interface that conforms to the Display Serial Interface (DSI) standard specified by MIPI. Further, a multiphase, multi-wire physical layer standard MIPI C-PHY may be utilized to provide high throughput performance over bandwidth-limited channels for connecting displays and cameras to the application processor. 
     In particular, the multiphase, multi-wire (C-PHY) interface defined by the MIPI Alliance uses three wires or conductors to transmit information between devices. Each of the three wires may be in one of various voltage signaling levels during transmission of a symbol over the C-PHY interface, where clock timing is encoded into each symbol requiring that the combination of voltages driven onto the wires must change at every symbol boundary. During the initial time period of the symbol after a symbol boundary, the voltages on the lines will be in transient states where each of the wires is transitioning between various different voltage levels or steps, with this period having uncertainty of signal states. After this initial transition period, the voltages on the lines will move toward a steady state where the symbol and encoded information may be determined. This steady state period may be characterized with an eye diagram or pattern, which is a useful tool to specify the signal characteristics. In order to support higher data rates in a three-wire signaling system such as a C-PHY system, a need exists for improved test features to properly monitor the eye formation, which becomes smaller with increased data rates, as well as measure skew between the three wires as seen at the receiver end of the 3-wire interface. 
     SUMMARY 
     Embodiments disclosed herein provide systems, methods and apparatus that enable improved communications on a multi-wire and/or multiphase communications link. 
     In an aspect of the disclosure, a method for characterizing a waveform received in a data communication device coupled to a 3-line communication interface is disclosed. The method includes sampling signals present on each line of the 3-line communication interface using a plurality of first samplers, where a respective first sampler of the plurality of first samplers samples each line, wherein each sampler includes at least one of a programmable time delay and a programmable voltage offset configured to respectively delay the sampled signal by a time amount or determine a voltage offset. Further, the method includes comparing the outputs of each first sampler with respective direct line samples of signals on each of the three lines of the 3-line communication interface to generate error signals, where the direct line samples are sampled using a plurality of second samplers, each of which is coupled to a respective line of the 3-line communication interface, and then generating an array of error signal data over time using the comparison of the sample outputs and the line samples. The method also includes then determining waveform characteristics based on the array of error signal data. 
     According to another aspect, an apparatus for characterizing a waveform received in a data communication device coupled to a 3-line communication interface is disclosed. The apparatus includes means for sampling signals present on each line of the 3-line communication interface using a plurality of first samplers, where a respective first sampler of the plurality of first samplers samples each line, wherein each sampler includes at least one of a programmable time delay and a programmable voltage offset configured to respectively delay the sampled signal by a time amount or determine a voltage offset. Further, the apparatus features means for sampling direct line signals using a plurality of second samplers. Moreover, the apparatus includes means for comparing the outputs of each first sampler with respective direct line samples from each of the second samplers for each of the three lines of the 3-line communication interface to generate error signals, and means for generating an array of error signal data over time using the comparison of the sample outputs and the line samples from the means for comparing. Additionally, the apparatus includes means for determining waveform characteristics based on the array of error signal data. 
     In yet another aspect, the present disclosure provides for a non-transitory processor readable storage medium comprising code for sampling signals present on each line of a 3-line communication interface using a plurality of first samplers, where a respective first sampler of the plurality of first samplers samples each line, wherein each sampler includes at least one of a programmable time delay and a programmable voltage offset configured to respectively delay the sampled signal by a time amount or determine a voltage offset. The medium also includes code for comparing the outputs of each first sampler with respective direct line samples of signals on each of the three lines of the 3-line communication interface to generate error signals, where the direct line samples are sampled using a plurality of second samplers, each of which is coupled to a respective line of the 3-line communication interface. Still further, the medium includes code for generating an array of error signal data over time using the comparison of the sample outputs and the line samples, and determining waveform characteristics based on the array of error signal data. 
     In still another aspect, a system for characterizing a waveform received in a data communication device coupled to a 3-line communication interface is disclosed. The system includes a first sampling circuitry in a receiver that is configured to sample signals present on each line of the 3-line communication interface wherein the first sampling circuity includes at least one of a programmable time delay and a programmable voltage offset configured to respectively delay the sampled signal by a time delay amount or determine a voltage offset. Further, the system includes a second sampling circuitry in the receiver that is configured to directly sample the signal on each line of the 3-line communication interface. Furthermore, the system includes a comparison circuitry configured to compare the outputs of the first sampling circuitry and the second sampling circuitry over a plurality of time delay amounts and a plurality of voltage offsets to generate a plurality of error signals, and an error signal array generator configured to generate an array of error signal data over time using the plurality of error signals. Finally, the system includes waveform characteristic determination circuity configured to determine waveform characteristics of the signals received on the 3-line communication interface based on the array of error signal data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an apparatus employing a data link between IC devices that selectively operates according to one of plurality of available standards. 
         FIG. 2  illustrates a system architecture for an apparatus employing a data link between IC devices that selectively operates according to one of plurality of available standards. 
         FIG. 3  illustrates a C-PHY 3-phase data encoder. 
         FIG. 4  illustrates signaling in a C-PHY 3-phase encoded interface. 
         FIG. 5  is a state diagram illustrating potential state transitions in a C-PHY 3-phase encoded interface. 
         FIG. 6  illustrates a C-PHY 3-phase decoder. 
         FIG. 7  is an example of the effects of signal rise times on transition detection in a C-PHY decoder. 
         FIG. 8  illustrates transition detection in a C-PHY decoder. 
         FIG. 9  illustrates one example of signal transitions occurring between pairs of consecutive symbols transmitted on a C-PHY interface. 
         FIG. 10  illustrates transition regions and eye regions in an eye-pattern. 
         FIG. 11  illustrates an example of an eye-pattern generated for a C-PHY 3-Phase interface. 
         FIG. 12  illustrates an example of an exemplary circuitry for measuring voltage and time based error values in a receiver according to aspects of the present disclosure. 
         FIG. 13  illustrates an exemplary consolidated error curve that may be generated by the apparatus of  FIG. 12 . 
         FIG. 14  illustrates a flow diagram of a methodology for measuring error statistics used for eye pattern determination. 
         FIG. 15  illustrates a plot of the error curves of the three A, B, and C lines in a C-PHY system over time. 
         FIG. 16  illustrates a flow diagram of a methodology for measuring or determining inter-line skew using the apparatus of  FIG. 12 . 
         FIG. 17  is a block diagram illustrating an example of an apparatus employing a processing circuit that may be adapted according to certain aspects disclosed herein. 
         FIG. 18  is a flow chart of a method of clock generation according to certain aspects disclosed herein. 
         FIG. 19  is a diagram illustrating an example of a hardware implementation for an apparatus employing a processing employing a processing circuit adapted according to certain aspects disclosed herein. 
         FIG. 20  is a diagram illustrating an example of another hardware implementation for an apparatus employing a processing employing a processing circuit adapted according to certain aspects disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring such concepts. 
     As used in this application, the terms “component,” “module,” “system” and the like are intended to include a computer-related entity, such as, but not limited to hardware, firmware, a combination of hardware and software, software, or software in execution. For example, a component may be, but is not limited to being, a process running on a processor, a processor, an object, an executable, a thread of execution, a program and/or a computer. By way of illustration, both an application running on a computing device and the computing device can be a component. One or more components can reside within a process and/or thread of execution and a component may be localized on one computer and/or distributed between two or more computers. In addition, these components can execute from various computer readable media having various data structures stored thereon. The components may communicate by way of local and/or remote processes such as in accordance with a signal having one or more data packets, such as data from one component interacting with another component in a local system, distributed system, and/or across a network such as the Internet with other systems by way of the signal. 
     Moreover, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or.” That is, unless specified otherwise, or clear from the context, the phrase “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, the phrase “X employs A or B” is satisfied by any of the following instances: X employs A; X employs B; or X employs both A and B. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from the context to be directed to a singular form. 
     Overview of C-PHY Interface 
     Certain aspects of the invention may be applicable to a C-PHY interface specified by the MIPI Alliance, which may be deployed to connect electronic devices that are subcomponents of a mobile apparatus such as a telephone, a mobile computing device, an appliance, automobile electronics, avionics systems, etc. Examples of a mobile apparatus include a cellular phone, a smart phone, a session initiation protocol (SIP) phone, a laptop, a notebook, a netbook, a smartbook, a personal digital assistant (PDA), a satellite radio, a global positioning system (GPS) device, a multimedia device, a video device, a digital audio player (e.g., MP3 player), a camera, a game console, a wearable computing device (e.g., a smartwatch, a health or fitness tracker, etc.), an appliance, a sensor, a vending machine, or any other similarly functioning device. 
