Abstract:
A system and method is presented for reducing skew between the positive and negative components of a differential signal in a high speed communications link. The communications link includes a signal generator producing and transmitting complementary positive and negative signals over separate transmission lines and a receiver receiving the complementary signals. The communication link further includes a skew compensation circuit having a skew detector, a controller, and separate delay and buffer elements for both the positive and negative component of the differential signal. The controller separately controls each of the delay or buffer elements in response to the detected skew between differential signal components.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on U.S. Provisional Patent Application Ser. No. 61/019,716, filed on Jan. 8, 2008, and entitled “DIFFERENTIAL SKEW-COMPENSATION CIRCUIT”, the contents of which are fully incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not applicable. 
       BACKGROUND 
       [0003]    1. Field of the Invention 
         [0004]    The invention relates generally to skew correcting circuits for differential signals and, in particular, to circuits for high-speed differential communication links that can measure and adaptively compensate for differential signal skew. 
         [0005]    2. Description of Related Art 
         [0006]    Today&#39;s high-speed communication systems often rely on differential signal protocols to enhance noise immunity, reduce transient currents, and increase effective signal amplitude. As depicted in  FIG. 1 , a conventional differential communication link  10  includes a transmit end  12  having a transmitter  14 , a receive end  16  having a receiver  18 , and a pair of transmission lines  20 A, B extending therebetween. Both the transmitter  14  and receiver  18  utilize differential signal generation and transmission protocols known in the art. 
         [0007]    An illustrated differential signal  19  is comprised of two complementary intra-pair signal components, referred to herein as the P-side signal  21 A and the N-side signal  21 B. The P-side signal  21 A is also known as the positive, true, or non-inverted signal, or component, of the differential signal while the N-side signal  21 B is also known as the negative, complement, or inverted signal, or component. Each signal  21 A, B includes an AC portion, or signal pulse,  22 A, B, having a fixed width. 
         [0008]    To effectively transmit and reconstruct the differential signal  19 , the signal polarity of the P-side and N-side signals  21 A, B must be mirrored copies of one another and the relative timing of the two sides must be identical, or in phase. As is known to those in the art, several design, material, manufacturing, and circuit anomalies can unpredictably and/or undesirably alter the relative intra-signal/inter-side phase relationship between the P-side and N-side signals  21 A, B as they propagate through the transmission lines  20 A, B. This typically results in a sub-optimal signal relationship between the P-side and N-side  26 A, B of the received differential signal  25 . As measured with a standard eye diagram (widely used for qualitatively analyzing digital transmission signals), these anomalies reduce the effective eye opening in both the voltage and timing margins at the receive end  16  of the high speed communication link  10 . This performance-degrading phenomenon is referred to as differential signal skew. 
         [0009]    Recent studies have demonstrated differential signal skew to be significant in printed circuit board (PCB) environments due to what is referred to as the weave effect. Substrates for printed circuit boards are often constructed with a glass weave to provide mechanical reinforcement. Because the dielectric constant of glass is typically higher, or at least different, than the surrounding resin, differential traces routed over such a glass weave structure typically exhibit a measurable amount of trace-to-trace variations in characteristic impedance and propagation delay, thus causing differential signal skew. 
         [0010]    To compensate for weave effect, a number of design methods for routing traces to reduce the impact on differential signal skew have been presented. Examples include methods taught by U.S. Pat. App. Pub. Nos. 2004/0262036—Printed Circuit Board Trace Routing Method, 2004/0181764—Conductor Trace Design to Reduce Common Mode Cross-Talk and Timing Skew, and 2007/0223205—Shifted Segment Layout for Differential Signal Traces to Mitigate Bundle Weave Effect. These methods are specific to the passive interconnect within the circuit board substrate and therefore are believed to be effective in reducing the amount of differential signal skew specifically attributable to laminate-related weave effects. 
         [0011]    Currently, no known computer-based circuit board design tools employ these design methods to prevent differential signal skew caused by the weave effect. Thus, these methods must be manually implemented by a knowledgeable and skilled board designer. Several of these design methods, such as non-orthogonal and disjoint routing techniques, are likely to require additional routing space, increasing the size, complexity, and cost of the circuit board. Even after these methods become automated and common in high speed communication circuit boards, differential signal skew is also caused by other mechanisms including, for example, imbalanced trace routing, vias, connectors, return paths, and active circuitry. Using these methods to minimize differential signal skew caused by the weave effect does not alleviate the skew-causing effects of these other mechanisms, and therefore are necessarily limited in application and value. 
