Patent Publication Number: US-2011075761-A1

Title: Differential signal transmission system and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-222396, filed on Sep. 28, 2009, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present invention relates to a transmission system and method of transmitting a differential signal. 
     BACKGROUND 
     A bit rate (speed) between boards is increased in an information processing apparatus such as a server system as a processing speed of a central processing unit (CPU) increases. A differential signal, having advantages such as noise immunity or low radiation of electro-magnetic interference (EMI), is used in the information processing apparatus that exchanges an electrical signal at a high-speed communication (Refer to Japanese Laid-open Patent Publication No. 10-303708). 
     Differential signals include two signals, i.e., a positive-side signal and a negative-side signal. Depending on variations in manufacturing accuracy of a printed circuit, and variations in material, a delay time difference takes place between two transmission lines. The delay time difference between the two transmission lines is not so problematic when the bit rate is low. The higher the bit rate, the more severe the waveform distortion of a transmission signal becomes. 
     In particular, if a high-speed transmission of 20 gigabits per second (Gb/s) or higher is performed, a time width of a signal waveform becomes short, and a delay time difference in excess of 1 unit interval (UI: one period of a bit clock) can take place over a travel distance of about tens of centimeters over a printed board. As a result, a margin of the time delay difference between the differential signals is reduced, and it is difficult to receive correctly a data signal. As a preventive step, a technique of detecting and then compensating for a skew of the differential signals on the receiver is used. 
     If the delay time difference is large between the transmission paths for transferring the differential signals in the above-described related art, it is difficult to maintain a differential state between the differential signals received by the receiver. It is thus difficult to detect the skew (phase difference) of the differential signals. The compensation for the skew of the differential signals is thus difficult, and an erratic operation may take place in a subsequent circuit of the receiver. 
     SUMMARY 
     According to an aspect of the invention, a transmission system for transmitting a first differential signal includes a transmitter, a transmission path, and a receiver. The transmitter transmits the first differential signal and a second differential signal. The transmission path transfers the first differential signal and the second differential signal transmitted by the transmitter. The receiver receives the first differential signal and the second differential signal having transferred through the transmission path. The transmitter includes a generator circuit and a switch. The generator circuit generates the second differential signal lower in baud rate than the first differential signal. The switch selects between the second differential signal generated by the generator circuit and the first differential signal to output the selected differential signal to the transmission path. The receiver includes a detector circuit and a corrector circuit. The detector circuit detects a skew of the second differential signal transmitted by the transmitter and transferred through the transmission path. The corrector circuit corrects a skew of the first differential signal transmitted by the transmitter and transferred through the transmission path, based on the skew detected by the detector circuit. 
     The object and advantages of the invention will be realized and attained by the elements, features, and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an exemplary transmission system of a first embodiment; 
         FIG. 2  illustrates an example of differential signal to be transmitted by a transmitter; 
         FIG. 3A  illustrates an example of the differential signal (Δt=0.3) received by a receiver; 
         FIG. 3B  illustrates an example of the differential signal (Δt=0.6) received by the receiver; 
         FIG. 3C  illustrates an example of the differential signal (Δt=0.8) received by the receiver; 
         FIG. 4A  illustrates an example of waveform of the differential signals without skew; 
         FIG. 4B  illustrates an example of the waveform of the differential signals with skew; 
         FIG. 4C  illustrates an example of the waveform of a skew detection signal; 
         FIG. 5  is a graph illustrating an exemplary relationship between a bit rate of the skew detection signal and UI; 
         FIG. 6  is a flowchart illustrating an exemplary operation of the transmitter according to the first embodiment; 
         FIG. 7  is a flowchart illustrating an exemplary operation of the receiver according to the first embodiment; 
         FIG. 8  is a block diagram illustrating an exemplary structure of a transmission system according to a second embodiment; 
         FIG. 9  is a flowchart illustrating an exemplary operation of a transmitter according to the second embodiment; 
         FIG. 10  is a flowchart illustrating an exemplary operation of a receiver according to the second embodiment; 
         FIG. 11  is a block diagram illustrating an exemplary structure of a transmission system according to a third embodiment; 
         FIG. 12  is a flowchart illustrating an exemplary operation of a transmitter according to the third embodiment; 
         FIG. 13  is a flowchart illustrating an exemplary operation of a receiver according to the third embodiment; 
         FIG. 14  is a sequence chart illustrating an exemplary operation of the transmission system according to the third embodiment; 
         FIG. 15  is a block diagram illustrating an exemplary structure of a transmission system according to a fourth embodiment; 
         FIG. 16  is a flowchart illustrating an exemplary operation of a transmitter according to the fourth embodiment; 
         FIG. 17  is a flowchart illustrating an exemplary operation of a receiver according to the fourth embodiment; 
         FIG. 18  is a sequence chart illustrating an exemplary operation of the transmission system according to the fourth embodiment; 
         FIG. 19  is a block diagram illustrating an exemplary structure of a transmission system according to a fifth embodiment; 
         FIG. 20  is a flowchart illustrating an exemplary operation of a transmitter according to the fifth embodiment; 
         FIG. 21  is a flowchart illustrating an exemplary operation of a receiver according to the fifth embodiment; and 
         FIG. 22  is a sequence chart illustrating an exemplary operation of the transmission system according to the fifth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Referring to the drawings, transmission systems and transmission methods according to the embodiments are described in detail below. In accordance with the transmission systems and the transmission methods discussed herein, a differential signal from a transmitter is switched from a data signal to a signal lower in a bit rate than the data signal. In accordance with the transmission systems and the transmission methods discussed herein, a differential state is maintained between the differential signals received by the receiver even if a large delay time difference takes place between transmission lines transferring the differential signals. In accordance with the transmission systems and the transmission methods discussed herein, a skew of the differential signal is accurately detected. 
