Patent Publication Number: US-8971468-B1

Title: Increasing the resolution of serial data recovery units (DRUs) based on interleaved free running oversamplers

Description:
TECHNICAL FIELD 
     The present disclosure relates to data recovery units (DRUs). In particular, the disclosure relates to increasing the resolution of serial DRUs based on interleaved free running oversamplers. 
     BACKGROUND 
     Currently, two or more free running oversamplers may be interleaved with one another to obtain sampled data with higher resolution. However, the hardware employed must provide a negligible interleaving phase error for the process, voltage, and temperature (PVT) variations. This requirement appears to be a serious limitation, because oversamplers typically are not designed with this goal in mind. Thus, this requirement is typically very difficult to fulfill, due to many impairments that can arise at the silicon and package levels. 
     Therefore, there is a need for a solution that aims at measuring and compensating the relative skew of two oversamplers during runtime. 
     SUMMARY 
     A system for increasing resolution of recovery of serial data includes X free running oversamplers, and X−1 fractional-unit interval (UI) compensation unit(s) coupled to receive a serial data input signal, wherein X is at least two. The fractional-UI compensation unit(s) is associated with respective one(s) of the free running oversamplers, wherein one of the free running oversamplers not associated with any of the fractional-UI compensation unit(s) is coupled to receive the serial data input signal, and the remaining one(s) of the free running oversamplers is coupled to receive respective output signal(s) from the respective fractional-UI compensation unit(s). The system also includes a plurality of data recovery units coupled with respective ones of the free running over samplers, wherein the data recovery units are configured to provide respective output signals based at least in part on output signals from the respective free running oversamplers. The system also includes a skew detector configured to difference the output signals from the data recovery units and to generate a skew measurement signal, wherein the fractional-UI compensation unit(s) is configured to provide the output signal(s) for the respective one(s) of the free running oversamplers based at least in part on the skew measurement signal. The system further includes a multi-UI compensation unit coupled to receive output signals from the free running oversamplers, and a user application configured to output a sampled data output signal based at least in part on an output signal from the multi-UI compensation unit. 
     Optionally, the system further includes a reference clock configured to synchronize, with a reference frequency, at least one of the free running oversamplers or the user application. 
     Optionally, the system further includes a low pass filter (LPF) configured to filter the skew measurement signal. 
     Optionally, the user application comprises a data recovery unit. 
     Optionally, the user application is further configured to output an enable signal. 
     Optionally, the fractional-UI compensation unit(s) is configured to compensate a skew from the skew measurement signal. 
     A system for increasing resolution of recovery of serial data includes X first free running oversamplers, and X second free running oversamplers coupled with respective ones of the first free running oversamplers to mirror the respective ones of the free running oversamplers, wherein X is at least two. The system also includes X−1 first fractional-unit interval (UI) compensation unit(s) coupled to receive a serial data input signal. The first fractional-UI compensation unit(s) is associated with respective one(s) of the first free running oversamplers, wherein one of the first free running oversamplers not associated with any of the first fractional-UI compensation unit(s) is coupled to receive the serial data input signal, and the remaining one(s) of the first free running oversamplers is coupled to receive output signal(s) from the respective first fractional-UI compensation unit(s). The system further includes X−1 second fractional-UI compensation unit(s) coupled to receive a free running pattern signal. The second fractional-UI compensation unit(s) is coupled with respective one(s) of the second free running oversamplers, wherein one of the second free running oversamplers not associated with any of the second fractional-UI compensation unit(s) is coupled to receive a free running pattern signal, and the remaining one(s) of the second free running oversamplers is coupled to receive output signal(s) from the respective second fractional-UI compensation unit(s). The system also includes a plurality of data recovery units coupled with respective ones of the second free running over samplers, wherein the data recovery units are configured to provide respect output signal(s) based at least in part on output signals from the respective second free running oversamplers. The system further includes a skew detector configured to difference output signals from the data recovery units to generate a skew measurement signal. The first fractional-UI compensation unit(s) is configured to provide the output signal(s) for the respective one(s) of the first free running oversamplers based at least in part on the skew measurement signal. The second fractional-UI compensation unit(s) is configured to provide the output signal(s) for the respective one(s) of the second free running oversamplers based at least in part on the skew measurement signal. The system further includes a multi-UI compensation unit coupled to receive output signals from the first free running oversamplers, and a user application configured to output a sampled data output signal based at least in part on an output signal from the multi-UI compensation unit. 
