Abstract:
A voltage controlled delay loop and method are disclosed for clock and data recovery applications. The voltage controlled delay loop generates clock signals having similar frequency and different phases. The voltage controlled delay loop comprises at least one delay element to generate at least two phases of a reference clock; a central interpolator for interpolating the at least two phases of the reference clock to generate an interpolated signal; and an input that injects the interpolated signal into a delay stage. The central interpolator provides a fine phase control. In addition, a coarse phase control can optionally be achieved by selectively injecting the interpolated signal into a given delay stage. A further voltage controlled delay loop is disclosed with coarse and fine phase control using a number of interpolators.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present invention is related to U.S. patent application Ser. No. ______, entitled “Voltage Controlled Delay Loop and Method With Injection Point Control,” filed contemporaneously herewith and incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is related to techniques for clock and data recovery (CDR) and, more particularly, to methods and apparatus for digital control of the generation and selection of different phases of a clock signal.  
       BACKGROUND OF THE INVENTION  
       [0003]     In many applications, including digital communications, clock and data recovery (CDR) must be performed before data can be decoded. Generally, in a digital clock recovery system, a reference clock signal of a given frequency is generated together with a number of different clock signals having the same frequency but with different phases. In one typical implementation, the different clock signals are generated by applying the reference clock signal to a delay network. Thereafter, one or more of the clock signals are compared to the phase and frequency of an incoming data stream and one or more of the clock signals are selected for data recovery.  
         [0004]     A number of existing digital CDR circuits use voltage controlled delay loops (VCDL) to generate a number of clocks having the same frequency and different phase for data sampling (i.e., oversampling). For example, published International Patent Application No. WO 97/14214, discloses a compensated delay locked loop timing vernier. The disclosed timing vernier produces a set of timing signals of similar frequency and evenly distributed phase. An input reference clock signal is passed through a succession of delay stages. A separate timing signal is produced at the output of each delay stage. The reference clock signal and the timing signal output of the last delay stage are compared by an analog phase lock controller. The analog phase lock controller controls the delay of all stages so that the timing signal output of the last stage is phase locked to the reference clock. Based on the results of the oversampled data, the internal clock is delayed so that it provides data sampling adjusted to the center of the “eye.” The phase of the VCDL is adjusted to keep up with phase deviations of the incoming data.  
         [0005]     While such voltage controlled delay loops effectively generate the sampling clocks and control the delay stages to maintain alignment of the reference clock signal and the last timing signal, they suffer from a number of limitations, which if overcome, could further improve the utility of such voltage controlled delay loops. For example, the analog implementation of the phase lock controller is complex and generally cannot be easily ported from one technology to another. In addition, digital-to-analog conversion is required to convert lo the digital phase adjustment control into analog signal control. A need therefore exists for voltage controlled delay loops with digital phase control.  
       SUMMARY OF THE INVENTION  
       [0006]     A voltage controlled delay loop and method are disclosed for clock and data recovery applications. The voltage controlled delay loop generates clock signals having the same frequency and different phases. The voltage controlled delay loop comprises at least one delay element to generate at least two phases of a reference clock; a central interpolator for interpolating at least two phases of the reference clock to generate an interpolated signal; and an input that injects the interpolated signal into a delay stage. The central interpolator provides a fine phase control. In addition, a coarse phase control can optionally be achieved by selectively injecting the interpolated signal into a given delay stage.  
         [0007]     According to another aspect of the invention, coarse and fine phase control is provided in a voltage controlled delay loop using a number of interpolators. The voltage controlled delay loop comprises a plurality of delay elements; an input that selectively injects a reference clock into any one of the plurality of delay elements to provide a coarse phase control; and a plurality of interpolators associated with the delay elements that interpolate at least two adjacent phases to generate the desired clock signal.  
         [0008]     A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  illustrates an exemplary conventional clock recovery circuit;  
         [0010]      FIG. 2  illustrates the transitions in a data stream;  
         [0011]      FIG. 3  illustrates a VCDL having coarse phase control;  
         [0012]      FIG. 4  illustrates a VCDL having the coarse phase control provided by the injection point control of  FIG. 3  and fine phase control provided by an interpolator associated with each delay stage; and  
         [0013]      FIG. 5  illustrates a VCDL incorporating features of the present invention with coarse phase control provided by the injection point control of  FIG. 3  and fine phase control provided by a central interpolator. 