         [0012]    Another known design method related to skew compensation of single-ended and differential signals include U.S. Pat. No. 6,335,647—Skew Adjusting Circuit, U.S. Pat. No. 6,937,681, Skew Correction Apparatus, and U.S. Pat. No. 6,374,361—Skew-Insensitive Low Voltage Differential Receiver. The design methods taught by these references include adjusting the delay of a data or clock signal relative to a distinctly different signal such as a reference clock or other data signals in parallel busses. Such methods improve the timing of sampling circuits at the receiving end of a differential signal link by adjusting the skew of the signal in its entirety. These methods do nothing to improve the integrity, enhance the value, or actually adjust the differential signal skew of the P and N sides within differential signals themselves. Accordingly, these references do not provide a method of adaptively compensating for differential signal skew. 
         [0013]    Still other design methods, believed to be suitable for their intended purposes, do not provide adequate correction of differential signal skew. For example one design method taught by U.S. Pat. App. Pub. No. 2006/0256880—Automatic Skew Correction for Differential Circuits monitors the P and N sides of a transmitted differential signal reflected back to the transmitter and correspondingly adjusting the relative timing of the transmitter&#39;s P and N outputs. Due to losses in PCB interconnect products (i.e., up to 30 dB for a one-way loss and up to 60 dB [1000:1 voltage reduction] for a round-trip loss), the reflected signal strength can be drastically reduced, thus severely impacting the effectiveness of the internal feedback loop. In low-loss environments where the reflected signal strength can be appreciable, this method assumes that the reflected signals are generated solely at the receiving end of the communication link. However, intermediate structures such as solder balls, connectors, and vias, present larger reflected signals back to the transmitter, thus further reducing the effectiveness of this method. 
         [0014]    Other methods disclosed in U.S. Pat. No. 6,909,980—Auto Skew Alignment of High-Speed Differential Eye Diagrams and U.S. Pat. App. Pub. No. 2004/0064765—Differential Detector Employing Analog-to-Digital Converter compensate for signal skew in the receiver. However, each of these methods sample the P and N sides independently using analog-to-digital (A/D) converters, thus eliminating the benefits of using differential signals. In addition, these methods compensate for differential signal skew by delaying internal digital samples, either by shifting the digital samples within the processor or by adjusting the time points where the signals are sampled. As such, these methods do nothing to enhance the actual differential signal at the input to the receiver, but simply improve the displayed waveforms. 
         [0015]    A yet further method for adjusting skew in a differential signal is taught by U.S. Pat. No. 6,353,340—Input and Output Circuit With Reduced Skew Between Differential Signals. In several disclosed embodiments, when the individual input P and N sides have opposite polarity, the output P and N sides are allowed to toggle. However, when the differential signal skew is sufficient enough that the input P and N sides have the same polarity, the P and N output sides are held at a steady state or high-impedance state until the trailing side toggles states. Another disclosed embodiment discloses the use of delay lines; however, the delay is again determined by the logic state of the individual P and N sides. In all the disclosed embodiments, the differential signal skew adjustment is determined by the logic state of the individual P and N sides of the input signal and not by the difference between the P and N sides. As such, the benefits of differential signal protocols are lost. Also, the differential signal skew compensation is adjusted instantaneously for each and every signal transition, which can severely impact jitter and duty cycle distortion; thus, degrading the resulting differential signal. 
         [0016]    A still further method taught by U.S. Pat. App. Pub. No. 2006/0244505—Intra-Pair Differential Skew Compensation Method and Apparatus for High-Speed Cable Data Transmission Systems, attempts to overcome the issues aforementioned associated with the method of U.S. Pat. No. 6,353,340. This method requires an initial training sequence wherein the differential signal skew compensation is adjusted once and, thereafter, the compensation is fixed. Such a method is only valuable to compensate for static causes of differential signal skew such as the PCB weave effect. However, this method cannot compensate for dynamic causes such as power droop or intersymbol interference. Also, using training patterns is not always feasible in communication links, and the compensation adjustments are dependent on the data pattern. Because any training pattern will have different characteristics than actual data streams, compensating based on a training pattern produces sub-optimal results. 
