Abstract:
A method of synchronizing a clock signal to a data signal, comprising the steps of (A) detecting a first edge of the data signal, (B) determining a first value indicating a position of the first edge, (C) adding the first value to a second value, wherein the second value indicates a position of a second edge of the data signal and (D) adjusting the clock signal, based on the result of step (C), if the result is greater than a predetermined value.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/203,676, filed May 12, 2000 and is hereby incorporated by reference inits entirety. 
     CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application may relate to application Ser. No. 09/745,660, filed Dec. 21, 2000, Ser. No. 09/747,281, filed Dec. 21, 2000, Ser. No. 09/747,257, filed Dec. 22, 2000, Ser. No. 09/746,802, filed Dec. 22, 2000, and Ser. No. 09/747,188, filed Dec. 22, 2000, which are each hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and/or architecture for implementing phase-locked loops (PLLs) generally and, more particularly, to a method and/or architecture for implementing linearized digital PLLS. 
     BACKGROUND OF THE INVENTION 
     Conventional approaches for implementing PLLs include the bang-bang approach which comprises taking snapshots of the phase error with respect to edges of incoming data. The bang-bang approach corrects on every data edge based solely on the direction (polarity) of the offset. As a result, a bang-bang system is never truly “locked”. In the best case, a bang-bang system is nearly locked and makes a correction at every data edge (i.e., clocks are either switched-clockwise or counter clockwise depending on the polarity of the phase offset). The bang-bang approach has the disadvantage of introducing excessive jitter in the resulting recovered clock since the clock is being shrunk or expanded at every edge. 
     Referring to FIG. 1, a circuit  10  implementing a conventional bang-bang approach for constructing digital phase locked loops is shown. The circuit  10  involves the use of over sampling methods to determine in which quadrant of the clock the data edge resides. The quadrant information is then applied to an adjustment mechanism which moves the clock the appropriate direction at each interval. No information associated with the magnitude of phase error is retained or utilized. Polarity of the error and presence of a data transition are the only information used to adapt the phase of the clock to the incoming datastream. 
     Referring to FIG. 2, a flow diagram  30  illustrating the operation of the conventional bang-bang circuit  10  is shown. The circuit  10  checks for a data edge and determines the relative polarity between the data and clock. If the polarity of the data relative to the clock is positive, the clocks are switched counterclockwise. If the polarity of the data relative to the clock is negative, the clocks are switched clockwise. 
     Since the circuit  10  does not use magnitude information, a transfer function is exhibited at the phase detector which has the characteristics typical of a bang-bang approach. Such detectors have an inability to tolerate large input signal distortion, such as the distortion that may be found at the end of typical wired media. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a method of synchronizing a clock signal to a data signal, comprising the steps of (A) detecting a first edge of the data signal, (B) determining a first value indicating a position of the first edge, (C) adding the first value to a second value, wherein the second value indicates a position of a second edge of the data signal and (D) adjusting the clock signal, based on the result of step (C), if the result is greater than a predetermined value. 
     The objects, features and advantages of the present invention include providing a method and/or architecture for implementing a linearized digital PLL that may (i) reduce the sorts of distortion associated with media induced effects, (ii) reduce duty-cycle-distortion (DCD) and/or (iii) reduce data-dependant-jitter (DDJ), (DCD and DDJ may be lumped into the single category of systematic jitter). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
     FIG. 1 is a block diagram of a conventional bang-bang system; 
     FIG. 2 is a flow diagram illustrating the operation of the conventional bang-bang circuit of FIG. 1; 
     FIG. 3 is a block diagram of a preferred embodiment of the present invention; 
     FIG. 4 is a block diagram of the logic block of FIG. 3; 
     FIG. 5 is a timing diagram illustrating example waveforms of the circuit of FIG. 3; and 
     FIG. 6 is a flow diagram illustrating an example operation of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 3, a block diagram of a circuit  100  is shown in accordance with a preferred embodiment of the present invention. The circuit  100  generally comprises a logic block (or circuit)  102  and a control block (or circuit)  104 . The circuit  104  may be implemented as a control circuit configured to adjust the frequency of an output clock. 
     The circuit  104  generally comprises a circuit  110 , a circuit  112 , a circuit  114  and a circuit  116 . The circuit  104  may also comprise a number of memory elements  118   a - 118   n  and a number of buffers  120   a - 120   n . The circuit  110  may be implemented as an edge detection circuit. The circuit  110  may present a signal (e.g., DATAPULSE) to the logic block  102 . The signal DATAPULSE may be generated in response to a signal (e.g., DI_N) and a signal (e.g., DI_P). In one example, the circuit  110  may be configured to generate a pulse signal in response to a transition of a data signal. The circuit  114  may be implemented, in one example, as a phase lock loop (PLL). The PLL circuit  114  may present a number of clock signals (e.g., PLL_CLK_ 0 -PLL_CLK_N) to the circuit  116 . The circuit  116  may be implemented as a multiplexer circuit. The circuit  116  may present a number of signals (e.g., CLK(A:D)). In one example, the circuit  116  may be implemented as a multiple input multiplexer that may present an output signal based on a control signal (e.g., SEL) generated by the logic block  102 . The circuit  116  may be configured to select a number of the signals PLL_CLK_ 0 -PLL_CLK_N for presentation as the signals CLK(A:D) in response to the signal SEL. 