     First Embodiment 
       FIG. 1  is a block diagram illustrating an exemplary transmission system of a first embodiment. Referring to  FIG. 1 , a transmission system  100  of the first embodiment includes a transmission line  10 , a transmitter  110 , and a receiver  120 . The transmission system  100  transmits a data signal (first differential signal) from the transmitter  110  to the receiver  120  via the transmission path  10 . The transmission system  100  is an information processing apparatus such as a server system. 
     The transmission line  10  is a transmission path that permits the differential signals (electrical signal) to transfer therethrough. The transmission line  10  includes a positive-side transmission path  11  for transferring a positive-side signal of the differential signals and a negative-side transmission path  12  for transferring a negative-side signal of the differential signals. The transmission path  10  is preferably constructed such that the positive-side transmission path  11  and the negative-side transmission path  12  are approximately equal to each other in path length. 
     The transmitter  110  includes a generator circuit  111 , a switch (SW)  112 , and a differential output circuit  113 . The generator circuit  111  generates a skew detection signal (second differential signal) as a differential signal lower in baud rate (bit rate) than the data signal to be transmitted from the transmitter  110  to the receiver  120 . The skew detection signal may be a signal having a particular pattern or an alternating signal (clock signal). The baud rate of the skew detection signal is low to the extent that 1 UI of the skew detection signal is larger than a permissible skew. The permissible skew is a skew quantity that can be detected by a skew detector circuit  123 . 
     The generator circuit  111  sets a speed B1 of the skew detection signal to be B0/n (n=2, 3, 4, . . . ) where B0 is a speed of the data signal (bit rate). The generator circuit  111  may set a speed B1 of the skew detection signal to be B0/2n (n=1, 2, 3, 4, . . . ). Since the speed (bit rate) of the skew detection signal may be set in accordance with the bit rate of the transmitter  110 , a structure of the generator circuit  111  generating the skew detection signal may be simplified. The generator circuit  111  outputs the generated skew detection signal to the switch  112 . 
     The switch  112  receives the skew detection signal from the generator circuit  111  and the data signal the transmitter  110  transmits to the receiver  120 . The switch  112  switches between the skew detection signal and the data signal and outputs the selected signal. The differential signal output by the switch  112  is differentially amplified by the differential output circuit  113  and then output to the transmission line  10 . The differential signal input to the transmission line  10  is transferred through the transmission line  10  to the receiver  120 . 
     The switching of the switch  112  is controlled by a control circuit of the transmitter  110 . Alternatively, the switching of the switch  112  may be controlled by a control circuit of the transmission system  100 . The control circuits of the transmitter  110  and the transmission system  100  may be constructed of a processing circuit such as a digital signal processor (DSP). 
     The receiver  120  includes a skew corrector circuit  121 , a differential circuit  122 , and a skew detector circuit  123 . The skew corrector circuit  121  sets a skew correction value based on the skew of which detector circuit  123  has notified the skew corrector circuit  121 . In response to the set skew correction value, the skew corrector circuit  121  corrects the skew of the differential signal transferred by the transmission line  10 . For example, the skew corrector circuit  121  varies a delay time difference between a positive-side signal and a negative-side signal of the differential signal transferred by the transmission line  10 , in accordance with the set skew correction value, and outputs the negative-side signal and the positive-side signal with the delay time difference thereof varied. 
