Concern about the performance of wires wires in scaled technologies has led to research exploring other communication methods. This paper examines wire and gate delays as technologies migrate from 0.18-/spl mu/m to 0.035-/spl mu/m feature sizes to better understand the magnitude of the the wiring problem. Wires that shorten in length as technologies scale have delays that either track gate delays or grow slowly relative to gate delays. This result is good news since these "local" wires dominate chip wiring. Despite this scaling of local wire performance, computer-aided design (CAD) tools must still become move sophisticated in dealing with these wires. Under scaling, the total number of wires grows exponentially, so CAD tools will need to handle an ever-growing percentage of all the wires in order to keep designer workloads constant. Global wires present a more serious problem to designers. These are wires that do not scale in length since they communicate signals across the chip. The delay of these wives will remain constant if repeaters are used meaning that relative to gate delays, their delays scale upwards. These increased delays for global communication will drive architectures toward modular designs with explicit global latency mechanisms.

The future of wires

This paper describes an algorithm for generating provably passive reduced-order N-port models for RLC interconnect circuits. It is demonstrated that, in addition to macromodel stability, macromodel passivity is needed to guarantee the overall circuit stability once the active and passive driver/load models are connected. The approach proposed here, PRIMA, is a general method for obtaining passive reduced-order macromodels for linear RLC systems. In this paper, PRIMA is demonstrated in terms of a simple implementation which extends the block Arnoldi technique to include guaranteed passivity while providing superior accuracy. While the same passivity extension is not possible for MPVL, comparable accuracy in the frequency domain for all examples is observed.

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PRIMA: passive reduced-order interconnect macromodeling algorithm

The various arguments for introducing optical interconnections to silicon CMOS chips are summarized, and the challenges for optical, optoelectronic, and integration technologies are discussed. Optics could solve many physical problems of interconnects, including precise clock distribution, system synchronization (allowing larger synchronous zones, both on-chip and between chips), bandwidth and density of long interconnections, and reduction of power dissipation. Optics may relieve a broad range of design problems, such as crosstalk, voltage isolation, wave reflection, impedence matching, and pin inductance. It may allow continued scaling of existing architectures and enable novel highly interconnected or high-bandwidth architectures. No physical breakthrough is required to implement dense optical interconnects to silicon chips, though substantial technological work remains. Cost is a significant barrier to practical introduction, though revolutionary approaches exist that might achieve economies of scale. An Appendix analyzes scaling of on-chop global electrical interconnects, including line inductance and the skin effect, both of which impose significant additional constraints on future interconnects.

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Rationale and challenges for optical interconnects to electronic chips

This paper provides an introductory overview of the Cell multiprocessor. Cell represents a revolutionary extension of conventional microprocessor architecture and organization. The paper discusses the history of the project, the program objectives and challenges, the disign concept, the architecture and programming models, and the implementation.

Introduction to the cell multiprocessor

Brains perform with remarkable efficiency, are capable of prodigious computation, and are marvels of communication. We are beginning to understand some of the geometric, biophysical, and energy constraints that have governed the evolution of cortical networks. To operate efficiently within these constraints, nature has optimized the structure and function of cortical networks with design principles similar to those used in electronic networks. The brain also exploits the adaptability of biological systems to reconfigure in response to changing needs.

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Communication in Neuronal Networks

Short, medium, and long on-chip interconnections having linewidths of 0.45-52 /spl mu/m are analyzed in a five-metal-layer structure. We study capacitive coupling for short lines, inductive coupling for medium-length lines, inductance and resistance of the current return path in the power buses, and line resistive losses for the global wiring. Design guidelines and technology changes are proposed to achieve minimum delay and contain crosstalk for local and global wiring. Conditional expressions are given to determine when transmission-line effects are important for accurate delay and crosstalk prediction.

When are transmission-line effects important for on-chip interconnections?

A global clock distribution strategy used on several microprocessor chips is described. The clock network consists of buffered tunable trees or treelike networks, with the final level of trees all driving a single common grid covering most of the chip. This topology combines advantages of both trees and grids. A new tuning method was required to efficiently tune such a large strongly connected interconnect network consisting of up to 6 m of wire and modeled with 50000 resistors, capacitors, and inductors. Variations are described to handle different floor-planning styles. Global clock skew as low as 22 ps on large microprocessor chips has been measured.

A clock distribution network for microprocessors

This paper reviews the status of present day on-chip wiring design methodologies and understanding. A brief explanation is given of the fundamental transmission-line properties that should be considered for accurate prediction of crosstalk, common-mode noise and clock skew. The deficiencies of RC-circuit representation are highlighted and design guidelines are given for using modeling and simulation techniques that have been previously used for package interconnections. Such techniques are believed to teach designers how to make better use of available technologies and help them architect systems that operate with many-GHz clock rates.

On-chip wiring design challenges for gigahertz operation

The Elmore delay is an extremely popular delay metric, particularly for RC tree analysis. The widespread usage of this metric is mainly attributable to it being the most accurate delay measure that is a simple analytical function of the circuit parameters. The only drawbacks to this delay metric are the uncertainty as to whether it is an optimistic or a pessimistic estimate, and the restriction to step response delay estimation. In this paper, we prove that the Elmore delay is an absolute upper bound on the 50% delay of an RC tree response. Moreover, we prove that this bound holds for input signals other than steps, and that the actual delay asymptotically approaches the Elmore delay as the input signal rise time increases. A lower bound on the delay is also developed using the Elmore delay and the second moment of the impulse response. The utility of this bound is for understanding the accuracy and the limitations of the Elmore delay metric as we use it for design automation.

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The Elmore Delay as a Bound for RC Trees with Generalized Input Signals

It is well understood that frequency independent lumped-element circuits can be used to accurately model proximity and skin effects in transmission lines [7]. Furthermore, it is also understood that these circuits can be synthesized knowing only the high and the low frequency resistances and inductances [4]. Existing VLSI extraction tools however, are not efficient enough to solve for the frequency dependent resistances and inductances on large VLSI layouts, nor do they synthesize circuits suitable for timing analysis. We propose a rules-based method that efficiently and accurately captures the high and low frequency characteristics directly from layout shapes, and subsequently synthesizes a simple frequency independent ladder circuit suitable for timing analysis. We compare our results to other simulation results.

Byron L. Krauter

Papers

When are transmission-line effects important for on-chip interconnections?

A clock distribution network for microprocessors

On-chip wiring design challenges for gigahertz operation

The Elmore Delay as a Bound for RC Trees with Generalized Input Signals

Layout based frequency dependent inductance and resistance extraction for on-chip interconnect timing analysis