         [0017]    A method taught by U.S. Pat. No. 7,085,337—Adaptive Per-Pair Skew Compensation Method for Extended Reach Differential Transmission, continuously adjusts differential signal skew compensation at the receiver. However, as with a method above, the P and N sides are treated as two different entities, instead of as a single lumped differential signal. As described above, this negates the benefits of using differential signals. Also, this method uses a slicer (i.e., an A/D converter) to digitize a version of the incoming data signal, then reconverts the digital data back into an analog waveform to compare the original and reconstructed analog signals and use the difference between these two signals to control the delay block. To be effective, the reconstructed signal needs to be converted to a digital format and then back to the analog domain within a fraction of the signal&#39;s edge rate, which severely limits the maximum data rate with which the method can be used. In addition, the described “delay block” does not directly adjust the delay of the incoming analog signal, but instead performs a complex filtering function on the individual P and N sides of the differential signal, which again reduces the benefits of using differential signals. 
         [0018]    Therefore, to ensure reliable performance of high-speed differential communication links, what is needed is a circuit and method for compensating for differential-skew that directly considers the received differential signal (i.e., not the P and N sides individually), does not place additional burden on the PCB design/designer, compensates for static and dynamic changes in differential signal skew, and compensates skew caused by many sources. 
       SUMMARY 
       [0019]    The present invention provides an active circuit that adaptively compensates for differential signal skew at the receiving end of a high-speed communication link. 
         [0020]    In accordance with one aspect of the present invention, a high speed differential communication link includes a transmitter generating a differential signal having a positive and a complementary negative component. The positive and negative components of the differential signal are transmitted over separate sides of a differential pair of transmission lines. A skew compensation circuit is provided that measures the skew between the positive and negative components of the differential signal and adaptively compensates for the skew. A receiver is included to receive the differential signal. 
         [0021]    Additionally, the skew compensation circuit includes a skew detector that determines the skew between the positive and negative components of the differential signal. Each of the positive and negative components of the differential signal is transmitted through a respective delay or buffer element. A controller receives the determined skew from the skew detector as an input and sends an output signal to each of the plurality of delay or buffer elements in response to the differential signal skew input. 
         [0022]    Furthermore, the high speed communications link further includes a terminator configured to reduce signal reflections on the transmission lines. The skew compensation circuit may be located between the terminator and the receiver or between the transmission lines and the terminator. 
         [0023]    In accordance with another aspect of the present invention, a method of minimizing skew in a high speed communication link includes terminating a differential signal having complementary positive and negative components received from a pair of transmission lines to reduce signal reflections, directing the terminated differential signal to a skew compensation circuit calculating a relative skew between the positive and negative components of the differential signal, directing a signal representative of the calculated skew to a controller, and generating output signals for at least one of the positive and negative components of the differential signal to minimize the relative skew therebetween. 
         [0024]    Various other features of the present invention will be made apparent from the following detailed description and the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1 , already described, is a schematic representation of a conventional high-speed differential communications link; 
           [0026]      FIG. 2  is a schematic representation of a first embodiment of a high-speed differential communication link with a skew compensation circuit in accordance with the present invention; 
           [0027]      FIG. 3  is a schematic representation of a second embodiment of a differential communication link with the skew compensation circuit of  FIG. 2  in accordance with the present invention; and 
           [0028]      FIG. 4  is a schematic representation of a third embodiment of a differential communication link with another skew compensation circuit in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    A high-speed differential signal communication link  110  according to a first embodiment of the present invention is shown in  FIG. 2 . The communication link  110  includes a skew compensation circuit  130  disposed between a terminator  132  and a conventional differential signal receiver  118 . A differential signal  119  generated by a conventional transmitter  114  is broadcast over a pair of differential transmission lines  120 A, B in accordance with known systems and methods. The transmission lines  120 A, B are terminated at the terminator  132  disposed in the receive end  116  of the link  110  to reduce signal reflections. Terminator  132  is shown as a single resistive element in  FIG. 2 , but those skilled in the art will recognize that transmission-line terminators can take many forms. The use of the term “terminator” is used herein to represent a circuit applied to the receive end  116  of a communication link  110  to provide optimal impedance match for minimizing signal reflections and/or optimizing the effective signal amplitude. 
         [0030]    The terminated differential signal  125  is fed into a skew-compensation circuit  130  configured to detect and reduce skew between the P-side and N-side signal  126 A, B of the received differential signal  125  as explained below. The skew-compensation circuit  130  outputs skew-corrected P-side and N-side signals  129 A, B with little to no relative skew between them. The corrected signals  129 A, B are subsequently fed into the differential receiver  118  at the receive end  116  of the link  110 . 
