Abstract:
A recovered clock signal is aligned (“eye centered”) with a data signal from which it is recovered by intentionally varying one of the factors or parameters that causes misalignment. For example, if a loop circuit (i.e., a phase-locked loop or a delay-locked loop) is used to recover the clock signal, charge pump current mismatch in the charge pump of the loop circuit is normally one factor in clock-data misalignment, and is also a parameter that can be manipulated. During a test mode, the current mismatch can be varied to obtain the best error rate, which signifies the best clock-data alignment. The test mode can be implemented using built-in self-test circuitry already on the device to transmit test data and then to receive it and analyze it for errors.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to the alignment of a recovered clock signal with a data signal, commonly referred to as “eye centering.” More particularly, this invention relates to eye centering on a programmable logic device. 
   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. 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, resulting in bit errors. However, when the clock must be recovered from the data, clock recovery errors, in combination with other errors such as process and temperature variations as well as clock/data delay variations, makes centering the sampling time in the data eye—“eye centering”—difficult or unreliable. 
   It would be desirable to be able to provide a method and circuitry for reliable eye centering in a recovered clock application. 
   SUMMARY OF THE INVENTION 
   The present invention achieves reliable eye centering by relying on the fact that at least one of the factors or parameters that give rise to clock-data alignment errors is adjustable. By deliberately adjusting that factor, the effects of the other factors can be cancelled out. In particular, in a clock data recovery application where the clock is recovered using a loop circuit, one of the factors that gives rise to alignment errors is charge pump current mismatch. Other factors include process and temperature variations as well as clock/data delay variations. Because charge pump current mismatch is not the only factor, charge pump current mismatch can be deliberately manipulated to control data alignment. In other types of applications, there may be other factors affecting data alignment that can be manipulated to control data alignment. 
   Thus, in a clock data recovery application, particularly in a programmable logic device, where a loop circuit (i.e., a phase-locked loop or a delay-locked loop) is used to recover the clock that is to be aligned with the data, the current sources in the charge pump of the loop circuit preferably are made variable, allowing the charge pump current mismatch to be manipulated. For example a variable current mirror as described below may be used. 
   The effectiveness of the present invention may be impacted by the type of phase detector used in the aforementioned loop circuit. The ability to change the clock-data alignment is more pronounced in the case of a linear phase detector (e.g., a Hogge detector). However, even nonlinear phase detectors (e.g., a bang—bang detector) typically have at least a small linear range, allowing the invention to operate. 
   In a preferred embodiment of a method according to the invention, a known data signal may be input to the clock data recovery circuit and the output signal monitored and compared to the known input signal. The charge pump current mismatch can be varied until the bit error rate between the input signal and the output signal is minimized. The charge pump current mismatch is then fixed at the value that minimized the bit error rate in the test signal. Preferably this comparison of input test data to output data is accomplished using built-in self-test circuitry on the device. 
   Therefore, in accordance with the present invention, there is provided a method of aligning a recovered clock with a data signal from which the clock was recovered, where misalignment of the data signal and the recovered clock results from at least one of a plurality of factors. The method includes varying the magnitude of one of the factors and analyzing the data signal as that one of said factors is varied. The varying and analyzing are repeated until error in the data signal is minimized. The magnitude of that one of the factors is then set to a value at which the error is minimized. 
   A clock data recovery circuit for use with the method, and a programmable logic device incorporating the 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 simplified schematic diagram of a charge pump for use in the circuit of  FIG. 2  in accordance with the present invention; 
       FIG. 4  is a schematic diagram of a variable current source for use in the charge pump of  FIG. 3 ; 
       FIG. 5  is a schematic diagram of circuitry in accordance with the present invention that can be used for automatic adjustment of clock-data alignment; and 
       FIG. 6  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 reliable eye centering—i.e., clock-data alignment—by manipulating one of the factors that causes clock-data misalignment. Although the factor is something that one ordinarily would want to eliminate, the fact that it is not the only factor means that manipulating it can cancel out the effect of the other factors. Preferably, the factors that cause clock-data misalignment in a clock data recovery environment are charge pump current mismatch in the loop circuit of the clock data recovery circuitry, process and temperature variations, and clock/data delay variations, and the factor that preferably is manipulated in accordance with the present invention is charge pump current mismatch. 
   The invention will now be described with reference to  FIGS. 1–5 . 