     The differential circuit  122  performs a differential process on the differential signal output by the skew corrector circuit  121 . The skew detector circuit  123  acquires the differential signal output from the skew corrector circuit  121  to the differential circuit  122  and detects a skew of the acquired differential signal. The skew detector circuit  123  notifies the skew corrector circuit  121  of the detected skew. 
     With the above-described arrangement, the transmitter  110  switches the switch  112  to transmit the skew detection signal to the receiver  120 , and the skew detector circuit  123  in the receiver  120  detects the skew of the skew detection signal. In response to the skew detected by the skew detector circuit  123 , the skew corrector circuit  121  sets the skew correction value. The transmitter  110  further switches the switch  112 , thereby transmitting the data signal to the receiver  120 . The skew corrector circuit  121  in the receiver  120  corrects the skew of the data signal in accordance with the set skew correction value. 
       FIG. 2  illustrates an example of the differential signal transmitted by the transmitter  110 . In  FIG. 2 , the abscissa represents time, while the ordinate represents amplitude. An eye pattern  200  in  FIG. 2  represents the differential signal input to the transmission line  10  from the transmitter  110 . As illustrated in the eye pattern  200 , the differential signal input to the transmission line  10  from the transmitter  110  is substantially free from wave degradation. 
       FIG. 3A  illustrates an exemplary differential signal (Δt=0.3) received by the receiver  120 .  FIG. 3B  illustrates an exemplary differential signal (Δt=0.6) received by the receiver  120 .  FIG. 3C  illustrates an exemplary differential signal (Δt=0.8) received by the receiver  120 . As illustrated in  FIGS. 3A-3C , the abscissa represents time while the ordinate represents amplitude. 
     Eye patterns  301 - 303  illustrated in  FIGS. 3A-3C  represent the differential signals received by the receiver  120  when the delay time differences Δt of the differential signals transferred through the transmission line  10  are 0.3 UI, 0.6 UI, and 0.8 UI. The delay time difference Δt is |tp-tn| where tp represents a delay time of the positive-side signal along the positive-side transmission path  11  and to represents a delay time of the negative-side signal along the negative-side transmission path  12 . 
     The waveform of the differential signals received by the receiver  120  becomes distorted as illustrated in the eye patterns  301 - 303  as the delay time difference Δt is large. If the rate of the differential signal is high (for example, as high as 20 Gb/s), the time of 1 UI of the differential signal becomes short. The delay time difference Δt becomes relatively large with respect to 1 UI of the differential signal. 
     For example, if a bit rate of the differential signal illustrated in the eye pattern  301  is doubled (for example, from 20 Gb/s to 40 Gb/s), the delay time difference Δt increases from 0.3 UI to 0.6 UI. The differential signals illustrated in the eye pattern  302  substantially result. The higher the bit rate of the differential signal, the larger the delay time difference Δt relatively becomes. 
       FIG. 4A  illustrates an example of waveforms of the differential signals free from skewing. A positive-side signal  411  and a negative-side signal  412  in  FIG. 4A  are respectively a positive-side signal and a negative-side signal of the differential signals free from skewing. A pattern of the positive-side signal  411  is “10110010001111010” and a pattern of the negative-side signal  412  is “01001101110000101.” A duration  413  indicates 1 UI of the differential signal. 
     If no large skew is present between the differential signals, a differential state is established between the differential signals. For example, a differential state is established between the positive-side signal  411  and the negative-side signal  412  at all the bits thereof. The differential circuit  122  thus operates normally. Also, if no large skew is present between the differential signals, a differential state is largely maintained between the differential signals. The skew detector circuit  123  may detect a skew between the differential signals. 
       FIG. 4B  illustrates an example of waveforms of the differential signals suffering from skewing. A positive-side signal  421  and a negative-side signal  422  are those of the differential signals suffering from a large skew. More specifically, a skew  423  shows that the negative-side signal  422  is delayed from the positive-side signal  421  by 1.5 UI. In a portion of the waveform diagram labeled number  424 , both the positive-side signal  421  and the negative-side signal  422  are at a high level (level 1), and the differential circuit  122  fails to operate normally. The skew detector circuit  123  has difficulty detecting the skew between the differential signals. 