     The C-PHY interface is a high-speed serial interface that can provide high throughput over bandwidth-limited channels. The C-PHY interface may be deployed to connect application processors to peripherals, including displays and cameras. The C-PHY interface encodes data into symbols that are transmitted in a three-phase signal over a set of three wires, which may be referred to as a trio, or trio of wires. The three-phase signal is transmitted on each wire of the trio in different phases. Each three-wire trio provides a lane on a communications link. A symbol interval may be defined as the interval of time in which a single symbol controls the signaling state of a trio. In each symbol interval, one wire is “undriven” while the remaining two of the three wires are differentially driven such that one of the two differentially driven wires assumes a first voltage level and the other differentially driven wire assumes to a second voltage level different from the first voltage level. The undriven wire may float, be driven, and/or be terminated such that it assumes a third voltage level that is at or near the mid-level voltage between the first and second voltage levels. In one example, the driven voltage levels may be +V and −V with the undriven voltage being 0 V. In another example, the driven voltage levels may be +V and 0 V with the undriven voltage being +V/2. Different symbols are transmitted in each consecutively transmitted pair of symbols, and different pairs of wires may be differentially driven in different symbol intervals. 
       FIG. 1  depicts an example of apparatus  100  that may employ a C-PHY 3-phase communication link. The apparatus  100  may include a wireless communication device that communicates through a radio frequency (RF) communications transceiver  106  with a radio access network (RAN), a core access network, the Internet and/or another network. The communications transceiver  106  may be operably coupled to a processing circuit  102 . The processing circuit  102  may include one or more IC devices, such as an application-specific IC (ASIC)  108 . The ASIC  108  may include one or more processing devices, logic circuits, and so on. The processing circuit  102  may include and/or be coupled to processor readable storage such as memory devices  112  that may include processor-readable devices that store and maintain data and instructions for execution or for other use by the processing circuit  102  and devices, and/or memory cards that support a display  124 . The processing circuit  102  may be controlled by one or more of an operating system and an application programming interface (API)  110  layer that supports and enables execution of software modules residing in storage media, such as the memory device  112  of the wireless device. The memory devices  112  may include read-only memory (ROM), dynamic random-access memory (DRAM), one or more types of programmable read-only memory (PROM), flash cards, or any memory type that can be used in processing systems and computing platforms. The processing circuit  102  may include or access a local database  114  that can maintain operational parameters and other information used to configure and operate the apparatus  100 . The local database  114  may be implemented using one or more of a database module, flash memory, magnetic media, electrically-erasable PROM (EEPROM), optical media, tape, soft or hard disk, or the like. The processing circuit may also be operably coupled to external devices such as an antenna  122 , the display  124 , operator controls, such as a button  128  and a keypad  126  among other components. 
       FIG. 2  is a block schematic illustrating certain aspects of an apparatus  200  that includes a plurality of IC devices  202  and  230 , which can exchange data and control information through a communication link  220 . The communication link  220  may be used to connect a pair of IC devices  202  and  230  that are located in close proximity to one another, or that are physically located in different parts of the apparatus  200 . In one example, the communication link  220  may be provided on a chip carrier, substrate or circuit board that carries the IC devices  202  and  230 . In another example, a first IC device  202  may be located in a keypad section of a flip-phone while a second IC device  230  may be located in a display section of the flip-phone. In another example, a portion of the communication link  220  may include a cable or optical connection. 
     The communication link  220  may include multiple channels  222 ,  224  and  226 . One or more channel  226  may be bidirectional, and may operate in half-duplex and/or full-duplex modes. One or more channel  222  and  224  may be unidirectional. The communication link  220  may be asymmetrical, providing higher bandwidth in one direction. In one example described herein, a first communications channel  222  may be referred to as a forward channel  222  while a second communications channel  224  may be referred to as a reverse channel  224 . The first IC device  202  may be designated as a host system or transmitter, while the second IC device  230  may be designated as a client system or receiver, even if both IC devices  202  and  230  are configured to transmit and receive on the communications channel  222 . In one example, the forward channel  222  may operate at a higher data rate when communicating data from a first IC device  202  to a second IC device  230 , while the reverse channel  224  may operate at a lower data rate when communicating data from the second IC device  230  to the first IC device  202 . 
     The IC devices  202  and  230  may each include a processor or other processing and/or computing circuit or device  206 ,  236 . In one example, the first IC device  202  may perform core functions of the apparatus  200 , including establishing and maintaining wireless communications through a wireless transceiver  204  and an antenna  214 , while the second IC device  230  may support a user interface that manages or operates a display controller  232 , and may control operations of a camera or video input device using a camera controller  234 . Other features supported by one or more of the IC devices  202  and  230  may include a keyboard, a voice-recognition component, and other input or output devices. The display controller  232  may include circuits and software drivers that support displays such as a liquid crystal display (LCD) panel, touch-screen display, indicators, and so on. The storage media  208  and  238  may include transitory and/or non-transitory storage devices adapted to maintain instructions and data used by respective processors  206  and  236 , and/or other components of the IC devices  202  and  230 . Communication between each processor  206 ,  236  and its corresponding storage media  208  and  238  and other modules and circuits may be facilitated by one or more internal bus  212  and  242  and/or a channel  222 ,  224  and/or  226  of the communication link  220 . 
     The reverse channel  224  may be operated in the same manner as the forward channel  222 , and the forward channel  222 , and the reverse channel  224  may be capable of transmitting at comparable speeds or at different speeds, where speed may be expressed as data transfer rate and/or clocking rates. The forward and reverse data rates may be substantially the same or differ by orders of magnitude, depending on the application. In some applications, a single bidirectional channel  226  may support communications between the first IC device  202  and the second IC device  230 . The forward channel  222  and/or the reverse channel  224  may be configurable to operate in a bidirectional mode when, for example, the forward and reverse channels  222  and  224  share the same physical connections and operate in a half-duplex manner. In one example, the communication link  220  may be operated to communicate control, command and other information between the first IC device  202  and the second IC device  230  in accordance with an industry or other standard. 
     The communication link  220  of  FIG. 2  may be implemented according to MIPI Alliance specifications for C-PHY and may provide a wired bus that includes a plurality of signal wires (denoted as M wires). The M wires may be configured to carry N-phase encoded data in a high-speed digital interface, such as a mobile display digital interface (MDDI). The M wires may facilitate N-phase polarity encoding on one or more of the channels  222 ,  224  and  226 . The physical layer drivers  210  and  240  may be configured or adapted to generate N-phase polarity encoded data for transmission on the communication link  220 . The use of N-phase polarity encoding provides high speed data transfer and may consume half or less of the power of other interfaces because fewer drivers are active in N-phase polarity encoded data links. 
     N-phase polarity encoding devices  210  and/or  240  can typically encode multiple bits per transition on the communication link  220 . In one example, a combination of 3-phase encoding and polarity encoding may be used to support a wide video graphics array (WVGA) 80 frames per second LCD driver IC without a frame buffer, delivering pixel data at 810 Mbps for display refresh. 
       FIG. 3  is a schematic diagram  300  illustrating a 3-wire, 3-phase polarity encoder that may be used to implement certain aspects of the communication link  220  depicted in  FIG. 2 . The example of 3-wire, 3-phase encoding is selected solely for the purpose of simplifying descriptions of certain aspects of the invention. The principles and techniques disclosed for 3-wire, 3-phase encoders can be applied in other configurations of M-wire, N-phase polarity encoders. 
     Signaling states defined for each of the 3 wires in a 3-wire, 3-phase polarity encoding scheme may include an undriven state, a positively driven state and a negatively driven state. The positively driven state and the negatively driven state may be obtained by providing a voltage differential between two of the signal wires  310   a ,  310   b  and/or  310   c , and/or by driving a current through two of the signal wires  310   a ,  310   b  and/or  310   c  connected in series such that the current flows in different directions in the two signal wires  310   a ,  310   b  and/or  310   c . The undriven state may be realized by placing an output of a driver of a signal wire  310   a ,  310   b  or  310   c  in a high-impedance mode. Alternatively, or additionally, an undriven state may be obtained on a signal wire  310   a ,  310   b  or  310   c  by passively or actively causing an “undriven” signal wire  310   a ,  310   b  or  310   c  to attain a voltage level that lies substantially halfway between positive and negative voltage levels provided on driven signal wires  310   a ,  310   b  and/or  310   c . Typically, there is no significant current flow through an undriven signal wire  310   a ,  310   b  or  310   c . Signaling states defined for a 3-wire, 3-phase polarity encoding scheme may be denoted using the three voltage or current states (+1, −1, and 0). 
     A 3-wire, 3-phase polarity encoder may employ line drivers  308  to control the signaling state of signal wires  310   a ,  310   b  and  310   c . The drivers  308  may be implemented as unit-level current-mode or voltage-mode drivers. In one example, each driver  308  may receive sets of two or more of signals  316   a ,  316   b  and  316   c  that determine the output state of corresponding signal wires  310   a ,  310   b  and  310   c . In one example, the sets of two signals  316   a ,  316   b  and  316   c  may include a pull-up signal (PU signal) and a pull-down signal (PD signal) that, when high, activate pull-up and pull down circuits that drive the signal wires  310   a ,  310   b  and  310   c  toward a higher level or lower level voltage, respectively. In this example, when both the PU signal and the PD signal are low, the signal wires  310   a ,  310   b  and  310   c  may be terminated to a mid-level voltage. 
