Data Handling Limits of On-Chip Interconnects

Abstract

With shrinking feature size and growing integration density in the deep sub-micrometer (DSM) technologies, the global buses are fast becoming the ldquoweakest-linksrdquo in VLSI design. They have large delays and are error-prone. Especially, in system-on-chip (SoC) designs, where parallel interconnects run over large distances, they pose difficult research and design problems. This paper presents a two-fold approach for evaluating the signal and data carrying capacity of on-chip interconnects. In the first approach, the wire is modeled as a linear time invariant (LTI) system and a frequency response is studied. The second approach addresses delay and reliability in interconnects from an information theoretic perspective. Simulation results for an 8-bit-wide bus in 0.1-mum technology are presented for both approaches. The results closely match to a similar optimal bus clock frequency that will result in the maximum data transfer rate. Moreover, this optimal frequency is higher than that achieved by present day designs which accommodate the worst case delays. The first approach achieves this higher transmission rate using ideal signal shapes, instead of square pulses, while the second approach uses coding techniques to eliminate high delay cases to generate a higher transmission rate. It is seen that the signal delay distribution has a long tail, meaning that most signals arrive at the output much faster than the worst case delay. Using communication theory, these ldquogoodrdquo signals arriving early can be used to predict/correct the ldquofewrdquo signals that arrive late.