Abstract:
A half-rate linear phase detector is particularly well-suited to clock data recovery in a serial data interface. The phase detector uses a quadrature clock to process different portions of the incoming data with different phases of the clock. The resulting component signals can be combined to provide the expected UP and DOWN phase detector output control signals. The phase detector output signals are balanced and of uniform width, minimizing oscillator control signal ripple in the clock data recovery circuit, while the linearity of the phase detector makes its output predictable.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to phase detection in clock data recovery applications. More particularly, this invention relates to a phase detector, for use in clock data recovery, that is predictable and stable. 
   It is almost axiomatic that digital systems are clocked. For a clock of period T, the clock signal is typically a series of square (or rectangular) pulses of durations T/2, separated by zero-amplitude intervals of durations T/2. Such a clock has a rate or frequency of 1/T. The clock is used to time data signals, with each data pulse having a duration T/2. However, there is no reason why consecutive data pulses need be separated by zero-amplitude intervals. Therefore, two or more (n) consecutive data pulses can be transmitted as a single continuous “high” signal of duration nT. In each clock period T, one data pulse can be transmitted. Thus, the data rate of the system is the same as that of the clock rate. In a double data rate system, data is sampled on both rising and falling clock edges, resulting in a data rate twice that of the clock rate, with each data pulse having a duration T/2. 
   Because any particular data pulse can be either high (“1”) or low (“0”), a train of unknown data pulses is commonly represented by two superposed waves, with the edges of the pulses are not purely vertical, so that the progression of pulses is distinctly visible. Each possible data position thus is shown as both high and low, signifying that either value is possible in a stream of actual data. Because the edges representing the pulse transitions in such a representation are not purely vertical, the intersecting inclined lines give each pulse position the appearance of an eye, and each pulse position is therefore referred to as a “data eye.” 
   When sampling data, it is best that the sampling occur as close as possible to the center of the data eye, as far as possible from the transitions, because sampling during a transition could provide a false reading of the data. This is relatively easy when the clock is sent along with the data. However, when the clock must be recovered from the data, clock recovery errors could make centering the sampling time in the data eye—“eye centering”—difficult or unreliable. Any such problems are compounded in a programmable logic device, where the circuit paths, as well as the clock recovery circuitry, differ from one user logic design to the next. 
   Clock recovery is commonly accomplished using a loop circuit—i.e., a phase-locked loop (PLL) or delay-locked loop (DLL)—in which a phase detector detects a phase variation between input and recovered signals, causing a charge pump to vary a control signal (i.e., voltage or current) of an oscillator (e.g., a voltage-controlled oscillator or current-controlled oscillator) to bring the recovered signal back into line with the input signal. Variation or ripple in the control signal may cause unacceptable jitter in the oscillator output, giving rise to clock recovery errors, thereby causing eye centering errors which in turn result in data read errors. 
   Known phase detectors contribute to jitter in different ways. For example, a full-rate linear phase detector, such as a Hogge phase detector, operates predictably but its performance degrades as the clock rate increases. A nonlinear phase detector such as a Bang Bang phase detector causes a large ripple on the control signal making the loop circuit output unpredictable. A half-rate linear phase detector is linear, therefore predictable, but produces a DOWN (or REFERENCE) signal having twice the pulse width of the UP (or ERROR) signal, resulting in a large ripple effect on the oscillator control signal. 
   It would be desirable to be able to provide a phase detection method and circuitry that minimizes jitter in a recovered clock application, thereby improving data reading. 
   SUMMARY OF THE INVENTION 
   The present invention minimizes jitter in a recovered clock application by providing a phase detector, for use in a clock recovery circuit, that is linear—and therefore predictable, that operates at half the clock rate—and whose performance therefore does not degrade as quickly at high speeds, and that produces UP and DOWN control pulses of equal width—thereby minimizing control signal ripple. This is achieved by using a quadrature clock to retime the data, and deriving different portions of the UP and DOWN control pulses from different quadrature phases of the retimed data. 
   Specifically, a quadrature clock preferably is derived from the recovered clock. In other words, four clock signals are derived from the recovered clock—the recovered clock itself, the recovered clock delayed 90°, the recovered clock delayed 180°, and the recovered clock delayed 270°. Each phase of the quadrature clock is used to retime the data and to derive a component of one of the UP and DOWN control signals from that retimed data. In one preferred embodiment, the control signal components are an UP signal component based on odd data transitions, an UP signal component based on even data transitions, a DOWN signal component based on odd data transitions, and a DOWN signal component based on even data transitions. The UP signal components preferably are combined to provide a single UP control signal, while the DOWN signal components preferably are combined to provide a single DOWN control signal. 
