Abstract:
A skew correction system incorporated into a transmitter forwarding a differential signal on a differential lane monitors returning signal reflections when the receiving end of the differential lane is appropriately terminated. Based on an analysis of the reflections, the skew correction system adjusts the relative timing of complementary edges of the differential signal departing the transmitter so as to substantially eliminate skew at the receiving end of the differential lane.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims benefit of U.S. Provisional Application No. 60/680,877 filed May 13, 2005, the entire disclosure of which is hereby incorporated by reference herein for all purposes. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates in general to methods and circuits for sensing and correcting skew between the two signals in a differential lane. 
   2. Description of Related Art 
   In high-speed digital applications it is common to use differential inputs and outputs to interconnect integrated circuits at the next higher level of integration (e.g. circuit board or multi-chip module). The reason for this is that differential signaling offers relative immunity to noise and drift that would otherwise have negative impact on the signal integrity of the lane carrying the high-speed information. 
   It is, however, known that this relative immunity to noise and drift can be compromised by skew. The reason for this is that when the skew between the differential signals is sufficiently large, the receiving stage may switch as a result of the transition of (substantially) one line of the differential pair. In such a case, the drift and noise canceling features of differential signaling are nearly eliminated. 
   U.S. Pat. Nos. 6,812,777 and 6,963,237 issued Nov. 2, 2004 and Nov. 8, 2005, respectively, to Tamura et al describe a method and circuits for controlling the differential skew of an output circuit. This method is illustrated in his  FIGS. 19 and 20  , included here as  FIGS. 1  and  2 , by example. Tamura adjusts the duty cycle of the output signals to eliminate the differential skew. Tamura, however, assumes that the differential skew arises as a consequence of problems in the driver circuit or the output circuit that is the subject of the patent. Tamura&#39;s invention does not deal with skew introduced by interconnect between the output circuit he has de-skewed and the input circuit of another, remote, integrated circuit. 
   U.S. Pat. No. 6,686,779 issued Feb. 3, 2004 to Takefumi Yoshikawa describes a method for controlling the differential skew of an output circuit. Yoshikawa uses independently programmable pull down capability for the true and complement versions of the differential output, thereby providing an ability to de-skew the output circuit. Yoshikawa, however, assumes that the differential skew arises as a consequence of problems in the driver circuit or the output circuit that is the subject of the patent. Yoshikawa&#39;s invention does not deal with skew introduced by interconnect between the output circuit he has de-skewed and the input circuit of another, remote, integrated circuit. 
   U.S. Pat. No. 6,909,980 issued Jun. 21, 2005 to Chenjing Fernando describes a method for deciding how an article of test equipment, e.g. an oscilloscope, should adjust the timing of input differential signals to obtain optimal skew values in an eye diagram. Fernando uses independently programmable “paired independent skew circuits” for the true and complement versions of a differential input, thereby providing an ability to de-skew the signal circuit. Fernando, however, assumes that the differential skew arises as a consequence of problems in the interconnection to the test equipment and that is the subject of the patent. Fernando&#39;s invention does not deal with skew introduced by interconnect between a source and the input circuit of another, remote, integrated circuit that is not a part of the test equipment that is the subject of his invention. 
   In contrast to the prior art cited in the patents by Tamura et al, Yoshikawa, and Fernando, the invention described here is a method for sensing skew at the receiving end of a differential signaling lane and automatically eliminating it independently of whether it was caused by problems in the driver IC, the interconnect, or the receiver input impedance. 
   The circuit driving the transmitter differential signals out of a transmitter IC may introduce skew because of asymmetric driving capability or other defects such as asymmetric loading of the output or of the input of the driver. But for the most part, skew is introduced by differences in the length, or effective length, of interconnects. In the construction of media for interconnecting integrated circuits there are a variety of opportunities for inadvertently introducing skew. If a pair of differential lines have to be routed between a transmitter and a receiver, it can at times be very difficult to ensure that the two lines have the same effective length because of length differences or differences in bends and corners in the two transmission paths, because the lines may have to be run in different layers of interconnect or may have spatial variations in dielectric constant, and because of non-homogeneous transmission media like glass fiber based laminates. A secondary, though still significant, source of skew is the difference in the impedance and/or frequency response of the two paths in a differential lane. Such differences can arise because of the differences in parasitics loading the paths caused by proximity of components, vias, and co-planar grounds, among others. 