     For each transmitted symbol interval in an M-wire, N-phase polarity encoding scheme, at least one signal wire  310   a ,  310   b  or  310   c  is in the midlevel/undriven (0) voltage or current state, while the number of positively driven (+1 voltage or current state) signal wires  310   a ,  310   b  or  310   c  is equal to the number of negatively driven (−1 voltage or current state) signal wires  310   a ,  310   b  or  310   c , such that the sum of current flowing to the receiver is always zero. For each symbol, the state of at least one signal wire  310   a ,  310   b  or  310   c  is changed from the symbol transmitted in the preceding transmission interval. 
     In operation, a mapper  302  may receive and map 16-bit data 310 to 7 symbols  312 . In the 3-wire example, each of the 7 symbols defines the states of the signal wires  310   a ,  310   b  and  310   c  for one symbol interval. The 7 symbols  312  may be serialized using parallel-to-serial converters  304  that provide a timed sequence of symbols  314  for each signal wire  310   a ,  310   b  and  310   c . The sequence of symbols  314  is typically timed using a transmission clock. A 3-wire 3-phase encoder  306  receives the sequence of 7 symbols  314  produced by the mapper one symbol at a time and computes the state of each signal wire  310   a ,  310   b  and  310   c  for each symbol interval. The 3-wire encoder  306  selects the states of the signal wires  310   a ,  310   b  and  310   c  based on the current input symbol  314  and the previous states of signal wires  310   a ,  310   b  and  310   c.    
     The use of M-wire, N-phase encoding permits a number of bits to be encoded in a plurality of symbols where the bits per symbol is not an integer. In the example of a 3-wire communications link, there are 3 available combinations of 2 wires, which may be driven simultaneously, and 2 possible combinations of polarity on the pair of wires that is driven, yielding 6 possible states. Since each transition occurs from a current state, 5 of the 6 states are available at every transition. The state of at least one wire is required to change at each transition. With 5 states, log e  (5) 2.32 bits may be encoded per symbol. Accordingly, a mapper may accept a 16-bit word and convert it to 7 symbols because 7 symbols carrying 2.32 bits per symbol can encode 16.24 bits. In other words, a combination of seven symbols that encode five states has 5 7  (78,125) permutations. Accordingly, the 7 symbols may be used to encode the 2 16  (65,536) permutations of 16 bits. 
       FIG. 4  includes an example of a timing chart  400  for signals encoded using a three-phase modulation data-encoding scheme, which is based on the circular state diagram  450 . Information may be encoded in a sequence of signaling states where, for example, a wire or connector is in one of three phase states S 1 , S 2  and S 3  defined by the circular state diagram  450 . Each state may be separated from the other states by a 120° phase shift. In one example, data may be encoded in the direction of rotation of phase states on the wire or connector. The phase states in a signal may rotate in clockwise direction  452  and  452 ′ or counterclockwise direction  454  and  454 ′. In the clockwise direction  452  and  454 ′ for example, the phase states may advance in a sequence that includes one or more of the transitions from S 1  to S 2 , from S 2  to S 3  and from S 3  to S 1 . In the counterclockwise direction  454  and  454 ′, the phase states may advance in a sequence that includes one or more of the transitions from S 1  to S 3 , from S 3  to S 2  and from S 2  to S 1 . The three signal wires  310   a ,  310   b  and  310   c  carry different versions of the same signal, where the versions may be phase shifted by 120° with respect to one another. Each signaling state may be represented as a different voltage level on a wire or connector and/or a direction of current flow through the wire or connector. During each of the sequence of signaling states in a 3-wire system, each signal wire  310   a ,  310   b  and  310   c  is in a different signaling state than the other wires. When more than 3 signal wires  310   a ,  310   b  and  310   c  are used in a 3-phase encoding system, two or more signal wires  310   a ,  310   b  and/or  310   c  can be in the same signaling state at each signaling interval, although each state is present on at least one signal wire  310   a ,  310   b  and/or  310   c  in every signaling interval. 
     Information may be encoded in the direction of rotation at each phase transition  410 , and the 3-phase signal may change direction for each signaling state. Direction of rotation may be determined by considering which signal wires  310   a ,  310   b  and/or  310   c  are in the ‘0’ state before and after a phase transition, because the undriven signal wire  310   a ,  310   b  and/or  310   c  changes at every signaling state in a rotating three-phase signal, regardless of the direction of rotation. 
     The encoding scheme may also encode information in the polarity  408  of the two conductors  310   a ,  310   b  and/or  310   c  that are actively driven. At any time in a 3-wire implementation, exactly two of the conductors  310   a ,  310   b , and  310   c  are driven with currents in opposite directions and/or with a voltage differential. In one implementation, data may be encoded using two bit values  412 , where one bit is encoded in the direction of phase transitions  410  and the second bit is encoded in the polarity  408  for the current state. 
     The timing chart  400  illustrates data encoding using both phase rotation direction and polarity. The curves  402 ,  404  and  406  relate to signals carried on three signal wires  310   a ,  310   b  and  310   c , respectively for multiple phase states. Initially, the phase transitions  410  are in a clockwise direction and the most significant bit is set to binary ‘1,’ until the rotation of phase transitions  410  switches at a time  414  to a counterclockwise direction, as represented by a binary ‘0’ of the most significant bit. The least significant bit reflects the polarity  408  of the signal in each state. 
     According to certain aspects disclosed herein, one bit of data may be encoded in the rotation, or phase change in a 3-wire, 3-phase encoding system, and an additional bit may be encoded in the polarity of the two driven wires. Additional information may be encoded in each transition of a 3-wire, 3-phase encoding system by allowing transition to any of the possible states from a current state. Given  3  rotational phases and two polarities for each phase, 6 states are available in a 3-wire, 3-phase encoding system. Accordingly, 5 states are available from any current state, and there may be log 2 (5)≅2.32 bits encoded per symbol (transition), which allows the mapper  302  to accept a 16-bit word and encode it in 7 symbols. 
     N-Phase data transfer may use more than three wires provided in a communication medium, such as a bus. The use of additional signal wires that can be driven simultaneously provides more combinations of states and polarities and allows more bits of data to be encoded at each transition between states. This can significantly improve throughput of the system, and reduce the power consumption over approaches that use multiple differential pairs to transmit data bits, while providing increased bandwidth. 
     In one example, an encoder may transmit symbols using 6 wires with 2 pairs of wires driven for each state. The 6 wires may be labeled A through F, such that in one state, wires A and F are driven positive, wires B and E negative, and C and D are undriven (or carry no current). For six wires, there may be: 
               C   ⁡     (     6   ,   4     )       =         6   !           (     6   -   4     )     !     -     4   !         =   15           
possible combinations of actively driven wires, with:
 
               C   ⁡     (     4   ,   2     )       =         4   !           (     4   -   2     )     !     ·     2   !         =   6           
different combinations of polarity for each phase state.
 
     The 15 different combinations of actively driven wires may include: 
                                                                        ABCD   ABCE   ABCF   ABDE   ABDF                   ABEF   ACDE   ACDF   ACEF   ADEF                   BCDE   BCDF   BCEF   BDEF   CDEF                        
Of the 4 wires driven, the possible combinations of two wires driven positive (and the other two must be negative). The combinations of polarity may include:
 
++−− +−−+ +−+− −+−+ −−++
 
     Accordingly, the total number of different states may be calculated as 15×6=90. To guarantee a transition between symbols, 89 states are available from any current state, and the number of bits that may be encoded in each symbol may be calculated as: log 2 (89)≅6.47 bits per symbol. In this example, a 32-bit word can be encoded by the mapper into 5 symbols, given that 5×6.47=32.35 bits. 
     The general equation for the number of combinations of wires that can be driven for a bus of any size, as a function of the number of wires in the bus and number of wires simultaneously driven: 
               C   ⁡     (       N   wires     ,     N   driven       )       =         N   wires     !           (       N   wires     -     N   driven       )     !     ·       N   driven     !               
one equation for calculating the number of combinations of polarity for the wires being driven is:
 
               C   ⁡     (       N   driven     ,       N   driven     2       )       =         N   driven     !         (       (       N   driven     2     )     !     )     2             
The equivalent number of bits per symbol may be stated as:
 
     
       
         
           
             
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                 1 
               
               ) 
             
           
         
       
     
       FIG. 5  is a state diagram  500  illustrating 6 states and 30 possible state transitions in one example of a 3-wire, 3-phase communication link. The possible states  502 ,  504 ,  506 ,  512 ,  514  and  516  in the state diagram  500  include and expand on the states shown in the circular state diagram  450  of  FIG. 4 . As shown in the exemplar of a state element  520 , each state  502 ,  504 ,  506 ,  512 ,  514  and  516  in the state diagram  500  includes a field  522  showing the voltage state of signals A, B and C (transmitted on signal wires  310   a ,  310   b  and  310   c  respectively), a field  524  showing the result of a subtraction of wire voltages by differential receivers (see the differential amplifiers/receivers  602  of  FIG. 6 , for example), respectively and a field  526  indicating the direction of rotation. For example, in state  502  (+x) wire A=+1, wire B=−1 and wire C=0, yielding output of differential receiver  702   a  (A-B)=+2, differential receiver  702   b  (B-C)=−1 and differential receiver  702   c  (C-A)=+1. As illustrated by the state diagram, transition decisions taken by phase change detect circuitry in a receiver are based on 5 possible levels produced by differential receivers, which include −2, −1, 0, +1 and +2 voltage states. 