     
    
     DETAILED DESCRIPTION  
       [0014]     The present invention provides voltage controlled delay loops with digital phase control. The present invention controls the phase offset from the reference clock to the data sampling clock by shifting the injection point of the reference clock into the voltage controlled delay loop.  
         [0015]      FIG. 1  illustrates an exemplary conventional clock recovery circuit  100 . As shown in  FIG. 1 , the clock recovery circuit  100  produces a clock signal with a predetermined number of phases, T 0 , S 0 , . . . T i , S i , discussed below in conjunction with  FIG. 2 . The exemplary clock recovery circuit  100  includes a reference clock signal (3 GHz, for example) generated by a phase locked loop (PLL)  110  and applied to the input of a voltage controlled delay line  120 . As shown in  FIG. 1 , the voltage controlled delay loop  120  interacts with two control loops  150 ,  160 . The first phase control loop  150  is comprised of a VCDL phase detector  130 , a digital filter  140  and a current steering DAC  145 . Generally, the first control loop  150  adjusts the delays of the voltage controlled delay loop  120 . The reference signal and the output of the VCDL  120  are applied to the VCDL phase detector  130  which provides phase detection by producing an output representative of the phase difference that is applied to a filter  140  whose digital output is converted to an analog current by the DAC  145  to control the delay in the stages of the voltage controlled delay loop  120 .  
         [0016]     The second data control loop  160  is comprised of a preamplifier  165 , a data sampling block  170 , a data decimator  175 , a parallel data and clock output block  180  and a second order proportional and integral (PI) filter  190 . The serial data is received and amplified by the preamplifier  165  and applied to the data sampling block  170 . The data sampling block  170  samples the data using the plurality of phases, T 0 , S 0 , . . . T i , S i . The data samples are then applied to the optional data decimator  175  that drops the data rate, for example, by a factor of two. In addition, the data sampling block  170  provides a recovered bit clock output that is applied to the data decimator  175 , parallel data and clock output block  180  and second order PI filter  190 . The parallel data and clock output block  180  outputs the sampled serial data and recovered lower frequency clock as parallel data (usually 16 or 20 bit wide) and clock. The second order PI filter  190  interprets the transition and sample information associated with the, T 0 , S 0 , . . . T i , S i  samples to generate phase control information for the VCDL  120 . Generally, the phase control information ensures that the transitions clocks are maintained close to the transition points in the serial data (see  FIG. 2 ).  
         [0017]      FIG. 2  illustrates the transitions in a data stream  200 . As shown in  FIG. 2 , the data is ideally sampled in the middle between two transition points. The phases T i , S i  generated by the VCDL  120  are adjusted to align with the transitions and sample points, respectively. Thus, the internal clock is delayed so that the data sampling is adjusted to the center of the “eye,” in a known manner.  
         [0018]      FIG. 3  illustrates a VCDL  300  having coarse phase control. In order to control the phase offset between the PLL frequency and data sampling (S i ) and transition sampling (T i ), the injection point of the PLL frequency into the VCDL  120  is shifted. As shown in  FIG. 3 , the exemplary VCDL  300  is generally comprised of a succession of  16  delay elements, for example,  310 - 1  through  310 - 16  interconnected in a loop. The exemplary VCDL  300  also includes  16  inputs  320 - 1  through  320 - 16  that are each connected to an associated delay line  310 - i . The correlation between the various phases T i , S i  generated by the VCDL  300  to the delay elements  310  is also shown in  FIG. 3 . As shown in  FIG. 3 , the injection point where the PLL signal is applied to the VCDL can be shifted in accordance with the present invention to any input  320 - i.    
         [0019]     The embodiment of  FIG. 3  moves the injection point by one full delay element, thereby producing significant quantization noise in the VCDL  300 . In the exemplary embodiment of  FIG. 3 , with four delay elements per unit interval (UI), the quantization noise would be ±¼ UI, which limits the jitter tolerance of the CDR to ½ UI. Thus, the movement of the injection point may not be fine enough to provide the necessary precision in the phase adjustment. In order to resolve this problem, a phase interpolator can be used, as discussed further below in conjunction with  FIGS. 4 and 5 . The interpolator provides a fractional delay between the output phases from the delay elements.  