     Optionally, the system further includes a free running pattern generator configured to generate the free running pattern signal. 
     Optionally, the system further includes a reference clock to synchronize, with a reference frequency, at least one of the first free running oversamplers, the second free running oversamplers, or the user application. 
     Optionally, the system further includes a low pass filter (LPF) configured to filter the skew measurement signal. 
     Optionally, the user application comprises a data recovery unit. 
     Optionally, the user application is further configured to output an enable signal. 
     Optionally, each of the first fractional-UI compensation unit(s) and the second fractional-UI compensation unit(s) is configured to compensate a skew from the skew measurement signal. 
     A method for increasing resolution of recovery of serial data includes: mirroring X first free running oversamplers with respective X second free running oversamplers, wherein X is at least two, and wherein there are X−1 first fractional-unit interval (UI) compensation unit(s) associated with respective one(s) of the first free running oversamplers, and X−1 second fractional-UI compensation unit(s) associated with respective one(s) of the second free running oversamplers. The method also includes: receiving a free running pattern signal at the second fractional-UI compensation unit(s); providing output signal(s) from the second fractional-UI compensation unit(s) for respective one(s) of the second free running oversamplers that is associated with the second fractional-UI compensation unit(s); receiving the free running pattern signal at one of the second free running oversamplers that is not associated with any of the second fractional-UI compensation unit(s); providing output signals from data recovery units that are associated with the respective second free running oversamplers based at least in part on output signals from the respective second free running oversamplers; differencing the output signals from the data recovery units to obtain a skew measurement signal; receiving serial data input signal(s) at the first fractional-UI compensation unit(s); providing output signal(s) from the second fractional-UI compensation unit(s) for respective one(s) of the first free running oversamplers that is associated with the first fractional-UI compensation unit(s); receiving the serial data input signal at one of the first free running oversamplers that is not associated with any of the first fractional-UI compensation unit(s); providing an output from a multi-UI compensation unit based at least in part on output signals provided from the first free running oversamplers; and outputting a sampled data output signal from a user application based at least in part on the output from the multi-UI compensation unit. The output signal(s) from the second fractional-UI compensation unit(s) is provided based at least in part on the skew measurement signal. The output signal(s) from the first fractional-UI compensation unit(s) is provided based at least in part on the skew measurement signal. 
     Optionally, the method further includes generating the free running pattern signal. 
     Optionally, the method further includes synchronizing, with a reference frequency, at least one of the first free running oversamplers, the second free running oversamplers, or the user application. 
     Optionally, the method further includes generating the reference frequency. 
     Optionally, the method further includes filtering the skew measurement signal with a low pass filter (LPF). 
     Optionally, the user application comprises a data recovery unit. 
     Optionally, each of the first fractional-UI compensation unit(s) and the second fractional-UI compensation unit(s) compensates a skew from the skew measurement signal. 
     Other features will be described in the detailed description below. The features, functions, and advantages can be achieved independently in various embodiments of the present inventions or may be combined in yet other embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate the design and utility of embodiments, in which similar elements are referred to by common reference numerals. These drawings are not necessarily drawn to scale. In order to better appreciate how the above-recited and other advantages and objects are obtained, a more particular description of the embodiments will be rendered, which are illustrated in the accompanying drawings. These drawings depict only typical embodiments and are not therefore to be considered limiting of its scope. 
         FIG. 1A  is a schematic diagram depicting an architecture of a free running data recovery unit (DRU). 