       FIG. 6  is a diagram illustrating certain aspects of a 3-wire, 3-phase decoder  600 . Differential receivers  602  and a wire state decoder  604  are configured to provide a digital representation of the state of the three transmission lines (e.g., the signal wires  310   a ,  310   b  and  310   c  illustrated in  FIG. 3 ), with respect to one another, and to detect changes in the state of the three transmission lines compared to the state transmitted in the previous symbol period. Seven consecutive states are assembled by the serial-to-parallel convertors  606  to obtain a set of 7 symbols to be processed by the demapper  608 . The demapper  608  produces 16 bits of data that may be buffered in a first-in-first-out (FIFO) register  610 . 
     The wire state decoder  604  may extract a sequence of symbols  614  from phase encoded signals received on the signal wires  310   a ,  310   b  and  310   c . The symbols  614  are encoded as a combination of phase rotation and polarity as disclosed herein. The wire state decoder may include a CDR circuit  624  that extracts a recovered clock  626  (RCLK) that can be used to reliably capture symbols from the signal wires  310   a ,  310   b  and  310   c . A transition occurs on least one of the signal wires  310   a ,  310   b  and  310   c  at each symbol boundary and the CDR circuit  624  may be configured to generate the clock  626  based on the occurrence of a transition or multiple transitions. An edge of the clock may be delayed to allow time for all signal wires  310   a ,  310   b  and  310   c  to have stabilized and to thereby ensure that the current symbol is captured for decoding purposes. 
     A 3-phase transmitter includes drivers that provide high, low and middle-level voltages onto the transmit channel. This results in some variable transitions between consecutive symbol intervals. Low-to-high and high-to-low voltage transitions may be referred to as full-swing transitions, while low-to-middle and high-to-middle voltage transitions may be referred to as half-swing transitions. Different types of transitions may have different rise or fall times, and may result in different zero crossings at the receiver. These differences can result in “encoding jitter,” which may impact link signal integrity performance. 
       FIG. 7  is an exemplary timing diagram  700  that illustrates certain aspects of transition variability at the output of a C-PHY 3-phase transmitter. Variability in signal transition times may be attributed to the existence of the different voltage and/or current levels used in 3-phase signaling. The timing diagram  700  illustrates transition times in a signal received from a single signal wire  310   a ,  310   b  or  310   c . A first symbol Sym n    702  is transmitted in a first symbol interval that ends at a time  722  when a second symbol Sym n+1    724  is transmitted in a second symbol interval. The second symbol interval may end at time  726  when a third symbol Sym n+2    706  is transmitted in the third symbol interval, which ends when a fourth symbol Sym n+3    708  is transmitted in a fourth symbol interval. The transition from a state determined by the first symbol  702  to the state corresponding to the second symbol  704  may be detectable after a delay  712  attributable to the time taken for voltage in the signal wire  310   a ,  310   b  or  310   c  to reach a threshold voltage  718  and/or  720 . The threshold voltages may be used to determine the state of the signal wire  310   a ,  310   b  or  310   c . The transition from a state determined by the second symbol  704  to the state for the third symbol  706  may be detectable after a delay  714  attributable to the time taken for voltage in the signal wire  310   a ,  310   b  or  310   c  to reach one of the threshold voltages  718  and/or  720 . The transition from a state determined by the third symbol  706  to the state for the fourth symbol  708  may be detectable after a delay  716  attributable to the time taken for voltage in the signal wire  310   a ,  310   b  or  310   c  to reach a threshold voltage  718  and/or  720 . The delays  712 ,  714  and  716  may have different durations, which may be attributable in part to variations in device manufacturing processes and operational conditions, which may produce unequal effects on transitions between different voltage or current levels associated with the 3 states and/or different transition magnitudes. These differences may contribute to jitter and other issues in C-PHY 3-phase receiver. 
       FIG. 8  includes a block schematic  800  illustrating certain aspects of CDR circuits that may be provided in a receiver in a C-PHY 3-phase interface. A set of differential receivers  802   a ,  802   b  and  802   c  is configured to generate a set of difference signals  810  by comparing each of the three signal wires  310   a ,  310   b  and  310   c  in a trio with the other of the three signal wires  310   a ,  310   b  and  310   c  in the trio. In the example depicted, a first differential receiver  802   a  compares the states of signal wires  310   a  and  310   b , a second differential receiver  802   b  compares the states of signal wires  310   b  and  310   c  and a third differential receiver  802   c  compares the states of signal wires  310   a  and  310   c . Accordingly, a transition detection circuit  804  can be configured to detect occurrence of a phase change because the output of at least one of the differential receivers  802   a ,  802   b  and  802   c  changes at the end of each symbol interval. 
     Certain transitions between transmitted symbols may be detectable by a single differential receiver  802   a ,  802   b  or  802   c , while other transitions may be detected by two or more of the differential receivers  802   a ,  802   b  and  802   c . In one example the states, or relative states of two wires may be unchanged after a transition and the output of a corresponding differential receiver  802   a ,  802   b  or  802   c  may also be unchanged after the phase transition. In another example, both wires in a pair of signal wires  310   a ,  310   b  and/or  310   c  may be in the same state in a first time interval and both wires may be in a same second state in a second time interval and the corresponding differential receiver  802   a ,  802   b  or  802   c  may be unchanged after the phase transition. Accordingly, a clock generation circuit  806  may include a transition detection circuit  804  and/or other logic to monitor the outputs of all differential receivers  802   a ,  802   b  and  802   c  in order to determine when a phase transition has occurred. The clock generation circuit may generate a receive clock signal  808  based on detected phase transitions. 
     Changes in signaling states of the 3 wires may be detected at different times for different combinations of the signal wires  310   a ,  310   b  and/or  310   c . The timing of detection of signaling state changes may vary according to the type of signaling state change that has occurred. The result of such variability is illustrated in the timing chart  850  of  FIG. 8 . Markers  822 ,  824  and  826  represent occurrences of transitions in the difference signals  810  provided to the transition detection circuit  804 . The markers  822 ,  824  and  826  are assigned different heights in the timing chart  850  for clarity of illustration only, and the relative heights of the markers  822 ,  824  and  826  are not intended to show a specific relationship to voltage or current levels, polarity or weighting values used for clock generation or data decoding. The timing chart  850  illustrates the effect of timing of transitions associated with symbols transmitted in phase and polarity on the three signal wires  310   a ,  310   b  and  310   c . In the timing chart  850 , transitions between some symbols may result in variable capture windows  830   a ,  830   b ,  830   c ,  830   d ,  830   e ,  830   f  and/or  830   g  (collectively symbol capture windows  830 ) during which symbols may be reliably captured. The number of state changes detected and their relative timing can result in jitter on the clock signal  808 . 
     The throughput of a C-PHY communications link may be affected by duration and variability in signal transition times. For example, variability in detection circuits may be caused by manufacturing process tolerances, variations and stability of voltage and current sources and operating temperature, as well as by the electrical characteristics of the signal wires  310   a ,  310   b  and  310   c . The variability in detection circuits may limit channel bandwidth. 
       FIG. 9  includes timing charts  900  and  920  representative of certain examples of transitions from a first signaling state to a second signaling state between certain consecutive symbols. The signaling state transitions illustrated in the timing charts  900  and  920  are selected for illustrative purposes, and other transitions and combinations of transitions can occur in a C-PHY interface. The timing charts  900  and  920  relate to an example of a 3-wire, 3-phase communications link, in which multiple receiver output transitions may occur at each symbol interval boundary due to differences in rise and fall time between the signal levels on the trio of wires. With reference also to  FIG. 8 , the first timing charts  900  illustrate the signaling states of the trio of signal wires  310   a ,  310   b  and  310   c  (A, B, and C) before and after a transition and second timing charts  920  illustrate the outputs of the differential receivers  802   a ,  802   b  and  802   c , which provides difference signals  810  representative of the differences between signal wires  310   a ,  310   b  and  310   c . In many instances, a set of differential receivers  802   a ,  802   b  and  802   c  may be configured to capture transitions by comparing different combinations for two signal wires  310   a ,  310   b  and  310   c . In one example, these differential receivers  802   a ,  802   b  and  802   c  may be configured to produce outputs by determining the difference (e.g. by subtraction) of their respective input voltages. 
     In each of the examples shown in the timing charts  900  and  920 , the initial symbol (−z)  516  (see  FIG. 8 ) transitions to a different symbol. As shown in the timing charts  902 ,  904  and  906  signal A is initially in a +1 state, signal B is in a 0 state and signal C is in the −1 state. Accordingly, the differential receivers  802   a ,  802   b  initially measure a +1 difference  924  and the differential receiver  802   c  measures a −2 difference  926 , as shown in the timing charts  922 ,  932 ,  938  for the differential receiver outputs. 