       FIG. 4C  illustrates an example of waveforms of skew detection signals. A positive-side signal  431  and a negative-side signal  432  in  FIG. 4C  are those of the differential signals having one-quarter the bit rate of the differential signals illustrated in  FIGS. 4A and 4B . The positive-side signal  431  and the negative-side signal  432  may have the same size of skew (skew  423 ) as that of the positive-side signal  421  and the negative-side signal  422  illustrated in  FIG. 4B . 
     The skew  423  is smaller than 1 UI occurring between the positive-side signal  431  and the negative-side signal  432 . As reference numbers  433 - 436  represent, a differential state is maintained between the positive-side signal  431  and the negative-side signal  432  at the bits thereof. The skew detector circuit  123  may thus detect the created skew. 
       FIG. 5  is a graph illustrating an exemplary relationship between a bit rate of the skew detection signal and UI. Referring to  FIG. 5 , the abscissa represents a skew quantity occurring between the differential signals along the transmission line  10 , and the ordinate represents a bit rate (baud rate) of the skew detection signal. The bit rate represented by the ordinate is a bit rate of the skew detection signal, generated by the generator circuit  111 , and rated on a scale of 1 as being the bit rate of the data signal. 
     The bit rate of the skew detection signal generated by the generator circuit  111  is determined based on the skew quantity between the differential signals caused along the transmission line  10 . A curve  511  illustrates a relationship established between the skew quantity of the differential signals caused along the transmission line  10  and a maximum bit rate of the skew detection signal if the skew quantity of the differential signals received by the receiver  120  is restricted to 0.5 UI or below. 
     A stepped curve  512  illustrates a relationship between the skew quantity and the maximum bit rate established when the bit rate of the skew detection signal is set to be B0/2n (n=1, 2, 3, 4, . . . ) in the curve  511  (B0 is the bit rate of the data signal). The larger the skew amount between the differential signals along the transmission line  10 , the lower the bit rate of the skew detection signal is preferably set. Even if a large skew is created between the differential signals, a created skew may be detected. 
       FIG. 6  is a flowchart illustrating an exemplary operation of the transmitter  110  of the first embodiment. When the transmitter  110  is powered on (or a reset signal is input to the transmitter  110 ) in step S 601 , the control circuit of the transmitter  110  switch-controls the switch  112  to start transmitting the skew detection signal (step S 602 ). The control circuit determines whether a specified constant time has elapsed since the start of the transmission of the skew detection signal in step S 602  (step S 603 ). If the specified constant time has not elapsed, the control circuit waits on standby until the specified constant time has elapsed (no branch from step S 603 ). 
     If the specified constant time has elapsed (yes branch from step S 603 ), the control circuit switch-controls the switch  112  to start transmitting the data signal (step S 604 ). Processing thus ends. For the specified constant time from the power-on or resetting, the skew detection signal is transmitted. After the elapse of the specified constant time, the data signal is transmitted. 
       FIG. 7  is a flowchart of an exemplary operation of the receiver  120  of the first embodiment. The receiver  120  is powered on (or a reset signal is input to the receiver  120 ) in step S 701 . The skew detector circuit  123  in the receiver  120  detects a skew of the skew detection signal transmitted by the transmitter  110  (step S 702 ). 
     The skew corrector circuit  121  sets a skew correction value in response to the skew detected in step S 702  (step S 703 ). A series of process steps thus ends. The skew of the skew detection signal is thus detected, and the skew of the data signal from the transmitter  110  is corrected in response to the detected skew. 
     Referring again to  FIG. 1 , the transmission system  100  of the first embodiment switches the signal from the transmitter  110  from data signal to the skew detection signal lower in bit rate than the data signal. The differential state is maintained in the signals received by the receiver  120  even if a delay time difference Δt is created between the positive-side transmission path  11  and the negative-side transmission path  12  of the transmission line  10 . The skew is accurately detected. The skew of the data signal is corrected by the skew correction value set based on the detected skew. The skew of the data signal is thus accurately compensated for. 
     Even if the data signal is high in bit rate, the skew of the data signal is accurately compensated for. For example, the data signal may be as high as 20 Gb/s or 40 Gb/s, and the delay time difference Δt may be 1 UI or larger between the positive-side transmission path  11  and the negative-side transmission path  12  of the transmission line  10 . Even under this condition, the skew of the data signal can be accurately compensated for. For example, high-rate differential signals can be transmitted in a backplane transfer within a server system even if the transmission lines of the differential signals fail to be accurately the same length over a backplane. 