   Therefore, in accordance with the present invention, there is provided a method for detecting phase error in a loop circuit based on relative alignment between a full-rate data signal and a clock signal derived by said loop circuit. The method includes deriving a half-rate quadrature clock from the derived clock signal, comparing respective phases of the half-rate quadrature clock with respective portions of the full-rate data signal to derive respective partial phase error signals, and combining respective partial phase error signals to derive at least one signal representing the phase error. A phase detector that operates according to this method, as well as a clock data recovery circuit incorporating such a phase detector, and a programmable logic device incorporating such a clock data recovery circuit, are also provided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
       FIG. 1  is a set of timing diagrams showing clock and data signals as may be found in the present invention; 
       FIG. 2  is a simplified schematic diagram of a clock data recovery circuit with which the present invention may be used; 
       FIG. 3  is a schematic diagram of a preferred embodiment of a phase detector in accordance with the present invention; 
       FIG. 4  is a set of waveforms illustrating the operation of the phase detector of  FIG. 3 ; and 
       FIG. 5  is a schematic representation of a system including a programmable logic device incorporating the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As described above, the present invention provides a reliable linear phase detector that operates at half the data rate but still provides UP and DOWN control pulses of substantially equal width, by using a half-rate quadrature clock. Specifically, even though the recovered clock operates at half the data rate, because a quadrature clock is derived from the recovered clock, different phases of the clock can operate on different transitions in the data. Thus, while a half-rate clock would ordinarily miss every other transition in the full-rate data signal, the different phases of the quadrature clock are able to monitor, in the aggregate, all transitions. Each phase of the quadrature clock results in a partial UP or DOWN control signal representing those transitions monitored by that phase, but the partial signals can be combined into a single UP or DOWN control signal. 
   The invention will now be described with reference to  FIGS. 1–4 . 
     FIG. 1  shows as waveform A a clock having period T (i.e., a clock rate or frequency of 1/T). The clock signal preferably includes high intervals each of duration T/2, separated by low, or zero-amplitude, intervals also each of duration T/2 (although it is possible to imagine a clock with a different duty cycle and still with a period T). Waveform B shows a data pulse train clocked by waveform A. As discussed above, the data rate is twice the clock rate because there is no reason to separate data pulses from one another. Therefore, each T/2 slot can accommodate a data pulse. Because when the data pulse train is considered in the abstract (as opposed to particular data), it is not known whether any particular pulse will be high or low, waveform B includes the superposition of trace B 1  and trace B 2 . As can be seen, the appearance of the two potential pulses in each slot, particularly in view of inclined trace portions  10 ,  11 , is that of an eye, and is referred to as a “data eye” as mentioned above. 
   Waveform C is a sampling clock having the same rate as the data rate of waveform B. Each data pulse is sampled on a rising edge  12 . Therefore, ideally waveform C, which is the recovered data clock, is offset from waveform A so that rising edges  12  occur in the center of each data pulse slot. If a rising edge  12  were to occur to close to transitions  10 ,  11 , then any uncertainty in the timing of the transitions, as indicated by dashed lines  13 , may cause read errors—e.g., if the transition is late, the previous value of the datum in that slot, rather than its current value, might be read if the read clock is early, as indicated by dashed line  14 . The same may occur if the read clock is late, as indicated by dashed line  15 . Therefore, it is important to be able to adjust the read clock so that it falls in the center of each data eye. 
     FIG. 2  shows a clock data recovery (“CDR”) circuit  20  with which the present invention may be used. CDR circuit  20  preferably includes a loop circuit  21  (such as a phase-locked loop circuit, shown) and a decision circuit  22 , which may be as simple as a D-type flip-flop (shown). As seen, data is input at  23  to both phase detector  210  of loop circuit  21  and to decision circuit  22 , which is timed by the clock recovered from the data by loop circuit  21 . The present invention provides an improved phase detector  210 , which can help improve clock-data alignment. 
     FIG. 3  shows a preferred embodiment  30  of an improved phase detector according to the present invention, while  FIG. 4  shows timing diagrams illustrating the operation of phase detector  30 . 
