A Spread Spectrum Clock Generator For SATA With Programmable Triangular Modulator

Abstract

As external storage devices are widely used, high-speed serial links connection host and external devices are becoming popular. Serial AT Attachment (SATA.) is one of the most promising technologies providing large bandwidth up to 3Gb/s with possible extension to 6Gb/s in the near future. As operating a high speed data rate, currents and voltages present in the circuit and the signal traces lead to great Electro-Magnetic Interference (EMI), Therefore, SATA systems require a wide spreading of 5000ppm and a 30~33KHz modulation rate to reduce EMI. Spread spectrum is a special frequency modulation technique, which is sacrificed the signal integrity of the clock to reduce the EMI. In this thesis, we proposed a novel spread spectrum clock generator (SSCG) to release the trade-offs between the jitter performance and the amount of EMI reduction for SATA appications. Besides considering the amount of EMI peak reduction in frequency-domain, it is more important to take care of the impact in time-domain for SSCGs since the signal integrity in time-domain is a very significant issue for serial data transmissions. This chip is implemented in the process of 0.18um CMOS. In chapter 4 we proposed a programmable triangular generator technique for SSCG (Down spread 2500ppm~ 5000ppm). The technique utilizes the digital data stored in up/down counter to control the spread spectrum amount. An integer-N frequency synthesizer (low frequency) supplies the up/down counter clock to maintain the modulation frequency. Therefore we can adjust spread spectrum amount dynamically to achieve an optimal value between EMI reduction amount and the performance of jitter.