         [0031]    As illustrated, the skew-compensation circuit  130  receives the P-side and N-side signals  126 A, B as inputs to respective delay elements  134 A, B. The delay elements  134 A, B are controlled by a controller  138  to adjust the relative skew between the two signals  126 A, B. The delay elements  134 A, B output the skew-corrected P-side and N-side signals  129 A, B which are subsequently received by both a skew detector  140  and the differential receiver  118  as inputs. When properly de-skewed by the skew-compensation circuit  130 , the P-side and N-side signals  129 A, B are phase aligned such that the receiver  118  can readily discern the intended differential signal  128 . 
         [0032]    The skew detector  140  monitors the skew-corrected P-side and N-side signals  129 A, B and quantifies the amount of relative skew between them. One method of quantifying the relative skew is by adding the AC components  150 A, B of the two signals  129 A, B together. When the two signals  129 A, B have no measurable skew between them, the sum of the AC components  150 A, B is zero. With this method of quantifying skew, the optimal output of the skew detector  140  is a zero-amplitude control signal. 
         [0033]    Another method for quantifying the relative skew between the P-side and N-sides  129 A, B is to subtract the respective AC components  150 A, B. When the difference between the signals  129 A, B is maximized, there is no skew in the differential signal  128 . With this method, the optimal output of the skew detector  140  is a signal with a relatively high AC voltage (or current) amplitude. Those skilled in the art will recognize that both the subtraction and addition methods are viable, and the selection of either method is dependent on the needs of the system application. 
         [0034]    The output of the skew detector  140  is fed into the aforementioned controller  138  via a signal line  142 . The controller  138  interprets the output signal from the skew detector  140 , filters the output to provide the desire frequency response and stability, generates appropriate control signals, and directs the control signals to the delay elements  134 A, B via control lines  144 A, B, respectively, thus forming a closed feedback control loop. 
         [0035]    In analog form, the controller  138  is a low-pass filter comprised of discrete inductors, capacitors, and resistors, which effectively time-averages the output of the skew detector  140  and provides a stable signal to delay elements  134 A, B. In digital form, the controller  138  consists of an analog-to-digital converter (ADC) and performs filtering using digital signal processing (DSP) algorithms that are well known in the art to generate the control signals to delay elements  134 A, B. Those skilled in the art will recognize that both analog and digital methods are viable, and the selection of either method is dependent on the needs of the system application. 
         [0036]    The skew compensation circuit  130  compensates for the differential signal skew caused by all of the sources upstream from the receiver  118 . The feedback provided by the skew detector  140  enables the skew compensation circuit  130  to dynamically adapt to changes in the differential signal skew. Although not illustrated, filter components may be utilized within the controller  138  to control the response time of the feedback loop, and to help ensure stability. 
         [0037]    Turning now to  FIG. 3 , a high-speed differential signal communication link  210  according to a second embodiment of the present invention is shown. The link  210  includes a skew compensation circuit  230  disposed between differential transmission lines  220 A, B and a terminator  232 . The configuration and operation of the skew compensation circuit  230  is similar to the circuit  130  of  FIG. 2 . The comparator skew detector  240 , controller  238 , and the delay elements  234 A, B all operate in the same manner. However, the skew compensation circuit  230  is disposed upstream from a terminator  232  such that the effective input differential-impedance match into the skew compensation circuit  230  is enhanced by ensuring the received P-side and N-side signals  229 A, B arrive simultaneously at the terminator, further improving the integrity of the differential signals  225 . 
         [0038]    Turning now to  FIG. 4 , a high-speed differential signal communication link  310  according to a third embodiment of the present invention is shown. Like the high speed communication link  110  of  FIG. 2 , a skew compensation circuit  330  is disposed between a terminator  332  and a differential signal receiver  318 . Unlike the links  110 ,  210  of  FIGS. 2 and 3 , no variable-delay elements are used in the skew compensation circuit  330  to compensate for skew in the differential signal. 
         [0039]    As illustrated, the differential transmission lines  320 A, B are terminated at the terminator  332  directed into the skew-compensation circuit  330 . The circuit  330  is configured to detect and reduce relative skew occurring between the P-side and N-side signals  329 A, B of the differential signal  328  that are subsequently fed into the differential receiver  318  at the receive end  316  of the link  310 . 