   SUMMARY OF THE INVENTION 
   A differential signal comprises a pair of complementary signals conveyed by a pair of lines of a differential lane, and when a differential lane forwards a differential signal from its transmitting end to its receiving end, edges of the complementary signals should preferably arrive concurrently at the receiving end. However, when a pair of lines have dissimilar path delays, the differential signal will be skewed at the receiving end in that edges of the complementary signals will not arrive concurrently at the receiving end. When a differential lane is terminated at its receiving end by a differential termination, any skew in the differential signal will produce signal reflections that affect the magnitudes, relative timing of edges, and the polarity of the complementary signals as viewed at the transmitting end in a manner indicating the nature of the skew at the receiving end. 
   The invention relates to a method or apparatus for reducing skew in the receiving end of a differential lane. In accordance with the invention, a skew correction system is incorporated into a transmitter supplying a differential signal as input to a differential lane. With a differential termination at the receiving end of the differential lane, the skew correction system monitors the differential signal at transmitting end to determine how, if at all, it is influenced by returning signal reflections, and based on the nature of the determined influence, adjusts the relative timing of complementary edges of the differential signal departing the transmitter so as to reduce skew at the receiving end of the differential lane. 
   The claims appended to this specification particularly point out and distinctly claim the subject matter of the invention. However those skilled in the art will best understand both the organization and method of operation of what the applicant(s) consider to be the best mode(s) of practicing the invention, together with further advantages and objects of the invention, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 and 2  are block and schematic diagrams illustrating a prior art method for controlling differential skew. 
       FIG. 3  is a block diagram illustrating the differential skew correction method of this invention. 
       FIG. 4  is a block diagram illustrating details of the skew sensor and skew adjuster of  FIG. 3 . 
       FIG. 5  is a timing diagram illustrating appearance of skewed differential signals at the driver of  FIG. 3 . 
       FIG. 6  is a timing diagram illustrating the appearance of the skewed differential signals at the receiver of  FIG. 3 . 
       FIG. 7  is a timing diagram illustrating the received signal. 
       FIG. 8  is a timing diagram illustrating the effect of a differential equalizer on the received signal. 
       FIG. 9  is a timing diagram illustrating the received signal. 
       FIG. 10  is a timing diagram illustrating the effect of two single ended equalizers on the received signal. 
       FIG. 11  is a timing diagram illustrating a de-skewed received signal. 
       FIG. 12  is a timing diagram illustrating the effect of a differential equalizer on the received signal. 
       FIG. 13  is a block diagram illustrating the response of a matched single ended transmission line. 
       FIG. 14  is a timing diagram illustrating the response of matched differential transmission lines. 
       FIG. 15  is a timing diagram illustrating the voltage on the lines resulting from skewed inputs. 
       FIG. 16  is a timing diagram illustrating the reflections on the lines after the signals have both arrived. 
       FIG. 17  is a timing diagram illustrating the reflections just before they arrive back at the driver. 
       FIG. 18  is a schematic diagram illustrating an ordinary differential driver stage. 
       FIG. 19  is a schematic diagram illustrating a differential driver stage for use with the skew corrector. 
       FIG. 20  is a block diagram illustrating details of a preferred skew sensor of  FIG. 19 . 
       FIG. 21  is a block diagram illustrating a circuit for calibrating of the skew adjuster. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  depicts a transmitter IC  44  for transmitting a differential signal to a receiver IC  60  via a differential lane formed by a pair of lines  20  and  21 . The receiver ends of the lines  20  and  21  are connected to a termination  26 , and a differential input circuit  62  of receiver IC  60 . Complementary edges of the differential signal on lines  20  and  21  ideally should arrive concurrently at receiver  60 , but due to differences in lines  20  or  21 , the complimentary edges can arrive at different times. When the timing of the differential signal arriving at receiver  60  exhibits such skew, termination  26  reflects the signal edges back toward transmitter  44 . 
   Transmitter IC  44  includes a skew correction system  46  in accordance with the invention that monitors the signal reflections returning to transmitter  44  to detect any skew in the differential signal input to receiver IC  60  and adjusts the relative timing of the complementary signal edges as they depart transmitter IC  44  to eliminate the detected reflections, thereby minimizing skew at the receiver end of the differential lane. 
   Skew correction system  46  includes a skew adjuster  52  receiving and adjustably delaying complementary edges of a differential signal from a pulse generator  16  to supply a differential signal to a pair of drivers  42  and  43  for buffering the signal onto lines  20  and  21  through a pair of reverse terminations  22  and  23 . A skew sensor  50  detects which of lines  20  and  21  conveys a reflection that increases the magnitude of the signal on that line thereby indicating that skew at the receiving end of the differential lane makes that line appear shorter than the other line. Skew sensor  50  therefore signals skew adjuster  52  to retard the input of that line and/or advance the input of the other line in order to eliminate the reflection. 
