Gate-size selection for standard cell libraries

The paper presents an algorithm to select a good set of gate sizes for the primitive gates of a standard cell library. A measurement error on a gate is defined to quantify the discrepancy resulting from replacing the size required by a synthesis sizing algorithm with a size available in a discrete cell library. The criterion for gate size selection is a set of gate sizes that minimizes the cumulative error of a prescribed measurement. Optimal solutions to the gate size selection problem targetting size and delay measurements are presented for cases when the probability distribution and the delay equations are simple. A realistic probability distribution is obtained using a sample space of gates derived from a group of designs that is synthesized under the semi-custom synthesis methodology (K. Shepard et al., 1997). A "delay match" (minimizing delay error) and a "size match" (minimizing size error) set of gate sizes are obtained numerically and are subsequently realized as discrete cell libraries. The previous group of designs are synthesized using the two selected cell libraries and two other cell libraries, one with "equal spacing" of cell sizes and the other with "exponential spacing" of cell sizes. The "size match" library gives the best overall slack and area results.