     The circuit  100  may implement a digital phase-detector (e.g., the logic block  102 ) that may be used as an integral part of a digital phase-locked loop for data and clock recovery circuits. Specifically, the digital phase-detector  102  may be used for linearization of the phase-detection and loop mechanisms to overcome the disadvantages associated with conventional systems (discussed in the background section of the present application). 
     Referring to FIG. 4, a more detailed diagram of the logic circuit  102  is shown. The logic circuit  102  generally comprises three major blocks, a phase-detector  122 , a filter  124 , and a phase-switcher  126 . A preferred embodiment of the present invention, in its basic form, presumes a multi-phase reference clock controlled by the phase-switcher  126 . The phase-detector  122  may be configured to detect the presence of a data-transition and compare the relative phase of the data-edge with that of the clock signals CLK(A:D). The relative phase is reduced to a numerical representation of the magnitude of the phase error between the data edge and the signals CLK(A:D), (e.g., between −N and +N, where N is the number of phases controlled by the phase-switcher  126 ). 
     The filter  124  may be implemented as a simple digital arithmetic accumulator that maintains an accumulated relative error and generates a signal to enable the movement of the phase-switcher clock-phase and a signal to indicate the direction (e.g., increment/decrement) of such phase-movement. By combining the functions, the phase of a clock out of the phase-switcher  126  is continually aligned to the incoming datastream allowing a simple sampling arrangement to recover the data bits. The functional architecture closely emulates an analog system, where the phase-detector and the filter block are similar, but represented by time-voltage-current analog circuits and the phase-switcher  126  is typically replaced by a VCO, or variable delay-line in a delay-locked loop (DLL). The phase detector  122  can transmit a number discrete digital levels, where a linear system may transmit a theoretically infinite resolution of signal into the filter  124 . 
     The filter  124  may accumulate digital numerical values. In a linear system, a capacitance element is utilized to integrate charge into voltage. The phase-switcher  126  combined with a multi-phase reference clock signals PLL_CLK 0 -PLL_CLK_N and CLK(A:D) effectively emulates VCO performance by allowing continual, though discrete-increment movement, of the clock phase edges into the system. 
     The phase detector  122  may comprise a register (e.g., REG 1 ) and a circuit  130 . The filter  124  may comprise a register (e.g., REG 2 ), a circuit  132 , a logic circuit  134 , and a register (e.g., REG 3 ). The phase switcher  126  may comprise a logic circuit  136 , a register (e.g., REG 4 ), a circuit  138  and a register (e.g., REG 5 ). The circuit  130  may be implemented as a coder circuit. The circuit  132  may be implemented as an enable look ahead circuit. The circuit  134  may be implemented as an accumulation logic circuit. The circuit  136  may be implemented as an increment/decrement logic circuit. The circuit  138  may be implemented as a decoder circuit. 
     The register REG 1  generally receives the signals DATAPULSE and CLK(A:D) from the circuit  104 . An output of the register REG 1  may be presented to an input of the circuit  130 . The circuit  130  may have an output that may present a signal to an input of the register REG 2 . The circuit  130  may generate the signal by encoding the polarity and magnitude of the phase differences between the data-edge and the signals CLK(A:D). The register REG 2  may have an output that may present a signal to a first input of the circuit  132  and a first input of the circuit  134 . The circuit  132  may have an output that may present a signal to the circuit  134  and a first input of the circuit  136 . The circuit  134  may have an output that may present a signal to an input of the register REG 3 . The register REG 3  may present a signal to inputs of the circuits  132 ,  134  and  136 . An output of the register REG 2  may be presented to an input of the circuit  136 . An output of the circuit  136  may be coupled to an input of the circuit  138  by the register REG 4 . The registers REG 2 , REG 3  and REG 5  generally have a control input that generally receives the signal CLK(A). The register REG 4  may have a control input that receives the signal CLK(B). The register REG 5  generally presents the signal SEL in response to an output of the circuit  138 . 
     The circuit  100  generally allows for the use of the detected phase error magnitude to emulate a linearized system having the characteristics at a macro level which approach a pure linear system. However, the circuit  100  may have resolution intervals allowing the simplicity of digital mechanisms to be implemented. 