         FIG. 1B  is a schematic diagram illustrating a structure of a DRU. 
         FIG. 2  is a schematic diagram showing an architecture of a free running DRU operating with double resolution. 
         FIG. 3  is a schematic diagram showing an architecture of a free running DRU using matched and interleaved free running oversamplers. 
         FIG. 4  is a schematic diagram of a system for increasing the resolution of serial DRUs based on interleaved free running oversamplers. 
         FIG. 5  is a flow chart showing a method of operation for the system of  FIG. 4 . 
         FIG. 6  shows graphs relating to the system of  FIG. 4 . 
         FIG. 7  is a schematic diagram of a system for increasing the resolution of serial DRUs based on interleaved free running oversamplers. 
         FIGS. 8A and 8B  depict a flow chart showing a method of operation for the system of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments are described hereinafter with reference to the figures. It should be noted that the figures are not drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the claimed invention or as a limitation on the scope of the claimed invention. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated. 
     The methods and apparatus disclosed herein provide an operative system for increasing the resolution of serial DRUs based on interleaved free running oversamplers. In particular, this system uses incoming data to measure and to compensate the skew between two or more oversamplers (e.g., serializer/deserializers or SerDes), without the need for any hardware design requirement relating to the precision of the relative skew of the oversamplers. Note that while some of the examples herein use SerDes in oversampling mode, or other transceivers, to implement the oversamplers, other circuits can be used in other embodiments. 
     It should be noted that in order to increase the resolution of data recovery units (DRUs) based on oversampling, it is possible to use two or more transceivers (e.g., SerDes), in parallel, to sample the same incoming data with an appropriate relative skew. The skew is typically not known, and may change over process, voltage, and temperature (PVT). The present disclosure allows for measuring, during runtime, the relative skew of any two or more transceivers in free running oversampling mode, and for using this information to correct the skew over time and to keep the skew at a desired target level. 
     The system provided by the present disclosure may provide distinct advantages over previous systems. For example, by using two or more SerDes, the system increases the resolution of DRUs for a given technology. When employing only one SerDes, some applications cannot be implemented because the resolution is not high enough. As such, the disclosed system allows for the use of some applications that otherwise are not currently possible in a given technology. Further, constraints on the hardware skew are reduced or removed. The system simply measures the skew during runtime and corrects the skew. Thus, there are no difficult constraints set at the SerDes hardware design level. Additionally, the system may simply employ standard oversamplers to measure during runtime and to compensate the skew. Since standard oversamplers may be utilized, the system can work with various different platforms in the industry. 
       FIG. 1A  is a schematic diagram  100  depicting an architecture of a free running data recovery unit (DRU). In this figure, a reference clock  110  has a reference frequency (f) in Hertz (Hz). The reference clock  110  synchronizes a DRU  130  by the reference frequency (f). The reference frequency (f) is multiplied, by a multiplier  120 , by N to produce an oversampling frequency (F), where F=N*f. A free running oversampler  140  is synchronized by the oversampling frequency (F) in Hz. As such, the free running oversampler  140  and the DRU  130  are operating synchronously to the reference clock  110 . 
     Also in this figure, at least one serial data signal  150 , containing data, is shown to be inputted into the free running oversampler  140 . The free running oversampler  140  samples data from at least one serial data signal  150 , and outputs at least one sampled signal  170 , via N wires  160 , to the DRU  130 . The DRU  130  selects the data samples that are the most closely located to the center of the eye diagram. The resolution (R) of the DRU  130  (in this context, higher resolution translates into higher high-frequency jitter tolerance) is limited by the oversampling frequency (F). It should be noted that the resolution (R) is equal to 1/F. The DRU  130  outputs at least one output data signal  180  and at least one enable signal  190 . The enable signal  190  is used to indicate which data bits in the output data signal  180  are valid sampled data. For example, when a data bit in the output data signal  180  is valid, the enable signal  190  will output a “1”, and when a data bit in the output data signal  180  is not valid, the enable signal  190  will output a “0”. 
