Pattern independent maximum current estimation in power and ground buses of CMOS VLSI circuits: Algorithms, signal correlations, and their resolution

Currents owing in the power and ground (P&G) buses of CMOS digital circuits a ect both circuit reliability and performance by causing excessive voltage drops. Excessive voltage drops manifest themselves as glitches on the P&G buses and cause erroneous logic signals and degradation in switching speeds. Maximum current estimates are needed at every contact point in the buses to study the severity of the voltage drop problems and to redesign the supply lines accordingly. These currents, however, depend on the speci c input patterns that are applied to the circuit. Since it is prohibitively expensive to enumerate all possible input patterns, this problem has, for a long time, remained largely unsolved. In this paper, we propose a patternindependent, linear time algorithm (iMax) that estimates at every contact point, an upper bound envelope of all possible current waveforms that result by the application of di erent input patterns to the circuit. The algorithm is extremely e cient and produces good results for most circuits as is demonstrated by experimental results on several benchmark circuits. The accuracy of the algorithm can be further improved by resolving the signal correlations that exist inside a circuit. We also present a novel partial input enumeration (PIE) technique to resolve signal correlations and signi cantly improve the upper bounds for circuits where the bounds produced by iMax are not tight. We establish with extensive experimental results that these algorithms represent a good time-accuracy trade-o and are applicable to VLSI circuits.