         [0020]      FIG. 4  illustrates a VCDL  400  having the coarse phase control provided by the injection point control of  FIG. 3 , as well as a fine phase control provided by an interpolator  420 - i  associated with each delay stage  410 - i . The input PLL signal, for example, having a frequency of 3 GHz, is applied to an optional frequency divider  405  that reduces the frequency, for example, in half. As discussed above in conjunction with  FIG. 3 , the PLL frequency can optionally be injected in any delay stage.  
         [0021]     In the exemplary embodiment of  FIG. 4 , there are  16  delay stages  410 . The entire delay through the bank of delay stages is equal to four unit intervals (UIs). Thus, each set of four consecutive delay stages  410  corresponds to one unit interval. For example, the clock phases TE 1 , T 1 , TL 1  and S 1 , correspond to one unit interval, as shown by the table  440  in  FIG. 4 . As shown in the table  440 , the interpolator  420  associated with phase TE 1  can produce phases in the range of 0 through ¼ UI.  
         [0022]     As shown in  FIG. 4 , each interpolator, such as the interpolator  420 - i , is comprised of an interpolator  415 - i  and a flip flop  430 - i . The flip flop  430 - i  has an external control (not shown) and serves as a synchronizer for the associated interpolator  420 - i  to ensure that the clock edge has passed through before the interpolation setting is changed. The interpolator  415 - i  takes the output of two adjacent delay elements  410  and produces a phase in between the phases associated with each adjacent delay element  410 , in a known manner (for example, using summing of the contributions of the two input signals in various ratios).  
         [0023]     Providing a separate interpolator  410 - i  for each delay stage is costly in terms of power and area. In addition, the interpolators must be well matched to each other to prevent clock phases from wandering in respect to each other. If the multiple interpolators are not well-matched, the interpolated phase of one interpolator may change it&#39;s phase offset in respect to the adjacent interpolator phase, which is not desirable. Finally, the timing requirements associated with switching all of the interpolators to a new phase setting while clock signals are propagating down the delay line are difficult.  
         [0024]      FIG. 5  illustrates a VCDL  500  incorporating features of the present invention and having the coarse phase control provided by the injection point control of  FIG. 3 , as well as a fine phase control provided by a central interpolator  530 . Thus, the PLL signal that is injected into the VCDL  500  is first interpolated to provide fine phase control. Following the fine phase control, the injection point may optionally be adjusted to provide a coarse phase control, using the approach of  FIG. 3 .  
         [0025]     As shown in  FIG. 5 , the input PLL signal, for example, having a frequency of 1-3 GHz, is applied to an optional frequency divider  510  that reduces the frequency, for example, in half. As discussed above in conjunction with  FIG. 3 , the output of the central interpolator can optionally be injected in any delay stage  310 . The output of the frequency divider  510  is then applied to a delay stage  520  having one or more delay elements (e.g., each providing a ¼ UI delay). The delay stage  520  is connected to the central interpolator  530  such that the left and right inputs to the central interpolator  530  are separated by at least one delay element, as shown in  FIG. 5 .  
         [0026]     The exemplary central interpolator  530  provides a number, for example  8 , distinct phases (over ¼ UI range), between each coarse phase setting. A multiplexer  540  selects the desired phase. If the phase must be adjusted beyond the granularity provided by the central interpolator  530  (i.e., more than a ¼ UI), then a coarse phase adjustment is made by adjusting the injection point (providing a granularity of ¼ UI).  
         [0027]     The single central interpolator  530  thus effectively provides the same number of precise clock phases as the method of  FIG. 4  with significantly reduced power and area requirements. In addition, the single, central interpolator  530  eliminates the matching requirements and timing requirements between the various interpolators of  FIG. 4 .  
         [0028]     If, for example, the central interpolator  530  generates seven additional phases between delay stages, quantization noise is improved by a factor of 8 to ± 1/32 UI, and thus jitter tolerance of the VCDL is significantly improved.  
         [0029]     A plurality of identical die are typically formed in a repeated pattern on a surface of the wafer. Each die includes a device described herein, and may include other structures or circuits. The individual die are cut or diced from the wafer, then packaged as an integrated circuit. One skilled in the art would know how to dice wafers and package die to produce integrated circuits. Integrated circuits so manufactured are considered part of this invention.  
         [0030]     It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.