A 3x9 Gb/s Shared, All-Digital CDR for High-Speed, High-Density I/O

This paper presents a novel all-digital CDR scheme in 90 nm CMOS. Two independently adjustable clock phases are generated from a delay line calibrated to 2 UI. One clock phase is placed in the middle of the eye to recover the data (“data clock”) and the other is swept across the delay line (“search clock”). As the search clock is swept, its samples are compared against the data samples to generate eye information. This information is used to determine the best phase for data recovery. After placing the search clock at this phase, search and data functions are traded between clocks and eye monitoring repeats. By trading functions, infinite delay range is realized using only a calibrated delay line, instead of a PLL or DLL. Since each clock generates its own alignment information, mismatches in clock distribution can be tolerated. The scheme's generalized sampling and retiming architecture is used in an efficient sharing technique that reduces the number of clocks required, saving power and area in high-density interconnect. The shared CDR is implemented using static CMOS logic in a 90 nm bulk process, occupying 0.15 mm². It operates from 6 to 9 Gb/s, and consumes 2.5 mW/Gb/s of power at 6 Gb/s and 3.8 mW/Gb/s at 9 Gb/s.