Report Number: CSL-TR-98-775
Institution: Stanford University, Computer Systems Laboratory
Title: Design of High-Speed Serial Links in CMOS
Author: Yang, Chih-Kong Ken
Date: December 1998
Abstract: Demand for bandwidth in serial links has been increasing as the communications industry demand higher quantity and quality of information. Whereas traditional gigabit per second links has been in bipolar or GaAs, this research aims to push the use of CMOS process technology in such links. Intrinsic gate speed limitations are overcome by parallelizing the data. The on-chip frequency is maintained at a fraction (1/16) of the off-chip data rate. Clocks with carefully controlled phases tapped from a local ring oscillator are driven to a bank of input samplers to convert the serial bit stream into parallel data. Similarly, the overlap of multiple-phased clocks are used to synchronize the multiplexing of the parallel data onto the transmission line. To perform clock/data recovery, data is further oversampled with finer phase separation and passed to digital logic. The digital logic operates upon the samples to detect transitions in the bit stream to track the bit boundaries. This tracking can operate at the cycle rate of the digital logic allowing robustness to systematic phase noise. The challenge lies in the capturing of the high frequency data stream and generating low jitter, accurately spaced clock edges. A test chip is built demonstrating the transmission and recovery of a 4.0-Gb/s bit streams with < 10 (minus superscript 14) bit-error rate using a 3x oversampled system in a 0.5-um MOSIS CMOS process.