         [0040]    The skew detector  340  monitors the P-side and N-side signals  326 A, B of the received differential signal  325  and quantifies the differential signal skew between them in a manner such as previously described. The signals  326 A, B are delayed through fixed-delay elements  334 A, B, described below. The outputs of the fixed-delay elements  334 A, B are delayed P-side and N-side signals  350 A, B. 
         [0041]    The skew detector  340  outputs a signal to a controller  338  via a signal line  342 . The controller  338  produces and outputs corresponding control signals via signal lines  344 A, B to buffers  345 A, B. The delayed P-side and N-side signals  350 A, B and buffered P-side and N-side signals  351 A, B are fed into adders  336 A, B where the signals are added together to produce de-skewed P-side and N-side signals  329 A, B of the skew-corrected differential signal  328 . 
         [0042]    Still referring to  FIG. 4 , in one method for skew compensation, the buffers  345 A, B produce P-side and N-side compensation signals  351 A, B that are of appropriate shape, polarity, and amplitude that when added to the delayed P-side and N-side signals  329 A, B, the resulting differential signal  328  is presented to the receiver  318  with minimal, if any, relative skew. 
         [0043]    One method of producing the control signals  351 A, B to correct for skew between the received P-side and N-side signals  326 A, B involves the controller  338  producing a signal that is expressed mathematically as P+N where the P-side and N-side signals  326 A, B are added together. The controller  338  and buffers  345 A, B then fraction this summation back to the differential input, where the P-side signal  326 A has (P+N)/2 added to it, and the N-side signal  326 B has (P+N)/2 subtracted from it. By injecting these fractional summations back into the input, the effective signal at the input to the receiver  318  is [P+(P+N)/2]−[N−(P+N)/2]=1.5P+0.5P+0.5N−0.5N=2P. In an ideal differential signal  325 , i.e., the N-side signal  326 B has the opposite polarity but is otherwise identical to the P-side signal  326 A, i.e., N=−P. The ideal received differential signal  325  is P−N=P−(−P)=2P, which is the same effective signal presented to the receiver  318  in this embodiment of a high speed communication link  310 . 
         [0044]    In addition to providing a de-skewed differential signal  328  to the receiver  318 , the skew compensation circuit  330  illustrated in  FIG. 4  also compensates for the differential reflected signal at the terminator  332 , thus presenting the transmitter  314  with an effective ideal termination, which significantly aids in improving the resulting signal quality as measured with an eye diagram. 
         [0045]    One design constraint with the skew compensation circuit  330  is the delay time of the feedback control loop through controller  338  and buffers  345 A, B. To be effective, the buffered P-side and N-side compensation signals  351 A, B should be injected into the adders  336 A, B within the transition period of the signal (e.g., rising or falling edge) of the delayed signals  350 A, B. By inserting fixed-delay elements  334 A, B into the signal path, the signals  326 A, B can be delayed an amount corresponding to the delay through the controller  338  and buffers  345 A, B. As such, the outputs of buffers  345 A, B are added to the delayed signals  350 A, B. 
         [0046]    In practice, the buffers  345 A, B and adders  336 A,B can take many forms and can often be incorporated into controller  338 . The primary purpose of buffers  345 A, B is to isolate the signals  351 A, B from being injected into the controller  338 , and to produce signals that are of appropriate shape and amplitude. The purpose of the adders  336 A, B is to add the delayed P-side and N-side signals  350 A, B and the compensation signals  351 A, B without affecting the respective driving circuitry. 
         [0047]    According to one aspect of the present invention, to ensure stability and minimize jitter effects, the dynamic response of the compensation may be controlled with respect to both amplitude and response time. Furthermore, and according to another aspect of the present invention, in an effort to obtain optimum compensation, the differential signal skew is monitored at the receiving end of the high-speed communication link rather than at the transmitting end as is done in the prior art. According to yet another aspect of the present invention and to maintain and enhance the benefits of differential signal protocols, the final logic-level detection circuit (a.k.a., receiver, sampler, or slicer) considers the difference between the P and N sides of the differential signal rather than the two sides independently. 
         [0048]    Accordingly, a system and method are provided that compensate for both static and dynamic differential-skew to ensure stability through filtering of the feedback loop, ensure peak performance by monitoring the differential signal at the receiving end of the communication link rather than at the transmitting end, and compensate for differential termination. 
         [0049]    The present invention has been described in terms of the various embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention. Therefore, the invention should not be limited to any particular described embodiment.