(1) Field of the Invention
The present invention relates to a skew compensation method and a skew compensation apparatus for automatically compensating for skew occurring among a plurality of serial transmission paths used for high-speed signal transmission with parallel-to-serial conversion.
(2) Description of the Related Art
Parallel-to-serial transmission is a technology which serializes a parallel signal by time division multiplexing, transmits the resultant serial signal, and then parallelizes the serial signal again at a receiver. The technology allows the elimination of the problem of bit-to-bit skew which is conspicuous when the rate of parallel transmission is increased. To implement parallel-to-serial transmission in a broader band, it is necessary to increase the rate of serial transmission, but the acceleration of serial transmission is limited. For broader-band parallel-to-serial transmission, therefore, an approach of using a plurality of serial signals in parallel has been proposed. However, the approach entails the occurrence of skew between the serial signals, similarly to the parallel transmission.
It is inherently possible to suppress the skew by installing each of the plurality of transmission paths over an equal distance. Due to reduced margin resulting from higher-speed signal transmission, however, it is difficult to install all the lines for serial signals such that they have equal lengths. In particular, tremendous skew occurs in proportion to a transmission distance in the case of WDM (Wavelength Division Multiplexing) used for optical communication. In the WDM, a plurality of, serial signals are transmitted in parallel by utilizing the property of a single optical fiber which carries a plurality of optical signals with different wavelengths. In this case, however, the different wavelengths cause tremendous skew irrespective of equal transmission distances.
Japanese Unexamined Patent Publication No. 2000-341135 discloses a technology which transmits a test pattern in equiphase from each of the transmitters for serial signals to eliminate skew occurring among a plurality of serial signal transmission paths. In accordance with the invention disclosed in the precedent document, each of receivers sends the timing of a synchronization signal to a controller. The controller sets the delay line of the transmitter with the most delayed one of the received timings and thereby matches the reception timings at the receivers.
However, since the foregoing method sends a test pattern by switching normal data to the test pattern, the skew cannot be monitored during the transmission of the normal data so that a sudden skew shift is beyond handling.