   In phase detector  30 , a quadrature clock is generated or derived from the input (recovered) clock  31 . For example, a series of delay lines  32  can be provided, with the different phases being tapped at taps  321 ,  322 ,  323 ,  324 . As seen in  FIG. 4  (waveforms B–E), and as in  FIG. 1 , the clock rate is half the data rate. 
   The input data  33  preferably are input to two D-type flip-flops  34 ,  35 , clocked by the undelayed clock phase, CLK 0 , and the 180°-delayed clock phase, CLK 180 , respectively. Because the respective outputs DEVEN (even data) and DODD (odd data) change only if DATA changes during a rising edge of the respective clock phase, DEVEN and DODD are as shown by waveforms F and G. DEVEN and DODD are ANDed with their respective clock phases in respective AND-gates  340 ,  350  to produce, respectively, DEVEN — fullrate and DODD — fullrate (waveforms H and I), which together carry the full-rate data. DEVEN — fullrate and DODD — fullrate are combined in OR-gate  36  to provide retimed data RDATA (waveform M), which is used as described below. Each of DEVEN — fullrate and DODD — fullrate also is combined with the original DATA by respective “Clocked Exclusive-OR” circuit  370 ,  371 , in which the exclusive-OR function is clocked by the respective quadrature clock phase. The output of clocked exclusive-OR circuit  370  is UPX (waveform J), and the output of clocked exclusive-OR circuit  371  is UPY (waveform K). It can be seen that UPX goes high whenever there is a transition in DATA between an even-numbered time slot and an odd-numbered time slot, while UPY goes high whenever there is a transition in DATA between an odd-numbered time slot and an even-numbered time slot. The combined phase detector UP signal (waveform L) is the sum, or OR, of UPX and UPY as provided by OR-gate  38 . 
   The even and odd data signals DEVEN and DODD preferably are input to two D-type flip-flops  341 ,  351 , clocked by the 90°-delayed clock phase, CLK 90 , and the 270°-delayed clock phase, CLK 270 , respectively, resulting, respectively, in retimed even data RDEVEN (waveform N) and retimed odd data RDODD (waveform O). Each of RDEVEN and RDODD is combined with the retimed data RDATA by respective “Clocked Exclusive-OR” circuit  372 ,  373 , in which the exclusive-OR function is clocked by the respective quadrature clock phase. The output of clocked exclusive-OR circuit  372  is DNX (waveform P), and the output of clocked exclusive-OR circuit  373  is DNY (waveform Q). It can be seen that DNX goes high whenever there is a transition in RDATA between an even-numbered time slot and an odd-numbered time slot, while DNY goes high whenever there is a transition in RDATA between an odd-numbered time slot and an even-numbered time slot. The combined phase detector DN signal (waveform R) is the sum, or OR, of DNX and DNY as provided by OR-gate  39 . 
   A comparison of the UP and DN signals shows that they are balanced—each UP pulse is followed by a DN pulse—and the UP and DN pulses are of equal width. Thus, when phase detector  30  is used in clock data recovery circuit  20 , there is very little ripple in the oscillator control voltage. Moreover, because phase detector  30  is linear, its output is predictable. 
   A programmable logic device (“PLD”)  50  incorporating clock data recovery circuitry according to the present invention may be used in many kinds of electronic devices. One possible use is in a data processing system  900  shown in  FIG. 5 . Data processing system  900  may include one or more of the following components: a processor  901 ; memory  902 ; I/O circuitry  903 ; and peripheral devices  904 . These components are coupled together by a system bus  905  and are populated on a circuit board  906  which is contained in an end-user system  907 . 
   System  900  can be used in a wide variety of applications, such as computer networking, data networking, instrumentation, video processing, digital signal processing, or any other application where the advantage of using programmable or reprogrammable logic is desirable. PLD  50  can be used to perform a variety of different logic functions. For example, PLD  50  can be configured as a processor or controller that works in cooperation with processor  901 . PLD  50  may also be used as an arbiter for arbitrating access to a shared resources in system  900 . In yet another example, PLD  50  can be configured as an interface between processor  901  and one of the other components in system  900 . It should be noted that system  900  is only exemplary, and that the true scope and spirit of the invention should be indicated by the following claims. 
   Various technologies can be used to implement PLDs  50  as described above and incorporating this invention. 
   It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention, and the present invention is limited only by the claims that follow.