       FIG. 1B  is a schematic diagram illustrating a structure of a DRU  105 . The DRU  105  may be the DRU  130  of  FIG. 1A . As shown in  FIG. 1B , at least one input signal, via N wires  115 , is inputted into a phase detector (PD)  125 . At least one output signal from the phase detector  125  is inputted into a low pass filter (LPF)  135 . Also, at least one output signal from the LPF  135  is inputted into a numerically controlled oscillator (NCO)  145 . At least one output signal from the NCO  145  is fed back (i.e., via a negative feedback loop) into the phase detector (PD)  125 . At least one output signal from the NCO  145  and at least one input signal are inputted into a sample selector  155 . The sample selector  155  selects the data samples from the signal that are the most closely located to the center of the eye diagram, and outputs at least one enable signal  165  and at least one data out signal  175 . 
       FIG. 2  is a schematic diagram  200  showing the architecture of a free running DRU operating with double resolution. This figure illustrates a simple way to improve the resolution of a DRU by a factor of two (2) by simply oversampling the data at double the rate (i.e., the oversampling frequency F=2*N*f). However, this implementation may not always be possible due to of hardware limitations, as was previously discussed above. 
       FIG. 3  is a schematic diagram  300  showing the architecture of a free running DRU using matched and interleaved free running oversamplers  340   a ,  340   b . In this figure, a reference clock  310  has a reference frequency (f) in Hertz (Hz). The reference clock  310  synchronizes a DRU  330  by the reference frequency (f). The reference frequency (f) is multiplied, by a multiplier  320 , by N to produce an oversampling frequency (F), where F=N*f. Two free running oversamplers  340   a ,  340   b  are synchronized by the oversampling frequency (F) in Hz. As such, the two free running oversamplers  340   a ,  340   b  as well as the DRU  330  are operating synchronously to the reference clock  310 . Although X=2 free running oversamplers  340   a ,  340   b  are shown in  FIG. 3 , in other embodiments there may be more than two free running oversamplers. 
     Also in this figure, at least one serial data signal  350 , containing data, is shown to be inputted into the free running oversampler  340   a . In addition, at least one serial data signal  350  is shown to be inputted into a skew unit  355  to skew the serial data signal(s)  350  by 0.5 unit interval (UI). At least one output signal from the skew unit  355  is inputted into the free running oversampler  340   b.    
     The free running oversampler  340   a  samples data from at least one serial data signal  350 , and outputs at least one sampled signal  370   a , via N wires  360   a , to the DRU  330 . Also, the free running oversampler  340   b  samples data from at least one output signal from the skew unit  355 , and outputs at least one sampled signal  370   b , via N wires  360   b , to the DRU  330 . The DRU  330  selects the data samples that are the most closely located to the center of the eye diagram. The DRU  330  outputs at least one output data signal  380  and at least one enable signal  390 . The enable signal  390  is used to indicate which data bits in the output data signal  380  are valid sampled data. For example, when a data bit in the output data signal  380  is valid, the enable signal  390  will output a “1”, and when a data bit in the output data signal  380  is not valid, the enable signal  390  will output a “0”. The enable signal  390  may have other values in other embodiments. 
     The oversampled data from the two free running oversamplers  340   a ,  340   b  is interleaved (e.g., oversampler  340   a  generates odd samples, and oversampler  340   b  generates even samples, or vice versa). In the illustrated example, the resolution (R) is 1/(N*F). However, the resolution (R) may be anywhere from 1/(N*F) to 1/(2*N*F) (the lower the resolution, the better). The optimal condition (e.g., R=1/(2*N*F)) may be achieved if the skew between the two free running oversamplers  340   a ,  340   b  is controlled to be half a unit interval (i.e., 0.5 UI), which is equal to R/ 2 . 
     In some applications, the delay R/2 may be implemented by shifting appropriately the sampling phase of oversampler  340   b , but it may be difficult to guarantee that the skew between the two free running oversamplers  340   a ,  340   b  is matched over PVT variations. The embodiment shown in  FIG. 4  is designed to overcome this challenge. 
