Optimization of custom MOS circuits by transistor sizing

Abstract

Optimization of a circuit by transistor sizing is often a slow, tedious and iterative manual process which relies on designer intuition. Circuit simulation is carried out in the inner loop of this tuning procedure. Automating the transistor sizing process is an important step towards being able to rapidly design high-performance, custom circuits. JiffyTune is a new circuit optimization tool that automates the tuning task. Delay, rise/fall time, area and power targets are accommodated. Each (weighted) target can be either a constraint or an objective function. Minimax optimization is supported. Transistors can be ratioed and similar structures grouped to ensure regular layouts. Bounds on transistor widths are supported. JiffyTune uses LANCELOT, a large-scale nonlinear optimization package with an augmented Lagrangian formulation. Simple bounds are handled explicitly and trust region methods are applied to minimize a composite objective function. In the inner loop of the optimization, the fast circuit simulator SPECS is used to evaluate the circuit. SPECS is unique in its ability to efficiently provide time-domain sensitivities, thereby enabling gradient-based optimization. Both the adjoint and direct methods of sensitivity computation have been implemented in SPECS. To assist the user, interfaces in the Cadence and SLED design systems have been constructed. These interfaces automate the specification of the optimization task, the running of the optimizer and the back-annotation of the results on to the circuit schematic. JiffyTune has been used to tune over 100 circuits for a custom, high-performance microprocessor that makes use of dynamic logic circuits. Circuits with over 250 tunable transistors have been successfully optimized. Automatic circuit tuning has been found to facilitate design re-use. The designers' focus shorts from solving the optimization problem to specifying it correctly and completely. This paper describes the algorithms of JiffyTune, the environment in which it is used and presents a case study of the application of JiffyTune to individual circuits of the microprocessor.