     For the specified constant time from the power-on or resetting, the skew detection signal is transmitted. After the elapse of the specified constant time, the data signal is transmitted. Before the transmission of the data signal, the skew is detected to set the skew correction value. The skew of the data signal is accurately compensated for from the start of the transmission of the data signal. 
     Second Embodiment 
       FIG. 8  is a block diagram illustrating an exemplary structure of a transmission system  100  of a second embodiment. Referring to  FIG. 8 , elements that may be substantially identical to those illustrated in  FIG. 1  are designated with the same reference numerals, and the discussion thereof is omitted here. As illustrated in  FIG. 8 , the receiver  120  in the transmission system  100  includes a signal ditector circuit  821  in addition to the arrangement illustrated in  FIG. 1 . 
     The signal sensor circuit  821  senses the skew detection signal output by the transmitter  110 . More specifically, the signal sensor circuit  821  acquires the differential signal output from the skew corrector circuit  121  to the differential circuit  122 , and determines whether the acquired differential signal is a skew detection signal. If the acquired differential signal is a skew detection signal, the signal sensor circuit  821  outputs a sense signal to the skew detector circuit  123 . 
     The signal sensor circuit  821  then measures a bit rate of the acquired differential signal, and determines whether the measured bit rate is lower than a specified threshold value. The specified threshold value is equal to or lower than the bit rate of the data signal and higher than the bit rate of the skew detection signal. If the measured bit rate is equal to or higher than the specified threshold value, the signal sensor circuit  821  determines that the differential signal is not a skew detection signal. If the measured bit rate is lower than the specified threshold value, the signal sensor circuit  821  determines that the differential signal is a skew detection signal. 
     Alternatively, the signal sensor circuit  821  may measure the bit rate of the acquired differential signal, and determine whether an amount of change in the measured bit rate is equal to or higher than a specified threshold value. If the amount of change in the measured bit rate is lower than the specified threshold value, the signal sensor circuit  821  determines that the differential signal is not a skew detection signal. If the amount of change in the measured bit rate is equal to or higher than the specified threshold value, the signal sensor circuit  821  determines that the differential signal is a skew detection signal. 
     Alternatively, the signal sensor circuit  821  may sense a pattern of the acquired differential signal (such as an alternating pattern), and determine whether the detected pattern is a specified pattern. The specified pattern is a pattern of the skew detection signal generated by the generator circuit  111 . If the detected pattern is not the specified pattern, the signal sensor circuit  821  determines that the differential signal is not a skew detection signal. If the detected pattern is the specified pattern, the signal sensor circuit  821  determines that the differential signal is a skew detection signal. 
     The skew detector circuit  123  does not detect the skew of the differential signal until the sense signal is output from the signal sensor circuit  821 . Δt the moment (or after) the signal sensor circuit  821  outputs the sense signal, the skew detector circuit  123  detects the skew of the differential signal. Alternatively, even if the skew detector circuit  123  has detected a skew of the differential signal, the skew detector circuit  123  may not notify the skew corrector circuit  121  of the detected skew until the sense signal is output. After the sense signal is output, the skew detector circuit  123  may notify the skew corrector circuit  121  of the detected skew. 
       FIG. 9  is a flowchart illustrating an exemplary operation of the transmitter  110  of the second embodiment. The control circuit of the transmitter  110  switch-controls the switch  112  to start transmitting the skew detection signal (step S 901 ). Step S 901  may be performed at the timing of power-on or resetting of the transmitter  110 , or at the timing at which a user enters a command. Steps S 902  and S 903  may be respectively substantially identical to steps S 603  and S 604  illustrated in  FIG. 6 , and the discussion thereof is omitted herein. With this arrangement, the skew detection signal is transmitted for a specified constant time from the specified timing, and the data signal is then transmitted after an elapse of the specified constant time. 
       FIG. 10  is a flowchart illustrating an exemplary operation of the receiver  120  of the second embodiment. The signal sensor circuit  821  in the receiver  120  measures the bit rate of the signal received from the transmitter  110  (step S 1001 ). In response to the bit rate measured in step S 1001 , the signal sensor circuit  821  determines whether the signal received from the transmitter  110  is a skew detection signal (step S 1002 ). 
     If it is determined in step S 1002  that the received signal is not a skew detection signal (no branch from step S 1002 ), processing returns to step S 1001 . If the received signal is a skew detection signal (yes branch from step S 1002 ), the skew detector circuit  123  detects a skew of the skew detection signal (step S 1003 ). 