       FIG. 4  shows a system  400  for increasing the resolution of serial DRUs based on interleaved free running oversamplers. In this figure, a reference clock  410  has a reference frequency (f) in Hertz (Hz). The reference clock  410  synchronizes a user application  430  (e.g., a DRU) by the reference frequency (f). The reference frequency (f) is multiplied, by a multiplier  420 , by N to produce an oversampling frequency (F), where F=N*f. Two (X=2) free running oversamplers  440   a ,  440   b  are synchronized by the oversampling frequency (F) in Hz. As such, the two free running oversamplers  440   a ,  440   b  as well as the user application  430  are operating synchronously to the reference clock  410 . 
     Also in this figure, the free running oversampler  440   a  outputs at least one sampled signal  470   a , via N wires  460   a , to a DRU  475   a . The free running oversampler  440   b  outputs at least one sampled signal  470   b , via N wires  460   b , to a DRU  475   b . Output signals from the DRUs  475   a ,  475   b  are differenced by a skew detector  495  to produce a skew measurement signal  497 . The skew measurement signal  497  is inputted into a low pass filter (LPF)  485 . The filtered skew measurement signal from the LPF  485  is then inputted into a fractional-unit interval (UI) compensation unit  455 . 
     In addition, at least one serial data signal  450  is inputted into the fractional-UI compensation unit  455 . The fractional-UI compensation unit  455  skews the serial data signal(s)  450  to be 0.5 UI (i.e., by a fractional skew). Also, at least one serial data signal  450  is shown to be inputted into the free running oversampler  440   a . Additionally, an output signal from the fractional-UI compensation unit  455  is inputted into the free running oversampler  440   b.    
     Also in this figure, the free running oversampler  440   a  outputs at least one sampled signal  470   a , via N wires  460   a , to a multi-UI compensation unit  465 . The free running oversampler  440   b  outputs at least one sampled signal  470   b , via N wires  460   b , also to the multi-UI compensation unit  465 . The multi-UI compensation unit  465  adjusts the delay to compensate for any skew that is a multiple of R (i.e., the coarse skew). At least one output signal from the multi-UI compensation unit  465  is inputted into the user application  430 . The user application  430  outputs at least one output data signal  480  and at least one enable signal  490 . The enable signal  490  is used to indicate which data bits in the output data signal  480  are valid sampled data. For example, when a data bit in the output data signal  480  is valid, the enable signal  490  will output a “1”, and when a data bit in the output data signal  480  is not valid, the enable signal  390  will output a “0”. The enable signal  490  may have other values in other embodiments. 
     It should be noted that in  FIG. 4 , the two additional DRUs  475   a ,  475   b , operating on the same reference clock  410  as the user application  430  (e.g., the main DRU), are operating independently on the data from the free running oversamplers  440   a ,  440   b , with a resolution of R/ 2 . Each DRU  475   a ,  475   b  operates independently from each other. In some embodiments, each DRU  475   a ,  475   b  may have the architecture shown in  FIG. 1B , and may have an NCO (e.g., NCO  145 ). Referring back to  FIG. 4 , when locked, the NCO inside each of the DRUs  475   a ,  475   b  will output the phase of the incoming data signal compared to the phase of the clock oversampling the data. Of course, the difference between the two phases (NCO phase 0 and NCO phase 1) is a measure of the fractional portion of the skew (i.e., the fractional skew) difference between the free running oversamplers  440   a ,  440   b , including the delay in the path. When the skew is different from the target value (e.g., 0.5), the delay element is tuned slowly during runtime to maintain the overall skew equal to the target value. The negative feedback depicted in  FIG. 4  compensates for any PVT variation that might occur. 
