Khaled R. Heloue

Efficient block-based parameterized timing analysis covering all potentially critical paths

In order for the results of timing analysis to be useful, they must provide insight and guidance on how the circuit may be improved so as to fix any reported timing problems. A limitation of many recent variability-aware timing analysis techniques is that, while they report delay distributions, or verify multiple corners, they do not provide the required guidance for re-design. We propose an efficient block-based parameterized timing analysis technique that can accurately capture circuit delay at every point in the parameter space, by reporting all paths that can become critical. Using an efficient pruning algorithm, only those potentially critical paths are carried forward, while all other paths are discarded during propagation. This allows one to examine local robustness to parameters in different regions of the parameter space, not by considering differential sensitivity at a point (which would be useless in this context) but by knowledge of the paths that can become critical at nearby points in parameter space. We give a formal definition of this problem and propose a technique for solving it that improves on the state of the art, both in terms of theoretical computational complexity and in terms of run time on various test circuits.

Full-Chip Model for Leakage-Current Estimation Considering Within-Die Correlation

In this paper, we present an efficient technique for finding the mean and variance of the full-chip leakage of a candidate design, while considering logic structures and both die-to-die and within-die (WID) process variations, and taking into account the spatial correlation due to WID variations. Our model uses a ldquorandom-gaterdquo concept to capture high-level characteristics of a candidate chip design, which are sufficient to determine its leakage. These high-level characteristics include information about the process, the standard cell library, and expected design characteristics. We show empirically that, for large gate count, the set of all chip designs that share the same high-level characteristics have approximately the same leakage, with very small error. Therefore, our model can be used as either an early or a late estimator of leakage, with high accuracy. In its simplest form, we show that full-chip-leakage estimation reduces in finding the area under a scaled version of the WID channel length autocorrelation function, which can be done in constant time.

Efficient Block-Based Parameterized Timing Analysis Covering All Potentially Critical Paths

In order for the results of timing analysis to be useful, they must provide insight and guidance on how the circuit may be improved so as to fix any reported timing problems. A limitation of many recent variability-aware timing analysis techniques is that, while they report delay distributions, or verify multiple corners, they do not provide the required guidance for re-design. We propose an efficient block-based parameterized timing analysis technique that can accurately capture circuit delay at every point in the parameter space, by reporting all paths that can become critical. Using an efficient pruning algorithm, only those potentially critical paths are carried forward, while all other paths are discarded during propagation. This allows one to examine local robustness to parameters in different regions of the parameter space, not by considering differential sensitivity at a point (that would be useless in this context) but by knowledge of the paths that can become critical at nearby points in parameter space. We give a formal definition of this problem and propose a technique for solving it, which improves on the state of the art, both in terms of theoretical computational complexity and in terms of runtime on various test circuits.

Quantifying robustness metrics in parameterized static timing analysis

Process and environmental variations continue to present significant challenges to designers of high-performance integrated circuits. In the past few years, while much research has been aimed at handling parameter variations as part of timing analysis, few proposals have actually included ways to interpret the results of this parameterized static timing analysis (PSTA) step. In this paper, we propose a new post-variational analysis metric that can be used to quantify the (timing) robustness of designs to parameter variations. In addition to helping designers diagnose if and when different nodes can fail, this metric can guide optimization and can give insights on what to fix, by identifying nodes with small robustness values and proceeding to fix those nodes first. Inspired by the rich literature on design centering, tolerancing, and tuning (DCTT), we use distance as a measure for robustness. Our analysis thus determines the minimum distance from the nominal point in the parameter space to any timing violation, and works under the assumption that parameters are specified as ranges rather than statistical distributions. We demonstrate the usefulness of this distance-based robustness metric on circuit blocks extracted from a commercial 45 nm microprocessor.

Early Analysis and Budgeting of Margins and Corners Using Two-Sided Analytical Yield Models

Manufacturing process variations lead to variability in circuit delay and, if not accounted for, can cause excessive timing yield loss. The familiar traditional approaches to timing verification, such as the use of process corners and predefined timing margins, cannot readily handle within-die variations. Recently, statistical static timing analysis (SSTA) has been proposed as a way to deal with variability. Although many powerful techniques have been proposed, the fact that SSTA requires a significant change of methodology has delayed its wide adoption. In this paper, we propose a framework whereby the familiar concepts of corners and margins, which are generally meaningful at the transistor or cell level, are elevated to the chip level in order to handle within-die variations. This is achieved by using high-level models, such as the generic path model or the generic circuit model with different classes of paths, to represent the behavior of typical designs. These models allow us to determine ldquoyield-specificrdquo margins (setup and hold margins) and virtual corners, which, if applied during standard (deterministic) timing analysis, would guarantee the desired yield. Our framework can be used at an early stage of circuit design and is consistent with traditional timing verification methodology.

Effect of statistical clock skew variations on chip timing yield

Integrated circuit design with sub-100nm technology requires closer attention to the effect of process variations on circuit timing. In a previous work, we had developed a method of statistical timing analysis in which the effect of process variations on circuit timing is assessed, given a generic logic path in a target design technology. In this work, we extend that previous work in an important way by incorporating into the analysis the effect of clock skew. The resulting model captures both die-to-die and within-die process variations, in both logic and clock paths, it handles within-die correlation using principal component analysis, and it leads to an expression for the resulting timing yield. Among other uses, this allows one to compute how much reduction one will see in the timing yield, for a given clock skew variance.

Clock skew optimization via wiresizing for timing sign-off covering all process corners

Manufacturing process variability impacts the performance of synchronous logic circuits by means of its effect on both clock network and functional block delays. Typically, variability in clock networks is either handled early in the design flow by assigning margins to clock network delays, or at a later stage through post-processing steps that only focus on achieving minimal skew, without regard to functional block variability. In this work, we present a technique that alters clock network lines so that the circuit meets its timing constraints at all process corners. This is done near the end of the design flow while considering delay variability in both the clock network and the functional blocks. Our method operates at the physical level and provides designers with the required changes in clock network line widths and/or lengths. This can be formulated as a linear programming (LP) problem, and thus can be solved efficiently. Empirical results for a set of ISCAS-89 benchmark circuits show that our approach can considerably reduce the effect of process variations on circuit performance.