   Because there are limits to the detectability of narrow pulses in signals which are created by relatively small amounts of skew, skew adjuster  52  may be calibrated and the measurements of the limits of detectable skew on both sides of the zero skew point are noted. The noted measurements may then be averaged to yield the setting required for zero skew. Calibration establishes the relationship between the control signal latched in the counters and the variable delay in the respective channel. This calibration can be accomplished in many ways, but preferentially as shown in  FIG. 21  by selecting the feedback inputs of multiplexers  98  and  99 , causing the delay lines  62  and  63  to oscillate in a closed loop at a frequency equal to 1/(2*TD), where TD is the time delay of the delay line. In this manner, the time delay for each control code supplied on lines  86  and  87 , through multiplexers  82  and  83  respectively, on both delay lines can be easily determined by observing the output frequency from buffers  42  and  43 , respectively, for each such supplied code. 
     FIG. 4  is an example implementation of skew sensor  50  and skew adjuster  52 . Drivers  42  and  43  drive the inputs of lines  20  and  21  through back terminating resistors  22  and  23 , respectively. Buffers  2  and  3  are used to generate a static voltage equal to the high and low levels of the outputs from drivers  42  and  43 , respectively. The resistor networks connected to the outputs of 2 and 3 cause these buffers to have the same load impedance as  42  and  43 , but they are computed to generate a voltage at the midpoints of each of the two resistor pairs that is mid-way between the normal driven levels on lines  20  and  21  and the level that exists when there is a reflection on these lines as a result of skew. Schmidt comparators,  72  and  73 , sense any reflections that increase the voltage magnitude to threshold levels 1.25 times the magnitude of the transmitted voltage level at the inputs of lines  20  and  21 . The up/down counters are crafted to count only once for each pulse edge originally transmitted down lines  20  and  21 . The first such circuit of  72  or  73  to respond to a reflection will switch and cause the up/down counters to advance or retard their counts by one, changing the delay of the programmable delays in such a way as to reduce the reflection duration, thereby reducing the skew which caused the reflection. (Note that the up/down counters are preset to mid-range at initialization.) The sequence of operations is repeated with the emission of the next differential pulse signal from the pulse generator  16 . The sequence continues to repeat until the detectable limit of skew is reached. 
     FIG. 5  illustrates skewed differential signals as they emerge from transmitter  44  of  FIG. 3 . As the length of the differential paths  20  and  21  grow the rise times at the receiving side of the paths also grow, as illustrated in  FIG. 6 , because of high frequency skin effect and dielectric losses. Thereby the skew between the differential paths may become a much smaller and seemingly insignificant part of the signal&#39;s rise time. Under these conditions one might deduce that the skew was no longer a problem. However, it has become customary in many applications to use receiver equalization on differential signals in an attempt to restore the rise time and pulse width of the received signal and thereby improve the signal to noise ratio and the resulting bit error rate. In  FIG. 8 , when such equalization is applied, the rise time does recover to a limited extent. In fact, it is found that the functionality of a differential equalizer (one that works on the difference between the two signals in a differential lane) is degraded by skew, so that it can not equalize the lane as well as two single ended equalizers, as shown in  FIG. 10 . But in using two single ended equalizers, the equalizers restore high grade signals that have significant skew between them, which is undesirable, as described above. In  FIG. 11 , the received signal is de-skewed. In  FIG. 12 , the de-skewed signal is equalized by a differential equalizer, creating a high quality signal with no skew. This invention solves the skew problems described in this paragraph. 
     FIG. 13  depicts the response of a matched transmission line,  20 , to a unit step input supplied by pulse generator  18 . Since the line is matched at the input and output by source termination  22  and load termination  24  respectively, the response consists of a voltage wave  30 , of amplitude 0.5 that propagates from the input to the output, without any reflections. 
     FIG. 14  depicts the response of a matched differential transmission lane (consisting of two transmission lines,  20  and  21 , or a differential two conductor transmission line—e.g. parallel wire line) to differential unit step inputs supplied by generators  18  and  19 , which together constitute a differential pulse generator,  16 . Since the lines are matched at the input, by source terminations  22  and  23 , and output, by load terminations  24  and  25 , the response consists of voltage waves  30  and  31 , of amplitude +/−0.5, respectively, that propagate from the input to the output, without any reflections. Note here that the far end termination  26 , consists of two series resistors  24  and  25 , of value Zo, but since the lines and signals are balanced, there is a virtual ground at the junction of the two resistors, and the circuit behaves as if each of the resistors were connected from their respective lines to ground. 