     The depicted structure of  FIG. 4  may employ standard free running oversamplers (i.e., for which no special care has been taken at the design level to minimize the relative skew) for the two free running oversampler  440   a ,  440   b . It should be noted that the fractional-UI compensation unit  455  can measure and correct for the fractional portion of the skew (i.e., below one (1) unit interval (UI), which is synonymous with 1/F in this context). For example, if the skew is 4.3 UI, the 0.3 UI will be measured and compensated by the fractional-UI compensation unit  455 . 
     It should also be noted that each of the free running oversamplers  440   a ,  440   b  may have divider(s) and FIFO(s), which can insert a relative skew that is a multiple of the period R (i.e., a coarse skew; in the previous example, this would be 4 UI). This portion of the relative skew changes at each startup, but remains constant over PVT variations. In some embodiments, this skew is compensated for each time after startup. Being a multiple of 1/F, the skew may be compensated for digitally in the field-programmable gate array (FPGA) fabric. This is the functionality of the multi-UI compensation unit  465 . At startup, the multi-UI compensation unit  465  adjusts its delay to compensate for any skew that is a multiple of R (i.e., the coarse skew). 
     The fractional-UI compensation unit  455  compensating for the fractional skew and the multi-UI compensation unit  465  compensating for the coarse skew may operate independently. The precision of the fractional-UI compensation unit  455  is negatively affected if the incoming data signal  450  has a data rate that is frequency locked to the reference clock  410 . Thus, it may be desirable to select a reference clock  410  which is asynchronous to the frequency of the incoming data signal  450 . 
     In the illustrated embodiments, there are two free running oversamplers  440  (i.e., X=2). However, in other embodiments, there may be more than two free running oversamplers  440  (i.e., X&gt;2). It should be noted that for X free running oversamplers  440  (X is an integer of at least two), there are X−1 fractional-UI compensation units  455 . 
       FIG. 5  is a flow chart showing a method  500  of operation of the system of  FIG. 4 . At the start (Item  505 ) of the method  500 , a reference clock generates a reference frequency (f) (Item  510 ). The reference frequency (f) is used to synchronize at least two free running oversamplers  440   a ,  440   b  and/or a user application  430  (Item  515 ). Then, an output signal from each of the free running oversamplers  440   a ,  440   b  is inputted into a respective data recovery unit  475  (e.g.,  475   a ,  475   b ) associated with each of the free running oversamplers  440   a ,  440   b , where there are X free running oversamplers (Item  520 ). Output signals from the data recovery units  475   a ,  475   b  are then differenced to obtain a skew measurement signal  497  (Item  525 ). Then, the skew measurement signal  497  is filtered with a low pass filter (LPF)  485  (Item  530 ). 
     The skew measurement signal and a serial data input signal are then inputted into at least one fractional-unit interval (UI) compensation unit  455 , where there are X−1 fractional-UI compensation unit(s)  455  (Item  535 ). The output signal from each fractional-UI compensation unit  455  is inputted into the free running oversampler  440   b  associated with the fractional-UI compensation unit  455  (Item  540 ). Also, the serial data input signal is inputted into the free running oversampler  440   a  that is not associated with any of the fractional-UI compensation units  455  (Item  545 ). 
     Then, output signals from the free running oversamplers  440   a ,  440   b  are inputted into a multi-UI compensation unit  465  (Item  550 ). An output signal from the multi-UI compensation unit  465  is inputted into the user application  430  (Item  555 ). Then, a sampled data output signal  480  is outputted from the user application  430  (item  560 ). Also, an enable signal  490  is outputted from the user application  430  (Item  565 ). Then, the method  500  ends at Item  570 . 
     In the above example, there are X=2 free running oversamplers. In other embodiments, X may be larger than 2. In either case, the number of fractional-UI compensation units  455  is equal to X−1. 
       FIG. 6  contains a series of graphs  600 ,  610 ,  620 ,  630 ,  640  relating to the system  400  of  FIG. 4 . In this figure, graph  600  depicts an exemplary incoming data signal. Graph  610  shows the output digital signal from free running oversampler  440   a  after it has sampled the data signal of graph  600 , and graph  620  shows the output digital signal from free running oversampler  440   b  after it has sampled the data signal of graph  600 . For this example, free running oversampler  440   a  and free running oversampler  440   b  are interleaved, which is achieved by the negative loop in  FIG. 4 . As such, the free running oversamplers  440   a ,  440   b  can only have a residual reciprocal multi-UI skew. 
