Off-chip 400 Mbps signal transmission: noise reduction using non-resonant lengths and other techniques

Abstract

A 400 Mbps single-ended off-chip link including an open-drain driver and quasi-differential receiver has been designed, simulated, and measured. In this work, the design choices, in particular as related to noise tolerance, are discussed. To reduce the reflections inherent in signal propagation at a 400 Mbps rate, both the data and clock interconnects are restricted to non-resonant lengths. To further improve impedance matching, a daisy-chain termination is evaluated in which the net termination occurs after rather than before, the package pin. Other methods of reducing reflection noise are discussed, such as providing resistor terminations on an SCM or MCM package in order to reduce stub lengths. Measurements indicate good agreement with simulation; when the link is implemented with the noise-reducing features described the measured noise and jitter is sufficiently low for reliable operation at 400 Mbps.