     In a first example corresponding to the timing charts  902 ,  922 , a transition occurs from symbol (−z)  516  to symbol (−x)  512  (see  FIG. 8 ) in which signal A transitions to a −1 state, signal B transitions to a +1 state and signal C transitions to a 0 state, with the differential receiver  802   a  transitioning from +1 difference  924  to a −2 difference  930 , differential receiver  802   b  remaining at a +1 difference  924 ,  928  and differential receiver  802   c  transitioning from −2 difference  926  to a +1 difference  928 . 
     In a second example corresponding to the timing charts  904 ,  932 , a transition occurs from symbol (−z)  516  to symbol (+z)  506  in which signal A transitions to a −1 state, signal B remains at the 0 state and signal C transitions to a +1 state, with two differential receivers  802   a  and  802   b  transitioning from +1 difference  924  to a −1 difference  936 , and differential receiver  802   c  transitioning from −2 difference  926  to a +2 difference  934 . 
     In a third example corresponding to the timing charts  906 ,  938 , a transition occurs from symbol (−z)  516  to symbol (+x)  502  in which signal A remains at the +1 state, signal B transitions to the −1 state and signal C transitions to a 0 state, with the differential receiver  802   a  transitioning from a +1 difference  924  to a +2 difference  940 , the differential receiver  802   b  transitioning from a +1 difference  924  to a −1 difference  942 , and the differential receiver  802   c  transitioning from −2 difference  926  to a −1 difference  942 . 
     These examples illustrate transitions in difference values spanning 0, 1, 2, 3, 4 and 5 levels. Pre-emphasis techniques used for typical differential or single-ended serial transmitters were developed for two level transitions and may introduce certain adverse effects if used on a MIPI Alliance C-PHY 3-phase signal. In particular, a pre-emphasis circuit that overdrives a signal during transitions may cause overshoot during transitions spanning 1 or 2 levels and may cause false triggers to occur in edge sensitive circuits. 
       FIG. 10  illustrates an eye pattern  1000  generated as an overlay of multiple symbol intervals, including a single symbol interval  1002 . A signal transition region  1004  represents a time period of uncertainty at the boundary between two symbols where variable signal rise times prevent reliable decoding. State information may be determined reliably in a region defined by an eye mask  1006  within an “eye opening” that represents the time period in which the symbol is stable and can be reliably received and decoded. The eye mask  1006  masks off a region in which zero crossings do not occur, and the eye mask is used by the decoder to prevent multiple clocking due to the effect of subsequent zero crossings at the symbol interval boundary that follow the first signal zero crossing. 
     The concept of periodic sampling and display of the signal is useful during design, adaptation and configuration of systems which use a clock-data recovery circuit that re-creates the received data-timing signal using frequent transitions appearing in the received data. A communication system based on Serializer/Deserializer (SERDES) technology is an example of a system where an eye pattern  1000  can be utilized as a basis for judging the ability to reliably recover data based on the eye opening of the eye pattern  1000 . 
     An M-wire N-Phase encoding system, such as a 3-wire, 3-phase encoder may encode a signal that has at least one transition at every symbol boundary and the receiver may recover a clock using those guaranteed transitions. The receiver may require reliable data immediately prior to the first signal transition at a symbol boundary, and must also be able to reliably mask any occurrences of multiple transitions that are correlated to the same symbol boundary. Multiple receiver transitions may occur due to slight differences in rise and fall time between the signals carried on the M-wires (e.g. a trio of wires) and due to slight differences in signal propagation times between the combinations of signal pairs received (e.g. A-B, B-C, and C-A outputs of differential receivers  802   a ,  802   b  and  802   c  of  FIG. 6 ). 
       FIG. 11  illustrates an example of an eye-pattern  1100  generated for a C-PHY 3-phase signal. The eye-pattern  1100  may be generated from an overlay of multiple symbol intervals  1102 . The eye-pattern  1100  may be produced using a fixed and/or symbol-independent trigger  1130 . The eye-pattern  1100  includes an increased number of voltage levels  1120 ,  1122 ,  1124 ,  1126 ,  1128  that may be attributed to the multiple voltage levels measured by the differential receivers  802   a ,  802   b ,  802   c  an N-phase receiver circuit (see  FIG. 8 ). In the example, the eye-pattern  1100  may correspond to possible transitions in 3-wire, 3-phase encoded signals provided to the differential receivers  802   a ,  802   b , and  802   c . The three voltage levels may cause the differential receivers  802   a ,  802   b , and  802   c  to generate strong voltage levels  1126 ,  1128  and weak voltage levels  1122 ,  1124  for both positive and negative polarities. Typically, only one signal wire  310   a ,  310   b  and  310   c  is undriven in any symbol and the differential receivers  802   a ,  802   b , and  802   c  do not produce a 0 state output. The voltages associated with strong and weak levels need not be evenly spaced with respect to a zero crossing level. For example, the weak voltage levels  1122 ,  1124  represent a comparison of voltages that may include the voltage level reached by an undriven signal wire  310   a ,  310   b  and  310   c . The eye-pattern  1100  may overlap the waveforms produced by the differential receivers  802   a ,  802   b , and  802   c  because all three pairs of signals are considered simultaneously when data is captured at the receiving device. The waveforms produced by the differential receivers  802   a ,  802   b , and  802   c  are representative of difference signals  810  representing comparisons of three pairs of signals (A-B, B-C, and C-A). The eye pattern shown has four received signal levels that are the result of three transmitted single-ended levels (e.g., ¼ V, ½ V, ¾ V) of the driver circuit in the C-PHY transmitter. Combinations of the three single-ended levels from the drivers on the three signals will cause strong and weak 1s and 0s to appear across the three differential receiver inputs (3 ways to receive 2 signals at a time out of a total of 3 signals). Only the center of the eye between the weak 0 and weak 1 are considered by a receiver. The eye pattern is thus generated or drawn by overlapping the three waveforms of all three pairs of signals, which are: A minus B, B minus C, and C minus A. The eye pattern is drawn in this manner because all three pairs of signals are used simultaneously when the clock is recovered and data is captured at the C-PHY receiver. 
     Drivers, receivers and other devices used in a C-PHY 3-Phase decoder may exhibit different switching characteristics that can introduce relative delays between signals received from the three wires. Multiple receiver output transitions may be observed at each symbol interval boundary  1108  and/or  1114  due to slight differences in the rise and fall time between the three signals of the trio of signal wires  310   a ,  310   b ,  310   c  and due to slight differences in signal propagation times between the combinations of pairs of signals received from the signal wires  310   a ,  310   b ,  310   c . The eye-pattern  1100  may capture variances in rise and fall times as a relative delay in transitions near each symbol interval boundary  1108  and  1114 . The variances in rise and fall times may be due to the different characteristics of the 3-Phase drivers. Differences in rise and fall times may also result in an effective shortening or lengthening of the duration of the symbol interval  1102  for any given symbol. 
     A signal transition region  1104  represents a time, or period of uncertainty, where variable signal rise times prevent reliable decoding. State information may be reliably determined in an “eye opening”  1106  representing the time period in which the symbol is stable and can be reliably received and decoded. In one example, the eye opening  1106  may be determined to begin at the end  1112  of the signal transition region  1104 , and end at the symbol interval boundary  1114  of the symbol interval  1102 . In the example depicted in  FIG. 11 , the eye opening  1106  may be determined to begin at the end  1112  of the signal transition region  1104 , and end at a time  1116  when the signaling state of the signal wires  310   a ,  310   b ,  310   c  and/or the outputs of the three differential receivers  802   a ,  802   b  and  802   c  have begun to change to reflect the next symbol. 
     The maximum speed of a communication link  220  configured for N-Phase encoding may be limited by the duration of the signal transition region  1104  compared to the eye opening  1106  corresponding to the received signal. The minimum period for the symbol interval  1102  may be constrained by tightened design margins associated with the CDR circuit  624  in the decoder  600  illustrated in  FIG. 6 , for example. Different signaling state transitions may be associated with different variations in signal transition times corresponding to two or more signal wires  310   a ,  310   b  and/or  310   c , thereby causing the outputs of the differential receivers  802   a ,  802   b  and  802   c  in the receiving device to change at different times and/or rates with respect to the symbol interval boundary  1108 , where the inputs of the differential receivers  802   a ,  802   b  and  802   c  begin to change. The differences between signal transition times may result in timing skews between signaling transitions in two or more difference signals  810 . CDR circuits may include delay elements and other circuits to accommodate timing skews between the difference signals  810 . 
     Eye Monitoring and Skew Measurement 
     As mentioned before, the increases in speed for MIPI C-PHY protocols, in particular, have made monitoring of the eye pattern or characteristics at a receiver, such as receivers  600  or  800 , more important. The monitoring of the eye pattern as experienced by the receiver may be used to plot an eye diagram that is helpful for characterizing the incoming signal and the signal&#39;s integrity. This information may, in turn, be used for debugging receiver circuitry or board design problems, as examples. Additionally, it may be further beneficial to measure the timing skew between the three wires A, B, and C as experienced at the receiver to characterize the incoming signal with more accuracy. 