     Graph  630  shows a signal exhibiting correct interleaving of the signals in graphs  610  and  620 . Graph  640  shows a signal exhibiting the wrong interleaving of the signals in graphs  610  and  620 , as is evidenced by the “010” pattern  650  in the interleaved signal. The “010” pattern indicates the presence of a multi-UI compensation error of the skew (i.e., a coarse skew error). The multi-UI compensation unit  465  will find the correct interleaving skew based on an identification of the “010” and/or “101” pattern, because all incorrect positions will exhibit a “010” or “101” in the interleaved pattern. 
       FIG. 7  illustrates another system  700  for increasing the resolution of serial DRUs based on interleaved free running oversamplers. In this figure, a reference clock  710  has a reference frequency (f) in Hertz (Hz). The reference clock  710  synchronizes a user application  730  (e.g., a DRU) with the reference frequency (f). The reference frequency (f) is multiplied, by a multiplier  720 , by N to produce an oversampling frequency (F), where F=N*f. Two free running oversamplers  740   a ,  740   b  are synchronized by the oversampling frequency (F) in Hz. As such, the two free running oversamplers  740   a ,  740   b  as well as the user application  730  are operating synchronously to the reference clock  710 . 
     Also in this figure, a mirror free running oversampler  745   a  mirrors free running oversampler  440   a , and a mirror free running oversampler  745   b  mirrors free running oversampler  440   b . The mirror free running oversampler  745   a  outputs at least one sampled signal  771   a , via N wires  761   a , to a DRU  775   a . Also, the mirror free running oversampler  745   b  outputs at least one sampled signal  771   b , via N wires  761   b , to a DRU  775   b . Output signals from the DRUs  775   a ,  775   b  are differenced by a skew detector  795  to produce a skew measurement signal  797 . The skew measurement signal  797  is inputted into a low pass filter (LPF)  785 . 
     The filtered skew measurement signal is then inputted into a fractional-unit interval (UI) compensation unit  755  as well as a mirror fractional-UI compensation unit  735 . The fractional-UI compensation unit  755  and the mirror fractional-UI compensation unit  735  skews the signals to be 0.5 UI (i.e., by a fractional skew). A free running pattern generator  725  generates at least one free running pattern signal. At least one free running pattern signal is outputted from the free running pattern generator  725 , and inputted into mirror free running oversampler  745   a , and into the mirror fractional-UI compensation unit  735 . At least one output signal from the mirror fractional-UI compensation unit  735  is inputted into the mirror free running oversampler  745   b.    
     Also in this figure, at least one serial data signal  750  is shown to be inputted into the free running oversampler  740   a . Also, at least one serial data signal  750  is shown to be inputted into the fractional-UI compensation unit  755 . Additionally, an output signal from the fractional-UI compensation unit  755  is inputted into the free running oversampler  740   b . The free running oversampler  740   a  (i.e., the oversampler not associated with the fractional-UI compensation unit  755 ) outputs at least one sampled signal  770   a , via N wires  760   a , to a multi-UI compensation unit  765 . Also, the free running oversampler  740   b  (i.e., the oversampler associated with the fractional-UI compensation unit  755 ) outputs at least one sampled signal  770   b , via N wires  760   b , to the multi-UI compensation unit  765 . The multi-UI compensation unit  765  adjusts the delay to compensate for any skew that is a multiple of R (i.e., the coarse skew). At least one output signal from the multi-UI compensation unit  765  is inputted into the user application  730 . The user application  730  outputs at least one output data signal  780  and at least one enable signal  790 . The enable signal  790  is used to indicate which data bits in the output data signal  780  are valid sampled data. For example, when a data bit in the output data signal  780  is valid, the enable signal  790  will output a “1”, and when a data bit in the output data signal  780  is not valid, the enable signal  790  will output a “0”. The enable signal  490  may have other values in other embodiments. 