     The advantage of the linearized system  100  over the pure digital PLL may be demonstrated by observation of the operation of the system  100  under high-levels of data stream distortion. Particularly, the operation of the circuit  100  may be observed under the sorts of distortion associated with media induced effects, (e.g., systematic jitter, duty-cycle-distortion (DCD) and data-dependant-jitter (DDJ)). 
     Systematic jitter has the characteristics that the predominant effect is one of having few data transitions at the average location of the data edge. Rather, the data transitions may have a bi-modal distribution of the edge placements of the datastream at some −M1/+M2 location. When the data edges predominantly occur at locations −M1 and +M2 relative to the average location (or zero-phase) then any misalignment with the local clock cannot be determined by any single data edge placement. 
     The operation of the present invention may be easily demonstrated by considering a simple sequence. Presume an incoming datastream DI_N and DI_P is distorted such that the edges occur at −J nS and +K nS, where 0 nS is the ideal non-distorted location of the edges, or the ‘average’ location of the edges. Further presume that mechanisms associated with real systems during acquisition and normal operation are such that the magnitude of J and K are not necessarily equal. The conventional ‘bang-bang’ digital PLL would see −J1, +K1, −J2, +K2, −J3, +K3, etc. and generate a response, as a control to the internal phase-switcher, which would cause the clock to decrement in phase, then increment, decrement, increment, etc, no matter what the values of J and K. 
     In contrast, the present invention may accumulate (or sum) the magnitude as −J1+K1−J2+K2−J3+K3 and respond when the accumulation goes beyond some threshold. If J=K then the accumulation would net zero on a continuous basis. For magnitudes of −J+K greater than (clock period)/2N (where 2N is the number of clock phases available for selection by the phase-switcher, as mentioned above) the system  100  may accumulate a small numerical average corresponding to the ‘average’ alignment ‘around’ the ideal zero-phase location, just as does a linear system. Thus, the system  100  would be able to adapt to frequency-tracking conditions associated with real systems, whereas the conventional approaches discussed in the background section would fail beyond some level of distortion magnitude. 
     The theoretical fail point for the conventional system is ½ the clock period of distortion of the incoming datastream, then reduced by addition of general system non-idealities, matching, and the presence of random jitter components in the datastream. The theoretical limits of operation of the circuit  100  are generally limited only by the numerical resolution N, associated with the detection resolution increments, and for cases of N=4, about ¾ clock-period, also as above reduced by system non-idealities, matching, and random jitter in the datastream. The ability to tolerate an additional ¼ clock-period of data distortion can make the difference between a device that is marginal or does not function with a particular media, and one that exhibits infinitely low bit-error-rates. 
     For the USB 2.0 specification (published April 2000 and hereby incorporated by reference in its entirety), a conventional bang-bang digital PLL will be marginal, if operable, to the system specifications for datastream distortion. Alternative implementations of the phase-detector may vary primarily in the exact construction of the numerical slicing/detection method or conversion of phase-alignment to a numerical value or input to the accumulator. Variants of the filter block  124  are ordinarily limited to the magnitude of the accumulator threshold level detection for enabling a phase-adjustment of the phase-switcher block  126 . Other filter clock variants may allow for the effective detection limit to adapt to acquisition conditions to allow for combination of fast acquisition and maximum tolerance when acquired. The implementation variants of the phase-switcher  126  and reference clock functions are predominantly associated with the number of raw clock phases available (e.g., 2N) for selection-switching, and the incrementer/decrementer and associated clock-mux design and timing. 
     The circuit  100  implements a dual bandwidth linearized digital PLL similar to that described in co-pending provisional application (Serial No. 60/203,678) which is hereby incorporated by reference in its entirety. The system  100  additionally implements the clocks sampled by data method described in co-pending provisional application (Serial No. 60/203,616), which is hereby incorporated by reference in its entirety. 
     A detailed description of an operation of the logic block  102  will now be described. An incoming serial data signal DI_N and DI_P may be sampled on the rising and falling edges to generate the signal DATAPULSE. The signal DATAPULSE may be used to clock the current values of the clocks CLK(A:D) into the register REG 1 . The value of the register REG 1  may be encoded into a 3-bit signal (via the coder  130 ) comprising one bit of polarity information and two bits of magnitude information. The coded value generally represents the offset of the sampled clocks to the ideal sample point in the serial data stream. The coded value is generally clocked into the register REG 2  on the falling edge of the signal CLKA (e.g., A(fall)). 