       FIG. 12  illustrates an exemplary circuit diagram  1200  of an apparatus at a receiver device for implementing improved eye monitoring and skew measurement according to aspects of the present disclosure. The circuit  1200  implements eye monitoring using auxiliary sampling that is offset in time from the signals on the A, B, and C wires in order to create comparison or error signals based on the comparison there between that can be used to plot an eye diagram. To this end, the circuit  1200  includes auxiliary samplers  1202 ,  1204 , and  1206  that respectively receive inputs from the A, B, C wires  1208 ,  1210 , and  1212 . The auxiliary samplers  1202 ,  1204 , and  1206  may implemented with D Flip flops, or any other suitable circuitry configured to hold an input value at its output (e.g., a Q output in the case of a D flip flop) for a clocked period of time. 
     The samplers  1202 ,  1204 , and  1206  are clocked with an auxiliary recovered clock signal (Aux_RCLK)  1216  generated by a clock delay line or circuit  1214 , which is configured to delay a recovered clock signal (RCLK)  1218  by various predetermined or programmable delay times (See e.g., delay code  1220  received from a processor implementing code or software that determines the delay time, although the present disclosure is not limited to such and in other embodiment the circuitry  1214  may include logic configured to set various delay times). In an aspect, the circuit  1214  may effectuate an N number of clock delay steps over a number of unit interval (UIs) or similar time periods. In one example, the N number of steps may be 64 different time delay steps over which the circuitry  1214  sweeps the clock delay. It is further noted that the RCLK signal  1218  is received from CDR circuitry within the receiver, such as CDR  624  or  806  as discussed earlier. The outputs of the auxiliary samplers  1202 ,  1204 , and  1206  are thus clocked with a clock  1216  that is delayed from the CDR RCLK  1218 , resulting in the output of sampled signals of the A, B, and C wires that are delayed from the signals on the A, B, and C wires by the predetermined or programmed time delay. 
     In addition to the auxiliary samplers  1202 ,  1204 , and  1206 , the circuitry  1200  includes normal functional path samplers  1222 ,  1224 , and  1226  in the receiver, which respectively sample the A, B, and C wire signals  1208 ,  1210 , and  1212  according to the time clocked by the CDR RCLK signal  1218 . It will be appreciated by those skilled in the art that the output sampled signals Rx_A, Rx_B, and Rx_C ( 1228 ,  1230 ,  1232 ) from samplers  1222 ,  1224 , and  1226  will ahead of the delayed sampled signals  1234 ,  1236 , and  1238  from auxiliary samplers  1202 ,  1204 , and  1206 . Pairs of the sampled signals and delayed signals corresponding to the A, B, and C wires (i.e., signals  1228  and  1234 , signals  1230  and  1236 , and signals  1232  and  1238 ) are input to comparators  1240 ,  1242 , and  1244 , which compare the signals to obtain error signals or states  1246 ,  1248 , and  1250  for each line (i.e., Error_A, Error_B, and Error_C) when the voltage of one sampled signal is less than the corresponding delayed sampled signal during a sampled time step. Thus, during a time of transitions of the signals on the A, B, or C wires, such as during time of the transition Region (t ΔJ ) discussed earlier, differences will likely exist between at least some of the samples  1228 ,  1230 ,  1232  and the delayed samples  1234 ,  1236 , and  1238  as signal voltage changes will be occurring between the sampled time and the delayed sampled time. In an aspect, the comparators  1240 ,  1242 , and  1244  may be implemented using XOR logic gates as illustrated in  FIG. 12 , but the present disclosure is not necessarily limited to such. In the example of XOR logic, when a difference is present between the samples  1228 ,  1230 ,  1232  and the delayed samples  1234 ,  1236 , and  1238 , the output of the comparators  1240 ,  1242 , and  1244  using XOR logic will be a logic “HIGH” or “1”, whereas if there is no difference (or little difference according to some predetermined threshold) in the levels of the signals, the output of the XOR logic will be “LOW” or “0”. In this example, logic “1” will then indicate the likelihood of error and, thus, a time period where the eye pattern would be not be indicated, whereas logic “0” will indicate the likelihood of less or no error (i.e., a steady state of the signals), which would, in turn, indicate the appropriateness of the eye pattern being mapped in this time period. 
     Additionally, the apparatus  1200  includes a voltage threshold circuitry  1252  that is configured to set a voltage offset or threshold for the auxiliary samplers  1202 ,  1204 ,  1206 . In an aspect, the voltage threshold circuitry  1252  may be programmable and configured to receive a voltage threshold code from a processor running an algorithm or software that is used to set the voltage threshold, although the present disclosure is not limited to such and in other embodiments the circuitry  1252  may include logic configured to set various voltage thresholds. In an example, the voltage threshold circuitry may input an “M” number of voltage offset values that the threshold circuitry  1252  will sweep the voltage to be able to measure the characteristics of the incoming signals on lines A, B, and/or C. The voltage threshold or offset signal  1256  is provided to the auxiliary samplers  1202 ,  1204 ,  1206 , which may be configured to output the input value dependent upon whether the voltage input to the sampler is above or below the threshold to determine the value of the input signal. For example, if the input signal voltage is higher than threshold, then the value is captured as 1. If the input signal voltage is less than threshold, the value is captured as 0. If the input signal is equal to threshold (within noise range) then value may be randomly either 0 or 1. 
     As further illustrated in  FIG. 12 , the error signals  1246 ,  1248 , and  1250  are input to a digital engine  1260  or similar circuitry to accumulate the error signals over time; e.g., an error signal array generator. In one example, the accumulated error signals may be over the “N” number of time steps, which may be N delay time steps or samples over a number of unit intervals (UIs) or equivalent time periods in one example. Furthermore, the apparatus  1200  and voltage threshold circuitry  1252 , in particular, will also sweep through the “M” number of voltage steps. Thus, the digital engine  1260  may be configured to accumulate an array of M voltage×N time values of the error signals occurring at each M, N combination of values that may be used for calculating the eye pattern of the system. For example, the apparatus  1200  may run through a number of cycles and accumulate the M, N values to populate the array based on a continuous data stream input on the A, B, and C wires for a number of UIs, such as approximately 2000 or more UIs as an example. The accumulation or counting of error signal  1246 ,  1248 , and  1250  may be accomplished using an error counter (not shown) in digital engine  1260 , which could be implemented with registers, memory devices, or other suitable circuitry for counting or storing data. 
     It is noted that the apparatus of  FIG. 12  may be implemented on a same chip as the receiver. Thus, the apparatus  1200  may be implemented such that testing used for characterization of the eye diagram and skew measurements may be performed easily without the need for external testing apparatus. 
       FIG. 13  illustrates an exemplary eye diagram that may be determined along with a consolidated error curve that may be generated by the apparatus  1200 . In this example, the generated consolidated error curve  1302  is determined from the measured array of error signals determined over M voltage steps and N time delay steps over a number of cycles of receiver operation. As may be seen, the error curve  1302  will show a measured higher number of errors during the transition period, with the number of errors diminishing over time after the transition period. From the data in the M×N array, the eye pattern interior  1304  may be accurately calculated and plotted for the three incoming received A, B, and C wire signals in a particular receiver. In a further aspect, it is noted that an eye diagram can be plotted either for each wire alone or for all three wires combined. 
       FIG. 14  illustrates an exemplary flow diagram of a method  1400  for measuring error signals of one or more of the wires A, B, and/or C using the apparatus  1200 . As illustrated, the method  1400  includes first setting initial time and voltage steps (i.e., a particular M voltage offset value and a particular N time delay value) as shown at block  1402 . After the initial settings and after the reception of signals on one or more of the A, B, and C wires, flow proceeds to block  1404  where an error counter, which may be implemented within the digital engine  1260 , is started for accumulating the error signals  1246 ,  1248 ,  1250  or error statistics as further illustrated at block  1406 . 
     Flow next proceeds to determination block  1408  where a determination is made whether the error collection has been completed, i.e., whether the N time delay steps and M voltage offset steps have been completed. If not, flow proceeds to block  1410  where at least one of the time delay steps and voltage steps are incremented to move to a next data point (e.g., (M+1, N)) in the M, N array of collected error statistics. Flow then proceeds back to block  1404  where the error counter is started for collecting the next error statistics. This process continues until all of the data points in the M, N matrix or vector are accumulated. 
     As mentioned above, the apparatus  1200  may also be utilized to measure the skew between the A, B, and C lines which, in this case, is a measurement of the time difference or delta between the error signals generated for the respective A, B, and C wires. As a visual example of how the time skew might occur a receiver,  FIG. 15  illustrates a plot of the error curves or vectors of the three A, B, and C lines over time. As shown, the three error curves  1502 ,  1504 , and  1506  generated by the apparatus of  FIG. 12  for each respective A, B, and C line may have skew or time differential over the N number of delay steps swept through by the clock time delay circuitry  1214  in apparatus  1200 . In order to determine the time differences between the error signals, apparatus  1200 , and digital engine  1260 , in particular, may be configured to set a particular error threshold  1508 . This error threshold is programmable to any of a number of desired thresholds. In one example, error threshold  1508  may be set to approximately an error level that corresponds to the start of the eye pattern or interior  1304  as illustrated in  FIG. 13 . In an aspect, it is noted that the error threshold  1508  can be determined from previous analyses of eye patterns and may correspond to an error level at the mid-point in the eye transition. As further illustrated, if there is skew between the A, B, and C lines, the error curves  1502 ,  1504 , and  1506  will cross the error threshold  1508  at different times (or particular time delay steps) demarcated as ts 1 , ts 2 , and ts 3 , respectively in this example. The skew may be determined based on the differences between this time or transition time steps ts 1 , ts 2 , and ts 3 . In certain aspects, it is noted that in logic within apparatus  1200 , such as digital engine  1260 , the error signals  1246 ,  1248 , and  1250 , for example may trigger a logic level “1” when the error signal is at or above the threshold  1508 , and then a logic level “0” when the error signal falls below the threshold  1508 . Thus in this example, the first 1 to 0 transition for each error curve or vector indicates the time step of transition (i.e., ts) of the error signal. Accordingly, the difference among the three transition steps ts 1 , ts 2 , and ts 3  indicates the A, B and C skew. 