     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 that nevertheless has a period T). Waveform B shows a data pulse train clocked by waveform A. As discussed above, the data rate may be twice the clock rate. However, where the data rate equals the clock rate, each slot of duration T can accommodate a data pulse. Because when the data pulse train is considered in the abstract (as opposed to considering particular actual 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 bit errors—e.g., the previous value of the datum in that slot, rather than its current value, might be read if the recovered clock is early, as indicated by dashed line  14 . The same may occur if the recovered clock is late, as indicated by dashed line  15 . Therefore, it is important to be able to adjust the recovered 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 . As discussed above, various factors may affect the alignment of the recovered clock with the data. Among those factors is current mismatch between the two current sources  212  in charge pump  211  of loop circuit  21 . While normally such current mismatch is something to be eliminated, in accordance with the present invention it is used to advantage. This is done by replacing charge pump  211  with adjustable charge pump  311  of  FIG. 3 . 
   In charge pump  311 , current sources  212  are replaced with adjustable current sources  312 . In addition, unit gain buffer  313  and additional switches  314 ,  315  are provided to prevent charge sharing. Each adjustable current source  312  may be implemented as a variable current mirror  412  as shown in  FIG. 4 , where each leg B n  can be turned on to increase the current. Each leg B n  may have the same weight, so that the effect of adding an additional leg B n  is linear, but preferably legs B n  are binarily weighted, so that the effect of each additional leg B n  is to double the current. Normally, the UP and DOWN current sources may be of NMOS and PMOS architectures, respectively. Current mirror  412  is an NMOS structure. The corresponding PMOS structure for the DOWN current source will be clear to one of ordinary skill in the art. 
   As stated previously, charge pump current mismatch is only one of several factors that contribute to clock-data misalignment. Other factors include process and temperature variations, as well as clock/data delay variations. However, once a particular device has been fabricated, and it is in its operating environment, most of the other factors become constants. Therefore, varying the charge pump current mismatch can be used to adjust the clock-data alignment. An arrangement for making that adjustment automatically, preferably using self-test circuitry that preferably is already present on the device, is shown in  FIG. 5 . 
     FIG. 5  shows a high-speed serial interface  50  of a type that is becoming common in many electronic devices, including programmable logic devices, for implementing high-speed serial communications protocols. Interface  50  includes a transmitter stage  51  and a receiver stage  52 . In transmitter stage  51 , parallel data  500  from elsewhere on the device are selected by multiplexer  510  (whose purpose will become clear below) and are serialized by FIFO  511  and multiplexer  512  in accordance with a serial clock generated by phase-locked loop  513 . The data are then transmitted off the device by transmit buffer  514 , which is shown as a differential buffer because some of the high-speed signaling schemes are differential schemes, such as LVDS. A first-in, first-out (FIFO) buffer, also called an elastic buffer, may be used to compensate for phase variations across different clock domains. 
   In receiver stage  52 , serial data are received from off the device by (differential) receive buffer  524 . A clock is recovered from the data at  523 , and used to deserialize the data in demultiplexer  522  and FIFO  521 . The now-parallel data  501  are then selected by demultiplexer  520  (whose purpose will become clear below) and communicated to elsewhere on the device. 
   For self-test purposes, interface  50  preferably includes a pseudo-random bit sequence generator  515  which generates test data in a self-test mode. Those data are selected by multiplexer  510  and transmitted like real data  500  through a loop-back path  53  of known characteristics to receiver stage  52 , where it is deserialized and directed by demultiplexer  520  to a verifier circuit  525  that compares it to the data generated by generator  515  and reports the bit error rate in the received data. 
   In accordance with the present invention, in order to determine the optimum charge pump current mismatch (or “offset,” where, as here, the mismatch is intentional), the self-test circuitry can be operated while all values of current offset are tried, under control of a suitable controller  526 . The bit error rate results can be stored, for each offset current value (each B n  that is turned on), in look-up table  527 . Afterwards, controller  526  can select from look-up table  527  the offset current value associated with the lowest bit error rate, and can adjust the offset current to that value. This procedure preferably should be repeated each time the device is powered up, as environmental factors contributing to clock-data misalignment—e.g., temperature—could be different. 
   A programmable logic device (“PLD”)  60  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. 6 . 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  60  can be used to perform a variety of different logic functions. For example, PLD  60  can be configured as a processor or controller that works in cooperation with processor  901 . PLD  60  may also be used as an arbiter for arbitrating access to a shared resources in system  900 . In yet another example, PLD  60  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  60  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.