Steven G. Walker

Bio: Steven G. Walker is an academic researcher from IBM. The author has contributed to research in topics: Equivalent circuit & Partial element equivalent circuit. The author has an hindex of 6, co-authored 7 publications receiving 1266 citations.

First-order incremental block-based statistical timing analysis

Abstract: Variability in digital integrated circuits makes timing verification an extremely challenging task. In this paper, a canonical first order delay model is proposed that takes into account both correlated and independent randomness. A novel linear-time block-based statistical timing algorithm is employed to propagate timing quantities like arrival times and required arrival times through the timing graph in this canonical form. At the end of the statistical timing, the sensitivities of all timing quantities to each of the sources of variation are available. Excessive sensitivities can then be targeted by manual or automatic optimization methods to improve the robustness of the design. This paper also reports the first incremental statistical timer in the literature which is suitable for use in the inner loop of physical synthesis or other optimization programs. The third novel contribution of this paper is the computation of local and global criticality probabilities. For a very small cost in CPU time, the probability of each edge or node of the timing graph being critical is computed. Numerical results are presented on industrial ASIC chips with over two million logic gates.

First-Order Incremental Block-Based Statistical Timing Analysis

Abstract: Variability in digital integrated circuits makes timing verification an extremely challenging task. In this paper, a canonical first-order delay model that takes into account both correlated and independent randomness is proposed. A novel linear-time block-based statistical timing algorithm is employed to propagate timing quantities like arrival times and required arrival times through the timing graph in this canonical form. At the end of the statistical timing, the sensitivity of all timing quantities to each of the sources of variation is available. Excessive sensitivities can then be targeted by manual or automatic optimization methods to improve the robustness of the design. This paper also reports the first incremental statistical timer in the literature, which is suitable for use in the inner loop of physical synthesis or other optimization programs. The third novel contribution of this paper is the computation of local and global criticality probabilities. For a very small cost in computer time, the probability of each edge or node of the timing graph being critical is computed. Numerical results are presented on industrial application-specified integrated circuit (ASIC) chips with over two million logic gates, and statistical timing results are compared to exhaustive corner analysis on a chip design whose hardware showed early mode timing violations

Full-wave PEEC time-domain method for the modeling of on-chip interconnects

Abstract: With the advances in the speed of high-performance chips, inductance effects in some on-chip interconnects have become significant. Specific networks such as clock distributions and other highly optimized circuits are especially impacted by inductance. Several difficult aspects have to be overcome to obtain valid waveforms for problems where inductances contribute significantly. Mainly, the geometries are very complex and the interactions between the capacitive and inductive currents have to be taken into account simultaneously. In this paper, we show that a full-wave partial element equivalent circuit method, which includes the delays among the partial elements, leads to an efficient solver enabling the analysis of large meaningful problems. Applying this method to several examples leads to helpful insights for realistic very large scale integration wiring problems. It is shown in this paper that the impact overshoot, reflections, and inductive coupling are critical for the design of critical on-chip interconnects.

Dealing with inductance in high-speed chip design

Abstract: Inductance effects in on-chip interconnects have become significant for specific cases such as clock distributions and other highly optimized networks. Designers and CAD tool developers are searching for ways to deal with these effects. Unfortunately, accurate on-chip inductance extraction and simulation in the general case are much more difficult than capacitance extraction. In addition, even if ideal extraction tools existed, most chip designers have little experience designing with lossy transmission lines. This tutorial will attempt to demystify on-chip inductance through the discussion of several illustrative on-chip examples analyzed using full-wave extraction and simulation methods. A specialized PEEC (partial element equivalent circuit) method tailored for chip applications was used for most cases. Effects such as overshoot, reflections, frequency dependent effective resistance and inductance will be illustrated using animated visualizations of our full-wave simulations. Simple examples of design techniques to avoid, mitigate, and even take advantage of on-chip inductance effects will be described.

System and method for derivative-free optimization of electrical circuits

Abstract: The present invention is a system and method for optimizing electrical circuits by means of derivative-free optimization. Tunable parameters such as component values, transistor sizes or model parameters are automatically adjusted to obtain an optimal circuit. Any method of measuring the performance of the circuit, including computer simulation, can be incorporated into the optimization technique, with no derivative requirements. An arbitrary continuous optimization problem can be posed, including an objective function, equality and inequality constraints, and simple bounds on the tunable parameters. The optimization technique is efficient and guarantees that it will find a locally optimal solution from any starting point. Further, the procedure includes a method of automatically recovering from electrical failure to enable automatic and productive circuit optimization. A set of measurement widgets is provided to automatically introduce the checking required to recover from electrical failure. The automated circuit optimization leads to higher quality circuits, increases designer productivity, results in a better understanding of the tradeoffs inherent in the circuit and lifts the thinking of the circuit designer to a higher level.

Thermal Modeling, Analysis, and Management in VLSI Circuits: Principles and Methods

Abstract: The growing packing density and power consumption of very large scale integration (VLSI) circuits have made thermal effects one of the most important concerns of VLSI designers The increasing variability of key process parameters in nanometer CMOS technologies has resulted in larger impact of the substrate and metal line temperatures on the reliability and performance of the devices and interconnections Recent data shows that more than 50% of all integrated circuit failures are related to thermal issues This paper presents a brief discussion of key sources of power dissipation and their temperature relation in CMOS VLSI circuits, and techniques for full-chip temperature calculation with special attention to its implications on the design of high-performance, low-power VLSI circuits The paper is concluded with an overview of techniques to improve the full-chip thermal integrity by means of off-chip versus on-chip and static versus adaptive methods

A reconfigurable design-for-debug infrastructure for SoCs

Abstract: In this paper we present a design-for-debug (DFD) reconfigurable infrastructure for SoCs to support at-speed in-system functional debug. A distributed reconfigurable fabric inserted at RTL provides a debug platform that can be configured and operated post-silicon via the JTAG port. The platform can be repeatedly reused to configure many debug structures such as assertions checkers, transaction identifiers, triggers, and event counters.

Keynote Paper Statistical Timing Analysis: From Basic Principles to State of the Art

Abstract: Static-timing analysis (STA) has been one of the most pervasive and successful analysis engines in the design of digital circuits for the last 20 years. However, in recent years, the in- creased loss of predictability in semiconductor devices has raised concern over the ability of STA to effectively model statistical variations. This has resulted in extensive research in the so-called statistical STA (SSTA), which marks a significant departure from the traditional STA framework. In this paper, we review the recent developments in SSTA. We first discuss its underlying models and assumptions, then survey the major approaches, and close by discussing its remaining key challenges.

Statistical Timing Analysis: From Basic Principles to State of the Art

Abstract: Static-timing analysis (STA) has been one of the most pervasive and successful analysis engines in the design of digital circuits for the last 20 years. However, in recent years, the increased loss of predictability in semiconductor devices has raised concern over the ability of STA to effectively model statistical variations. This has resulted in extensive research in the so-called statistical STA (SSTA), which marks a significant departure from the traditional STA framework. In this paper, we review the recent developments in SSTA. We first discuss its underlying models and assumptions, then survey the major approaches, and close by discussing its remaining key challenges.

A clock distribution network for microprocessors

Abstract: 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.