       FIG. 16  illustrates a method  1600  for determining skew measurement between lines according to aspects of the present disclosure. As illustrated, method  1600  begins with starting error counters at block  1602 . It is noted that the error counters may be implemented within digital engine  1260  in one example. Also, the error signal or vector is for each line A, B, and C may be simultaneously counted, or counted one at a time. Still further, the error counter is determining the error value of the error signals for a particular time step of the N delay steps being swept through. 
     Flow next proceeds to decision block  1604  where a determination is made whether the error at the particular N delay time step is less than the predetermined threshold amount. If not, a value of “1” is stored as shown at block  1606 , whereas if the error level is below the threshold, a value of “0” is stored as shown at block  1608 . From both blocks  1606  and  1608 , flow proceeds to decision block  1610  to determine whether all of the N delay steps have been swept through. If not, flow proceeds to block  1612  where the time step is incremented and flow proceeds back to block  1602  to measure the error signal at the next time step. The flow of method  1600  continues until all of the N number of time steps have been considered. 
     Examples of Processing Circuits and Methods 
       FIG. 16  is a conceptual diagram  1700  illustrating an example of a hardware implementation for an apparatus employing a processing circuit  1702  that may be configured to perform one or more functions disclosed herein. In accordance with various aspects of the disclosure, an element, or any portion of an element, or any combination of elements as disclosed herein may be implemented using the processing circuit  1702 . The processing circuit  1702  may include one or more processors  1704  that are controlled by some combination of hardware and software modules. Examples of processors  1704  include microprocessors, microcontrollers, digital signal processors (DSPs), field programmable gate arrays (FPGAs), programmable logic devices (PLDs), state machines, sequencers, gated logic, discrete hardware circuits, and other suitable hardware configured to perform the various functionality described throughout this disclosure. The one or more processors  1704  may include specialized processors that perform specific functions, and that may be configured, augmented or controlled by one of the software modules  1716 . The one or more processors  1704  may be configured through a combination of software modules  1716  loaded during initialization, and further configured by loading or unloading one or more software modules  1716  during operation. 
     In the illustrated example, the processing circuit  1702  may be implemented with a bus architecture, represented generally by the bus  1710 . The bus  1710  may include any number of interconnecting buses and bridges depending on the specific application of the processing circuit  1702  and the overall design constraints. The bus  1710  links together various circuits including the one or more processors  1704 , and storage  1706 . Storage  1706  may include memory devices and mass storage devices, and may be referred to herein as computer-readable media and/or processor-readable media. The bus  1710  may also link various other circuits such as timing sources, timers, peripherals, voltage regulators, and power management circuits. A bus interface  1708  may provide an interface between the bus  1710  and one or more transceivers  1712 . A transceiver  1712  may be provided for each networking technology supported by the processing circuit. In some instances, multiple networking technologies may share some or all of the circuitry or processing modules found in a transceiver  1712 . Each transceiver  1712  provides a means for communicating with various other apparatus over a transmission medium. Depending upon the nature of the apparatus, a user interface  1718  (e.g., keypad, display, speaker, microphone, joystick) may also be provided, and may be communicatively coupled to the bus  1710  directly or through the bus interface  1708 . 
     A processor  1704  may be responsible for managing the bus  1710  and for general processing that may include the execution of software stored in a computer-readable medium that may include the storage  1706 . In this respect, the processing circuit  1702 , including the processor  1704 , may be used to implement any of the methods, functions and techniques disclosed herein. The storage  1706  may be used for storing data that is manipulated by the processor  1704  when executing software, and the software may be configured to implement any one of the methods disclosed herein. 
     One or more processors  1704  in the processing circuit  1702  may execute software. Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, algorithms, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. The software may reside in computer-readable form in the storage  1706  or in an external computer readable medium. The external computer-readable medium and/or storage  1706  may include a non-transitory computer-readable medium. A non-transitory computer-readable medium includes, by way of example, a magnetic storage device (e.g., hard disk, floppy disk, magnetic strip), an optical disk (e.g., a compact disc (CD) or a digital versatile disc (DVD)), a smart card, a flash memory device (e.g., a “flash drive,” a card, a stick, or a key drive), a random access memory (RAM), a ROM, a PROM, an erasable PROM (EPROM), an EEPROM, a register, a removable disk, and any other suitable medium for storing software and/or instructions that may be accessed and read by a computer. The computer-readable medium and/or storage  1706  may also include, by way of example, a carrier wave, a transmission line, and any other suitable medium for transmitting software and/or instructions that may be accessed and read by a computer. Computer-readable medium and/or the storage  1706  may reside in the processing circuit  1702 , in the processor  1704 , external to the processing circuit  1702 , or be distributed across multiple entities including the processing circuit  1702 . The computer-readable medium and/or storage  1706  may be embodied in a computer program product. By way of example, a computer program product may include a computer-readable medium in packaging materials. Those skilled in the art will recognize how best to implement the described functionality presented throughout this disclosure depending on the particular application and the overall design constraints imposed on the overall system. 
     The storage  1706  may maintain software maintained and/or organized in loadable code segments, modules, applications, programs, etc., which may be referred to herein as software modules  1716 . Each of the software modules  1716  may include instructions and data that, when installed or loaded on the processing circuit  1702  and executed by the one or more processors  1704 , contribute to a run-time image  1714  that controls the operation of the one or more processors  1704 . When executed, certain instructions may cause the processing circuit  1702  to perform functions in accordance with certain methods, algorithms and processes described herein. 
     Some of the software modules  1716  may be loaded during initialization of the processing circuit  1702 , and these software modules  1716  may configure the processing circuit  1702  to enable performance of the various functions disclosed herein. For example, some software modules  1716  may configure internal devices and/or logic circuits  1722  of the processor  1704 , and may manage access to external devices such as the transceiver  1712 , the bus interface  1708 , the user interface  1718 , timers, mathematical coprocessors, and so on. The software modules  1716  may include a control program and/or an operating system that interacts with interrupt handlers and device drivers, and that controls access to various resources provided by the processing circuit  1702 . The resources may include memory, processing time, access to the transceiver  1712 , the user interface  1718 , and so on. 
     One or more processors  1704  of the processing circuit  1702  may be multifunctional, whereby some of the software modules  1716  are loaded and configured to perform different functions or different instances of the same function. The one or more processors  1704  may additionally be adapted to manage background tasks initiated in response to inputs from the user interface  1718 , the transceiver  1712 , and device drivers, for example. To support the performance of multiple functions, the one or more processors  1704  may be configured to provide a multitasking environment, whereby each of a plurality of functions is implemented as a set of tasks serviced by the one or more processors  1704  as needed or desired. In one example, the multitasking environment may be implemented using a timesharing program  1720  that passes control of a processor  1704  between different tasks, whereby each task returns control of the one or more processors  1704  to the timesharing program  1720  upon completion of any outstanding operations and/or in response to an input such as an interrupt. When a task has control of the one or more processors  1704 , the processing circuit is effectively specialized for the purposes addressed by the function associated with the controlling task. The timesharing program  1720  may include an operating system, a main loop that transfers control on a round-robin basis, a function that allocates control of the one or more processors  1704  in accordance with a prioritization of the functions, and/or an interrupt driven main loop that responds to external events by providing control of the one or more processors  1704  to a handling function. 
       FIG. 18  is a flow chart of a method  1800  for characterizing a waveform received in a data communication device coupled to a 3-line communication interface, such as a C-PHY interface. Method  1800  illustrates a process block  1802  that includes sampling signals present on each line of the 3-line communication interface using a plurality of first samplers (e.g., auxiliary samplers  1202   2014 ,  1206 ), where a respective first sampler of the plurality of first samplers samples each line A, B, and C. Additionally, each sampler includes at least one of a programmable time delay and a programmable voltage offset configured to respectively delay the sampled signal by a time amount or determine a voltage offset. As explained before, the programmable time delay may be determined by the clock delay line  1214  and the voltage offset may be determined by the voltage threshold circuit  1252 . 
     Additionally, method  1800  includes comparing the outputs of each first sampler with respective direct line samples of signals on each of the three lines of the 3-line communication interface to generate error signals (e.g., error signals  1246 ,  1248 ,  1250 ) where the direct line samples are sampled using a plurality of second samplers (e.g., samplers  1222 ,  1224 , and  1226 ), each of which is coupled to a respective line of the 3-line communication interface as illustrated at block  1804 . 