     A decision is then made depending on the current operation mode of the system. When the system  100  is in the high bandwidth (or acquire) mode, if the magnitude of the offset value is zero then no further action is taken (e.g., the Inc/Dec logic  136  is not enabled). However, if the magnitude of the offset is non-zero then the polarity of the offset is passed directly to the Inc/Dec logic  136 , (e.g., the Inc/Dec logic  136  is enabled). The value of the register REG 4  is then incremented or decremented as indicated by the polarity of the offset value on the next rising edge of the clock signal CLKB (e.g., B(rise)). The register REG 4  and the Inc/Dec logic  136  may be implemented as a 3-bit counter with wrap around and single adjustment limits. The value of the register REG 4  may be decoded into a 1 of 8 value that is clocked into the register REG 5  on the next rising edge of the signal CLKB. 
     When the register REG 5  is updated the select values into the PLL clock select multiplexer(s)  116  are changed, thus changing the mapping between the input PLL clocks (PLL_CLK_ 0 -PLL_CLK_N) and the internally sampled clocks CLK(A-D). For example, where the input PLL clocks are all 480 MHz clocks with 1/8 bit of phase difference, the selection may result in a 1/8 bit time phase adjustment on the sample clock CLKA. 
     When the system  100  is in the low bandwidth (or tracking) mode, the offset magnitude value is added to the value currently in the accumulator  134 . The result is clocked into the register REG 3 . The logic circuit  132  generally performs a look-ahead function and if the offset being added to accumulator  134  will cause either an overflow or underflow then the Inc/Dec Logic  136  is enabled. The Inc/Dec logic  136  generally updates the register REG 4  as determined by the value of the most significant bit of the register REG 3 , which represents the polarity of the value currently stored in the accumulator. 
     The value in the register REG 4  is generally decoded into a 1 of 8 value that is clocked into the register REG 5  on the next falling edge of CLKA. When the register REG 5  is updated, select values into the PLL clock select multiplexers are changed, thus changing the mapping between the input PLL clocks PLL_CLK_ 0 -PLL_CLK_N and the internally sampled CLK(A-D). Using the example where the input PLL clocks PLL_CLK_ 0 -PLL_CLK_N are all 480 MHz clocks with 1/8 bit of phase difference, a 1/8 bit time phase adjustment on the sample clock CLKA may be made. The 1/8 time phase adjustment may determine when the clocks are locked and the data is valid. 
     The counter  112  may assert the signal DATAVALID at a first predetermined count (e.g., seven bit times). The circuit  100  may present the output clock as the inversion of the current CLKA. The data is generally recovered by sampling the data stream with a falling edge of the signal CLKA (e.g., through two D flip-flops) and then again with a rising edge of the signal CLKA (e.g., through a third D flip-flop) to ensure that it is synchronized with the output recovered clock. 
     Referring to FIG. 6, a method (or process)  200  is shown. The method  200  generally comprises a decision state  202 , a state  204 , a state  206 , a state  208 , a decision state  212 , a decision state  210 , a decision state  212 , a state  214  and a state  216 . The decision state  202  generally determines if a data edge is present. If a data edge is not present, the decision state  202  continues to check for such a condition. If a data edge is present, the state  204  determines a relative polarity and phase-offset magnitude for the data and clock. The state  206  adds the polarity and magnitude to a previously accumulated value stored in the state  208 . Next, the state  208  stores the next accumulated value from the state  206 . The state  210  compares the magnitude of the stored value from the state  208  to C. If the magnitude is greater than C, the state  214  switches clock counter clockwise and returns to the state  202 . If the state  210  determines that the magnitude is less than c, the state  212  determines if the magnitude in the state  208  is less than −M 1 . If the magnitude is not less than −M 1 , the method  200  returns to the state  202 . If the magnitude of the value of the state  208  is less than −M 1 , the state  216  switches the clocks clockwise and returns to the state  202 . 
     To improve systematic jitter (noise) rejection, in a low bandwidth linear system where the error between transmit and receive time bases is known to be small, the low bandwidth correction method  200  may be employed. The low bandwidth correction method  200  may be such that a phase adjustment occurs only when an accumulated magnitude exceeds a pre-determined threshold value. Thus, several data edges may occur with no adjustment. Therefore when the magnitude of the accumulator reaches the threshold value, the offset is more likely the result of long term buildup of time base differences than of systematic jitter. 
     The function performed by the flow diagram of FIG. 6 may be implemented using a conventional general purpose digital computer programmed according to the teachings of the present specification, as will be apparent to those skilled in the relevant art(s). Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will also be apparent to those skilled in the relevant art(s). 
     The present invention may also be implemented by the preparation of ASICs, FPGAs, or by interconnecting an appropriate network of conventional component circuits, as is described herein, modifications of which will be readily apparent to those skilled in the art(s). 
     The present invention thus may also include a computer product which may be a storage medium including instructions which can be used to program a computer to perform a process in accordance with the present invention. The storage medium can include, but is not limited to, any type of disk including floppy disk, optical disk, CD-ROM, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, Flash memory, magnetic or optical cards, or any type of media suitable for storing electronic instructions. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.