     The skew corrector circuit  121  sets the skew correction value in response to the skew detected in step S 1003  (step S 1004 ). Step S 1004  completes a series of process steps. The skew detection signal transmitted from the transmitter  110  is thus detected. When the skew detection signal is sensed, the skew of the skew detection signal may also be detected. 
     In the transmission system  100  of the second embodiment as illustrated in  FIG. 8 , the receiver  120  senses the skew detection signal transmitted from the transmitter  110 , and detects the skew of the skew detection signal after sensing the skew detection signal. The transmitter  110  may transmit the skew detection signal at any time, and the receiver  120  may detect the skew of the skew detection signal. 
     The skew of the skew detection signal may not be detect if the skew detection signal is not sensed. This arrangement prevents or at least inhibits the skew detector circuit  123  from detecting erratically the skew of the data signal, and prevents or at least reduces the skew corrector circuit  121  from malfunction. 
     Third Embodiment 
       FIG. 11  is a block diagram illustrating an exemplary structure of a transmission system  100  of a third embodiment. Referring to  FIG. 11 , elements that may be substantially identical to those illustrated in  FIG. 1  are designated with the same reference numerals and the discussion thereof is omitted here. Referring to  FIG. 11 , the transmission system  100  of the third embodiment includes a control circuit  1110  in addition to the structure of  FIG. 1 . The control circuit  1110  may be a processing circuit such as a digital signal processor (DSP). The control circuit  1110  may output concurrently a correction command to the transmitter  110  and the receiver  120  at the time of power-on or resetting of the transmitter  110 , or at the time at which a user enters a command. 
     If no correction command is output from the control circuit  1110 , the switch  112  in the transmitter  110  outputs the data signal. If a correction command is output from the control circuit  1110 , the switch  112  in the transmitter  110  outputs the skew detection signal. If no correction command is output from the control circuit  1110 , the skew detector circuit  123  in the receiver  120  detects no skew from the differential signal. If a correction command is output from the control circuit  1110 , the skew detector circuit  123  in the receiver  120  detects a skew from the differential signal. 
       FIG. 12  is a flowchart illustrating an exemplary operation of the transmitter  110  of the third embodiment. The control circuit of the transmitter  110  determines during the transmission of the data signal whether a correction command has been received from the control circuit  1110  (step S 1201 ). If no correction command has been received, the control circuit of the transmitter  110  waits on standby until a correction command has been received (no branch from step S 1201 ). 
     If it is determined in step S 1201  that a correction command has been received (yes branch from step S 1201 ), processing proceeds to step S 1202 . Steps S 1202 -S 1204  illustrated in  FIG. 12  may be respectively substantially identical to steps S 602 -S 604  illustrated in  FIG. 6 , and the discussion thereof is omitted here. If a correction command is output from the control circuit  1110 , the skew detection signal is thus output. 
       FIG. 13  is a flowchart illustrating an exemplary operation of the transmitter  110  of the third embodiment. The receiver  120  determines during the transmission of the data signal whether a correction signal has been received from the control circuit  1110  (step S 1301 ). If no correction command has been received, the receiver  120  waits on standby until a correction command has been received (no branch from step S 1301 ). Upon receiving a correction command (yes branch from step S 1301 ), the receiver  120  proceeds to step S 1302 . 
     Steps S 1302 -S 1303  illustrated in  FIG. 13  may be respectively substantially identical to steps S 702 -S 703  illustrated in  FIG. 7 , and the discussion thereof is omitted here. With this arrangement, a skew is detected from the skew detection signal if the correction command is output from the control circuit  1110 . 
       FIG. 14  is a sequence chart of an exemplary operation of the transmission system  100  of the third embodiment. The control circuit  1110  outputs a correction command to each of the transmitter  110  and the receiver  120  (step S 1401 ). The transmitter  110  transmits the skew detection signal (step S 1402 ). The receiver  120  detects the skew of the skew detection signal transmission-started in step S 1402  (step S 1403 ). 
     The receiver  120  sets the skew correction value based on the skew detected in step S 1403  (step S 1404 ). The transmitter  110  transmits the data signal (step S 1405 ). A series of process steps are thus complete. 
     Referring again to  FIG. 11 , in the transmission system  100  of the third embodiment, the transmitter  110  outputs the skew detection signal in response to the correction command output from the control circuit  1110 , and the receiver  120  detects the skew of the skew detection signal. The control circuit  1110  may transmit the correction command at any time, and the receiver  120  may detect the skew of the skew detection signal. 