     It should be noted that the user application  730  of  FIG. 7  (as well as the user application  430  of  FIG. 4 ) may be various different items in different embodiments. For example, in some embodiments, the user application  430 / 730  may be a DRU. In addition, the user application  430 ,  730  may have various different functions including, but not limited to: burst data synchronization (for this application, data appears in burst, where each burst has a different phase), and interval measurement (for this application, the length of an event may be measured with high resolution). 
     In the illustrated embodiments, there are two free running oversamplers  740  (i.e., X=2). In other embodiments, there may be more than two free running oversamplers (i.e., X&gt;2). It should be noted that for X free running samplers  740  (first free running samplers  740 ), there are X mirror free running oversamplers  745  (second running samplers  745 ), X−1 fractional-UI compensation units  755  (first fractional-UI compensation units  755 ), and X−1 mirror fractional-UI compensation units  735  (second fractional-UI compensation units  735 ). 
       FIGS. 8A and 8B  depict a flow chart showing an exemplary method  800  of operation for the system  700  of  FIG. 7 . At the start (Item  805 ) of the method  800 , the reference clock  710  generates a reference frequency (f) (Item  810 ). The reference frequency (f) is used to synchronize at least two free running oversamplers  740  and/or a user application  730  (Item  815 ). Then, each of the free running oversamplers  740  are mirrored with a mirror free running oversampler  745  associated with each of the free running oversamplers  740 , where there are X free running oversamplers  740  (Item  820 ). An output signal from each of the mirror free running oversamplers  745  is inputted into a respective data recovery unit  775  associated with each of the mirror free running oversamplers  745  (Item  825 ). 
     Then, output signals from the data recovery units  775  are differenced to obtain a skew measurement signal  797  (Item  830 ). The skew measurement signal  797  is then filtered with the low pass filter (LPF)  785  (Item  835 ). The free running pattern generator  725  then generates a free running pattern signal (Item  840 ). 
     Then, the skew measurement signal and the free running pattern signal are inputted into at least one mirror fractional-unit interval (UI) compensation unit  735 , where there are X−1 mirror fractional-UI compensation unit(s)  735  (Item  845 ). An output signal from each of the mirror fractional-UI compensation unit(s)  735  is then inputted into a respective mirror free running oversampler  745  (mirror free running oversampler  745   b  in the example) associated with the mirror fractional-UI compensation unit  735  (Item  850 ). Then, the free running pattern signal is inputted into the mirror free running oversampler  745  (the mirror free running oversampler  745   a  in the example) that is not associated with any of the mirror fractional-UI compensation units  735  (Item  855 ). 
     The filtered skew measurement signal from the LPF  785  and a serial data input signal  750  are inputted into at least one fractional-UI compensation unit  755 , where there are X−1 fractional-UI compensation unit(s)  755  (Item  806 ). An output signal from each of the fractional-UI compensation unit(s)  755  is inputted into the respective free running oversampler  740  (free running oversampler  740   b  in the example) associated with the fractional-UI compensation unit  755  (Item  865 ). The serial data input signal is inputted into the free running oversampler  740  (the free running oversampler  740   a  in the example) that is not associated with any of the fractional-UI compensation units  755  (Item  870 ). 
     Output signals from the free running oversamplers  740  are then inputted into the multi-UI compensation unit  765  (Item  875 ). An output signal from the multi-UI compensation unit is inputted into the user application  880 . A sampled data output signal is outputted from the user application  730  (Item  885 ). Also, an enable signal is outputted from the user application  730  (Item  890 ). The illustrated method  800  ends at item  895 . 
     Although particular embodiments have been shown and described, it will be understood that they are not intended to limit the claimed inventions, and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the claimed inventions. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. The claimed inventions are intended to cover alternatives, modifications, and equivalents.