     Further, method  1800  includes generating an array of error signal data over time using the comparison of the sample outputs and the line samples as shown in block  1806 . In an aspect, the array is stored and generated within digital engine  1260 , but is not limited to such and may be implemented in a processor, ASIC, or equivalents thereof. Finally, method  1800  includes determining waveform characteristics based on the array of error signal data as shown at block  1808 . The processes of block  1808  may also be implemented within digital engine  1260 , but is not limited to such and may be implemented in a processor, ASIC, or equivalents thereof. As described herein, the waveform characteristic that is determined includes plotting an eye pattern or eye interior as seen at the receiver in order to better understand the present receiver and 3-wire interface for testing and/or tuning purposes. 
     According to further aspects, the method  1800  may include generating the error signal using one or more XOR logic circuitry configured to compare the outputs of each first and second sampler with respective direct line samples of signals on each of the three lines of the 3-line communication interface to generate the error signals as illustrated by elements XOR elements  1240 ,  1242 , and  1244  shown in  FIG. 12 . It is noted, however, that the comparison functionality is not limited to an XOR operation, and other comparison functions may be implemented to determine an error signal between the outputs of the first and second groups of samplers. 
     In still further aspects, method  1800  includes the programmable time delay being based on a recovered clock signal (RCLK) determined in a clock and data recovery (CDR) circuit within a receiver receiving the signals on the 3-line communication interface, such that the time delay is at least one delay step of a predetermined number of delay time steps of the recovered clock signal (e.g., one of the N number of time delay steps). Additionally, method  1800  may further include each delay time step of the predetermined number of time steps being a prescribed delay time behind the recovered clock signal in time. Moreover, the method  1800  may include the programmable voltage offset including a plurality of predetermined voltage steps of voltages at which a first sampler will sample a respective line of the 3-line communication interface. 
     In still a further aspect, method  1800  may include a mode for determining skew between at least two lines of the three lines of the 3-line interface. While the examples above illustrated determining skew between all 3 lines of the 3-line interface, it is also possible just to consider two of the three, such as the difference between the times of the threshold transition steps of error signal of line A and line B, line B and line C, or line A and line C in some aspects. In particular the method  1800  may include first setting an error signal threshold, and then determining at which time steps of a plurality of time delay steps the error signal from each of the at least two lines respectively falls below the error signal threshold. From these determined time steps, the skew is then determinable between the at least two lines based on the difference of the determined time steps of the respective lines. 
       FIG. 19  is a diagram illustrating an example of a hardware implementation for an apparatus  1900  employing a processing circuit  1902 . In the illustrated example, processing circuit  1902  may be implemented within a transmitter for a 3-line, multi-phase interface, such as a C-PHY interface. In further aspects, the apparatus  1900  may be implemented as part of a transmitter in a master device, but could also be implemented in a transmitter within a slave device as well. 
     The processing circuit  1902  typically contains a processor or processing circuitry  1916  that may include one or more of a microprocessor, microcontroller, digital signal processor, a sequencer and a state machine. The processing circuit  1902  may be implemented with a bus architecture, represented generally by the bus  1920 . The bus  1920  may include any number of interconnecting buses and bridges depending on the specific application of the processing circuit  1902  and the overall design constraints. The bus  1920  links together various circuits including one or more processors and/or hardware modules, represented by the processor  1916 , specific modules or circuits such as a signal data determination module  1904 , transmitter/line interface circuits  1912  that send signaling over the various lines, connectors, or wires  1914 , and computer-readable storage medium  1918 . The bus  1920  may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further. 
     The processor  1916  is responsible for general processing, including the execution of software stored on the computer-readable storage medium  1918 . The software, when executed by the processor  1916 , causes the processing circuit  1902  to perform the various functions described before for any particular apparatus. The computer-readable storage medium  1918  may also be used for storing data that is manipulated by the processor  1916  when executing software, including data encoding for symbols transmitted over the connectors or wires  1914 , which may be configured as data lanes. The processing circuit  1902  further includes at least module  1904 , which may be used to provide the continuous source of data to be sent over wires  1914  that is, in turn, used at a receiver to determine measurements of the eye pattern and skew. The modules including module  1904  may be software modules running in the processor  1916 , resident/stored in the computer-readable storage medium  1918 , one or more hardware modules coupled to the processor  1916 , or some combination thereof. The modules including module  1904  may include microcontroller instructions, state machine configuration parameters, or some combination thereof. 
     In one configuration, the apparatus  1900  may be configured for data communication over a C-PHY 3-phase interface. The apparatus  1900  may include module and/or circuit  1904  that is configured to generate and cause transmission of the calibration pattern discussed above in connection with  FIG. 13 . Additionally, processor-readable storage medium  1918  may include code  1906  that is configured for causing the processing circuitry  1916  to generate the disclosed calibration pattern. 
       FIG. 20  is a diagram illustrating an example of a hardware implementation for an apparatus  2000  employing a processing circuit  2002 . In the illustrated example, processing circuit  2002  may be implemented within a receiver for a 3-line, multi-phase interface, such as a C-PHY interface. In a further example, the apparatus  2000  may be implemented as part of a receiver in slave device, but could also be implemented in a receiver within a master device as well according to certain examples. 
     The processing circuit  2002  typically contains a processor  2018  that may include one or more of a microprocessor, microcontroller, digital signal processor, a sequencer and a state machine. The processing circuit  2002  may be implemented with a bus architecture, represented generally by the bus  2020 . The bus  2020  may include any number of interconnecting buses and bridges depending on the specific application of the processing circuit  2002  and the overall design constraints. The bus  2020  links together various circuits including one or more processors and/or hardware modules, represented by the processor  2018 , modules or circuits  2004 ,  2006 ,  2008 ,  2010 , and  202 , difference receiver circuits  2014  that determine difference signaling state between different pairs of the connectors or wires  2016  and a computer-readable storage medium  2022 . The bus  2020  may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further. 
     The processor  2018  is responsible for general processing, including the execution of software or code stored on the computer-readable storage medium  2022 . The software or code, when executed by the processor  2018 , causes the processing circuit  2002  to perform the various functions described before for any particular apparatus. The computer-readable storage medium  2022  may also be used for storing data that is manipulated by the processor  2018  hen executing software, including data decoded from symbols transmitted over the connectors or wires  2016 , which may be configured as data lanes and clock lanes. The processing circuit  2002  further includes at least one of the modules  2004 ,  2006 ,  2008 ,  2010 , and  2012 . The modules  2004 ,  2006 ,  2008 ,  2010 , and  2012  may be software modules running in the processor  2018 , resident/stored in the computer-readable storage medium  2022 , one or more hardware modules coupled to the processor  2018 , or some combination thereof. The modules  2004 ,  2006 ,  2008 ,  2010 , and  2012  may include microcontroller instructions, state machine configuration parameters, or some combination thereof. 
     In one configuration, the apparatus  2000  may be configured for data communication over a C-PHY 3-phase interface. The apparatus  2000  may include a module and/or circuit  2004  that is configured for sampling the signals received over wires  2016  from a transmitter, such as apparatus  1900 . The module  2004  may be configured as at least a portion of the circuitry illustrated in  FIG. 12 ; namely, the auxiliary or first samplers  1202 ,  1204  and  1206  and the normal line or second samplers  1222 ,  1224 , and  1226 , and the comparison circuitry (e.g., XOR circuits  1240 ,  1242 , and  1244 ). Apparatus  2000  may further include a module and/or circuit  2006  for delay timing generation and a module and/or circuit  2008  for voltage offset generation. In an aspect, modules  2006  and  2008  may be implemented, at least in part, by circuitry  1214  and  1253 , respectively. It is further noted that modules  2006  and  2008  may also include implementation with software or in conjunction with processor  2018  running software that determines the various time delays and voltage offsets. 
     In yet further aspects, apparatus  2000  may include a module and/or circuit  2010  for determining signal characteristics including plotting an eye pattern or interior such as illustrated in  FIG. 13 . It is noted that in certain examples, the module and/or circuit  2010  may be implemented with the digital engine  1260  or suitable equivalents thereof, as well as in conjunction with processor  2018 , for example. Furthermore, apparatus  2000  may include a skew measurement module  2012  used for implementing a skew measurement mode as discussed earlier. Skew measurement module may be implemented with the digital engine  1260  or suitable equivalents thereof, as well as in conjunction with processor  2018 , for example. 
     In other examples, the processor-readable storage medium  2022  may include various code or instructions including code for causing the processor  2018  to perform the variously disclosed functions used for measuring the signal characteristics and skew. The apparatus  2000  may be configured for various modes of operation, such as MIPI C-PHY low power mode and high speed data mode. 
     It is understood that the specific order or hierarchy of steps in the processes disclosed is an illustration of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the processes may be rearranged. Further, some steps may be combined or omitted. The accompanying method claims present elements of the various steps in a sample order, and are not meant to be limited to the specific order or hierarchy presented. 
     The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” Unless specifically stated otherwise, the term “some” refers to one or more. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed as a means plus function unless the element is expressly recited using the phrase “means for.”