     The skew of the skew detection signal may not be detected if the correction command is not output. This arrangement prevents or at least reduces the skew detector circuit  123  from detecting erratically the skew of the data signal, and prevents or at least inhibits the skew corrector circuit  121  from malfunction. 
     Fourth Embodiment 
       FIG. 15  is a block diagram illustrating an exemplary structure of a transmission system  100  of a fourth embodiment. Referring to  FIG. 15 , elements that may be substantially identical to those illustrated in  FIG. 11  are designated with the same reference numerals and the discussion thereof is omitted here. Referring to  FIG. 15 , the skew corrector circuit  121  of the transmission system  100  of the fourth embodiment sets the skew correction value in response to the skew of which the skew detector circuit  123  has notified the skew corrector circuit  121 , and outputs a correction complete notification to the control circuit  1110 . 
     The control circuit  1110  outputs to the transmitter  110  the correction complete notification output by the receiver  120 . If the control circuit  1110  outputs the correction complete notification, the switch  112  in the transmitter  110  outputs the data signal. Alternatively, the skew corrector circuit  121  may output directly the correction complete notification to the transmitter  110  rather than via the control circuit  1110 . 
       FIG. 16  is a flowchart illustrating an exemplary operation of the transmitter  110  of the fourth embodiment. Steps S 1601  and S 1602  illustrated in  FIG. 16  may be respectively substantially identical to steps S 1201  and S 1202  illustrated in  FIG. 12 , and the discussion thereof is omitted here. When the transmission of the skew detection signal starts in step S 1602 , the control circuit of the transmitter  110  determines whether the correction complete notification has been received from the receiver  120  (step S 1603 ). If no correction complete notification has been received, the control circuit of the transmitter  110  waits on standby until a correction complete notification has been received (no branch from step S 1603 ). 
     If it is determined in step S 1603  that a correction complete notification has been received (yes branch from step S 1603 ), the control circuit of the transmitter  110  switch-controls the switch  112  to start transmitting the data signal (step S 1604 ). A series of process steps are thus completed. The data signal is output when the correction complete notification is output from the receiver  120 . 
       FIG. 17  is a flowchart illustrating an exemplary operation of the receiver  120  of the fourth embodiment. Steps S 1701 -S 1703  illustrated in  FIG. 17  may be respectively substantially identical to steps S 1301 -S 1303  illustrated in FIG.  13 , and the discussion thereof is omitted here. If the skew correction value is set in step S 1703 , the skew corrector circuit  121  outputs the correction complete notification to the transmitter  110  via the control circuit  1110  (step S 1704 ). A series of process steps are thus complete. The correction complete notification is output to the transmitter  110  if the skew corrector circuit  121  sets the skew correction value. 
       FIG. 18  is a sequence chart of an exemplary operation of the transmission system  100  of the fourth embodiment. Steps S 1801 -S 1804  illustrated in  FIG. 18  may be respectively substantially identical to steps S 1401 -S 1404  as illustrated in  FIG. 14 , and the discussion thereof is omitted here. If the skew correction value is set in step S 1804 , the receiver  120  transmits the correction complete notification to the transmitter  110  (step S 1805 ). The transmitter  110  then transmits the data signal (step S 1806 ) and a series of process steps is thus complete. 
     Referring again to  FIG. 15 , in the transmission system  100  of the fourth embodiment, the correction complete notification is output to the transmitter  110  if the skew correction value is set by the skew corrector circuit  121 . The transmitter  110  then outputs the data signal. The transmission of the data signal starts when the receiver  120  sets the skew correction value. The skew of the data signal is accurately compensated for. 
     If the receiver  120  sets the skew correction value, the transmitter  110  may start transmitting the data signal without waiting until the elapse of the specified constant time. The period of time throughout which the skew detection signal is transmitted is reduced in this way, and the transmission efficiency of the data signal is increased. 
     Fifth Embodiment 
       FIG. 19  is a block diagram of a transmission system  100  of a fifth embodiment. Referring to  FIG. 19 , elements that may be substantially identical to those illustrated in  FIG. 15  are designated with the same reference numerals and the discussion thereof is omitted here. As illustrated in  FIG. 19 , the skew detector circuit  123  in the transmission system  100  of the fifth embodiment outputs a rate reduction command to the control circuit  1110  if the skew of the differential signal has not been detected. 
     The control circuit  1110  outputs to the transmitter  110  the rate reduction command output by the receiver  120 . If the control circuit  1110  outputs the rate reduction command, the generator circuit  111  in the transmitter  110  reduces the bit rate of the skew detection signal generated thereby. It is noted that the skew detector circuit  123  can directly output the rate reduction command to the transmitter  110  rather than via the control circuit  1110 . 