     FIG. 15  depicts the response of a matched differential transmission lane, consisting of transmission lines  20  and  21 , to differential unit step inputs supplied by differential pulse generator  16 , which have become skewed. The signals are depicted just as the first signal  30 , reaches the far end of the line. 
     FIG. 16  depicts the response of a matched differential transmission lane to differential unit step inputs, which have somehow become skewed. The signals are depicted just as the second signal reaches the far end of the line. What has occurred is that the signal that arrived first saw a termination of 3Zo to ground because of the physical 2Zo termination resistance and the Zo of the other transmission line—which has zero volts on it at this time. This led to a reflection voltage coefficient of:
   Kr= ( Zt−Zo )/( Zt+Zo )=(3 Zo−Zo )/(3 Zo+Zo )=0.5 
   This reflection  26 , travels back towards the input of the line. Simultaneous with the reflection on transmission line  20 , which increases the voltage on line  20 , a signal is created on transmission line  21  by the termination current in line  20 . This reflection,  28 , is the same size as the reflected signal in line  20 , but it reduces the magnitude of the voltage on line  21 , rather than increasing it—as is the case with line  20 . At time T 2  plus deltaT, line  21  finally receives the input step, the virtual ground occurs at the junction of the two terminating resistors  24 , and further reflections from the receiving end of the line are terminated. 
     FIG. 17  depicts the progress of the reflected waves,  26  and  28 , towards the transmitting end of lines  20  and  21 , where they will be sensed by the skew sensor  50  in the transmitter  44 . 
   The logic circuits in the main signal path of  FIG. 4  are shown as single ended, for simplicity, however for best signal integrity they should all be implemented as differential.  FIG. 18  shows a common output driver for differential applications, including back terminations  22  and  23 . The preferred differential implementation  42 , is shown in  FIG. 19 . Note that the circuit of  FIG. 18  works well when the two inputs have identical timing. In the present instance, however, the inputs will generally have markedly different timing because they are being adjusted to compensate for skew. 
   The circuit of  FIG. 19  works well for this situation and has the additional advantage that the total power supply current is constant, irrespective of timing. 
   The system is shown using a unit step input. After the reflection and skew correction action appropriate to a step input, the counters are frozen or disabled, the step is reset to zero, and after a short settling time (to allow for reflections that occur during the reset) the counters are re-enabled and a new step can be issued. By repetitive action, the skew at the receiver is thereby eliminated. Those of skill in the art will appreciate that a variety of other means could be used to implement the sequencing of the skew correction, and other types of signals could be used—other than a step. For example, rather than sensing the magnitude of the reflections, skew sensor  50  could observe the polarities of the reflections or could observe an order of arrival of reflection edges and determine how to adjust skew adjuster  52  to minimize skew based on either of those observations. 
   Clearly, skew sensor  50  of  FIG. 4  must sense a combination of the sequence of receipt of the reflections and the magnitude of reflections—otherwise, the receipt of two reflections in rapid sequence would confuse it. An example of an alternative skew sensor is shown in  FIG. 20 . Here skew sensor  50  uses detected and filtered versions of the reflected signals to activate the up/down counter controls. Detector diode  86  is coupled to line  20  by coupling capacitor  78  and the resistor (100Z0) connected from capacitor  78  to node  99 , which is set at the common mode voltage of the logic signals on the lines  20  and  21 . The positive edge of the step emitted from driver  22  charges capacitor  92  to the equivalent of the output voltage level on line  20  just after the step transition. A subsequent positive reflection from the differential termination between lines  20  and  21  will charge capacitor  92  to a higher voltage than is created by network created by the two resistors marked 1.25Z0, 0.75Z0, and diode  96 . (Note that diode  96  compensates for the voltage drop created by diode  86  when it detects a reflected signal. Diode  97  has the same function respecting diode  87 .) This will cause comparator  72  to switch, forcing the U/D line at its output to the U mode, thereby increasing the delay in delay line  62  and reducing the skew at the receiver. Similar circuitry containing diode  87  capacitor  79 , and  93  and a resistor diode network, similar to the one associated with line  20 , monitors line  21 . However, on line  21  only negative reflections can activate the D/U line of delay line  63  because of the polarity of the diodes involved. This means that comparator  73  can only switch if line  21  carries the signal that arrives at the differential termination before that of line  20 . The switching action of this comparator will result in an increase in the delay setting of delay line  63  thereby reducing the skew at the receiver end of lines  20  and  21 . 
   The system described above can be used off-line, but it is also capable of being used on-line to correct for dynamic changes in skew caused by temperature drifts and power supply variation. One of many ways this can be done is through short interrupts of the normal flow of data, during which the skew correction is activated as described above.