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A Prototype Framework for Knowledge-Based Analog Circuit Synthesis* Abstract 1. Introduction 2. Background

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A Prototype Framework for Knowledge-Based Analog Circuit Synthesis* Abstract 1. Introduction 2. Background A Prototype Framework for Knowledge-Based Analog Circuit Synthesis* Ramesh Harjani, Rob A. Rutenbar and L. Richard Carley Department of Electrical and Computer Engineering Carnl:gie Mellon University Pittsburgh, Pennsylvania 15213 Abstract Analog design is commonly perceived to be one of the most knowledge-intensive of design tasks: the techniques needed to An organization for a knowledge-based analog (circuit build good analog circuits seem to exist solely as expertise in- synthesis tool is described. Analog circuit topologies are vested in individual designers. represented as a hierarchy of functional blocks; a planning This paper describes a knowledge-based framework for an mechanism is introduced to translate performance specifica- analog circuit synthesis tool. Although “knowledge-based” tions between levels in this circuit hierarchy. A prototype im- has come to be synonymous with “rule-based” in CAD ap- plementation, OASYS, synthesizes sized transistor schematics plications, our prototype implementation relies more heavily for simple CMOS operational amplifiers from performance on planning mechanisms than on rule execution. We attack specifications and process parameters, and demonstrates the the behavior-tc+structure portion of the synthesis task; our workability of the a.pproach. goal is to produce circuit schematics including device sizes, from performance specifications for common analog functional 1. Introduction blocks. This approach is motivated by the lack of tools to Design automation ideas from digital VLSI have only support the design of custom analog circuits. In particular, recently begun to migrate into analog circuit design. In part there are emerging semi-custom methodologies to lay out a this reflects the inherent complexities of the analog design given circuit schematic, but as yet no real tools to help design process. But it also reflects the success with which some clas- this schematic of sized interconnected devices from a set of sical analog applications have been supplanted by digital tech- performance specifications. niques; for example, digital signal processing is frequently The paper is organized as follows. Section 2 contrasts used in place of analog filtering. There are limits to this re- the analog and digital domains, and summarizes related syn- placement process, however. Also, even though many of the thesis research. Section 3 clescribes the structure of the transistors in an ostensibly analog circuit such as an analog- framework in detail, and how it is motivated by traditional to-digital converter may actually belong to digital sections of analog design approaches. We introduce the critical role of the circuit, design of the analog sections typically is most hierarchy in analog circuits, and describe methods to structure time-consuming. Moreover, there is large demand for analog and exploit analog design knowledge. Section 4 describes ex- circuitry in system;3 applications such as telecommunications perience with a small prototype implementation called and robotics, where analog interfaces to an external er,viron- ment are coupled with digital processing systems. Hence, OASYS, which designs simple CMOS operational amplifiers. there is growing interest in analog design tools. We analyze the architecture of OASYS, and examine some automatically synthesized circuits it has produced. Finally, Unfortunately, the state of analog synthesis tools is quite Section 5 presents some concluding observations on the primitive in comparison to digital synthesis tools. In the digi- workability of the approach. tal domain, structured abstractions and hierarchy are com- monplace, and are relied upon to make seemingly large syn- 2. Background thesis tasks tractable by breaking them into smaller steps. Before describing the components of a framework for Such abstractions and hierarchy do not currently play a analog synthesis, we review the salient differences between central role in analog design. Analog circuits are still analog and digital design problems, and some previous ap- designed, largely by hand, by experts intimately familiar with proaches to these problems. nuances of the tarl;et application and IC fabrication process. 2.1 Analog Design versus Digital Design - Consider the task of designing a functional block to be * This research was supported in part by the Semiconductor implemented as a single, perhaps large, cell on a VLSI circuit. Research Corporation, and by a grant from the Gould Foundation. The overall high-level synthesis task, for either analog or digi- tal circuits, is to interconnect a set of appropriately designed primitive components (e.g., transistors) to produce the correct behavior for the cell. We informally partition the differences permission to copy without fee all or part of this material is granted between analog and digital design tasks into four categories: provided that the copies are not made or distributed for direct commercial size, hierarchy, process, and performance constraints. We dis- advantage, tbe ACM copyright notice and the title of the publication and cuss each of these below. its date appear, and noi:ice is given that copying is by permission of the Association for Computing Machinery. To copy otherwise, or to The size difference is easily stated: analog circuits tend republish, requires a fw and/or specific permission. to have fewer transistors than digital circuits. However, there 24th ACM/IEEE Design Automation Conference Paper 3.3 42 0 1987 ACM 07:18-100X/87/0600-0042$00.75 is typically much more design time invested in each transis- with precisely matched electrical characteristics. If such tor. A complex analog cell might have 100 transistors, while precision cannot be attained with this process, it is simply im- a digital cell of similar functional complexity might have possible to pursue this circuit approach. Moreover, design of 1000. The difference derives from the assumed functionality individual device characteristics is largely based on of each transistor. In digital circuits, devices tend to work as parameters extracted or predicted for the process. Inaccurate switches, moving 1s and 0s between storage elements at consideration of the effects of a few subtle process parameters known times. Such devices can often have the minimum size can easily compromise the behavior of the circuit. The in- permissible in the process (to switch faster) and are electri- fluence of process is much stronger during device-by-device cally identical in their performance; only those devices on design for analog circuits. critical paths or driving large loads need to have radically dif- Performance constraints on the behavior of analog cir- ferent electrical properties. Analog circuits, on the other cuits also differ radically from those of digital circuits. Digi- hand, often exploit the full spectrum of capabilities exhibited tal circuits are often specified using a behavioral language by individual devices. The electrical characteristics of in- such as ISPS 131, which can capture the dataflow for digital dividual transistors are more carefully designed, and in- quantities moving through functional blocks and storage ele- dividual devices in a circuit may differ substantially. ments. For common analog circuits, the qualitative behavior Moreover, macroscopic properties of the entire circuit, such as is often known implicitly: an A/D converter digitizes con- gain, frequency response, or stability, may hinge on the tinuous signals; a phase-locked loop synchronizes the phase of relationship among currents, voltages, resistances, and different signals. The specification may take the form of a set capacitances controlled by individual devices. Indeed, the of performance parameters that must be achieved, such as generally smaller size of analog circuits is partially a reflection gain, bandwidth, input noise, or phase margin. That these of the difficulty of managing more than several tens of parameters constrain continuous quantities (e.g., voltages or devices, each of which contributes a dozen interacting electri- currents) and typically depend on the careful design of several cal constraints to the synthesis task. mutually interacting devices at potentially different levels of Digital and analog circuits both employ hierarchy, but in the analog hierarchy, all conspire to make the analog syn- substantially different ways. In digital circuits, there is tacit thesis task difficult. agreement on the abstraction levels through which a design must pass: behavioral, functional-block, register-transfer, 2.2 Previous Approaches to Analog Design gate-network, switch-network, circuit, mask. These abstrac- We distinguish here three approaches to tools for analog tions play a central role in the organization of synthesis tools, synthesis tasks: layout-based approaches, parameterized- which usually help translate from one abstract level to the structure approaches, and knowledge-based approaches. next slightly more concrete level. There is not such a well- The layout-based approaches show the most direct in- developed hierarchy for analog circuits, but hierarchy is im- fluence of digital design ideas. Semi-custom analog styles, portant nevertheless. System-level designs, such as analog-to such as transistor arrays (analogous to gate arrays) and digital (A/D) or digital-to-analog (D/A) converters are com- analog standard cells provide a rapid path to silicon for posed of common functional blocks, performing analog func- analog functions already designed to the level of the primitive
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