       FIG. 20  is a flowchart illustrating an exemplary operation of the transmitter  110  of the fifth embodiment. Steps S 2001 -S 2003  illustrated in  FIG. 20  may be respectively substantially identical to steps S 1601 -S 1603  illustrated in  FIG. 16 , and the discussion thereof is omitted here. If no correction complete notification has been received in step S 2003  (no branch from step S 2003 ), the control circuit of the transmitter  110  determines whether a rate reduction command has been received from the receiver  120  (step S 2004 ). 
     If it is determined in step S 2004  that no rate reduction command has been received from the receiver  120  (no branch from step S 2004 ), processing returns to step S 2003 . If a rate reduction command has been received (yes branch from step S 2004 ), the generator circuit  111  reduces the bit rate of the skew detection signal generated thereby (step S 2005 ). Processing returns to step S 2003 . 
     If it is determined in step S 2003  that a rate reduction command has been received (yes branch from step S 2003 ), the control circuit switch-controls the switch  112  to transmit the data signal (step S 2006 ). A series of process steps are thus complete. The bit rate of the skew detection signal is reduced if the rate reduction command is output from the receiver  120 . 
       FIG. 21  is a flowchart illustrating an exemplary operation of the receiver  120  of the fifth embodiment. Steps S 2101  and S 2102  illustrated in  FIG. 21  may be respectively substantially identical to steps S 1701  and S 1702  illustrated in  FIG. 17 , and the discussion thereof is omitted here. The control circuit of the transmitter  120  determines whether a skew has been detected in step S 2102  (step S 2103 ). If no skew has been detected (no branch from step S 2103 ), the skew detector circuit  123  outputs the rate reduction command to the transmitter  110  (step S 2104 ). Processing then returns to step S 2102 . 
     If it is determined in step S 2103  that a skew has been detected (yes branch from step S 2103 ), processing proceeds to step S 2105 . Steps S 2105  and S 2106  illustrated in  FIG. 21  may be respectively substantially identical to steps S 1703  and S 1704  illustrated in  FIG. 17 , and the discussion thereof is omitted here. The rate reduction command can be output to the transmitter  110  in this way if no skew has been detected by the skew detector circuit  123 . 
       FIG. 22  is a sequence chart illustrating an exemplary operation of the transmission system  100  of the fifth embodiment. Steps S 2201 -S 2203  illustrated in  FIG. 22  may be respectively substantially identical to steps S 1801 -S 1803  illustrated in  FIG. 18 , and the discussion thereof is omitted here. However, it is presumed in this case that the receiver  120  fails to detect a skew in step S 2203  (detection failure). 
     The receiver  120  outputs the rate reduction command to the transmitter  110  (step S 2204 ). The transmitter  110  reduces the bit rate of the skew detection signal to be generated (step S 2205 ), and outputs the skew detection signal with the bit rate thereof reduced (step S 2206 ). The receiver  120  then detects the skew of the skew detection signal transmission in step S 2206  (step S 2207 ). 
     It is assumed that a skew has been detected in step S 2207  (detection success). The receiver  120  sets the skew correction value based on the skew detected in step S 2207  (step S 2208 ). Steps S 2209  and S 2210  illustrated in  FIG. 22  may be respectively substantially identical to steps S 1805  and S 1806  illustrated in  FIG. 18 , and the discussion thereof is omitted here. 
     Referring again to  FIG. 19 , if the skew detector circuit  123  fails to detect a skew in the transmission system  100  of the fifth embodiment, the receiver  120  transmits the rate reduction command to the transmitter  110  to reduce the bit rate of the skew detection signal. The bit rate of the skew detection signal may be automatically reduced if the bit rate of the skew detection signal is not low enough. The skew of the data signal is accurately compensated for. Even if the bit rate of the data signal varies, the skew of the data signal may be accurately compensated for even by automatically reducing the bit rate of the skew detection signal. 
     In accordance with the transmission systems and the transmission methods as described above, the differential state is maintained between the received signals at the receiver by switching the signal from the transmitter from the data signal to the low bit rate signal even if a large delay time difference occurs between the transmission paths of the differential signals. The skew is accurately detected and corrected. The skew of the data signal is accurately compensated for. In addition the above-described embodiments, the following technique is also described. 
     The transmission system and the transmission method provide the advantage that the skew of the differential signals is accurately compensated for. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.